Zoom lenses that don't change focus point when you change the focal length

<p>Are there any Nikon zoom lenses that retain the focal point irrespective of the zoom level? </p>

<p>Example: With the 18-105 lens, if you zoom to 105 mm, focus on something, and then zoom out to 18 mm, without changing focus (keeping the shutter button half pressed) and then take a snap, will the focal point be the same?</p>

<p>Even true zooms (that aren't varifocal designs that need to be refocused) may not retain accurate focus when changing focal lengths. I wouldn't trust it unless the lens was stopped down enough to offset minor focus errors.</p>

<p>Thanks Lex. That's what I thought. It's harder to focus on a small area with a wide focal length though. So it'd have been a neat feature if you could zoom in close, set focus, and zoom out and the camera intelligently adjusts the focal point proportionate to the new zoom level. </p>

<p>I'm not sure whether the variations are enough to be of any practical concern. Nikon's FV Lock flash option relies on maintaining focus while changing focal lengths. I've used it with three different Nikkor zooms to focus and change focal lengths and recompose without refocusing to maintain a consistent flash exposure. I don't recall seeing any focus problems, although I don't recall whether I've ever used the f/2.8 zoom wide open in that situation.</p>

<p>Interesting. With the 18-105 I mentioned above, there's also the complexity of max aperture changing across focal lengths. </p>

<p>Thanks Lex. </p>

<p>Parfocal lenses are are more complicated and expensive designs that were generally dropped as AF cameras took over. They are still important for video/movie applications.<em><br /></em></p>

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<p>I've used it with three different Nikkor zooms to focus and change focal lengths and recompose without refocusing to maintain a consistent flash exposure.</p>

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<p>I doubt that focus distance changes in varifocal zooms are significant enough to manifest themselves in a change in flash exposure. I doubt the distance information delivered by the chip is on a fine enough grid to even pick up the difference.</p>

<p>I read somewhere that the 70-180 is parfocal - I never bothered to check though (and now I can't since I sold the lens earlier this year). In any case, the effort necessary to make a zoom parfocal seems to exclude that any cheap kit lens will be of that type.</p>

<p>Dieter, a 'varifocal zoom' is an oxymoron. A true zoom lens should not change focus with focal length, whereas a varifocal lens, by definition, will show a marked focus change.<br>

That said, there are very few zoom lenses that hold their focus exactly when zoomed. This used to be pretty much immaterial in the days of smudgy old film, but now that digital pixel-peeping has become the norm, we can easily see the shortcomings of all our lenses.</p>

<p>FWIW, the technique that used to be taught was: Zoom in to maximum focal length; focus; and then zoom out to the required framing. That way any change in focus is made less obvious, since the depth-of-field increases as the lens is zoomed out. Still a valid way of using manually focused zooms, but AF and magnified Live View have made it almost redundant these days.</p>

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<p>a 'varifocal zoom' is an oxymoron</p>

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<p>I am just using the term zoom as it commonly is used - wrongly - and needed a way to indicate that I wasn't talking about a parfocal. A "true" zoom is indeed ideally parfocal - but have you seen a lot of mentioning of "varifocals" instead of "zooms" on manufacturers websites or elsewhere? Everyone call those not parfocal lenses zooms - there is no "truth in advertising".</p>

<p>Setting the Wayback Machine to 1925, we find the definition among lens designers for "zoom" meant a changeable focal length while maintaining constant focus. A lens that doesn't retain its focus when "zooming" was simply a variable focal length lens. The terminology fell by the wayside over the ensuing decades and the word "zoom" is applied to any lens that changes focal length. Designing and building a true zoom meeting the original definition is nasty expensive, so very few are made. Most are used in the motion picture industry.</p>

<p>As a matter of curiosity, the term "telephoto" originally meant a lens that with a physical length shorter than its focal length. Since making a wide angle lens physically shorter than its focal length was effectively impossible, the term evolved to mean longer focal length lens, which is probably a more practical use of the word anyway.</p>

<p>"So it'd have been a neat feature if you could zoom in close, set focus, and zoom out"</p>

<p>If your goal is to zoom in with a medium to wide lens to adjust focus and then zoom out to recompose, try Live View. Works quite well if your camera body has that function. Many macro shooters use this regularly to make certain of focus. </p>

<p>The accuracy of parfocal lenses is a compromise, like any other optical parameter of a lens. The focal point moves fore and aft as the lens is zoomed, crossing zero at 3 or fewer positions. Some ENG and most studio cameras have an adjustable backfocus, which allows you to optimize the focusing accuracy. With autofocus, it becomes less of an issue, so parfocal performance can be sacrificed in order to improve other parameters, including the cost.</p>

<p>My Nikon f/2.8 zooms are pretty good, but not perfect. When autofocus is engaged, the adjustments are very quick, and you don't really notice them because they occur just as you press the shutter release. Try zooming in and out on a nearby object, then pressing the shutter release (or AF button). You'll hear it happen, and possibly see the results in the viewfinder.</p>

<p>The most accurate zoom lenses are probably specialized for cinematic use. The cost of these lenses is staggering - exceeding $150K. Even single focus cinematic lenses cost $50K and up. Most production companies rent lenses, cameras and other gear, depending on their needs.</p>

<p>Zoom lenses designed for movie hold their focus when zoom as cinematographer often focus on a subject and then zoom in or out. I have used some enlarging zoom lens in the Noritsu enlarger. You focus it with very finely threaded tube but then you don't touch the focus any more. When you want to change the magnification you simply use the power zoom feature. The lenses I used were made by Schneider. </p>

<p>I'm not sure I'd agree with Rodeo Joe that parfocal lenses were less important back in the days of "smudgy old film." When manually focusing , metering and adjusting flash settings, it's pretty nice to have both focus accuracy and F-stop consistency. The old Nikkor 80-200 F4 AIS lens was especially nice to use for this reason, with little if any focus correction required. Auto focus and auto exposure would seem to make this less important.</p>

<p>AFAIK most modern zooms maintain focus at least roughly while focussing, but in the C-mount surveillance lens market there are some varifocal lenses that don't have this property. </p>

<p>This thread has been a treasure trove of useful information so far. Thanks a lot to everyone who contributed! </p>