I know when I change focal length of a zoom lens the physical length of it is changed. It's understandable: the optical parameters of the lens were changed.

But what is going on when (auto-/manual-)focus is changed? What is changes in my primary fix lens if I focus on different distances? The physical length of the prime lens is the same.

In general, how does (auto-/manual-)focus changing work and what is difference between focus changing and focal length changing?

UPDATE: this question isn't actually about how manual focus works but about similarities and differences between focus process and changing focal length in zoom lens


SONY alpha lenses, like many other brands, are internal focusing.

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.

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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


There are a lot of different ways lenses are designed.

Some zoom lenses change length when zoomed.
Some of those zoom with a push/pull motion. (EF 100-400mm f/4-5.6 L IS)
Others zoom by twisting a zoom ring. They might (EF 70-300mm f/4-5.6 IS) or might not (EF-S 55-250mm f/4-5.6 IS II) rotate the front element as they zoom.

Other zoom lenses zoom using a zoom ring that moves internal elements and the overall physical length of the lens doesn't change when the lens is zoomed. (EF 16-35mm f/2.8 L II, EF 17-40mm f/4 L, the EF 70-200mm series with various maximum apertures in both non-IS and IS versions)

There are zoom lenses which both extend to zoom and move the front element in and out to focus.
Some of those rotate the front element when focusing (EF 75-300mm f/4-5.6, EF-S 18-55mm f/3.5-5.6 IS II).

There are zoom lenses which extend to zoom but have internal focus.
They usually don't rotate the front of the lens when zooming. (EF 24-105mm f/4 L IS, EF-S 17-55mm f/2.8 IS, EF-S 18-135mm f/3.5-5.6 IS, etc.)

There is also at least one lens with internal zoom but with front elements that move in and out with focus. But the front element is recessed in the lens barrel enough that the overall length of the lens is not affected by the movement of the front lens during focus adjustments. (EF 17-40mm f/4 L)

There are other zoom lenses with both internal zoom and internal focus.
The only external movements with such lenses is when the zoom or focus rings are turned by the user. (The Canon EF 70-200mm f/2.8 and f/4 series, EF 200-400mm f/4 IS Extender 1.4X)

Some lenses, including some prime lenses (lenses that don't zoom), move the front elements to focus and the physical length of the lens changes.
Some of those, such as most macro lenses, will change length significantly. (MP-E 65mm 1-5X Macro).
For other lenses the length only changes slightly with focus adjustments. (EF 50mm f/1.4)

Other lenses, including both zooms and primes, have internal focus and the overall length of the lens does not change at all with focus adjustments. (EF 70-300mm f/4-5.6 L IS, EF 28mm f/1.8, the vast majority of Canon's prime and zoom lenses other than the cheapest entry level lenses)

There are prime lenses that have internal focus and don't change length when focusing. (EF 100mm f/2, most Canon prime lenses other than the cheapest few)

There are prime lenses that do change length when focusing.
Some don't rotate the front element. (EF 50mm f/1.4)
Others may also rotate the front element. (EF 50mm f/1.8 II)

It just all depends on how each lens is designed. Some designs are cheaper to make, others are more expensive to make.

  • 1
    Thank you for explanation! Honestly, I haven't known about this internal conception of changing optical characteristics of lens at all. – Andriy Kryvtsun Sep 4 '16 at 12:11

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