Based on one of the comments/questions of the readers of my blog, the MTF of a camera is characterized as a function of the distance between the “target” and the camera. Again the slanted edge method was used, the light input was a green LED (525 nm), F5.6, Tamron fixed focus lens f = 8 mm.
Figure 1 shows the obtained results.
Figure 1 : MTF as a function of the distance between the test target and the camera (Tamron 8mm lens, 2/3”, F5.6).
As can be seen from the graph, distances of 60 cm and larger, give the best results as well as consistent results. A distance of 40 cm gives already a lower MTF compared to the larger distances, but for 30 cm and 20 cm the MTF is drastically reduced.
The reason for this fall-off in MTF is twofold :
- Below 30 cm distance between object and lens, the focusing capability of the lens is limited, and the image is becoming blurry (= less contrast and lower MTF),
- Between 60 cm and 30 cm the reduction in MTF can be explained by the fact that the incoming rays deviated more and more from the normal, and optical as well as electrical cross-talk will become larger. As a result the MTF is reduced.