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This is a kind of continuation of this question.

I'm shooting a poorly lit object. Camera has fastest ISO value of 3200. I set aperture to some specific value and don't alter it, the object is unchanged and the camera stands still. I first shoot with ISO 1600 and 1/125 second shutter speed, then with ISO 3200 and 1/250 second shutter speed. Pictures taken with ISO 1600 turn out rather clean and pictures with ISO 3200 turn out rather noisy. Both images look identically exposed.

The reason is with 1/250 second shutter speed lower amount of light enters the camera and higher ISO value causes more amplification of the signal and also more amplification of the noise and so more noise gets into the resulting image. With 1/125 second shutter speed twice as much light enters the camera and so less amplification is required and so noise is also amplified less and this causes less noise in the resulting image.

So I'm shooting a specific scene with a specific camera and all settings are left constant and I only vary either:

  • the aperture and ISO value or
  • the shutter speed and ISO value

so basically I vary the amount of light and the ISO value.

How do I know which amount of light (and so aperture or shutter speed) is the minimum required to obtain an image with low noise? How would I know without making the test shots that I need 1/125 shutter speed or slower?

  • Please define minimal noise and low noise. What are your thresholds for each? – Michael C Oct 10 '17 at 1:16
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Define 'low noise'... In general (disregarding long exposure noise) the lower the ISO setting, the lower the noise. In your case, with a stable camera and a non-moving subject I'd suggest setting the ISO to its default (100 or 200), setting aperture for desired DOF, and adjusting the shutter speed for a proper exposure according to the camera's light meter.

In your case, you would want to leave your aperture where it is, reduce your ISO to 100 (for example) which is 4 stops lower than 1600, and reduce your shutter speed to 1/8 of a second- 4 stops lower than 1/125.

  • Not a good idea either. Note that with long exposures, the chip itself heats up, generating more noise of its own. – yo' Oct 7 '17 at 15:48
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    1/8 second isn't that long in terms of heat buildup. It's not even relevant if the camera is a mirrorless or is being used in Live View so that the sensor is already continuously energized. – Michael C Oct 10 '17 at 0:45
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I think you are looking at this from the wrong angle. Simply put, the lower the ISO you can use to get a properly exposed image, the less noise you will get. So your goal is to set the conditions in which you are able to use the lowest ISO possible and still get the shot you want.

The simplest solution to this is to use auto ISO and ignore that setting altogether. Then you are left to simply determine the DOF and shutter speed you want to use to capture your image. Set DOF for your artistic tastes, and then the lowest shutter speed you can get away with to capture your image. That may mean using a tripod or monopod for a stationary or slow moving subject to eliminate camera shake.

ISO will set itself for a proper exposure with the parameters you have defined. If the noise (due to high ISO) is unacceptable to you, you need to reevaluate if using a shallower DOF (larger aperture) would be acceptable (assuming the shutter speed is already at it's lowest limit). If it is not, you simply can't get the image you want with the equipment and you have under those lighting conditions.

So my answer is that your 2 options to vary are the wrong ones. Ultimately you have to set the ISO to whatever will give you the proper exposure, since they camera will do that anyway (assuming you have auto ISO), then ignore that setting altogether. Once you set your aperture, then it's simply a matter to determine the slowest shutter speed you can use.

  • The various ways auto ISO and fractional stops of the ISO setting are implemented in a particular camera also come into play. ISO 125 can be noisier than ISO 1250 for some cameras due to the way 1/3 stop ISO settings may be push/pulled or pulled/pushed between exposure and developing. Is it really better to shoot at full-stop ISOs? In such cases manual selection of ISO may be a better choice. – Michael C Oct 10 '17 at 0:49
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Well you trade two things:

  • high ISO means high electronic amplification and this is inaccurate, generating noise.
  • low ISO means long exposure time and three issues:
    • movement (you can cancel this by using a solid tripod and a remote trigger);
    • sensor overheating as heat increases the noise, and the longer you expose, the more heat is produced by the chip (I describe below how to overcome this);
    • battery life as the long exposures drain your camera battery (have a spare battery with you!).

So, the optimum is difficult. If there's plenty of time on your hands, you can try combining multiple shots. Let's say that ISO 400 at 1/4 gives a balanced exposure. Then you have two options for combining:

  1. Shoot 8 pictures at ISO 400 and 1/32 and sum them up.
  2. Shoot 8 pictures at ISO 3200 and 1/32 and make an average.

Which works better may depend on how high ISO is implemented in your camera and you should experiment with this or do some research. Note that you want to wait some time between the shots to let the chip cool down. In general, you surely want to capture RAW as the JPEG compression, decompression, combining and recompression can be killing your picture quality.

You can of course choose any number of images and not precisely 8, but note that the quality increases with sqrt(n) where n is the number of picture you take, so going from 1 to 4 is a great improvement, while going from 4 to 8 is not as great. And the more you take, the more other noise (e.g. lights moving, your tripod digging into the grass and changing the capture angle, you accidentally moving something, ...) can be present.

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A couple of things to consider:

  1. ISO for a digital camera directly correlates with ISO for a film camera in regard to proper camera exposure. Proper camera exposure relates to the reproduction of the tonal values in the scene relative to the tonal values in the latent image on the film/sensor. In other words, ISO plays the same role in the exposure triangle for film and digital cameras.

  2. The way in which ISO correlates to noise in a digital image is different than the way ISO correlates to grain in a film image. With a film image, film ISO roughly correlates directly with grain on an absolute scale, i.e. ISO 3200 film will tend to have similar levels of grain independent of manufacturer. On a digital camera, higher ISO's correlate to higher noise but the amount of noise at ISO 3200 will vary greatly among digital cameras. On one camera noise may be consistently barely perceptible at ISO 3200 while another camera may consistently produce unacceptable images at ISO 3200.

It is a matter of software and electronics rather than chemistry. One way to think about the maximum ISO setting of a digital camera is as the amount of amplification that will produce an image that might possibly be acceptable to some photographers but is unlikely to be acceptable to many photographers. The highest ISO might provide an image that is sometimes good enough under adverse conditions.

It is probably reasonable to expect a digital camera with a maximum ISO of 3200 to produce very noisy images at ISO 3200 while a camera with a maximum ISO of 104k will tend to produce less noisy images at ISO 3200 due to better electronics and better signal processing. Humans have finer control over digital devices than photosensitive chemical processes.

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Choosing exposure settings is an exercise in trade offs. You need to balance the depth of field that the aperture settings give with the amount of movement that the shutter speed provides with the amount of noise the ISO setting provides. The dreaded trade off triangle

If you're concerned about one of these factors more than the others for a particular shot you should try choose that setting you want and adjust the other two settings to get the correct exposure.

In your particular case you're interested in reducing the amount of noise, so you should select the highest ISO that provides an acceptable amount of noise and then us your camera's meter to select aperture and shutter speed settings for the correct exposure. In general most people try and keep the ISO setting as low as they can, for example my camera goes down to 100. Most modern cameras I have used have acceptably low noise up to about 800. At that point it becomes too noticeable for me taste, but that's really a judgment call that only you can make for your photos.

If there isn't much light available using a tripod and a cable release will allow you to have slow shutter speeds without introducing shaking from your hand, which can greatly increase the range of shutter speeds that'll work. Of course tripods don't prevent all movement, such as wind if you're shooting landscapes. If you're still getting to much movement you can start tweaking the ISO and aperture and see if there's an acceptable compromise. If you're shooting a particularly troublesome subject it might not be an acceptable compromise.

The ability to make this type of judgment call comes with experience. The best way to get an understanding for this is to get out and experiment!

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What you are essentially asking is what is the highest ISO value you can use on a given scene that will give acceptable noise levels, since it directly determines the amount of light you will need.

And the answer is that it depends, both on your level of tolerance for noise and on the scene itself. Indeed, since noise is mostly visible in dark areas (where there is little light and the SNR ratio is thus lowest), if the scene, or at least the subject, is bright, you can have a lot of noise that will be mostly invisible. For example the three images below were shot with the same camera and settings; I think most people would agree, even if in a naive sense, that they differ in their "noise level".

Assuming you know your maximal acceptable ISO value for what you are about to shoot, you can just look at the settings suggested by your camera's meter (e.g., in Program mode). If the suggested ISO value is too high for you, for example if it is 3200 and you want to shoot at 800, you know that you will need to adjust the shutter speed and/or aperture to get four times as much light if you want a similarly exposed shot to what P mode would have produced. Note that this will not always be possible: at some point, a slower shutter speed will cause motion blur, while a wider aperture will not be physically possible (or, if it is, will cause a drop in sharpness unless your lens is very, very good). Indeed, that's basically why ISO exists in the first place.


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How do I know which amount of light (and so aperture or shutter speed) is the minimum required to obtain an image with low noise? How would I know without making the test shots that I need 1/125 shutter speed or slower?

Until you define a specific SNR as the threshold for low noise, the answer will vary by where you place the line between low noise and not-low noise.

To get the least amount of noise use the ISO setting with the least fixed pattern/read noise for your camera that allows you to expose the scene as you wish. For some specific cameras this would normally be the lowest full stop ISO setting that allows you to properly expose the image. For other cameras it might be the lowest ISO setting including the 1/2 or 1/3 stop settings that allow you to properly expose the image. It all depends upon how the various ISO settings are implemented in your specific camera.

The more light you can allow into your camera via either a larger aperture, a longer shutter time, or both without blowing the highlights the better your signal-to-noise ratio (SNR) will be. This will usually be at the camera's native (that is, the lowest non-extended) ISO setting.

Only when we are limited by the amount of available light, the camera's maximum aperture width, the aperture setting needed to get a desired depth of field, or the shutter time needed to prevent blurring due to a moving subject or due to camera motion would we increase the ISO setting to maintain proper exposure.

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