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Apr 27, 2022 at 5:44 comment added xiota "I’d like to understand why it is so much lower at 40ft." – That's why I want to see the images you're looking at.
Apr 26, 2022 at 22:23 comment added Mark Robin I’d like to understand why it is so much lower at 40ft. Let’s just agree it is roughly F8 at longer distances.
Apr 26, 2022 at 7:59 comment added xiota I don't know what explains your long distance looks like F10 results. My experience with mirror lenses is that DOF is always poor, at all distances. When you have access to a card reader and are able to upload examples, please notify me. I'm interested to see what you're looking at.
Apr 25, 2022 at 10:56 vote accept Mark Robin
Apr 25, 2022 at 10:46 comment added Mark Robin 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
Apr 25, 2022 at 3:57 comment added xiota It's not really clear what you're doing or calculating. Can you add sample images to your question?
Apr 25, 2022 at 1:08 comment added Mark Robin 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?
Apr 24, 2022 at 23:36 comment added xiota 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.
Apr 24, 2022 at 23:30 history edited xiota CC BY-SA 4.0
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Apr 24, 2022 at 14:56 vote accept Mark Robin
Apr 24, 2022 at 14:56
Apr 24, 2022 at 14:54 comment added Mark Robin 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.
Apr 24, 2022 at 1:42 history edited xiota CC BY-SA 4.0
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Apr 24, 2022 at 1:31 history answered xiota CC BY-SA 4.0