It might seem shocking at first to see that your full-frame camera can have poorer lens resolution scores than the cheapo APS-C sensor. Here’s what is probably going on.
Your small-frame sensor probably has smaller pixels than that full-frame camera does. Smaller pixels means more pixels per millimeter on the sensor, and hence more lines per millimeter, too.
Here’s a specific example. Let’s compare the D7100 to the D610. Both cameras are roughly 24MP, so you’d think that they would score about the same.
4000 X 6000 pixels, and has dimensions of 3.92 microns for each pixel. The D7100 sensor itself is 15.6 mm X 23.6 mm.
4016 X 6016 pixels, with 5.95 micron pixels and is 24.0 mm X 35.9mm.
Let’s look at some resolution measurements, using the same lens at the same f/stop. Here, I’m using the Sigma 150-600 at 600mm, f/6.3.
D7100 with Sigma 150-600 at 600mm f/6.3 tops out at MTF50 of 40 lp/mm
D610 with Sigma 150-600 at 600mm f/6.3 tops out at MTF50 of 34 lp/mm
Did I get ripped off?? Why is the resolution so much worse with the D610 compared to the D7100? The secret lies in the pixels. There are less pixels per millimeter in the D610, and therefore less MTF50 lp/mm resolution.
But there are more millimeters in the D610 sensor! The key measurement of interest here is in line pairs per picture height (lp/ph).
The math: lp/ph = lp/mm * mm_of_height
The D7100 sensor height is 15.6 mm. The D610 sensor height is 24.0 mm.
For the above measurements, the D7100 measures 40 lp/mm * 15.6 mm = 624 lp/ph
The D610 measures 34 lp/mm * 24.0 = 816 lp/ph.
So, the D610 wins after all. It has a ‘score’ of 816 and the D7100 has a ‘score’ of 624. More total lines of resolution in the photo for the D610.
But wait, there's more:
D610 pixel area = 35.4 square microns
D7100 pixel area = 15.4 square microns
This means that the D610 pixels can drink up more than twice the light per pixel, which makes for vastly superior low-light and high-ISO abilities.
You can also achieve narrower depth of focus with the full-frame sensor, so you have more options photographically.
Feel better now?