There are all sorts of misconceptions about image quality in digital photography. For instance, people commonly believe that the resolution, or the number of megapixels, define image quality.
I’ll illustrate this with a screenshot from dpreview’s comparometer:
As you can tell, the top left image is from Sony A7II, the one I’ve been using since 2016. The bottom left is from the one I just ordered, the Sony A7RV. The top right is from the Canon 5d, the camera I’ve been using since 2006 and which Biljana was using until very recently. The bottom right is from the Pentax 645Z, the medium format camera.
As you can tell, other than some white balance differences, they are all basically the same image with different amounts of magnification. This means that the difference in resolution determines how big you can print the image without perceivable loss of fine detail.
This means that doubling of the resolution means that the image is printable on double the paper size, and we happen to have a standard of paper size measurements, in fact two, A and B:
Basically, every larger size (smaller number) is produced by mirroring the smaller size along the longer side, thus doubling the surface.
If, for example, a 12.7MP image from Canon 5d can produce a high quality B2 print (which I have done), an image of double the resolution, 24-26MP, can produce a B1 print of same apparent quality. This, of course, assumes that everything else, like noise and the amount of actual resolution measured in line pairs, scales equally.
Other than printability on large paper sizes, image quality is not affected by sensor resolution. There are, however, several other factors that determine image quality: noise, color depth, and dynamic range. Noise is obvious – it can degrade the image in appearance if it is excessive. Dynamic range is also easy to understand – it’s the ability to resolve greater number of brightness levels. In essence, one ev (exposure value, or aperture value) is twice the amount of light. With every ev of dynamic range, there’s a 100% increase in the level of brightness. This means that the dynamic range is 2 to the power of n, the same way binary numbers are defined by the bit depth of the variable type; 8-bit means 256 possible values, 16-bit means 65536 and so on. Today’s sensors can resolve over 14 ev of dynamic range, where slide film resolved 5 ev, and best BW and color negative emulsions resolved 10 ev. This means that everything above 10 ev is excellent, but using it in a real picture might require tonal compression in processing.
Color depth, however, is somewhat less clear as parameters of image quality go, but I would define it the same way I would dynamic range, because it’s the same thing: the ability to define gradient of primary colors, where every pixel is defined by three binary components of certain bit depth, for red, green and blue. 8-bit color depth means a color gradient of 256 shades for each of the 3 components. 16-bit color depth means 65536 shades for each of the 3 components, and so on; again, it’s the 2 to the n-th power. Of course, the ability to convert a signal from the sensor into a n-bit format doesn’t mean there’s actuall n-bits of data in the source, assuming the analog to digital converter doesn’t introduce its own issues. You can read the analog data from a small smartphone sensor into the 16-bit numberspace, but there won’t be 16 bits of color data in there. So, the ability to define discrete shades of colors across the large dynamic range is what differentiates between sensors with “thin” and “thick” colors. The difference in color depth is visible at any image size and is much more important for the perception of image quality than resolution, which only becomes relevant when you enlarge the image. So, the luminance and chrominance dynamic range is what defines the number of brightness levels and color tones a sensor can capture. When we introduce the noise, which contaminates both luminance and chrominance data, we get all the parameters of image quality.
So, what does this mean, translated to the world of actual cameras? It means that the pictures from my current A7II and A7RV will look exactly the same, unless I decide to print over a meter wide, in which case the A7RV images will look more detailed if you come so close that you no longer perceive the whole picture. As for the color depth and dynamic range, there will be no perceivable difference, because both cameras are extremely capable.
The difference is that the autofocus on the new A7RV is extremely capable, while the autofocus on the A7II is rudimentary and unable to deal with things that move. Also, the viewfinder on the A7II is adequate, while the viewfinder on the A7RV is excellent, which contributes nothing to the image quality, but should reduce my eye strain significantly, which matters to me since my eyes are not what they used to be. Also, the fact that A7RV has 60 MP resolution means that it has 26MP of resolution within the APS-C circle, which means I can magnify the telephoto range by the factor of 1.5x and still retain the same print size that I have on the A7II, which is a much more tangible functional difference than the ability to print larger than a meter in width, which I almost never do. The ability to turn 400mm of range into 600mm is extremely useful.
Now for the drawbacks. The old camera is free since I already own it, while the new camera cost 3500 € used. This is a significant cost, since all of my lenses probably cost less than that; alternatively, I could get several GM grade lenses for that amount of money. This means that I needed to have very good reasons for the upgrade. Also, the new camera produces bigger files, and more of them because it’s faster, which means greater requirements on memory cards and storage drives, not to mention computer processing power. My computers and storage drives are already adequate, but I had to buy an extremely expensive cf-express memory card, which is a NVMe gen-3 1 lane drive. Yes, they now have memory cards that are NVMe drives, because apparently you need that in order to record video and clear the buffer quickly. In essence, all the drawbacks are a matter of money, while all the benefits are a matter of user comfort and the ability to actually get the kind images that I otherwise wouldn’t be able to get, for instance by turning a 400mm 35mm system into a 600mm APS-C system temporarily and tracking a bird in flight so accurately that its closer eye is continuously kept in focus.