UKH

If you could only take one camera and one lens on a trip, what would your choose?

For me it's the Olympus OMD EMI MKII + 17mm prime.

Pretty simple for me as its what i have and what i use, Panasonic Lumix G9 with the Panasonic Leica 12-60, although i could be swayed by the 12-35 as its a bit smaller and constant aperture.

I have a similar camera to yours, the GH5. I have the 12-35 and 35-100 lenses.

I probably use the 35-100 lens maybe once a year while the 12-35 is practically glued to my camera.

If I need to zoom in, then I use my 100-400mm lens. The 35-100 is just redundant. If you have a zoom lens then you usually either have it wide open or you have it zoomed in, which means that the 100-400 does the job of the smaller lens.

If I had to choose one lens only then the 12-35 would be my workhorse/walk-around choice.

With so many different cameras and sensor sizes, would it be a good idea to have this discussion having a level field to compare focal lengths from different sensor sizes?

What trip; Font? Alps? Masai Mara safari? Caribbean scuba?

"Camera" Landsat 5

"Lens" Thematic Mapper

Not sure if this really counts, but it has taken a lot of my favourite images.

https://www.usgs.gov/media/images/roof-world

https://www.usgs.gov/media/images/impact

https://www.usgs.gov/media/images/landsat-5-image-showing-seattle-washingto...

Must’ve read my mind. I’m going for a wee wander tomorrow and can’t decide between my Sony 18-135, 70-350 or Sigma 56.  The 56 is by far my favourite lens but I’m thinking my location might give me some landscape or wildlife opportunities. They’d all be on my recently purchased A6400, which I’m loving.

If it was a once in a lifetime trip somewhere, Yosemite maybe, and I absolutely had to get the shot, it would be my Canon 1DX with my 16-35mm f2.8 lens. If it was somewhere I could go back to easily, Torridon boulders say, then it would still be my 1DX. I love shooting with it.

I have been using a Sony RX10 for a couple of years now: 24-600mm lens and no opportunity to get dust on the sensor - can't fault it!

Chris

Fuji XT-4 w/23mm prime

does it have to be digital? 

How would you implement this “level field” into the “discussion” exactly? 

I think he means full frame equivalent focal length.

E.g. a 25mm micro 4/3rds would be 50mm full frame, a 23mm APSC would be (approx) a 35mm full frame etc....

X-T1 and the 18-55mm for versatility.

My favourite lense is my 35mm F2 though.

Canon EOS 80d and their 70-200 F/4 L.  Fallback would be a good 50 mm prime.

I can always stitch landscapes back together when I get home...

I'll assume the one lens is a prime and not a zoom, so I'd probably go for a Canon R5 with an EF100L Macro f2.8. I own the latter but not the former. 
Though I'd be tempted by a Fuji GFX50R with the GF50 f3.5 for portability and IQ.

Given that anything from wide landscapes to close-up wildlife, but also want to be able to carry stuff very lightly my compromise 1-camera-1-lens setup for traveling is my Olympus em5 mark iii  + Olympus 12-200 superzoom. 

Quality is good enough for me and it suits my range and weight/bulk requirements.

I used to have an Olympus OM1 (analogue) on which the 28mm lens was more or less a fixture. I’d had it so long the shutter & aperture controls were (are) completely instinctive to me. And it still works without a battery. So if I could replicate that in the digital age I’d be like a pig in ****. Zoom lenses give you another unnecessary decision, more weight and less definition.

I assume you are aware of the Fuji X series cameras? Wonderful tactile controls for those of us who miss our old manual film SLRs. Some lovely primes for them too and the IQ is good.

This doesn't account for other factors like depth of field for a given field of view. To be honest I thought that a thread like this would be "nerdy" enough that people would just "know" and not get confused by the physical focal lengths  

Thanks for the hint. I don’t really keep up to date, I have  a Pentax K7 that I’ve never completely bonded with so I’ll have a look at those.

If I'm taking a camera then I'll have a pretty good idea of what I want to photograph.

So it might be  G9 with Leica 100-400 (small wildlife, good light), or a 6d with 70-200mm f2.8 (big wildlife, poor light), or 6d with Sigma Art 35 f1.4 (just 'cos I like the lens) or G9 with Leica 12-60 (winter sun climbing in Spain). 

Totally depends on what sort of 'trip' we're talking about.

Leica rangefinder & 50mm lens?

Good enough for Henri Cartier Bresson!

The people that actually printed from his negatives may have preferred him to have had a different set-up

Viewfinder versus darkroom?

We could all probably do with composing our shots better rather than relying on post processing

No. Viewfinder is utterly irrelevant to this. Assume perfect focus, composition and timing, otherwise the printing wouldn't be happening in the first place. 

Lazy metering, requiring massive salvage work in the darkroom (dodging, burning, contrast filters etc) was the bane of those who printed H C-B, apparently (just a vague recollection of reading something somewhere sometime - sorry I can't provide a link)

How does post-processing improve composition (aside from a teeny bit of crop, rotate and "correcting" so-called distortion like converging verticals, all of which would be rather limited? 

If it was a landscape/general photography trip (rather than climbing) and money were no object (i.e. someone else was paying for it!), the Fujifilm GFX100. Probably with the 32-64mm lens.

17mm? Your images would all be samey. Why not a zoom 16-35mm, or 18-200mm or similar You'd get a greater variety of images with an option of focal lengths, from grand landscapes, portraits, to quasi macro.

Similarly many of my best pictures were taken on my old fully mechanical and manual Pentax MX film SLR with 28mm prime lens.

The best camera you have is the one you have with you. Some of the responses with full frame dslrs and/or bulky long range zooms would defeat this for taking on long mountain trips or routes. For me it would be my outstanding and compact Pentax Limited 20-40mm Zoom mounted on a compact Pentax dslr  body (can’t do without optical viewfinder) currently K-S2 but soon a KP body. However the Ricoh GR III APS-C pocket camera with fixed 28mm lens would be a tempting and worthy successor to the Pentax MX/Olympus OM1 and prime wide angle lens.

Nikon D850 with 24-70 f2.8.  There is not much you can't do with it - bit it is a bit heavy!

So true. I have some great memory photos, and some OK pictures with my phone. The only thing possible when running

True, but it boils down to what you are prepared to carry for your 'art'. I've been out running on the moors with a G9 with 100-400 in a 15L pack when I had hopes of spotting a rare bird. It's less pack weight than on a mountain marathon. 

I almost always have my phone camera with me. It is crap.

Yes, if you're only taking one lens, it would seem odd to me not to take a zoom unless you had one particular subject in mind.

If I were only taking one lens with my X-T2, I'd struggle to choose between my 18-55 and 55-200. I might have to splash out on the 18-135.

wow, that looks really impressive! as if these are pictures of another planet, very exciting and beautiful!
especially the first shot, the Tibetan plateau, huh? unfortunately I've never seen anything like it in my life, but I hope to fix it!

A fast 50mm prime. 

A fast 50mm prime. 

Difficult.  I’m mostly taking shots of my young daughter nowadays and my go to is a canon 85mm 1.4L on my 5D Mk IV.  A great portrait lens and quite flexible if you’re prepared to use your legs.... As a walk about I use a 24-70mm 2.8L but it’s not the same as a prime. 

Similar thing here! I have spent some time getting the right setup for run-photography. M3/4 camera with zoom lense, GorillaPod style tripod, and sometimes even a Mavic Mini drone.  Makes it possible to travel fast to some off the beaten track sunrise spot, throw on a windproof, and set up for photography. Just don't expect to beat any strava records. 

Thoroughly enjoying the Sigma 24-35 f2 bolted on to a 1DX at the moment. Having a zoom (albeit limited range) and the wide aperture is great fun.

I politely disagree (with this comment and with others who seem to be demanding that it must be a zoom). I interpreted the OP as asking what we would PERSONALLY like, rather than what we would recommend generally. And "pleasure of use" comes into this a bit. 

I had a three week road trip (rental cars) in Canada and the USA (tiny pockets thereof!) in September 2019. I took my Canon EOS 600D, the Canon 18-135mm, a Sigma 55-200mm cheapie, and a Canon 24mm f/2.8 "pancake" lens. 
By far the most enjoyable one in use was the 24mm prime. Maybe I am only saying this because it was light and compact, making a neat set up and not feeling so much like "middle aged punter wandering around pretending to be a photographer", and I haven't gone and checked the albums from this trip to see how many of the photos I liked were made with this lens, but if I had to choose only one camera and lens for "a trip", this combination is likely. 
The lack of zoom can make things more interesting sometimes; there are plenty of photography articles discussing exactly this. 
On my work jollies to Italy I've several times only taken a prime, and enjoyed working around the "limitations". 

from a purely personal point of view I'm very happy with my Canon Eos 70D with an EF-L 24-105 zoom for general walking round photographing random stuff. I very rarely use any other lenses unless I'm doing something very specific like macro or zooming closer on birds than the 105 will go. 

I am sure that there can be interest in working round limitations, but I'd rather not be limited in the first place and put my efforts into other aspects of photography than working round self-imposed shortcomings!

That was my point, Robert. That's you, and that's fine.  It doesn't have to apply to everyone, and nor does my choice.

Nikon F2AS and 50mm f/1.2 or 55mm f/3.5 macro for enjoyment of use.

One of the Fujifilm X-T bodies with 16-55mm f/2.8 for practical purposes.

Is there any point in very fast primes in these days of high ISO low noise sensors unless you really need very shallow depth of field?  It's not as if we have to deal with Kodachrome 25 or 64 any more.  I'm a fan of prime lenses (Pentax limited) which are great optically without the bulk of having excessively wide apertures.

Well, my most used lens is an f2 135mm (often used wide open), so I would argue YES! No, I don't care about depth of field, I'm just interested in collecting as many photons as possible. I like photons - they're much better than electrons (spawn of the devil, most of them are a waste of space). As for noise reduction algorithms, don't get me started!

No prizes for guessing what I photograph.

If you have a very clear vision of the picture you want to capture, then you may well be right. But sometimes putting some soft road-blocks in the 'easy' way is a good way of expanding your vision and seeing that there are other ways of pursuing your art than repeating what worked well yesterday and the day before and the day before that. It helps to see subject differently and to get a different perspective. Refreshing, even.

One of the reasons I enjoy photography is that even when I don't have a camera in my hand, I look at the world in different ways and see things I hadn't seen before. For me, photography is a means to and end and the end is NOT the photograph.

Interesting comment which funnily enough ties into my above example of my Canada/US trip. I went to a small concert (the band Ex Hex) in San Francisco and I usually photograph my concerts but Inhad no idea what the venue policy on cameras might be, so to keep things discreet I just took the Canon with the 24mm lens. I am used to cropping in pretty tight on performers but this wasn’t an option in this case, and I actually enjoyed getting a series of pictures pretty different to what I normally have. There was a woman right at the front at this show, with a fancy fast zoom, and she was pissing about with ALL THE TIME, she must have had so many close ups of fingers on guitar strings etc. To each their own and all that, but I think I enjoyed the actual concert more than she did 😃

a few iPhone pics in this gallery too

https://www.flickr.com/gp/blue-straggler/rJ52i8

I see it the other way round. If I knew the only shot I wanted, I would take the appropriate prime (hypothetical because I can't justify the cost of owning any!). But even when I do have a shot in mind, very often the best shot turns out to be something else so I want the flexibility of the three zooms I normally carry when focussed on photography.

I agree absolutely. Since I got into photography, I definitely see things differently and in new ways. Often something that I could frame with my telephoto zoom! A friend accused me on a trip of not seeing things she saw, because I was always thinking of a photo - I argued that I saw more.

I certainly enjoy the process of being in the mountains for photography as much as the end result of a photo.

But even with minimum iso of 100 on many cameras you will need much less light than used to be the case for good quality films. With your fast lens wide open the number of photons will be limited by automatic or manual exposure control of shutter speed. Most fast lenses are only at their best when stopped down, eg from f1.8 to f4. 

Noise reduction algorithms are destructive but only come into play at very high ISO and don’t apply if shooting in RAW when noise is indiscernible at say iso 400. 

I agree that good lenses are very important but this does not correspond to ever wider apertures. Pentax Limited Prime lenses are a good example

By the way I get the hint that you still enjoy shooting on film rather than digital and therefore may actually need fast lenses

Good try, but no. Haven't use film in decades. But I can see how you might have got there ("electrons, spawn of the devil"). No, my liking for fast primes like the Samyang 135mm f2 is for astrophotography. Most electrons I get in my bucket are noise - of one sort or another -  and I spend a lot of effort sifting through them to find the few that actually carry useful data (from those nice photons).

Not my experience, sadly. I do shoot a fair bit of wildlife (mainly birds), so my system of choice is and M4/3 camera (G9) with a longish zoom (100-400), mainly at 400mm f6.3. Portability and image stabilisation are really important (no tripod). I'm typically using ISO800-1600 depend on the situation and I'm constantly treading a fine line between noise reduction being necessary and damaging. The G9 has very good in-camera noise reduction (generally better than what I can do in processing a RAW file), but it's still a tricky compromise. I think I would have preferred a 400mm f5.6 prime just so I could get the ISO down a notch (or exposure time down), assuming it would be about the same weight/size. Fortunately it doesn't exist, so I don't worry about it.

It can be whatever you want

How is the EM5 MKIII? I've got a EM5 MKII and was thinking of upgrading. 

Depends on what I'm shooting. I shoot a lot of photojournalism stories and found when arranging photostories online or in print shooting on a single lens provides a level of consistency for the narrative. Plus, I hate faffing with loads of kit! 

I didn't have the old version, so cannot compare directly. One feature that has probably improved is image stabilization. I can do 1-2 second handheld exposures. If this is a desirable feature for you, it might be worth the upgrade. For me, it's very nice to have since it means I can get away without a tripod for many things that I like photographing.
There are also a lot of new in-camera processing presets, which I never use. Video has also improved a lot, I am told.

The menu system is a bit tricky to navigate. I think I will never get used to it. Otherwise, it is a very nice little camera. 
 

I've got a film OM-2 with a 50mm I love. Shame it's so bloody hard to find a 35mm to fit!

I like a prime as I use my feet to move rather than zooming in. I have zooms and use them when required, but enjoy having one less thing to think about it when the composition is what I really want. 

That's Olympus menus for you!

Nice catch-phrase, but it really doesn't make sense. The perspective with a 50mm @ 5m is quite different than a 200mm at 20m.

Can be a bit impractical with mountains!

I fail to see how a prime improves composition - it just narrows the choice.

Dare I suggest that you just agree to disagree, and let people state their personal preferences without constantly arguing and banging on and on about zoom lenses?

Dare I suggest you let me make reasonable points in this discussion without you accusing me of "banging on". Obviously it is about personal preferences, but I am interested in discussing and in people explaining those preferences.

simple.

It makes you think more about composition, the element often missing from many images, and of course I'm not citing the excellent pictures in your gallery here.  It's just why for some people and types of photography primes have an advantage over zooms.

Unless you have a very expensive zoom the multiple elements can lead to increased flare etc which can impair image quality particularly in difficult mountain lighting compared to a good prime lens or small selection of primes.

Good quality zooms are usually bulky and heavy, for a day in the mountains I will have a good idea of the lens(s) I will need and pack accordingly 

Having said that I have a very good 100-300mm f4 zoom which is big and bulky but well worth it for climbing shots (not taken when I’m actually climbing obviously)

Fair enough, but I'm still struggling to see how it does that. Is it that it limits the number of good compositions in a situation and so makes you work harder to find them?

Yes, I realise that primes do sometimes improve image quality - a trade off for flexibility.

Fair enough, if you know. I tend to not know and like to be ready for the unforeseen. 

I don't see why, I think zooms make thinking about composition very immediate and encourage you to think about it as much as a prime. That is my experience anyway.

Really good zooms are more affordable now though and offer IQ that meets the needs of the vast majority of people [me anyway!].

Not really - I have a Fuji zoom and the IQ is great [I think] and it's not particularly big or heavy. 

I do love a prime lens. But I think there is a lingering snobbery [not saying from you] about primes as compared to zooms - 'proper' photographers use primes - but I don't think it is justified any longer. 

I've an M6 and a 35mm 1.4 Summilux. Perfection.

If “discussing” means “repeatedly telling people they are wrong”, sure 😃

I have found that most wildlife is just more patient when zooming in with my lens instead of with my feet

I would also be worried about zooming of a cliff.

Different people are giving different opinions and listening to others' opinions. Some opinions might even be changed. That is how discussion works. Nobody is telling anybody they are wrong FFS.

Sorry Robert FFS

That 85mm 1.4L is a belter of a lens. Only problem is since I got it I think all the other lenses I have are very average in comparison

That's the problem with a lovely lens - make you greedy for others!

I would take my Canon EOS R5 that bits easy but the lens is a lot harder. I think I would go with the  Canon RF 24-240 its not the best lens but pretty versatile lens for a trip. 

Thinking about this, the one prime lens I am tempted to get at the moment is the Samyang 12mm F2 for nighscapes. At the moment I have been using the Fuji XF 10-24 at 10mm and its max aperture of F4. I wonder if anyone who knows a bit about astrophotography (I don't really!) could advise me on whether the shorter exposures or lower ISO I could use with the Samyang should significantly improve results (especially if they have experience of using the Samyang lens - it seems to get pretty good reviews, though some queries over quality control). 

I've only just started to get into astro-photography in the last few months but i can give you the benefit of what i've learned so far.

My interest is taking pictures of galaxies, nebulae etc through my eight inch telescope using a DSLR (a canon 1200d).

There is considerable scope to vary exposure time and ISO.  If you use a low ISO, say 100, exposure time will be long with stars trailing (unless you use a guided mount).  This issue will reduce as you increase ISO, so why not just increase ISO?

The issue is signal-to-noise ratio (SNR).  Astro-photographers play around with exoosure time and ISO to try and maximise the SNR so that faint objects don't disappear into the bacground noise.  The noise comes from the electronics in the sensor.  Apparently,  the sweet spot for my camera is around ISO 400-1600.

ISO also affects the dynamic range of the sensor, which is another reason to avoid very high ISO.

Astro-photographers also play around with light frames, dark frames and stacking multiple frames to improve SNR but that may be going a bit off-piste for your needs.  There's lots of stuff on the Internet about it.

I have the Samyang14mm, which I guess will be similar (but different). Overall I'm pretty happy with it, but it definitely has it's flaws. Fortunately I can work round them.

Focusing is challenging. The optimal focus point (for infinity) varies wildly with aperture. Bear in mind focusing on stars is very unforgiving - you don't just get a bit of blur, you'll get either a hard edged disc (horrible) or a doughnut (really horrible) depending on which way you are off. I spent some time measuring the optimal focus points at f2.8, f4 and f5.6 and have marked then on the barrel. So far so good. The other nastiness is hysteresis in the focus control: The focus point are quite different depending on which direction you move the barrel. In my case I have a system whereby I always move to the focus point TOWARDS infinity. The upside is that once you've figured all this out, it works pretty reliably.

Optically the lens is brilliant. Wide open I get excellent sharp stars (FWHM < 3px) in the centre and a bit of astigmatism in the far corners, but it's perfectly printable (without showing the distortion at A2). At f5.6 (with I rarely bother with) it's pin sharp across the whole sensor.

Star trails - personally I don't like them, so maximum exposure time is limited if you don't use a tracker. 30s is the absolute limit before trailing becomes obvious (in an A2 print). In the peak district, light pollution will limit me to 10-20s (ISO1600 on a 6d). In the Outer Hebrides you could expose for minutes, but you really have to have a tracker.

Another thing to bear in mind is that ultra-wide astrophotography is really dependent on foreground. Once you have a good wide-angle shot of the Milky Way, they're all much of a muchness. The interest is in the foreground. It more about nighttime landscapes than astrophotography (which is a completely different world of pain). Untracked, that's OK, but if you use a tracker you'll have to blend tracked images of the sky (blurred moving foreground) with an untracked image of the foreground (blurred moving sky). If the foreground 'horizon' is complex, that's a non-trivial task to do convincingly.

In the UK the air is pretty humid - that means as your lens cools down in the night you have to think about condensation. You'll probably want to either get or make a 'dew heater' - a battery powered band that goes round the lens hood and keeps the front of the lens a bit warmer than the air. Probably don't need it for the odd Milky Way picture, but if you are capturing meteor showers, then definitely.

The final thing I'd mention - nothing to do with astrophotography - is that the geometric distortion in the lens is really bad by the standard of modern lenses. Probably doesn't matter if you are not photographing buildings, but it's worth being aware or it and knowing how to correct it (e.g., Hugin). For astro, you'll need it if you start stacking unaligned images - they won't align if you don't correct the distortion first.

Bit longer than I intended - hope it helps.

Thanks for your full reply!

That is really useful information!

Nor do I. Just seems gimmicky to me.

Yes, I am interested in photographing night (probably moonlit) landscapes scenes as realistically as posssible.

Untracked, that's OK, but if you use a tracker you'll have to blend tracked images of the sky (blurred moving foreground) with an untracked image of the foreground (blurred moving sky). If the foreground 'horizon' is complex, that's a non-trivial task to do convincingly.

I can't see myself tracking. My thoughts on the Samyang lens is that a shot which I currently take on my Fuji lens at 40 sec at F4 and ISO 1600 ought to be possible on the Samyang lens at 40 sec at F2 and ISO 400 so the lens would be worth the investment if this resulted in significantly less noise in the sky.

Interesting. My attempts so far have been been in the Namib desert where the night skies were awesome and humidity very much not an issue! Two photos attached with the Fuji lens. The Milky Way one with Jupiter (3 stitched shots) I am pretty happy with, but the landscape one (40sec, F4, ISO1600) certainly leaves a quite a bit to be desired in the sky, though this might be as much down to my processing skills as anything else. It was actually taken with about a quarter moon which exposes to look like a really bright full moon if the sky is not to end up too dark (but again this might be my lack of processing skills). I really dislike moonlit scenes looking unrealistically bright as day!

I'd always understood that for any camera, the lower the ISO better the SNR.......

Or just get a camera which is ISO Invariant. Job Done 😀

Smiles smugly from the sidelines.

Or just leave your camera at its base ISO?

Do you get less noise by sliding up the exposure when processing rather than letting the camera do it for you by setting an appropriate ISO?

Many Pentax DSLRs have a built in astro tracing function which uses the in-camera shake reduction and GPS to move the sensor during a long exposure and eliminate star trails.

On my "old" Nikon D5300 I just leave the ISO at the the bottom end of the scale for astro photographs and I stack lots of exposures with guiding. Guiding removes star trails. Stacking lots of exposures improves the signal to noise ratio. There is no benefit on my Nikon to increasing the ISO, it gains nothing beyond about ISO 200. 

Noisy images are due to insufficient signal to noise ratio which can only be improved with more exposure time.

On my camera, increasing the ISO above these low levels does nothing that can't be done by multiplying raw image by a scale factor and it degrades the dynamic range. I believe for a given scene brightness, on my Nikon, the signal to noise ratio is almost invariant with ISO, but this might not be the case for all cameras. As Dave highlighted, for his camera the number might be slightly higher. 

It's totally sensor dependent. There's plenty of info on Canon, but less so for Fuji. Your best bet is just to do some simple testing to see which is the best ISO for YOUR camera. Having said that I believe the Fuji sensor is ISO invariant, so it most likely to be base ISO (ISO 100 probably) and stretch in post-processing. Experiment!

Pedantic geek alert: Don't confuse guiding with tracking. Tracking (open loop earth rotation compensation) is quite useful in wide-angle astrophotography. Guiding (relatively complex and expensive closed loop compensation for 2nd order effects) less so.

On the subject of inexpensive trackers, I believe this clockwork tracker works quite well (although I have no first hand experience of it)...

https://www.omegon.eu/camera-mounts/omegon-mount-mini-track-lx2/p,55040

... particularly if you have dark skies like the Namib desert!

I believe that the issue is caused by noise in the analogue-to-digital converter and is at a fixed level.  So low signal or low ISO exacerbates it.

Yep, Fuji looks to be completely ISO invariant...

https://improvephotography.com/34818/iso-invariance/

https://www.dpreview.com/forums/post/61966520

https://fujilove.com/isoless-photography-with-the-fujifilm-x-series/

... So stick with base ISO.

Last year I had a stab at shooting the Milky Way for the first time ever while on holiday at a dark location. I took test shots at different exposures, apertures and ISO's.

I thought that I was doing something wrong because I was getting roughly the same noise levels at different ISO settings. And then I happened upon some websites and YouTube tutorials banging on about ISO Invariance.

My camera is one of those lucky enough to be almost ISO invariant.

youtube.com/watch?v=d8QV00mkJW4&

So, if I've understood correctly, if I want to use a single shot of the night sky, I should set to base ISO then take as long exposure as I can without getting noticeable star trails, then turn up the exposure in processing.

So, coming back to my original question, if I switched from my Fuji F4 lens to the Samyang F2 lens, I'll get 4 times as much light in and end up with 4 times the signal to noise ratio? but will the fact that I'll then need to turn up the exposure in processing less improve things further?

Correct.

Roughly. There are different noise sources you have to contend with and they scale differently. The three you generally have to worry about are read noise, dark current noise and photon (shot) noise. Read noise is fixed. Dark current noise scales with exposure time and very much with temperature. Photon noise scales as square root of the signal. Which dominates depends very much your system and on the particular conditions under which you captured the image. Also photon noise is pure random noise, but the others are not - they typically vary pixel-to-pixel in a semi-predicable manner ("fixed pattern noise") but also with a random component. There are various techniques to deal with each of them (stacking, dark frames, bias frames, dithering...).

The bottom line is that it's always better to collect as may photons as you can whether by having a faster lens (assuming it's optically up to scratch) and/or by having longer exposures. After that it gets a bit complicated.

Thanks. I think I'll try that with my 10-24 Fuji lens and, if noise seems an issue, I might well be tempted by the Samyang. 

I am aware of the difference between guiding and tracking.

I have an open loop tracker which I use with my 180mm lens to eliminate star trailing up to about 2 minute exposures. That is small and portable and the mini equatorial mount goes on a camera tripod.

I also use a second camera and 200mm lens to guide on my equatorial mount for exposures exceeding 5 minutes at 590mm. That is more complex and I run it from my laptop. I have not tried longer exposures because the sky is not dark enough where I am, so I lose the bottom end of the histogram. 

I accept that these are different, but are really just variations on the implementation with the objective of keeping the camera pointed at the same star as the earth rotates. 

I do not take images with the foreground in the shot. My tracker has a half rate setting to compromise between blurred stars and blurred foreground but I've not tried it.

Yes, you pretty obviously are, but my comment was aimed more at people like Robert doing 'night landscapes' and wanting to get rid of star trails. If they read your "guiding removes star trail" and went with that to Google they might have got quite goggle-eyed at the complexity of 'guiding'.

Wouldn’t it be double the SNR, not 4 times? Square root? 

That is fair, I could have made the distinction clearer

I see no reason why a clockwork mechanism can't be used for a simple tracker. It has to complete a revolution in 23 hour 56 minutes whilst carrying a camera.

Historically people have made trackers which are based around two pieces of wood hinged with a lead screw which pushes them apart. the idea is that you slowly turn the screw by hand to open the hinge which rotates the camera. 

Quite ingenious but it looks prone to wobbling the tripod. 

No, aperture is inversely proportional to the diameter of the opening, so area of opening (and therefore light captured) is inversely proportional to the square of the aperture. So halfing the aperture captures 4 times more light.

This is why aperture  often get marked in steps in ratio 1.4 (roughly the square root of 2), so that the step halfs the light csptured. Such as 2 - 2.8 - 4 - 5.6

It would really make more sense to use a scale where aperture is proportional to light captured!

For those that are interested, this page shows many different cameras and the effect of changing the ISO (gain before the ADC) versus the reduction in input referred noise.

The principle is that by increasing the gain before the ADC, noise contributions from it become smaller relative to the photon noise. Unfortunately this amplifier is noisy, and also it reduces the dynamic range in direct proportion to its gain. For many camera designs, the engineers have arranged it so that it is the noise of this amplifier is the main source of noise, so there is no significant reduction in input referred noise by increasing the gain.

The way to interpret the plots is to look at the slope of the noise versus the iso number. On many cameras (but not all, noticeably Canon) the slope is almost flat. This shows that increasing the ISO setting does not appreciably reduce the input noise level, but it does hurt the dynamic range. So leave it at the low setting!

The plot is logarithmic, so compresses a huge range. An "isoless" camera would have a flat line. Selecting a few models from the list on the webpage, that seems to be more cameras than not.

For Canons, there is a definite trade off - do you want to reduce the noise at the expense of some dynamic range?

In the plot, I added a Nikon D5300 which is about 8 years old and a Canon 760D from a similar era.

You can see from the plot that changing the ISO from 100 to 800 on the Nikon reduces the input referred noise by 0.3EV, but on the Canon, it reduces it by 1.5EV which is much more significant. Of course, increasing the ISO from 100 to 800 will also REDUCE the dynamic range by 3EV. On the Nikon, this is a no brainer - use the low ISO. On the Canon, more of a choice.

As Marek says, it varies from camera to camera due to the trade offs made by the design engineers. It might also be slightly temperature sensitive. 

https://www.photonstophotos.net/Charts/PDR_Shadow.htm#Canon%20EOS%20760D,Ni...

There's actually quite a range of 'barn-door' style trackers out there - some at quite significant prices (AstroTrac, >£1000). I built my own (somewhat over-engineered) one years ago that is probably a bit more sophisticated than most (does guiding as well as tracking, air-mass compensation, computerised polar alignment...). I like playing at engineering - it's more fun than moaning about cloudy skies. 

Ah sorry, no, I understand about aperture and agree that f/2 is 4 times as much light as f/4.

I was checking whether I am right or wrong in thinking that 4 times the light (or more generally SIGNAL - taking the aperture discussion completely outside of this) means just 2 times the SNR because SNR increases as square root of signal increase. 

That's only true for random (Poisson) noise. A lot of noise sources in photographs are non-random, so the 'square root' relationship doesn't apply.

thanks 

Many years ago I had an OM-2. It was mostly paired with a 28mm lens at the expense of 50mm and 135mm primes and an 80-200mm zoom.