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Putting taste/preference/opinion aside, I wonder whether I'm missing something, either on the technical side or the artistic side.

My question only addresses the context of digital cameras with "modern" sensors that have high resolutions and high ISO capability (e.g., the Nikon Z6; 24MP and ISO going up to 51200). It also relates to non-professional use by a hobbyist with somewhat advanced skills.

The argument I'm considering is: why would I need/want to buy an 85mm prime if I can get "exactly" the same with a 50mm with some cropping? The high resolution of the camera and the reportedly high optical quality (in particular, sharpness) of these new Z lenses — and in general high-quality prime lenses — means that cropping should not be an issue. I think there is no difference in terms of light — the focal ratio determines the "intensity" of light arriving at each pixel (relative to the amount of light in the scene, of course); so, all else being equal in both the scene and the settings, the pixels in the cropped image from the 50mm will be equally lit as the pixels in the full 85mm image (only difference will be that there are more pixels in the 85mm image). But even if it wasn't the same, these modern sensors with high ISO capability make the issue less relevant.

I understand that what I'm describing amounts to favoring digital magnification over optical magnification (which of course I understand is not right). But that would be a more relevant factor if we were talking of a wider difference — say, 50mm vs. 200mm or 20~24mm vs. 85~135mm. A digital magnification of 170%, with these super-sharp lenses should be nearly-indistinguishable from optical magnification (maybe? at least for most practical purposes?)

So, again, my question is: am I missing something, either on the technical or artistic side?

EDIT:
As an additional thought: I notice that bokeh (or other optical features) may constitute a relevant difference. However, I'm not really sure: at the same level of quality of the two lenses (assuming roughly the same quality of materials and construction, and same level of engineering skill and effort in the design of both lenses), I guess the larger focal distance will produce a higher-quality bokeh. On the other hand, the deficiencies in the bokeh are typically away from the center of the frame, no? So, when cropping the central area of the 50mm shot, we're removing the parts where the bokeh was lower-quality, making the two setups equivalent?
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As you already pointed out, there are basically three aspects that will differ with using the native focus length vs cropping in.

Amount of Bokeh

If you use a wider lens, the amount of bokeh will be less given the same distance. You may compensate partly by moving closer - however you asked about cropping instead.

You can use one of the online DOF calculators to see if that could be a problem for your photography. If you want more DOF it might be even an advantage. This is why APS-C Cams are liked by some wildlife photographers: More reach and a bigger DOF.

Background Compression

If the angle of view changes, the ratio of visible background behind an object in foreground changes, especially when very near to the subject.

This is a bit tricky: if you do not move, the angle of view should not change. So in your scenario with only cropping, this should not be a point.

However, it will become one if you instead mover closer. This will be very noticeable if you use narrow paper backgrounds.

Image Quality

You already said, that the middle of the image circle is usually the sharpest part. So of course cropping in will reduce IQ a bit, but you are having a bit more room for that by using the best part of the image. I doubt that it will be visible at all under normal circumstances. So you can disregard this one, as long as you start with a halfway decent lens.

Conclusion

So you could indeed argue that a wider lens is more flexible, as you can always crop in, but never "crop out". However, it comes at a price in image quality. The amount of cropping you can get away with, depends heavily on the image quality of the lens. Plus you need to center your shots to minimize distortion. Very wide or very tall motives might be more difficult to shoot due to that.

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  • \$\begingroup\$ The issue of centering may be solved in mirrorless cameras (or in general cameras with Live View, if you're willing to frame the shot looking at the screen on the back of the camera) by zooming in. It would be great if cameras allowed us to zoom in by 170% (or other ratios that match common focal lengths). My Nikon Z6 offers zoom-in at 200% (i.e., viewing what a 100mm lens would produce). Mind you, when zooming in, we lose the dials and parameters that the camera is normally showing — it would be great if cameras offered "normal viewing" at different zoom ratios! (pretty please, Nikon?) \$\endgroup\$
    – Cal-linux
    Commented Jan 10, 2023 at 1:08
  • \$\begingroup\$ @Cal-linux Good point! \$\endgroup\$ Commented Jan 11, 2023 at 7:36
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Lens choice is a matter of personal preference. One person might prefer framing an image in the viewfinder through a 50mm lens, another person might prefer framing in the viewfinder through an 85mm, and the same person might prefer 50mm sometimes and 85mm other times.

Sometimes seemingly small differences matter to one photographer and not to another. If the differences between using an 85mm and 50mm don’t matter to you, they don’t matter to you, and that is entirely rational, reasonable, and ok.

Same is true if they matter to somebody else.

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