How focusing changes a lens differently from zooming
Asked 9/2/2016
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What changes inside a lens when you adjust focus compared with when you change focal length on a zoom lens? On some prime lenses the barrel length seems to stay the same while focusing, so what is actually moving? I’m especially interested in the optical difference between changing focus and changing focal length, not just how the focus ring works mechanically.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
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SONY alpha lenses, like many other brands, are internal focusing.
Only the middle groups of the optical system are moved to achieve focusing, which leaves the total length of the lens intact. Benefits include fast autofocusing and a short minimum focusing distance. Also, the filter thread at the front of the lens does not rotate, which is convenient if you’re using a polarizing filter.
This means that the actual length of the lens does not change during focusing. This was not always true. Nikon for example, only began manufacturing internal focusing lenses in 1976. They are designated IF.
From Ken Rockwell's Nikon lens site:
"Internal Focusing." In the old days, the entire lens had to move in and out to focus. Telephoto lenses had to be designed with huge focusing tracks just to let them focus at all, and they couldn't focus very close because the helicoids just weren't long enough. The long focal lengths meant that there were long distances the lens had to move to focus.
Nikon discovered that one could focus the lens by just moving some elements around inside the lens barrel.
IF lenses focus closer and faster than conventional telephoto lenses. IF was a fantastic innovation for telephoto lenses when Nikon invented it in the 1970s for the manual-focus super teles. Today, most modern AF zooms, super teles and some macro lenses use this technique. It helps AF lenses focus quickly because there is less glass to have to move around.
The optical trick is that the internal elements move slightly to shorten the lens' actual focal length as one focuses closer. This lets these lenses focus very close. It also means that when compared to a traditional lenses that the IF lens will appear to have a slightly shorter focal length than marked at close distances. This discrepancy disappears at infinity.
Some prime lenses are not technically IF.
For example, the Nikon 50mm f/1.4 AF-S:
This isn't technically an internal-focusing lens, as the deeply recessed front element extends towards the front of the barrel at close focus distances. However, the overall length of the lens remains unchanged and the front element doesn't rotate during focusing
Originally by user49286. Source · Licensed CC BY-SA 4.0
user49286
9y ago
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Generated from our catalog & community — verify before relying on it.
Focusing and zooming change different things.
To focus, the lens changes the spacing or position of one or more optical groups so light from a subject at a chosen distance forms a sharp image on the sensor/film plane. In many modern lenses this is done by moving internal elements (“internal focusing”), so the outside length may not change at all. Older designs often moved the whole optical assembly or front section, so the lens could extend while focusing.
Zooming changes the lens’s effective focal length, which changes angle of view and magnification. That usually requires moving different lens groups in a coordinated way. Some zooms extend externally, while others also do this entirely with internal movement.
So the visible barrel movement is just a mechanical design choice. The key optical difference is:
- focus adjustment: sets subject distance for sharpness
- zoom adjustment: changes focal length / field of view
A prime lens keeps essentially one focal length, but it still focuses by shifting elements relative to each other or to the sensor plane. A zoom lens can also focus internally; the two functions are separate even though both involve moving elements inside the lens.
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