Please help me understand. I have the above F8 mirror lens. I know the difference between F stop and T stop. I have measured this T stop experimentally it is higher than F8. The DOF feels very narrow at close distances. It is very difficult for everyone to focus. I decided to measure DOF experimentally. I understand it is not an exact number only an estimate. When you look at a ruler almost parallel to the lens it is not black and white (binary true and false) whether it is in focus or not. At 40ft distance about 3-5” are in focus. Far away I counted the street signs. Pixel peeping between 1300 and 7000 feet is in focus. Calculating the F stop it is F 4-5 up close. F10 far away. Not so different from the F8 printed on the lens. Why is it so different at 40ft?
Depth of Field calculators assume that the Circle of Confusion is a consolidated circle that changes size in proportion to the F stop, but catadioptric lenses have a central obstruction, which creates a ring-shaped CoC.
Instead of reducing the size of the aperture from the outside in, as is typical of refractive lenses, the aperture is reduced from the inside out. So the overall diameter of the CoC is larger than would be expected from the F-stop used for exposure calculations. It is similar to using ND filters, where DOF corresponds to the size of the aperture, not the change in exposure.
The central obstruction also reduces sharpness and contrast, which further complicates DOF considerations. Ultimately, you have to use your own eyes and judgement to decide DOF, instead of relying on calculators.
Take a catadioptric lens with a front element with diameter of about 72mm with a central obstruction of about 34mm. The diameter of a circle with the same area is about 63.5mm. That's close enough to F8 for lens labeling and exposure calculations.
But DOF would correspond with F6.9, based on the full diameter of the aperture. Also, because the center is blocked, sharpness and contrast are reduced, which further reduces apparent sharpness. So DOF appearing to correspond with F4 is reasonable. In this example, DOF should not appear to correspond with any aperture narrower than F6.9.
No it does not make sense. The 500mm focal length is true when imaging an object at infinity (about 2000 focal lengths or more distance). When imaging at 40 feet, you must rack the lens forward to achieve focus. I calculate 526mm as the effective focal length for 40 feet. Now the working f-number becomes 8.3, not much but some loss of image brightness. Not the f/4 you measured. How did you work this problem for 40 feet object distance?