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11-07-2014, 01:00 AM   #1
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Focus distance on different lenses

I have a few lenses and each have different focus distance values shown. E.g the M135mm shows up to 20m, whilst my M50mm only up to 15. Heck, the Vivitar only shows up to 3m.
They all focus to infinity, but the value before infinity is different. Could someone explain to me what this means and what the difference is? I find that the vivitar with only 3m shown, focuses poorly at infinity whilst the M135mm does much better.

Because I'm looking at some of the newer lenses, where the focus distance shown on the lens is only 2m or less!!!


Please explain this!

11-07-2014, 01:56 AM   #2
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It's related to the hyperfocal distance and depth of field. Wider lenses will have a smaller number close to infinity, but that doesn't mean that you can be careless about the focus setting at infinity, as it will still have an impact.

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11-07-2014, 02:39 AM - 2 Likes   #3
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The focus ring can have any values the designers want. This is why old manual focus lenses have a long focus throw (more degrees to turn from minimum focus to infinity). That way manual focus is more precise.
But with AF lenses, you want a short focus throw, because that makes AF faster. It makes MF more imprecise, but the idea is that "nobody uses MF with modern gear anyway." Modern macro lenses might be an exception btw.
Anyway, the numbers listed on there don't matter, you can still focus on everything in between to some degree of precision. Wide angle lenses have a big depth of field, so the difference between focusing 50m and 5km might not be as big as it is with a 300mm lens, which will have a more shallow DoF and will have a noticeable difference between focusing on 5m and 6m. The DoF is shallowest at nearest focus, at lowest f-number, and with tele lenses (high mm number). Notice that most lenses only have distance numbers for the range that the designers assume they will be used the most. Nobody will use a 50mm lens and take photos where 200m and 205m will be critical. But in the near range, 60cm-5m? That is where focus must be very precise, with a 50mm lens.

Tl;dr: If you use AF, just forget it. If you use MF, the distance scales are not as important as the focus throw, which affects the precision of the focusing. But in theory, all distances from minimal focus to infinity should be attainable regardless of the labels.
11-07-2014, 03:04 AM   #4
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Well that cleared up quickly! So does that mean my Vivitar (28mm 3.5) isn't out of focus at infinity because its designed so, but because of my copy?

11-07-2014, 03:32 AM   #5
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QuoteOriginally posted by Anthen Quote
Well that cleared up quickly! So does that mean my Vivitar (28mm 3.5) isn't out of focus at infinity because its designed so, but because of my copy?
That probably means that it has not accurate infinity calibration at so called 'dead stop' - in other words the point when perfect infinity focus is achieved is just little bit earlier than focusing ring stops at infinity mark. That is common to many lenses and vast majority of my manual focus lenses ( and ALL autofocus lenses ) - are having infinity focus spot on just little bit before the stop mark on the focusing ring. Using liveview or focus magnifier will help to verify that.

I am not talking about the other scenario when the lens is not achieving sharp infinity focus at any point BEFORE the focusing stop is reached. In such cases the lens is not calibrated properly and the infinity is not possible without servicing the lens.
11-07-2014, 08:47 AM   #6
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The numbers printed on the distance scales are fairly arbitrary except for the infinity sign. On a related note, the DOF scales vary according to target format (APS-C vs. 35mm vs. whatever) and between manufacturers.


Steve
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