How should I read a lens distance scale when setting hyperfocal distance?
Asked 8/12/2014
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I use Nikon lenses with engraved distance scales so I can set hyperfocal distance manually. For example, at 28mm and f/16, the hyperfocal distance is about 1.66m, but the lens only shows marks at 1m and 2m with nothing between them. The scale does not look linear, so is halfway between 1m and 2m actually 1.5m? When a needed focus distance falls between marked values, how should the scale be interpreted?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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Okay, I'm pretty sure this checks out, but haven't yet sanity-checked it with any physical lenses.
Let's say 0 and 1 represent the distance scale / focus ring position at the MFD mark and infinity, respectively. Then 0.5, for instance, would mean turning the ring halfway between the extremities.
Then, we have a hyperbolic curve like this:
a = 1 - dmfd / df
where dmfd is the minimum focus distance and df is the desired focus distance. a is the number between 0 and 1 indicating how far to turn the focus ring. As you see, if we have dmfd=df, then a=0 as it should, and if df=∞, then a=1, also as expected.
Let's say your MFD is 0.3m and you want to focus at 1.6m. Then
a = 1 - 0.3m / 1.6m = 0.8125,
indicating you should turn the focus ring about four fifths of the way from MFD to infinity; if there are 1m and 2m marks in the distance scale they should be at a=0.7 and a=0.85 respectively, so the correct position would be about three fourths of the way from the 1m mark towards the 2m mark.
Originally by user14547. Source · Licensed CC BY-SA 4.0
user14547
12y ago
0
Generated from our catalog & community — verify before relying on it.
Lens distance scales are generally not linear. As focus approaches infinity, a small turn of the focus ring represents a large change in subject distance, so the markings get compressed.
That means the halfway point between the 1m and 2m marks is usually not exactly 1.5m. On many lenses, 1.66m will be somewhat closer to the 2m mark than to the 1m mark.
In practice, engraved distance scales are approximate, especially on modern autofocus lenses, so you usually estimate between the marks rather than expecting exact precision. For hyperfocal focusing, this is normally good enough because depth of field at 28mm and f/16 is quite forgiving.
If your lens also has depth-of-field marks, those are often more useful than trying to place the focus ring at an exact intermediate distance. Otherwise, set the focus a little short of the 2m mark rather than exactly halfway.
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