It is mainly the difference in the variation of the distances of the subjects and the magnification relationships and how they interact with the resolution limits of the lens.
The effects of the diffraction and the flowering of the sensor, although measurable in laboratory conditions, are more subtle. If an image is taken below the aperture limited by diffraction for a particular sensor and there are no fully saturated pixels, those effects will be shown much less in the resulting image than if taken in an aperture above the DLA and with a number significant of fully saturated pixels.
To fill the frame with the same flat test table, it is necessary to shoot at a distance greater than 1.6X with the trim body than with the full frame body. If you use 10 feet for the entire body of the box, you must shoot at 16 feet with the crop body. However, what does not increase is the number of lines per inch (as projected on the sensor) that the lens is capable of resolving. The image of the subject projected by the lens is smaller at 16 feet than at 10 feet, so the resolution limit of the lens is wider in relation to the characteristics of the surface of the subject and the size of each pixel (assuming the APS-C and the FF sensor have the same number of pixels).
To obtain the same display size, the image of a crop body should be magnified 1.6 times more than with an image of a full-frame body. For a 4×6 print, the full frame image only needs to be enlarged by approximately a factor of 4.23 compared to 6.77 for the body image of the cutout.
With the largest shooting distance (1.6X) and the largest magnification (1.6X), it is stretching the resolution limits of the lens to a greater degree (2.56X). To put it another way: to get the same sharpness with the trim body, you need a lens capable of solving 1,800 lines per inch to match the full-frame camera with a lens capable of resolving 700 lines per inch.
Even if you have an 80mm lens for the FF camera and a 50mm lens for the culture body to be able to shoot at the same distance, you would still need the 50mm lens used in the APS-C body to solve approximately 1125 lines by inches to match the 700 lines per inch 80 mm lens used in the FF body, because it is still expanding the result by 1.6X more to get the same screen size.
To simplify the mathematics, the following theoretical illustration assumes an APS-C sensor that is 1.5X smaller than the FF sensor (although the original question is about a camera with a 1.6X cut-factor sensor).
Imagine you have a lens with a theoretical resolution limit of 1000 line pairs per mm. With a 24 mm wide sensor it could project 24,000 pairs of lines. With a 36 mm wide sensor it could project 36,000 pairs of lines. Now take a test chart with 36,000 pairs of lines that fills the frame of the FF camera to ten feet. If you go back up to 15 feet to fill the camera frame of the trim body with the same test chart, then the 36,000 pairs of lines in the test chart will exceed the resolution capability of the lens because there are 36,000 pairs of lines trying to fit A 24mm wide sensor.
It does not back up because the lens is further extended when it is attached to a crop body. The lens projects the same size image in any way. The reason you make a backup is to allow the smaller sensor to capture the same frame. This reduces the angular size of the subject by 1 / 1.5X in the virtual image actually projected by the lens. But it does not reduce the angular size of the resolution limit of the lens by 1 / 1.5X when making a backup.
At 15 feet from the table, the angular difference between each pair of lines is 1 / 1.5X of the angular size when the camera was 10 feet from the table. But the lens still has the same resolution limit that is ultimately based on the angular size of the line pairs in the test chart. The line pairs per mm can only be significant when the distance from the entrance pupil of the lens to the sensor remains constant, as well as when the magnification factor of the virtual image projected on the sensor at a particular screen size remains constant.
Then, it enlarges the APS-C 1.5X image more than the FF image to see both images in the same screen size. This means that with the image of the APS-C sensor we can perceive blurry circles (measured in the sensor before the enlargement of the screen) that are 1 / 1.5X the size of the blurred circles at the limit of our perception in the FF image . The slightly fuzzy edges that would appear sharp in the FF image may be blurred due to the increased magnification of the APS-C image.
If the 1.5x crop body image of a 24K line pair graph taken from 15 is printed in 4×6 and the FF image of a 36K line pair graph taken in 10 & # 39; is printed at 6×9, then the sharpness should be the same because the line The pairs would be the same width in both prints. But when you print the body image of the 1.5 to 6×9 cutout, the line pairs (which are at the resolution limit of your lens) are now 1.5X wider. It does not gain any additional subject detail when enlarging more, because the lens can not resolve those details smaller than the width of the line pairs. At that point you are only revealing the blur.
The two effects are multiplied: the rewind for the same frame reduces the angular size of the subject details by 1.5X, then the magnification with 1.5X more to show the same size reduces the acceptable Confusion Circle by a factor of 1.5X.
Here's another way to look at it: if the 1.5x crop body image of a 24K line pair graph taken from 15 is printed in 4×6 and the FF image of a 36K line pair graph taken at 10 & # 39; is printed at 6×9, then the pairs of lines would be the same width in both prints. Note that the FF image is resolving the 36K line pairs shown at 6 x 9 inches, while the 1.5X clipping body is only solving the 24K line pairs shown at 4 x 6 inches. But when you zoom the body image from cutout 1.5 to 6×9, the line pairs (which are at the resolution limit of your lens) are now 1.5X wider.