# focus scales on lenses

3 replies to this topic

### #1 Matteo Cocco

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Posted 09 November 2007 - 03:45 AM

i was just wondering if there is some kind of trick to consider when focusing on the lens.
i mean, if my lens has focus marks for 5 feet and 7 feet, will the 6 feet mark be in the middle or slightly more towards the 7 (considering the logarithmic progression of focus)?
is ther any trick to find these kind of intermediate marks?
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### #2 Mitch Gross

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Posted 09 November 2007 - 10:34 AM

It would 2/3 of the way to wards the 7' mark. Best way to consider the log nature is to picture this: a scale from 1' to infinity. Where is two feet? Exactly in the middle. Where is four feet? Exactly halfway between 2' and infinity. And so on.
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### #3 Chris Keth

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Posted 09 November 2007 - 06:53 PM

A lot of ACs, myself included, make extra marks on the lens so we have those intermediate marks. Especially in the 5-15 foot range that is used a lot, lenses are usually lacking in precise marks. During checkout I just measure out the distance I want (like 7 feet in this case), eye focus, and add the mark with some chart tape.
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### #4 Chris Keth

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Posted 09 November 2007 - 07:00 PM

I edited too slowly:

2/3 is a good approximation, but it's not quite right on all lenses. See this photo of some master primes:

The 100mm on the right shows that the 11, 12, and 13 foot markings are almost equidistant. I usually just estimate a little farther than halfway on the barrel is halfway in realspace.

If you need something more accurate, a lot of ACs including myself make extra marks on the lens so we have those intermediate marks. Especially in the 5-20 foot range that is used a bunch, lenses are usually lacking in precise marks. During checkout I just measure out the distance I want (like 7 feet in this case), eye focus on a siemans star to be as precise as possible, and add the mark with some chart tape.
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