Proper exposure in digital photography

Started Sep 3, 2013 | Discussions thread
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Macx Senior Member • Posts: 1,433
Proper exposure in digital photography

In a recent thread it was argued, and forgive my paraphrasing, that nothing has changed significantly since film photography: We still expose the same way and with the same result. Just like film we meter a scene and dial in the usual shutter time, f-ratio aperture and ISO and we get the result we want.

Thus, it was argued, proper exposure really has nothing to do with what camera you are using. You can pick up practically any different model or any different format, and given the same circumstances and exposure values it will give you pretty much the same result any time: An image with the right levels of brightness in all the right places.

The problem is that that isn't true. Digital cameras can approximate or simulate film cameras, but underneath the surface it works significantly different and one of the important differences is in the way exposure works.

When we shot film, we metered for a certain level of brightness (often the middle grey) and dialled in the exposure values in order to recreate it in the final photo. The amount of exposure needed was determined by the chemical properties of the film, as was all the brightness levels from darkest to lightest.

That's not the case for digital photography. It is more than just changing the chemical properties of the film for the electronic properties of the sensor: In digital photography we set the brightness levels as we wish, independent of the exposure.

Let me try to give an example: I want to create a photo of a house. Now, how do I decide what shutter time and aperture I should choose? Well, I might be concerned with motion blur or camera shake, and I want to make sure I have enough depth of field. Those are valid concerns. But once they are taken care of, what then about "exposure"? Does it matter if I shoot at 1 s or 1/25 s shutter time? Don't I risk over- or underexposing the image?

The answer is, you do and you don't. It doesn't matter with regards to getting the image too bright or too dark. It makes no difference with the brightness. We can shoot the same photo at 1 s or at 1/25 s and still get the "right" exposure. The 1 s exposure won't be brighter than the 1/25 s one unless we want it to be.

This is a hard concept to swallow for some people, primarily because our cameras are built to fool us into thinking we're shooting film, but of course we're not. In digital photography the sensor is a counting machine, counting the light particles as they hit the sensor, but as we noted before, we have full control over how the data from the sensor is interpreted, what number should be white, middle grey, or black, and thereby how bright or dark the resulting image is.

There is a hard limit is when we oversaturate the sensor and fill the sensor pixels/sensels to capacity (and that limit might soon disappear). At that point the counting machine stops counting and we lose all the information from that point. This is what happens when we talk about having clipped highlights.

So, besides this, what does exposure matter? Well, even if it doesn't change the brightness of a photo when we change the exposure, the 1 s photo will still look different from the 1/25 s photo, because the 1/25 s photo will be noisier than the 1 s photo. I won't go into the physics, but this is basically because the stronger the signal, the less apparent the noise is. The less signal, the more we'll notice the noise until the image will be completely washed out in noise. Brightness level is unaffected, though.

In other words, exposure matters in terms of noise. And it follows, that a "proper exposure" is the one that gives you the least amount of noise.

This is the idea behind exposing to the right, which mirrorless cameras and other cameras with live readout from the sensor are particularly suited: To maximise exposure, while being careful not to reach the sensor's hard limit. (If you're interested in this, there are more thorough guides around, like gollywop's article her on this website.)

The idea of separating exposure from brightness is novel to most photographers, because it goes against everything we've been taught, like the idea of the "exposure triangle", and I'm not trying to force anybody to change what works for them. I would always argue that a good eye and good light trumps technical know-how. A racing driver doesn't need to know how a car works to be a world champion, but it can help him or her even so, and I hope that some might get a better understanding of the nature of digital photography exposure, because it can help them as well.

Understanding that exposure is about controlling noise and creating more noise free images is more than just losing a bit of "grain". Less noise means sharper images with better colours. Sometimes that's not what you want, but I think it's fair to say that in general that is a good thing.

I apologise belatedly for my long-windedness and thank you for taking the time to read thus far. I have another point to make, but I think that'll be in a follow up.

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