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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?

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    \$\begingroup\$ "Calculating the F stop it is F 4-5 up close. F10 far away." – How are you "calculating" F stop? \$\endgroup\$
    – xiota
    Commented Apr 23, 2022 at 1:33
  • \$\begingroup\$ On a full-frame sensor, one would expect a DOF of about 11 inches at 40 feet with this sort of lens. How are you calculating the aperture? I would do this by using exposure. \$\endgroup\$
    – qrk
    Commented Apr 23, 2022 at 2:32
  • \$\begingroup\$ dofmaster.com/dofjs.html I have an APSC camera. \$\endgroup\$
    – Mark Robin
    Commented Apr 23, 2022 at 3:45
  • \$\begingroup\$ With APSC I calculate 0.57ft at F8. Not what I observed. \$\endgroup\$
    – Mark Robin
    Commented Apr 23, 2022 at 3:54
  • \$\begingroup\$ DoF depends on what you consider "blurry". DoF calculators usually consider that blur occurs if a point results in a disk bigger than 1/1800th of the diagonal of the image. But if you pixel-peep on modern cameras your definition of "blurry" is a lot more stringent. \$\endgroup\$
    – xenoid
    Commented Apr 23, 2022 at 10:35

2 Answers 2

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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.

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  • \$\begingroup\$ Everything you said is true. With that in mind, does my observation make sense? It acts like f10 far away, and f4 at 40ft distance? I will not trust a DOF calculator with all that in mind so complex. But I can estimate the DOF using f4, even though it says f8 on the lens. \$\endgroup\$
    – Mark Robin
    Commented Apr 24, 2022 at 14:54
  • \$\begingroup\$ I added example measurements from a catadioptric lens. I wouldn't expect apparent F-stop based on DOF to improve at distance. Did you actually measure the physical distances with a laser or meter wheel, or are you guestimating distances? // Calculators are based on idealized lenses. Real lenses can behave differently. \$\endgroup\$
    – xiota
    Commented Apr 24, 2022 at 23:36
  • \$\begingroup\$ Great explanation thanks! The distances are exact I looked at the numbered street signs. But the result f10 is an estimate depending on the resolution and CoC. I believe the f-stop is greater than F8, based on the calculator and DOF. We’re not far apart your calculation and mine. Why would the lower sharpness and contrast only reduce the f number at close distances? But not far? \$\endgroup\$
    – Mark Robin
    Commented Apr 25, 2022 at 1:08
  • \$\begingroup\$ It's not really clear what you're doing or calculating. Can you add sample images to your question? \$\endgroup\$
    – xiota
    Commented Apr 25, 2022 at 3:57
  • \$\begingroup\$ Unfortunately I don’t have a card reader I’m traveling. I only repeated what you said in a question. Let me try again. At every F-stop we expect a certain absolute field depth in focus, given a specific focal length and distance. My F-stop is fixed for this lens. My FL is fixed. Let’s concentrate on these values at 40ft for now. With a normal lens 500mm I can estimate the length of a ruler in focus at F8. It is about a foot. Somewhere between 10-14 inches for certain. Longer when set to F10. With my Tamron only 3 inches. Why? Why not a foot? This value 3 in corresponds to F4 at 40ft calculator \$\endgroup\$
    – Mark Robin
    Commented Apr 25, 2022 at 10:46
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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?

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  • \$\begingroup\$ Not sure what rack the lens means. I guess moving from one extreme to the other. Perhaps you’re referring to focus breathing. It happens very little to none with this mirror lens. Let me know if you’re curious how I measured that. I used a few different DOF calculators in reverse starting with distance, DOF and focal length. At 40ft the total depth was about 3” in my observation. F10 at long distances. I even experimented with CoC noticing that it effects 40ft roughly the same as 7000ft. Please help me understand if you can. \$\endgroup\$
    – Mark Robin
    Commented Apr 24, 2022 at 23:16
  • \$\begingroup\$ Rack refers to the gearing used to move the mirror to achieve a focus. It may be just moving the mirror forward or backward or it may be a combination of mirror and lenes. Anyway, I think the movement is about 1 inch. I am not troubled by your depth of field observations: these are too subjective to get excited about. I am troubled by your belief that this instrument functions a f/4.5 when working at 40 feet. That would make the image gets brighter as you work closer and closer, not happening! \$\endgroup\$
    – Alan Marcus
    Commented Apr 25, 2022 at 1:39
  • \$\begingroup\$ I am not referring to T-stop which effects brightness. I measured that separately it is above F8 at all distances. I am referring to F-stop which only effects DOF independently from T-stop in this case only. As you know they are related. If you use an online calculator you can manually change the aperture until you get the results I’ve accurately observed. \$\endgroup\$
    – Mark Robin
    Commented Apr 25, 2022 at 10:31
  • \$\begingroup\$ Yes DOF is subjective. It depends on sharpness, resolution, and the CoC in one calculator. Still I am confident it is within a range at all resolutions even resizing to S on my Canon. I even tried resizing to 640x480 as an exercise. Still the ruler is in focus for about 3-4 inches. Between 7 and 10 to be clear. Not 6-8 inches as expected. It is so badly out of focus in that range you cant even read the numbers or tell there are tick lines at all. My Tamron lens has a very narrow DOF at 40ft. Very different from a traditional lens. Why? \$\endgroup\$
    – Mark Robin
    Commented Apr 25, 2022 at 11:08
  • \$\begingroup\$ Focus breathing decreases FL at closer distances. The opposite of what I am experiencing, regarding aperture. \$\endgroup\$
    – Mark Robin
    Commented Apr 25, 2022 at 11:41

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