If i underexpose an image and have to crank up the exposure in post, this will also amplify the noise, resulting in a lower quality image. In this case, it would've been better to expose correctly from the start. I was wondering if there is a similar downside to overexposing an image (from a pure quality perspective, leaving aside the additional postediting workload).
Of course, if I overexpose by using a higher ISO and then turn down the exposure later, the added noise from the initial exposure won't magically disappear. Also, if my image is so overexposed that the brightest areas are clipped, those area's wont be fixable either.
But assume I shoot in RAW on a sunny day. Sunny16 says I can use f/16, 1/100sec, ISO 100 for an even exposure. However, I decide to go with a shutter speed of 1/25sec instead, overexposing my image by roughly two stops. Since I'm shooting in RAW, I have a couple of stops of wiggle room in terms of dynamic range, so even with the slight overexposure, no parts of my image should be clipped (for the sake of argument, let's say this is the case, I know that shooting on the edge like that is not a good habit to get into).
In post, I crank down the exposure by two stops. Will the image quality be any worse than that of a photo shot in the same conditions at 1/100sec (same aperture and ISO)? If so, why? Would it be different if I had stopped down the aperture instead (assuming a lens that doesn't have considerable sharpness issues at lower apertures)? Leaving aside the obvious difference in DoF and movement freezing (also shaky hands, lets say I have a tripod and a shutter release cable) that the different aperture/shutter speed will have.
Answer
This is known as ETTR which stands for Expose To The Right. As you correctly described, this will improve image quality as long as there is no actual clipping. The name comes from the fact the the histogram will be skewed to the right without actually touching the right edge.
There is one more reason why this is good which you did not mention. Sensors measure light linearly, this means that every stop of exposure has an twice as many values to represent nuances within it. So by increasing exposure, you will use more of the higher precision stops. Here is why:
Let's assume a 12-bit sensor. It reads values as 0-4095. Each spot is twice as bright as the previous one but sensors measure light intensity linearly. So the highest stop uses values 2048-4095. The next lower stop uses values 1024-2047, going down until you get to a point where the signal is drowned by noise which is why not all 12-bit sensors can actually capture a dynamic-range of 12 stops.
The further right you expose, the higher the ratio between signal and noise becomes, so noise is less apparent. The same noise is still there but because the signal is stronger, it has a less impact. Also as you can see you have basically 11-bits to represent nuances the brightest stop and 10-bits for that of the stop before that and so on.
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