Underexposing at low ISO vs correct exposure at high ISO noise?

Started 7 months ago | Discussions thread
FingerPainter Forum Pro • Posts: 12,179
Re: Underexposing at low ISO vs correct exposure at high ISO noise?
15

dperez wrote:

I'm shooting Sony, but I don't think this is manufacturer specific. It seems like a question that would apply to most any brand of camera.

At some point in the semi-recent past I did some rummaging around on noise, dynamic range, ISO and ISO invariance. I found several articles that went into causes of noise and how it’s being improved.

In one article, among other things, they talked about different types of noise, and there was a lot of “stuff” noise, which added up to “The newer backlit, stacked, dual-base, hoo-hah sensors are better for noise…”

Somewhere in all this, it was implied (or I just inferred it from what they were saying) that if you capped the upper limit of ISO at a low enough value to have “reasonable” noise – say 1600 or 2000, you could shoot at higher ISO values by underexposing the images, get the benefit of the back-end noise reduction and thus overall reduced noise.

Yesterday, I tried it. Capped ISO at 2000, shot an image at 1000, 2000, 2000 -1 stop and 2000 -2 stop by changing shutter speed. Them uncapped the ISO and did the same thing so I got ISO 1000, 2000, 4000 and 8000. I selected those because they’re a direct multiple of the high Base of ISO 500.

Looked at the images. At 100% then at 300%. In Lightroom, changed only the exposure, kept everything else at zero. Comparing at 4000 and at 8000 I don’t see any difference. It may BE there, but I’m not seeing it.

In which case, I might as well set the ISO at some “reasonable” max I won’t normally hit (10,000 or something), and get images that are “properly” exposed as opposed to “underexposed”.

So, long journey to a couple (hopefully) simple questions.

1. SHOULD there be reduced noise by using a lower ISO and increasing exposure in post?

No.

If you don't care why, just skip to the next question and answer. If you want to know why, it would help to get some terms straight. You managed to use two terms in the above questions in very common but technically incorrect senses: "exposure" and "reduced noise". If we are going to get a proper understanding of how to minimise how noisy your images look, it would help a lot to use the terms correctly.

Exposure

You cannot increase exposure in post, even though the slider in Lightroom may be named "exposure". All the slider adjusts is how light or dark the image looks. This attribute is properly called "image lightness", not "exposure". The slider simulates the effect using a different exposure at capture time would have had on lightness, but does not simulate the effect using different exposure settings would have had on noise, sharpness, diffraction, or motion blur.

"Exposure" is the amount of light that fell on the sensor per unit area while the shutter was open. You can't change the exposure once the shutter closes. Despite widespread mis-use of the term, exposure does not actually mean how light or dark the image appears.

Exposure depends on three parameters:

  1. the amount of light in the scene ("scene luminance")
  2. the T-stop of the lens (approximated by f-stop),
  3. the length of time the shutter was open ("shutter speed")

You can sometimes affect the scene luminance by using flash, ,or other lights, opening or closing blinds, or using reflectors or shades, etc. .

You can control the f-stop and shutter speed directly on the camera. In an auto-exposure mode (P, A, or S modes) you might also change the f-stop or shutter speed if you adjust the ISO setting or the exposure compensation setting.

If you keep the f-stop, shutter speed and scene luminance constant, changing the ISO setting does not affect the exposure, just the image lightness.

Noise

Noise is a measure of the variation in pixel values. All photographs have some noise, because photographs are made from captured light and all light has variation in both photon arrival rate and colour.

We are concerned about noise because it can affect how our images look. It can make them look speckled where they should have solid colours, or indistinct where there should be higher contrast.

It is exceedingly common to refer to images that are more visibly affected by noise as having "more noise". This is, in fact, incorrect. What actually affects the visibility of noise is the Signal to Noise Ratio (SNR). The higher the SNR, the less noisy an image will look. While it is true that if you had less noise for a given signal, you'd have a higher SNR, in practice, one only achieves higher SNRs by increasing both the signal and the noise, but increasing the noise less than the signal. So generally, the images with the highest SNR - the images that look least noisy - also have the most noise.

So instead of talking about images having "less noise", we'll talk about images being "less noisy" or having a higher SNR.

The most effective way to increase the SNR is to increase the exposure. SNR varies closely with the square root of the exposure. Every two stops of increased exposure results in a near-doubling of SNR. Every two-stop reduction of exposure results in a near-halving of SNR.

A much less effective way to increase SNR is to increase the ISO setting for a given exposure. This produces a smaller benefit than an increase in exposure, and there is no benefit above the ISO setting at which the camera become ISO-invariant. Be aware that manually increasing the ISO setting in P, A or S modes, will cause the camera to reduce exposure. The loss in SNR from the reduction in exposure will be greater than the gain in SNR from the increase in ISO.

Since SNR depends on exposure, underexposing always results in a noisier image.

Since reducing ISO for a given f-stop and shutter speed doesn't reduce exposure, Doing so isn't "underexposing".

Since, on most digital cameras, increasing ISO a few steps above base while holding f-stop and shutter speed constant results in a a slight increase in SNR, reducing image lightness by reducing ISO setting results in s slightly noisier image.

Since adjusting "exposure" in post doesn't adjust actual exposure, it offers no improvement to SNR.

And is the improvement worth having underexposed images (anywhere from -1 to -3 stops) with whatever ramifications come with that?

No, not usually.

Since there is no improvement to noisiness, it isn't worth having if all you care about is noisiness.

2. With underexposure pushing the whole histogram down, making more pixels fall into the range that ends up being black, doesn’t the image it lose some shadow data that WOULD be there if properly exposed?

Yes.

"Properly exposed" can mean different things in different contexts. Properly exposed SOOC JPEGS are often exposed differently than properly exposed RAW files. I'm going to presume that by "properly exposed" you mean that the image is exposed so that a middle grey value in the scene is rendered as middle grey in the output without any post capture adjustment of lightness, and "underexposed" means exposed less than what would be properly exposed at base ISO.

Yes, underexposure may cause the image to loses some shadow data, but it may save some data in the highlights. This will happen if the dynamic range (DR) of the scene above middle grey is higher than the camera's DR above middle grey. This often occurs in brightly-lit scenes.

Preserving desired highlight data is really the only good reason to deliberately underexpose. Since it comes with the penalties of lost shadow detail and reduced SNR throughout the image, it should only be employed when there actually is highlight detail that would be lost at "proper exposure" and preserving the highlight detail is more important than preserving shadow detail or keeping a high SNR in the image.

Deliberately underexposing on all you shots is bad practice unless your camera has a low DR and all your shots are of brightly-lit scenes, and you always need the detail in the brightest parts of the scene.

Many modern cameras have a feature called "dual conversion gain." This is a switch in sensor capacitance that occurs at or above a particular ISO setting. It improves SNR at higher ISOs. On many modern digital cameras, the improvement in SNR from increasing the ISO setting is very small, except when you increase ISO to the setting where dual conversion gain first kicks in. Since the benefit to SNR of increased ISO is small on modern digital cameras, it can be worthwhile holding ISO to a value no larger than where the conversion gain switch occurs. When you don't blow highlight detail at the ISO setting where dual conversion gain is activated, reducing ISO below this point can reduce SNR almost as much as a reduction in exposure, for no benefit.

A better approach, when you can take it, is to:

  1. Set ISO to base
  2. Set the aperture to the largest f-stop that gives adequate lens sharpness and adequate DOF
  3. Set the shutter speed to the slowest value that adequately controls motion blur.
  4. If, at the above settings, desired highlight detail is blown, increase the shutter speed just enough to not blow that detail.
  5. If, at the above settings, there is highlight headroom, increase the ISO while holding f-stop and shutter speed constant, until the headroom is used up or you reach the ISO setting at which the camera becomes ISO invariant.

If light is low enough that you know step 4 will never occur and step 5 will always occur, you can substitute a higher ISO setting in step one.

Or even “exposed to the right”?

Sometimes.

Proper use of ETTR (Exposing To The Right) doesn't always mean using a higher exposure than "proper exposure". It just means using the largest exposure that doesn't blow desired highlight detail. In brightly-lit scenes, this may be an exposure lower than "proper exposure".

3. I know the danger of overexposing highlights (unrecoverable), but what OTHER factors are influenced by the underexposure? Does midtone or highlight detail benefit or suffer from the underexposure?

Midtone and shadow detail always suffer from underexposure. Highlight detail benefits from underexposure only if it would be blown at "proper exposure". If there is highlight headroom at "proper exposure" for the brightest highlights in the scene whose detail you care about, those highlights and everything else in the photo would benefit from just enough overexposure to use up that headroom.

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