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Hello, I've had a dilemma for a while now. If I'm imaging with a DSLR or a mirrorless camera: Short exposure times and high ISO will result in a noisy image, because of the high ISO. Long exposure times and lower ISO will also result in a somewhat noisy image, because of the warm sensor. I know high ISO impacts greatly the dynamic range and color accuracy So what's better? For example when imaging a target should I go for:
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| Longer exposure times are better. Have you tried taking dark calibration frames to remove the noise from the warm sensor? |
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Boyan Stiliyanov: High ISO images are not noisy on account of the high ISO, but on account of the fact that you really don't have much light in situations where you are using a high ISO. In fact, and especially for older cameras, high ISO images will have lower noise than low ISO images that are taken using the same exposure (time+aperture). For modern cameras, there tends to be little difference between low and high ISO as far as noise goes, so there is a dynamic range benefit to using lower ISOs. As far as sensor noise from heat, it is generally the total exposure time that matters regardless of ISO, unless you are using long gaps between exposures to allow the sensor to cool down. I should add - there is some benefit to longer exposures (lower total read noise for the same total exposure time) but you get rapidly diminishing returns beyond a minute or so, unless you are imaging from a very dark site. In most cases, read noise tends to be the least important noise contribution we deal with. |
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Hi Boyan, I guess it depends on the target you try to shoot. With higher ISO you typically lose a bit of dynamic range, but you would be able to get fainter details. If you try to photograph a really faint target, you may need to boost the data from an 400 ISO exposure quite heavily. This will amplify the noise as well. If you image with a higher ISO the signal will be stronger in that case and you may end up with less noise as with a 400 ISO image. This is just one example. In the end, it's all about the signal to noise ratio. On brighter subjects, it may be a different story. But I am no expert at all. What I typically recognize is that people tend to go crazy about this stuff. Sometimes, they would not take images at all, because they're not sure what to do. I typically use what it needs to get better images. If my mount didn't guide that well, I go for higher gains/ISO to reduce the exposure time. If it works well, I tend to go for longer exposure times. Narrowband is another story than LRGB. Faint targets are another story than bright ones. The list goes on and on. It all depends on the situation. Higher gain/ISO settings will look noisier if you look at only one sub frame. You will improve the signal (you didn't reduce the noise) mainly by taking many subframes. So look at the final image to make your own decision. CS Christian |
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As far as I Know, every camera has it's most efficient Iso in which the signal to noise ratio is great and if you go further you just get more noise without getting much more signal out of it. I shoot with a Canon 600D in which I only shoot with 800 Iso. I think I found that chart somewhere on here: https://www.photonstophotos.net/index.htm can't remember though which chart it was. It is an interesting website nevertheless. |
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Every camera brand is specific and often certain models need a different technique. Canon cameras like higher ISO always. ISO 1600 seems to be a favorite but some shoot best at ISO 800 or ISO 3200. Some Nikon cameras live and breath at ISO 200 no matter what. I am most familiar with Sony and 800-1600 is a safe choice with all Sony cameras but there are special cases to use a low (ISO 100) ISO but certain models are dual gain and require different calibration steps at low ISO. For some Sony's ISO between 100 and 800 (technically ISO 640) should never be used. Filters also change the math. So does you imaging location and sky brightness and scope or lens. Figure out the best ISO for your camera. There are tons of threads for that. Then take test shots to see when you sky glow starts to be a problem or use software like NINA to figure out when you are blowing out the stars. If you aren't able to do that then ISO 1600, 30s-1m without an NB filter and 3m with one should get you some results at around f/4 - f/5.6. If your lens is fast (f/2.8 or below) you may have to use 15s subs. Strange things happen with stacking and higher ISO usually have less read noise. This can be a benefit over low ISO shots. Also, shorter shots will usually make more heat than long shots. It isn't the collecting of photons that make heat but the read process usually. My cameras are much cooler shooting long subs than shooting video but the sensor is being used just as much. It's just being read less. I have never heard complaints from astro photographers about over heating their cameras but we all know that 4k video can roast a camera alive. |
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| I think with most Canon cameras ISO400 is too low. Exposure is a different story. |
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| With my EOS R the best results were with ISO 800, exposure as long as guiding allows, before clipping stars (180-300s). You have to gather as much light per frame as possible. The main problem is heat-related sensor noise. Not really a problem in this cold winter though… |
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| Higher ISO gives you lower readout noise. This is pretty much true for every camera. So 4 min at ISO 1600 will be the best, unless this leads to saturation of your main target. |
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With my Canon 5D MkIV there was a banding issue at anything below ISO 1600. Cheers, Scott |
