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Forum: Troubleshooting and Beginner Help 11-07-2014, 02:39 AM  
Focus distance on different lenses
Posted By Na Horuk
Replies: 5
Views: 1,141
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.
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