Why does a 500mm f/8 mirror lens seem to have much shallower depth of field at 40 feet?
Asked 4/22/2022
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I’m trying to understand the depth of field behavior of a Tamron SP 500mm f/8 mirror lens. I know mirror lenses are fixed-aperture and that T-stop can differ from the marked f-stop. When I tested mine, the measured transmission was slower than f/8, which I expected, but the depth of field at close distance seems much narrower than I’d expect.
At about 40 feet, only roughly 3–5 inches looks acceptably sharp. At long distances, I can get several distant signs in focus from about 1300 to 7000 feet. If I try to infer an f-number from those DOF observations, the close-distance result seems more like f/4–f/5, while the far-distance result seems closer to f/10. Why would the apparent DOF be so different at 40 feet on an f/8 mirror lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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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.
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
4y ago
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Generated from our catalog & community — verify before relying on it.
A mirror lens doesn’t behave quite like a normal refractive lens in DOF tests. The key reason is the central obstruction: its blur circles are ring-shaped rather than solid discs, so standard DOF calculators and visual DOF estimates can be misleading.
Also, T-stop affects exposure, not depth of field. A slower T-stop does not make the lens behave like a smaller aperture for DOF; it’s more like losing light to an ND filter.
At 40 feet, focus is achieved by extending the lens, which slightly increases the effective focal length and working f-number. But that change is small — not enough to explain an apparent shift to f/4.
What’s more likely is that at close distance the very long focal length already gives extremely shallow DOF, and the mirror lens’s ring-shaped blur, lower contrast, and reduced sharpness make the in-focus region harder to judge consistently. So your visual/experimental estimate can look very different from a simple calculator result.
In practice, for a 500mm mirror lens, trust real-world focus tests over standard DOF formulas, especially at close range.
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