Square, 4:3, and 3:2 aspect ratios
The dawn of serious medium-format wildlife photography got me thinking again about the effect that the sensor has on field of view and reach, and how we can use better terms to describe this effect.
It is typical to say that the D500 has a 1.5x crop factor, but the term "1.5x crop factor" is misleading unless you're cropping a sensor with 1.5x as many vertical pixels. My baseline is a 24.0 megapixel full-frame sensor. If you crop my baseline sensor by 1.5x, you would have the same field of view as on the D500, but only 10.7 megapixels compared to the D500's 20.9 megapixels. Therefore, let's use a term like field of view multiplier instead of "crop factor." The D500 has a field of view multiplier of 1.5 relative to my baseline.
Some people say the D500 is like a teleconverter. Indeed, if you put a 1.5x teleconverter on my baseline sensor, then you would have the same field of view as on the D500; but then you would have 24 megapixels. Instead of calling it a teleconverter, let's describe the D500 as having a reach multiplier of 1.41x relative to my baseline. Reach multiplier is proportional to pixel density: The D500 has 236.4 vertical pixels per mm while my baseline sensor has 167.4, and the ratio of those quantities is 1.41x.
Considering these quantities, it's fair to say that if you put a 1.41x teleconverter on my baseline and then cropped the result by 1.5/1.41 = 1.06x, then you'd have an image similar to what you'd get on a D500. We can use a similar analogy any time that 1 < reach multiplier < fov multiplier.
The 32 megapixel APS-C Canon R7 has a field of view multiplier of 1.61x and a reach multiplier of 1.87x. If you put a 1.61x teleconverter on my baseline, it would have the same field of view as the R7, but the R7 would still have more pixels, making its image more "croppable." If you instead put a 1.87x teleconverter on my baseline and cropped the R7 to the same field of view, then you would have comparable images. This is the case when 1 < fov multiplier < reach multiplier.
The venerable 10.2 megapixel Nikon D80 has a field of view multiplier of 1.51x and a reach multiplier of 0.98x. If you crop my baseline 1.51x, you would still have more pixels than the D80. This is the case when reach multiplier < 1 < fov multiplier.
The Panasonic G9 II has 25.2 megapixels on a Micro Four Thirds sensor with 13mm of height and the namesake 4:3 aspect ratio. You can think of the 3:2 aspect ratio as 4:3 with extra bars of pixels on the sides; for wildlife photography, these bars are arguably a "bonus" that often goes unused.
Square, 4:3, and 3:2 aspect ratios
If we ignore that bonus, then it makes sense to compare the sensors in the vertical dimension, where the G9 II has a field of view multiplier of 1.84x and a reach multiplier of 1.99x.
The new Fujifilm GFX 100S II has 102 megapixels on a medium-format sensor with 4:3 aspect ratio and 32.9mm of height. Based on vertical pixels, this is a field of view multiplier of 0.73x and a reach multiplier of 1.59x. If you put a 1.59x teleconverter on my baseline and cropped the GFX 100S II to the same field of view, then you would have comparable images.
As a concluding example, suppose you put a 500mm prime lens on the GFX 100S II. A 50 megapixel full-frame sensor would need a 365mm lens to get the same vertical field of view, or a 552mm lens to get the same reach.
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