Orquid "Phoenix"

Orquid "Phoenix"

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I heard someone say that after using a parfocal lens for the first time, they would never switch back. What is a parfocal lens, how is it different from any other lens, and what are the advantages and disadvantages for photography? Would you pay extra for this or prefer it over varifocal?

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Another question I am trying to answer with this is how much of a benefit is the parfocal Canon EF 24-70mm f/2.8L USM over the varifocal(I believe) Canon EF 24-105mm f/4 L IS USM. –  dpollitt Aug 16 '11 at 14:14
    
As far as I know, some (high-grade) lenses approach being parfocal, but no lens maker makes it a selling point because it's impossible to guarantee it during a lens's lifetime. –  Berzemus Aug 17 '11 at 8:48
    
Here is a question that might be related in some ways: photo.stackexchange.com/questions/10734/… –  Sean Aug 19 '11 at 3:52
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@dpolitt: Just one thing: What makes you assume that the 24-70 is actually parfocal? Nothing I have seen or heard about this lens indicates that it is better or worse than the 24-105 in this regard. There are some lists saying that this or that Canon lens is parfocal, but none are official and people have reported that their copies of those lenses are not, actually, quite parfocal. –  Staale S Aug 19 '11 at 7:39
    
@Staale - I figured as much from a google search, as Canon doesn't give us this info as far as I know. I agree, it is a bad assumption. So it "depends" on some factors if your given copy is parfocal or not... man this is confusing! –  dpollitt Aug 19 '11 at 18:21
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3 Answers

up vote 20 down vote accepted

Parfocal lens is a lens which remains in focus when you change the focal length. The non-parfocal lens is called varifocal.

It is very convenient to focus at the maximum focal length and change the zoom afterwards. It is more important for manual-focus lens because a well functioning auto-focus can quickly adjust the lens to keep it in focus.

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Why is it more important for manual-focus lenses? Why is it convenient to focus at the max focal length? Is the focus always perfectly held across the range? –  dpollitt Aug 16 '11 at 14:12
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@dpollit - i would imagine focusing at max range would be great (for example) when you want to make sure you're focused on the eyes, but then zoom out to capture the whole person –  rfusca Aug 16 '11 at 14:39
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@dpolitt: For autofocus lenses it does not really matter so much because refocusing is only a button-press away at all times. Zoom, focus, shoot. In fact, the default setting for most cameras is that pressing the shutter button half-way autofocuses, pressing it further down takes the photo, so you will refocus whether you want to or not! Focusing at max zoom is beneficial because, well, it is zoomed in, magnified, so you can better see what you are doing. Also helps with placing the autofocus point on the eye and not the ear. –  Staale S Aug 16 '11 at 15:14
    
@Staale - Thank you for a comment, I like your answer better then the one you commented on. –  dpollitt Aug 19 '11 at 1:33
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Parfocal lenses remain in focus when you change the focal length as previously stated. They are most useful for movie/video work - it's awkward to keep having to pull focus as your zoom changes.

See also 'focus breathing', where a change in the focus changes the apparent focal length slightly. Again this is something it's nice to avoid in movie/video work. In general the more you pay, the less they breathe...

Lens design is always a series of trade-offs, if money, size and weight are no object you can build a lens that will be parfocal, fast and have high quality optically. So in the movie business they end up with lenses like this: http://www.flickr.com/photos/mrmitch/2185341989/in/photostream

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A valid use case rom this article seems to be to shoot landscapes at wider aperture:

The key though, is, if you have a parfocal lens, use it to shoot at the hyperfocal distance (again, good old Wikipedia can help with an explanation of this - http://en.wikipedia.org/wiki/Hyperfocal_distance ) at a wider aperture than usual in landscapes. This gets us away from f22 and f16 down to sharper apertures like f13 and f11 - while still maintaining front to back sharpness. Many pro's, like David Noton, use this. Look at the technical info on many of his landscapes and they are shot at f11.

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I'm not visualizing something correctly maybe, why does a parfocal lens allow you to shoot at the hyperfocal distance at a wider aperture? –  rfusca Aug 19 '11 at 17:47
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@rfusca: From what I understand, if distance X is the closest distance in focus for shooting landscapes, opening the aperture increases X and moves it closer to a landscape at infinity (e.g. a mountain).. Without parfocal lens, you'd have to get a higher aperture value and focus on the closer object (harder to focus on a mountain peak at lower focal lengths).. and since shooting at hyperfocal distance, the mountain will be in focus. However If you have a parfocal zoom, you zoom in on the mountain peak, set the focus and zoom out.. you don't need a higher aperture value anymore. –  Sridhar Iyer Aug 19 '11 at 18:08
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seems like you could replicate this to a certain degree with live view for landscapes –  rfusca Aug 19 '11 at 21:37
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@rfusca: yes. its fairly new though.. I absolutely love the fact that I can do 20x mag via live view and get a sharp focus using manual focus. –  Sridhar Iyer Aug 19 '11 at 23:15
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