There’s been a lot of talk about using crop sensor cameras for subjects that need that “effective focal length” increase, for distant subjects like birds. For example, a 600mm lens has an effective focal length of (600 X 1.5) or 900mm. This may be close to the truth if both the full-frame and the crop sensors have the same overall resolution, but what about an FX camera that has more resolution than the DX camera, like the D850 versus the D500? Does the Nikon D500 beat the D850 if you crop the D850 picture down to the same field of view as the D500?
I decided to find out for myself just how good the D850 sensor is, and see if it can match the D500, even if you crop the D850 shots down to the same view you get with the D500. I know that the D500 can shoot faster and has a bigger frame buffer, but the D850 is no slouch, either. Both cameras are just as sensitive to light and can focus at the same speed, too. I’m not here to talk about the merits of one camera over the other; I’m only interested in knowing if I lose any quality using the D850 and crop the shots, when compared to un-cropped D500 shots.
If you like wide-angle shooting, there is of course no substitute for a full-frame camera. There are multiple “full-frame advantage” topics I could talk about, but I want to focus strictly on cropping here.
To get some answers, I set up a resolution target and then shot it from the exact same position and with the same lens; I just swapped camera bodies. After running it through my image analysis software, I took a look at the shots up close.
The shootout: D850 versus D500
I used my Sigma 70-200mm f/2.8 lens at 70mm and f/2.8 for these tests. I shot in raw format, and I didn’t post-process the shots in any way before analyzing them in my image analysis software.
I wanted to mention that I shot the chart with an exposure compensation of +0.7 stops with both cameras. The D500 meter consistently ended up with slightly darker images than the D850, but the image analysis software measurements are unaffected by that small difference.
The shot above is the MTF50 2-dimensional resolution plot from the D500. The measurements are in units of line pairs per millimeter (lp/mm). The 20.9 megapixel D500 sensor pixels are 4.22 microns, compared to the 45.7 megapixel D850, with pixels of 4.35 microns. The resolution chart is filling the frame left-to-right, and some of the chart goes outside of the frame top-to-bottom.
The shot above is the D850 using the same lens at the same distance. The plot looks a little funny, because the resolution chart no longer fills the frame. The actual resolution measurements of the little trapezoids in the target chart are unaffected by the framing difference, however. There are essentially the same number of sensor pixels on each little target trapezoid for each camera.
The resolution results between the D500 and D850 look quite close. Given the different pixel dimensions between the cameras, the D500 is expected to have slightly higher MTF50 lp/mm resolution numbers than the D850, if the little target trapezoids have the same “cycles per pixel” resolution. The resolution results are remarkably similar, here, and the D500 numbers are a tiny bit higher, but generally within experimental error.
D500 target center
The D500 edge measurements of the little trapezoids are in the range of 0.27 c/p to 0.32 c/p. For this sensor, a measurement of 0.31 c/p is equivalent to an MTF50 of 73.3 lp/mm.
The peak value of 0.32 equates to an MTF50 of 75.7 lp/mm, which matches the maximum value shown in the D500 MTF50 chart above.
D850 target center
The D850 edge measurements of the little target trapezoids are in the range of 0.28 c/p to 0.31 c/p. For this D850 sensor, a measurement of 0.31 c/p is equivalent to an MTF50 of 71.6 lp/mm.
At least in the lens center, then, there’s essentially no difference between the cameras.
Next, let’s take a look at the sensor right edge.
D500 right edge
D850 right edge
Comparing the cameras on the right edge, the D500 fared a little bit better on most of the target edges, but the measurements aren’t hugely different.
D500 top edge
D850 top edge
The readings between the D500 and D850 on the top of the frame are also comparable, but here I’d give the ‘edge’ to the D850 results. Without the little blue measurement values to guide me, I would have a hard time telling which shot was from which camera.
If I were to take a bunch of shots with both cameras and crop the D850 shots to match the D500 and then hand them to somebody to choose which was which, I’ll bet they couldn’t tell. The bottom line is that cropping the D850 shots gets me the same quality as the D500; there is no DX “effective focal length” advantage to be seen here.
I have always been on a big guilt trip when I crop a shot; this is definitely going to make me feel better about myself in that regard. At least with the D850.