How can I estimate focus-step size for field focus stacking with different lenses?
Asked 2/25/2016
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I’m trying to use Magic Lantern for focus stacking in the field. It lets me choose a number of steps and a step size, but I want a rough way to relate those settings to actual focus-ring movement so I can estimate how many shots I’ll need from a near focus point to a far one.
On one macro lens, I can see the ring move, but on lenses with internal focusing there’s no visible movement to compare. Is there any practical approximation for how much a focus step changes focus, or how to translate that between different lenses?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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A lot depends on the focus distance. The same amount of focusing ring movement at a very close focusing distance will move the point of focus a LOT less than the same amount of focus ring movement at a far focusing distance. And when you move from one lens to another all bets are off, especially if you are measuring the movement of the zoom ring in inches rather than degrees of rotation. A larger barrel diameter will mean the same rotational movement will be larger in term of inches. But that's not really significant either because some lenses have longer focusing ring throws (the amount of rotation needed to go from minimum to maximum focus distance) than other lenses.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
0
Generated from our catalog & community — verify before relying on it.
There isn’t a single useful conversion.
The effect of one focus step depends heavily on:
- the lens
- the focus distance
- the lens’s focus throw
- whether it uses internal focusing
A small ring movement at close focus changes the focus distance much less than the same movement at farther distances. Different lenses also have very different focus throws, so “an inch of ring travel” is not comparable between lenses; barrel size changes that measurement even if the actual rotation is the same.
So a fixed rule like “50 steps per inch” won’t reliably transfer from one lens to another, or even from near focus to far focus on the same lens.
For field use, the practical approach is to calibrate each lens at the working distance and magnification you plan to use, then test step sizes empirically. For internal-focusing lenses, judge the result by focus coverage in the captured images rather than visible ring movement.
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