Sensor Format Size and Image Quality

IMHO in many cases we don't reach saturation capacity, so I say clipping point. It's a bit arbitrary, and I caught a couple of Nikon camera models changing it with firmware upgrades (D2X and D4s).

And, in photography, there is a tendency to believe that a certain 'FWC' is a value above which no further photons can be captured and which applies exactly to each photosensor and applies exactly to all sensors of that model. I did not mean all photographers and I admit to the tendency myself ... for example, if a sensor has a stated FWC of 77,000e-, I think of mid-gray as being exactly 13,860e- and ignore any non-linearity or latitude - even if I knew it.

An analogy is diffraction which is often said to "set in" at some specific f/number for a given lens .

In my experience, it is rare to talk about clipping point in terms of electrons. But there's no reason why you can't if you make it clear what you're talking about.

That's not a bad thing to do if you understand FWC to be defined the way that I used it in the article. But, I admit that FWC has a different definition to a sensor designer.

In the Netting It Out bullet-point list, you write, "For IQ, size matters, and bigger is better." Isn't IQ more about the total light used to make the image than sensor size?

Suppose you make two photos, one with a MF body and the other with an APS-C body. During processing, you crop the MF image so the total light used to make the final version of the image is equivalent to what would be captured to make an uncropped image with an M4/3 body. The potential advantage of the larger MF sensor has been negated by the crop, which removes an excessive amount of light.

And when comparing equivalent images, it's often difficult - if even possible - to identify the photo made with the larger sensor camera without checking the metadata.

If this last point is one you'd dispute, I'd recommend adding equivalent photos to the article - images that can be viewed full-size by the reader - to illustrate the readily noticeable IQ difference between equivalent photos.

people buy larger sensor cameras because the image quality is better period. forget about all the equivalent jargon it doesn't exist.
when i switched to FF a7r2 from a em12 + 25mm 0.95 lens the sony with a cheap samyang 45 1.8 was in another league for image quality.

After that careful explanation, a member of the "gathers more light" crowd could be along shortly to put you straight, Bill.😉

How do you define IQ? If you define it as DR or noise than using suboptimal exposure or cropping is similar to using a smaller sensor. You can try it out.

To get the benefit of a larger sensor, you need to give it more light.

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Which is why I think the article would be enhanced by the addition of sample photos. It'd be interesting to see portraits of the family dog made with an iPhone, a medium format body, and a couple of different format cameras in-between.

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IQ is not a scalar. Some of its components are resolution, noise/DR, color crosstalk, blur, and lens aberrations. Those are discussed in the LR blog post.

In contexts like this, IQ refers to the technical recording of the image whatever it happens to be of; not the FOV and slice of time that the photographer chose to record.

Of course, there is subjectivity in the discussion of both, so "IQ" is not as straightforward as we might want it to be. One example is aliasing; some people like it (unless it results in bold moiré), and some people abhor it, even when it doesn't result in moiré.

Yes, I usually (but not always) can see more resolution coming from my D850 files than from my D500 files when viewed at 100%, but I've yet to see a difference on my 4K display between equivalent photos from those two cameras.

Anecdotal.

Obviously the pixel count is in the Sony's favor, but I entered the two cameras in the DPR studio comparison tool at ISOs 102.4K and 25.6K for the same total light (exposure times area), and visible noise was very similar.

They both have the same pixel size and no AA filter, so how much different could they really be? The D500 has slightly less pixel-level input-referred read noise than the D850, so the D500 should actually have an edge at the pixel level.

Well, if someone wants to make the case that sensor size matters when it comes to image quality, they should be able to illustrate the difference(s) claimed to exist. What better way to do that than by presenting photos for comparison? Words, math, and graphs only go so far. Ultimately, it's not the fact of differences existing (or not) that guides a photographer's decision about which format to shoot. It's the subjective decision of whether or not those differences matter enough to influence that choice. Rational people can look at the same sample photos and come to different conclusions on that question.