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10-19-2012, 08:09 AM   #1
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focus shift ?

what does this mean ? focus shift when stopped down , i don't understand : ( read this in a review and in a couple other places about pentax lenses recently )

Cons:some pf wide open, some focus shift when stopped down

10-19-2012, 08:35 AM   #2
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10-19-2012, 09:16 AM   #3
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To understand focus shift you need to understand open-aperture metering/focusing. (Apologies if you knew this already.) With most SLR and lens combinations, the camera keeps the lens at maximum aperture until you press the shutter release button. This gives you the brightest possible image in the viewfinder, and also makes focusing more precise because of the shallow depth of field when the lens is wide open. Look at the back of a lens and note the lever sticking out, and what happens when you flip this lever back and forth -- this is how the camera can keep the lens at maximum aperture even when the lens's aperture ring (or the camera) is set to a smaller aperture. Only when you press the shutter release (or use whatever depth-of-field preview function the camera has; in the case of a Pentax DSLR this is one of the functions of the Green Button) does the lens diaphragm actually close down to the desired setting.

Focus shift happens with certain lenses in certain situations, where stopping down the lens causes the focus distance to change a little. It's a result of spherical aberration and is more likely to be a problem with very fast lenses, when you are focusing on something very close, and stopping down by one or two stops. Not generally thought of as a problem with Pentax lenses, although nobody's perfect. What lens(es)?
10-19-2012, 10:25 AM   #4
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read this focus shifting , about the da 40 and some other lens or two i was looking into, but can't really remember which ones. does this affect images then when you refocus or just when you have something focused then change the aperture setting, not really sure i understand even though i appreciate baro-nite's explanation. also would a full frame ( the one that will be coming to us someday ) from pentax , use that same wide open brightness in the viewfinder or would only slr's do that ? thanks.

10-19-2012, 10:47 AM   #5
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The issue is that with some lenses, the focus distance changes slightly as the aperture changes. Since you focus with the lens wide open, this isn't apparent while you're actually shooting. You're right, the DA 40 and DA 70 are known to have this behavior. If you are in this situation -- using a lens with a focus shift, shooting at a moderately wide (but not fully open) aperture, and focusing on something close, other than working around the problem either by changing the aperture setting or guessing and deliberately focusing in front of the subject, you could use the camera's optical preview function to stop down the lens, then manually focus.
10-20-2012, 05:56 PM   #6
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ok, i found this article by googling focus shift : ( diglloyd - Focus - Focus Shift and Spherical Aberration ) but i really don't understand, will have to study this more , why bother buying fast lenses, if they don't focus properly ? here is an excerpt from the article :
Mitigating focus shift

There are multiple approaches to dealing with focus shift. One solution is to avoid f/1.2 and f/1.4 lenses entirely; shoot an f/1.8 or f/2 lens instead! But assuming an f/1.2 or f/1.4 lens exhibits focus shift, here are some options, none of which are entirely satisfactory.
Stop down — unfortunately, the entire zone of focus is shifted, and some lenses require f/5.6 or even f/8 to regain “lost ground”. If your zone of sharpness needs are precise, stopping down isn’t much of a solution. However, given the random variability of focus error, stopping down “kills two birds with one stone”, improving the odds. But didn’t you get that “fast” lens to shoot at wider apertures?
Compensate — deliberately focus slightly in front of the desired point (with most lenses). With practice this is feasible, but it’s a skill that takes time to acquire.
Shoot wide openwhat you see is what you get.
Focus at the shooting aperture — focus and shoot at f/1.4, focus and shoot at f/2, focus at f/2.8 to shoot at f/2.8, f/4, etc (by f/2.8 spherical aberration is all but eliminated).
Focusing at the shooting aperture is the only option for optimal results, but not always feasible. Stopped down lenses are not compatible with autofocus, so this means manual focus using the depth of field preview lever (or a lens with a manual diaphragm control egZeiss ZF on Canon EOS).
Live View (at the shooting aperture) is the most accurate choice of all, but requires a static subject and a tripod. Beware of Live View that automatically stops down the lens under bright conditions, and then exposing at a wider aperture than the focusing aperture. Canon EOS does this with Live View mode.
Autofocus compensation — some cameras now offer a lens-specific focus compensation adjustment. If you know you’re going to shoot (for example), the Canon EF 50mm f/1.2L at f/2, you can “dial in” a correction factor to front-focus slightly, correcting the focus for f/2. This feature is available on the Canon 5D Mark II. Nikon also offers the feature on the D3 and D700. Of course, correcting for f/2 will throw off other apertures.
10-20-2012, 06:52 PM   #7
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QuoteOriginally posted by glinda Quote
why bother buying fast lenses, if they don't focus properly ?
Lenses with focus shift are usually fast lenses. But not all fast lenses exhibit focus shift. That the DA 40 and DA 70 do, might have more to do with the fact that they are tiny pancake lenses with relatively few elements; after all, they aren't particularly fast. Of the truly fast Pentax lenses I haven't seen any reports of significant focus shift.

A perfect lens doesn't exist; part of the fun is exploiting what you have to best advantage. If you decide you'd like a DA 40 or DA 70 to have an extremely compact, capable, and beautifully-built lens, just recognize that you need to exercise care when shooting close subjects at moderately stopped-down aperture settings. If you anticipate shooting a lot in such conditions, consider a different lens.
10-20-2012, 07:24 PM   #8
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When you look really close most lenses shift focus some as they are stopped down, but on film either the increase in depth of field covers it, or you don't notice the slight unfocus of some areas. Film emulsion has some thickness, so an absolute plane of focus isn't exact on film.
Digital sensors have a more defined solid surface, and we tend to enlarge more (especially starting with a smaller sensor size), so slight shifts of focus can be detected, making it more of a "talking issue" among pixel-peepers.
Unless you tripod mount and focus on a test target, most lenses don't shift enough to notice, as your camera movement (hand held) can easily be more than the shift.
I have a couple of fast rangefinder lenses that are otherwise excellent, but show significant focus shift. I usually use them wide open in dim light, but have learned to compensate focus by slightly shifting the (manual) focus when I take them into bright light. That's just part of the skill of the craft.
I've even tested relative slow (f2.0) famous lenses and found some focus shift, but not enough to worry about. It's really one of those issues that is better to ignore.

10-21-2012, 08:46 AM   #9
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But you need to put this in perspective. We are talking tiny shifts that will never be noticeable in 99.99% of cases because the DOF also gets larger as you stop down. Bottom line: absolutely no way should this factor into a lens buying decison. It is of no real world significance to most photographers. It would like comparng cars based on a difference in gas mileage of 0.2 MPG that only shows up when driving in 2nd gear.
10-21-2012, 03:59 PM   #10
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ok, i guess this is a minor problem, then, and not one to be too concerned about; one more question on it : does this focus shift apply if i were to use a manual lens k or m , ( or even A- lens ) and use manual mode, the aperture ring and manual focusing , etc. and have the lens wide open or a stop or two down etc ? in other words is it more of an autofocus thing, not manual, or ? ( just trying to get a handle on this stuff ) thanks.
10-21-2012, 05:21 PM   #11
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No, not confined to autofocus -- it's an optical property of the lens itself.

I suppose one of these days I'll get around to testing some of my lenses to see if I can discern any focus shift -- would be good knowledge to have. Of course as Tom and Marc have pointed out, it has to be pretty substantial before it will really cause problems, owing to the increased depth of field as you stop down.
10-21-2012, 05:36 PM   #12
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I can assure you that I I've never seen this problem on the DA40 and DA70. I guess it's so small that it's an non-issue outside of the laboratory where slight missfocus, shaking and increased Depth Of Field really makes bigger differance than the slight shift in a controlled environment ever will be. The DA70 is a great lens and one of the top favorites of both me and my brother. You can't go wrong with it and I can promise you that you can't see the "issue" in real life.
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