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According to this answer, parfocal lenses (lenses that can zoom in/out without losing focus) are relatively rare and more expensive.

However, the original poster's kit-lens is parfocal. Additionally, the kit-lens that came with my Sony a390 and the kit lens with my brother's Nikon D3200 (both entry-level DSLR's, with presumably cheapo kit-lenses) are parfocal as well.

Does anyone know why, if this is a feature primarily of expensive lenses, so many cheapo kit-lenses seem to support it? What features of the lens cause it to be parfocal or not?

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Key phrase: typically. Making a parfocal lens is more expensive, but it doesn't make them prohibitively so. You'll probably also find that many of these aren't perfectly parfocal. –  John Cavan Apr 19 '13 at 18:56
Most Minolta lenses were parfocal, so I assume that the designs Sony got from Minolta were easy to keep that way. –  Itai Apr 19 '13 at 19:02
One reason will be video. In general, that's very desirable and video is a significant selling point now. –  John Cavan Apr 19 '13 at 19:13
Without knowing the intricacies of lens design, I would guess that shorter zooms are easier to make parfocal, which most kit lenses are. –  Itai Apr 19 '13 at 19:29
I'm only guessing here, but a cheap lens is not cheap on materials only, design is cheap too, which you can safely read as "simple". Then, if the lens is "simple" by design, I'd guess it must be so much easier to make it parfocal, or at least nearly parfocal. Designing a quality lens to be parfocal must be a question of higher mathematics, and perhaps getting high quality lens to stay affordable to as large group of buyers as possible, they don't bother with keeping it parfocal. Simple lens = easy. Quality lens = much harder. I'm only half awake in the middle of the night :( –  Esa Paulasto Apr 19 '13 at 21:24

1 Answer 1

One factor is the lens' maximum resolution. The less sharp a lens is at its peak resolution, the easier I think it is to label it parfocal. If a lens is razor sharp when focused properly, even a little movement one way or the other will be quite noticeable. Not quite so much if the lens is a little soft to begin with.

Another factor that may make it easier to consider a kit lens parfocal is the lens' maximum aperture. At f/3.5-5.6 the depth of field is a lot deeper than at f/2.8, so even if the focus moves a little between 18mm and 55mm there is a lot of overlap in the DoF at the two focal lengths of the EF-S 18-55mm f/3.5-5.6 IS II. There is nowhere near as much overlap between 17mm and 55mm at f/2.8 using the Ef-S 17-55mm f/2.8 IS. At 55mm with a subject distance of 10' the DoF at f/5.6 is twice the DoF at f/2.8: 2.15' vs. 1.07'.

I'm not convinced the cheaper kit lenses are truly parfocal as much as that they are effectively parfocal due to the narrower maximum apertures and lower peak resolution. In most cases you can focus at the longest focal length and then as you zoom out the increase in DoF is enough to compensate for any small movement in the true center of focus. With a true parfocal lens you could also focus at 18mm and zoom in without needing to refocus.

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Do you have any sources? From the tone, it sounds like these are all just educated guesses. –  BlueRaja - Danny Pflughoeft Apr 22 '13 at 19:54
It is fairly common knowledge that the increased DoF from narrower apertures will mask minor focus inaccuracies. Here is a quote from The-Digital-Picture.com's review of the EF-S 18-135mm f/3.5-5.6 IS STM: "And also helpful for video capture is that this lens tests to be essentially parfocal, meaning that focus distance settings do not change as focal lengths are changed. Of course, the depth of field at this lens' relatively narrower max apertures makes this feature less challenging to achieve - but it is still very helpful for shooting video." –  Michael Clark Apr 23 '13 at 0:59
Not to mention the fact that Canon does not list their kit lenses as parfocal, even though they are effectively so. lensrentals.com/blog/2011/02/photo-lenses-for-video/4 –  Michael Clark Apr 23 '13 at 1:05

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