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Hi all guys! I'm quite new this wonderful world of astrophotography, and I always had a lot of questions of which ISO value is "the best" for a shooting session. I did all my images in between ISO 800 and ISO 1600 using my Canon EOS 1300D Super UV/IR cut modified + CLS filter, but the question is: do I get a better image if I set the ISO to 400 and expose 30-45 seconds more than I would expose at ISO 800? Let's say that to get a good 1/3 histogram I shoot 120sec at ISO 800: do I get any benefit in image quality if I turn down at ISO 400 and expose at 150/160sec so I stay in the same histogram section? Thanks a lot for your help and sorry for my english ^^ Lorenzo |
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Hi Lorenzo, Based on this graph (I do not know if it is accurate) the sensor of 1300D becomes linear at ISO 800 but is already close to linear at ISO400. Linear means exposing 10 seconds at ISO1600 is *exactly* the same as exposing 20 seconds at ISO800 and increasing exposure 1EV in post processing. Zero practical difference. Why would you prefer 20 seconds at 800 instead of 10 seconds at 1600 if it is the same SNR? Because 800 has a higher dynamic range, meaning bright stars will not clip to white as fast. 400 is even better, but the difference between 400 and 800 is only 0.9 stops of dynamic range, not 1.1. So going from 20 seconds at 800 to 40 seconds at 400 will have an even better dynamic range, but only 90% improvement compared to going from 10 seconds 1600 to 20 seconds 800. It will also have a cost, because in DSLR sensors read noise increases when you lower the ISO. If you are shooting in a light polluted region you do not care so much about that. Because the sky background will make the read noise negligible. But if you are shooting somewhere dark, your darkest shadows will be hurt by the increase in read noise. This is particularly important if you do not expose two times more but less than that. 30 seconds at ISO400 will almost certainly be more noisy than 20 seconds at ISO800. They only reason to do it is if something is clipping to white and you really need to underexpose to create some kind of HDR. For better highlights, not better shadows. Which leads to the conclusion: the main reason why your image at ISO 400 will probably not be better than the image at ISO800 is duration. In order to have the same SNR (ignoring read noise), instead of 120 seconds at ISO800 you need to expose 240 seconds (2X) at ISO400. That is 4 minutes and many things can go wrong in tracking, guiding, wind, satellites passing by, neighboor turning on the lights. Even if everything works as good as possible, it is pretty much guaranteed the stars will be more blurry in the 240 second sub because tiny imperfections in tracking/guiding/seeing will add up. Dynamic range (as long as you are not clipping) can be restored by stacking many subs. Elongated stars cannot be fixed. Putting this all together, your optimum ISO is probably 800 (but if your tracking cannot take it you may also use 1600 with half the duration and take 2X the number of pics). Expose as much as you can without blowing the brightest stars. Let's say this is 120 seconds. If and only if there is a very bright star that you cannot avoid blowing out (for example Sadr in the Butterfly region), then do take a few 120 second subs (or even shorter) at ISO400 to restore its color in a HDR composition. You really don't gain anything by going to ISO400 and taking half the number of subs, you only increase read noise and tracking imperfections. Cheers, Dimitris PS For the record, that is why everybody is using ISO800 or more often ISO1600 with Canon cameras. With Nikon it is the same except sensors become linear at lower ISOs (sometimes even at ISO100 but typically around 400). |
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Go to ISO 1600 and expose as long as you can in a single sub. Canon cameras are almost all noisy (in terms of read noise) at ISO 800 and below. Look at this chart: (https://www.photonstophotos.net/Charts/RN_e.htm#Canon%20EOS%201200D_14) There is no 1300D in the database, but there is 1200D, and pretty much all Canon from the same period behave similarly. You can see that the read noise goes up substantially if you go from ISO 800 to ISO 400. So ISO 400 is definitely a no no, and even ISO 800 is questionable. If I have this camera in hand, I will go with ISO 1600 and even 3200 fo astrophotography, unless I have a super fast optics or I am overwhelmed by insanely strong light pollution. |
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What a big THANK YOU to both of you guys! Extremely precise answers and exactly to the point I needed, thanks! So I'll take this infos as golden rules for my next images sessions, tonight I tried M51 with a full moon from a Bortle 5 sky: probably an ISO 1600 instead of 800 would have been slightly better! Anyway it was just a try to get used to my gear as these are my very first shots, but a full moon isn't helpful on faint and super small (at least for my focal lenght ^^) galaxies! Here is the result, in 10 days I'll make another try with darker skies: M51 - Whirlpool Galaxy ( Lorenzo Scagnolari ) - AstroBin Thanks again! |
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What Wei-Hao said. The tradeoff is that noise is lower at higher ISOs, but remember that each stop of ISO also loses you one stop of dynamic range, which forces you to use shorter exposures if you care about saturating stars, which in turn increases the read noise contribution in your final image. This makes 1600 a sort of sweet spot for Canon DSLRs. This is actually where the newer astro CMOS cameras based on Sony sensors have huge advantages over Canon DSLRs with lower noise and larger full well depth that allow you to get the best of both worlds - low read noise and high dynamic range. |
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Great, thanks Arun! Yep I think my next upgrade will be a dedicated astrophotography camera, before levelling up the telescope and the mount ^^ I think that even with the star adventurer and my apo 60/360mm changing the camera will boost everything a little bit!  |
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| I have an error, where I say ten seconds iso 1600 is the same as 20 seconds iso 800 + 1ev in post, it is of course 10 seconds iso800 + 1ev and that goes for the rest of that paragraph. I just noticed it. |
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Hi Lorenzo, I' m also using an astromodified Canon 1300D in combination with a 200 mm tele lens, setting was mostly 1600 ISO. Recently I did some test shots on nebulas with 3200 ISO and reduced exposure time per sub, so I could do more subs in total, here is an example: https://www.astrobin.com/8dp7bu/?image_list_page=2&nc=&nce= For me the result looks ok, but this is of course subjective. I suggest doing test shots with both ISO settings (1600 and 3200) to find out if there is a difference in your results. Best wishes and clear skies Harald |
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| http://dslr-astrophotography.com/iso-dslr-astrophotography/ Great information on ISO and astrophotography. Deep read, but really helps. |
