Why do depth-of-field scales stay meaningful across a prime lens’s focus range?
Asked 9/27/2018
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On many manual-focus prime lenses, the engraved depth-of-field (DoF) scale seems to remain useful from close focus to infinity, even though the actual near/far distances covered at a given aperture change dramatically.
Why does this work? Is it because lenses are intentionally designed so the focus-ring rotation maps to subject distance in a special way, or is it mainly a consequence of optics and the non-linear distance scale on the lens?
Also, are there lenses where the same aperture marks would correspond to noticeably different angular spread at short versus long focus distances, making a fixed DoF scale less valid? If modern lenses often omit DoF scales, does removing that requirement make lens design easier, cheaper, lighter, or allow closer minimum focus?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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What you describe is not a "constraint" on the "design" of the lens. Rather, it's a consequence of physics.
The lines in the DOF graph from Points in Focus curve because the scales are linear. The curved lines indicate that the increase in DOF accelerates as distance increases. If you look at the focusing scale on a real lens, you'll see that it is not linear. Combine the non-linear change in distance with an accelerating DOF and you get something similar to the markings you see on physical lenses.
Every lens I've ever seen with a DOF scale works this way. Some old zoom lenses have curved DOF lines that correspond with the change in focal length. This also is a consequence of physics, not lens design.
This mapping could be anything... Without that constraint, would it be easier to design cheaper or lighter focusing elements? Would it be easier to design lenses with a shorter minimal focusing distance?
The mapping itself has nothing to do with optics, but movement of the relevant focusing mechanisms. The simplest design, linearly translates rotation with lens-element movements. The lens element movements are non-linearly related to focus distance. To linearly translate rotation to focus distance would require a non-linear translation from rotation to lens-element movements. Making the change in a lens that has distance and DOF markers would require introducing more complicated mechanics, perhaps a number of large, oddly-shaped gears. Great for steampunk cosplay, not so great for manufacturing, weight, and cost.
While the translation change could be done easily in modern cameras, they would do so via electronic control of the focusing motors, not by changing lens design.
Some cameras, such as the FujiFilm X-T3, have drive-by-wire focusing systems and allow switching from non-linear to linear mappings between focusing ring movements and distance. The mapping change is performed by the camera processor and electronic control of the focusing motor, not by changing the lens design. The physical relationship between lens element movements and focusing distance remains the same. Lenses for such cameras do not have distance or DOF markers.
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
7y ago
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It’s mainly a consequence of optics, not a special design constraint imposed to preserve the DoF scale.
Depth of field does not change linearly with subject distance, and lens distance scales are also non-linear. As focus distance increases, the distance markings bunch up toward infinity. Combined with the way DoF expands faster at longer distances, this is why a fixed engraved DoF scale can remain reasonably meaningful across much of a prime lens’s range.
So the mapping is not “anything”; it follows from the physics of image formation and focus. On real lenses with DoF scales, this behavior is normal. Older zoom lenses sometimes used curved DoF markings because focal length changes alter the relationship.
That said, DoF scales are only approximations. Their usefulness depends on assumptions about acceptable sharpness, viewing conditions, and lens performance. Different lenses are not perfectly identical, and aberrations mean the scale is never exact.
Modern lenses often omit DoF scales not because they are hard to preserve, but because cameras now offer better focusing aids and because the old scales were always approximate.
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AI7y ago
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