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In photo cameras, i.e. (D)SLRs, digital cameras, etc., when you focus on an object and then you zoom (in or out), the object gets out of focus. On the other hand, in movie or TV cameras, the focus is kept across the whole zoom range. It is even the best practice to focus while zoomed in the most so that the focus is the best possible, and then zoom out as needed.

What is the difference in construction of the lenses for photo cameras compared to the movie/TV cameras? Or is this effect achieved in some other way than lens construction?

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    Related question: How is focus-breathing controlled in lens design? – scottbb Nov 25 '17 at 12:34
  • Back in the Dark Ages, the intro course in the RTF department at UT Austin made a point of teaching us to focus a zoom lens at full zoom, then zoom out, and our focus target would remain in focus. – John R. Strohm Nov 27 '17 at 20:22
  • @JohnR.Strohm Or, more accurately, the focus error introduced by the zooming would remain within the depth of field given by the narrow apertures most zoom lenses had back then. Please seeWhy are so many kit-lenses parfocal if it's an expensive feature? for more. – Michael C Nov 27 '17 at 22:35
  • @MichaelClark Cinema camera lenses were designed to give the cinematographer complete control over EVERYTHING, so those things could be used to artistic effect. There's a cliche' move, where you use a shallow depth-of-field setting, and then change the focus distance, so that one point in the far scene comes into focus while a point in the near scene goes out of focus. Directors routinely use this one to shift the audience's viewpoint from one place to another. (One of the "Mission: Impossible" films used this trick to very good effect.) – John R. Strohm Nov 27 '17 at 22:46
  • @JohnR.Strohm My apologies. I took your above comment to be in reference to using non-Cine lenses. What does 'RTF' stand for in the context of your comment? – Michael C Nov 27 '17 at 23:03
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The effect's name is parfocality, we are speaking of parfocal lenses. Lenses that change their focus when zooming are called varifocal lenses.

Like reducing focus-breathing (change in focus also changes the focal length), parfocality is a premium-feature. Since photographers usually work with autofocus and since photographers tend to not recompose the image while taking a photo, photographers usually wouldn't want to pay the premium fee for these features. Or so I think.

True parfocality (i.e. absolutely no change in focus) cannot be achieved (or only with extreme effort) - it's all about perceived parfocality, as in "it seemingly does not change the focus, therefore it's parfocal".


How to achieve parfocality?

Here's a diagram I stole from PetaPixel's article on this subject which is a screenshot from Vistek's YouTube-video on the subject:

Difference varifocal vs parfocal lens Note that this diagram is only exemplary - not all lenses are built in the same way. Anyhow, as it explains, parfocal lenses usually have an independetly moving focus lens group that varifocal lenses lack - usually to keep costs, weight, and/or lens dimensions (as in length*diameter) low.

Some relatively cheap lenses (at least compared to the ones from Angénieux) achieve a similar effect with tricks, like the Canon 24-105 f/4L IS USM (I):

"There's a cam inside [...] that is designed to maintain an accurate focus when the lens is zoomed from tele towards wide." (Chuck Westfall, Canon USA)

Also, some lenses with focus-by-wire (like Canon's STM-lenses) might try to hold the focus automatically. For example, when I filmed with a loaned Sony FS700 and its kit-lens (18-200mm f/3.5-6.3), I noticed that when I zoomed in/out slowly, I could see the focus motor working to keep the focus point where it was. However, when zooming in/out more vigorously, the focus point "broke loose" because the motor could not keep up with the rate at which the focus point changed.


Is every video-lens parfocal?

From the my example with the FS700 above, we can also see that not even all lenses that are marketed as TV- and cinema-lenses are parfocal. So if you buy a very cheap zoom-lens that is marketed as "video", it is very unlikely to be a parfocal one (unless it says so).

From my limited experience, a good rule of thumb to determine if a lens is parfocal or not seems to be the existence of an adjustable backfocus - Note that backfocus, in video-terms, does not mean the same as in photography: it refers to the Flange focal distance.


Website XY says that my lens is parfocal, but I find that it is not. Is my lens defective?

Your lens probably is not broken. Absolute parfocality cannot be achieved, but rather, it's all about perceived parfocality.

To understand that, we need to understand the relation between spatial resolution and the perceived "sharpness" of an image.

E.g. in analog days, most people used standard films and never looked at their photos at more than, say, 20x30 cm (8x12"). Even good films are said to have about the same spatial resolution as a 20 MP digital sensor and 20x30 cm isn't what we today are looking at - an EOS 5D MkIII takes photos with a resolution of 5760×3840 px, so on my 1920x1200 px 24" monitor, when I look at a 50% scale of the picture, I have a picture size of more than A2. This means that I can see things that I could not see on my analog print. Now think of a portrait - I mistakenly focussed on the subject's nose instead of their eye. I can clearly see that on 100%, while at 10% scale (~15x10 cm), it will not even bug me any more.

The same happens with parfocality: If the image is so small that I can't distinguish between 5cm in front (or in the back) of the focal plane and the plane itself, the photo seemingly is "in focus" between ± 5cm of the focal plane. Therefore, if the focus changes just as much as the visual tolerance allows, one can say that the lens is parfocal.

With thanks to Jannik Pitt and Michael Clark.


I bought lens XY and it seemingly is parfocal, although everybody seems to disagree. Who is correct?

It's the same as the chapter above: You perceive your lens as parfocal, only now, it is because of depth of field (DOF).

You probably used a very small aperture / a very large aperture number (e.g. f/16) and/or some (ultra-)wide-angle lens (f/16 will not be sufficient for 400mm, for example) and/or you are focussing on something very distant.

If the lens changes its focus at a small enough rate, you cannot perceive that - this is especially true when zooming out, as this will increase the DOF. Technically, it's not parfocal - but you will perceive it as parfocal, which, as I keep writing here, is all that matters.

With thanks to Michael Clark.

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    It's also desired with telescope and eyepieces. If you change the eyepiece to zoom in, it's great if you don't have to focus again. – Eric Duminil Nov 25 '17 at 16:19
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    Also, look at that diagram. That parfocal lens looks heavier than the varifocal lens. – Loren Pechtel Nov 27 '17 at 2:41
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    Couldn't one just measure the change of focus with the zoom and then automatically compensate for it while zooming so that even with varifocal lenses you get no defocusing effect? Technology-wise it sounds so simple to do. The autofocus should be able to know exactly what to do for every zoom level. – Trilarion Nov 27 '17 at 8:24
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    @Trilarion as said, stepper-motor lenses could do that. however, it's difficult to do so as the change isn't linear; e.g. when zooming from 24-35mm, the focus will shift backwards by 0.5cm/mm, between 35-50mm, it will change by 5cm/mm, etc.p.p.. IMHO, that's why the Sony-lens wasn't able to cope with fast zooming. Also, regular photo lenses usually just don't care about this as it is assumed that you are recomposing your image in between shots, not in a shot, and that you will therefore simply start autofocussing anew, as this will be more precise, anyway. Also, this will consume less power. – flolilo Nov 27 '17 at 10:29
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    That being said even lenses marketed as parfocal sometimes aren't parfocal. Both my 17-40mm f4L and my 70-200mm f4L change focus when zooming. – Jannik Pitt Nov 27 '17 at 14:59
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A lens that maintains the same distance of focus as it is zoomed is said to be parfocal. This is a highly desirable quality in a lens if one is shooting film or video and wishes to maintain focus while zooming in or out during a continuous shot.

Some zoom lenses that are not really parfocal, particularly those with smaller maximum apertures, can appear to act like they are parfocal if the focusing error introduced by zooming is less than the depth of field at the lens' widest aperture.

Roger Cicala recently addressed the subject in his blog entry Mythbusting: Parfocal Photo Zooms (Bold type added by me).

About twice a week I get a call from a photo-videographer wanting to know which photo zoom lenses in a certain focal length are parfocal (stays in focus while changing focal lengths). It’s understandable why they would ask; a superb photo zoom is 1/10th the cost of a good video zoom.

They get pretty mad when I tell them that none of them are. Often, they’ll tell me they know this lens is or that one is, because LensGuruGod1232 on their favorite forum shoots with this lens and says it is. They may even – if they’ve done some research – pull up an old article I wrote years ago listing some photo lenses that were parfocal and add that to the argument.

I have to tell them that article doesn’t count anymore. For one thing, parfocal is not an absolute definition. What is acceptable as parfocal to one person in one situation is unacceptable in another. Also, equipment has changed. A lens that might have appeared parfocal on a small sensor shooting standard definition video may be obviously not parfocal on a large modern sensor shooting 4k video. Finally, I’ve gotten older and wiser. Some things I wrote about years ago, I now realize are, um, well, less correct than I would have liked.

He goes on to point out that one copy of a specific lens model can demonstrate more or less parfocality than another copy of the same lens model.

Similarly, most cinema lenses also demonstrate very little or no focus breathing. Focus breathing is a change in the lens' field of view as the focus distance is changed. This is also a desirable feature for cinema lenses so that the focus distance can be changed in a continuous shot while the exact same framing is maintained.

Lenses designed for still photos are generally used to take much higher resolution photographs than lenses designed for video. Still photographers generally care about absolute image quality more than whether a lens is parfocal or doesn't demonstrate focus breathing.

The emphasis on still lenses is absolute optical image quality. The emphasis on non-Cinema quality video lenses, particularly those in the same price ranges as lenses primarily intended for still photography, is being 'close enough' to parfocal and not demonstrating focus breathing while being 'good enough' optically for HD or 4K video.

See also:

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Lenses have to be specially designed in order to keep the same focus point when zooming. Making lenses this way is more complex and costs more, so such lenses are more expensive.

This feature is usually not important to photographers, so lenses intended for photographic use typically don't do this. But video lenses typically do have this feature as it allows for zooming while recording without the picture going out of focus, giving the videographer a wider range of creative effects that they can use.

There are, however, some photography lenses that do have this feature, although they are more expensive and usually aimed at a specific market that needs them. It's not uncommon to see telephotos with this feature, as it is very useful when shooting sporting events with a telephoto lens to not need to refocus all the time, and the shorter depth of field with a telephoto lens makes refocusing even more important.

Another feature that you'll see on video lenses is the ability for the lens to be focused without changing the focal length. This effect is less noticeable and has little impact on the photographer, but is again very useful for video as it allows changing the point of focus while recording and it's pretty common to see this effect used in video.

TL;DR Making lenses that can zoom without losing focus is more expensive. It's usually unnecessary for photography lenses to have this feature but it's very useful for video lenses.

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