Focus Shift is an optical problem that occurs due to Spherical Aberration, when an object is brought into focus at maximum aperture and captured with the lens stopped down. Focus shift can lead to blurry images and focus errors, when working with subjects at close distances and using fast aperture lenses. With the lens aperture fully open or “wide open”, incoming rays of light converge at different focal points due to spherical aberration along the optical axis, as shown in the top illustration below:
When the lens is stopped down (the size of the aperture is decreased), light rays no longer reach the edge or the “periphery” of the lens and only the ones close to the optical axis make it through. As a result, the point of best focus with the circle of least confusion is moved to the right, as shown in the second illustration above. If focus is not re-adjusted after this change of aperture, it will shift the sharpest focus plane (hence the name “focus shift”) away from the lens, essentially moving it slightly behind the focused area. Imagine focusing on an eye, only to find out later that you ended up with a nose in focus instead, just because you changed camera aperture.
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Focus Shift and Fast Aperture Lenses
Most lenses prone to focus shift problems have very fast maximum apertures of f/1.0, f/1.2 and f/1.4 for one major reason – a big portion of the lens surface is used to transmit the light. Fast prime lenses with uncorrected spherical aberration will always have focus shift problems for this reason. In fact, any fast lens with spherical aberration problems will exhibit focus shift at different apertures. Slower lenses with maximum aperture of f/1.8 and smaller that also suffer from spherical aberration problems will have a much less pronounced focus shift, so it will not be as noticeable in images.
Phase Detect AF Problem
What many photographers do not know, is that their AF lenses are always set to the maximum aperture when phase detection autofocus is used. The reason for this, is that phase detection uses two small sensors to measure AF accuracy (depending on the angle of light) and they require a lot of light to work effectively, which is why lens aperture stays wide open even when the lens is stopped down on the camera. The aperture changes only when you finally press the shutter button and opens back up at the end of the exposure. At smaller apertures, the light comes at a much smaller angle, making it harder for the AF sensors to see if the subject is in focus or not. On top of that, there is much less incoming light for the sensors to see much contrast, which also causes autofocus accuracy issues. These are the main reasons why camera manufacturers claim that autofocus does not work on lenses slower than f/5.6-f/8 (on the latest Nikon DSLRs). Interestingly, the most optimal lenses for the phase detect AF system are the ones that have a maximum aperture between f/2.0 and f/4.0. Large-aperture prime lenses with maximum aperture larger than f/2.0 have a very small depth of field, so they just take longer to obtain correct focus.
Why is this all important and what does phase detect AF have to do with the focus shift? As shown in the illustration above, focus shifts to the right (or away from the camera) when aperture is changed. Since phase detect AF leaves the lens aperture wide open while autofocusing, the sharpest focus plane is going to shift when you take a picture with the lens stopped down. This is potentially a big problem, because it means that you might not be able to achieve correct focus, unless you stop down enough to compensate the shift (by increasing depth of field). Sadly, even the AF Fine Tune function in some advanced cameras like Nikon D7000 is not going to help, because it does not allow micro-adjusting focus for different apertures. If you adjust focus for a lens at f/2.8, focus will certainly shift at f/1.4 and vice-versa.
Focus Shift: Phase Detect vs Contrast Detect
Contrast Detect (Live View) AF System, on the other hand, does not have these problems on advanced DSLRs that allow changing aperture in Live View mode. Since Live View shows everything through the lens and adjusts the lens focus based on image contrast, changing aperture and then refocusing will provide correct focus – that’s because aperture will actually change on the lens in contrast detect mode. Some entry-level DSLRs do not provide the ability to change aperture in Live View mode, which would again result in focus shift when changing lens aperture.
Reducing Focus Shift
As you can see from this article, focus shift can be a very problematic and annoying issue on fast aperture prime lenses. While there are some tricks you can employ to reduce or get rid of focus shift, most of them are unpractical. Unless camera manufacturers provide the ability to fine tune autofocus on each aperture, there is no easy way to deal with this problem. Unless, of course, manufacturers design lenses with aspherical elements to reduce spherical aberration and focus shift problems; or incorporate focus shift data into camera firmware, which will automatically adjust focus based on a lens and its aperture. Anyway, here is the list of tricks or workarounds to reduce focus shift:
- Use maximum aperture – take pictures at the maximum aperture and you won’t have to worry about focus shift. Might not be practical for most lenses, because they are soft wide open. See the next bullet point for an alternative solution.
- AF Fine Tune optimal aperture – if your camera has the ability to fine tune autofocus, set your lens to its optimal aperture that you will be primarily using, then fine tune autofocus. You will then have to shoot at this optimized aperture all the time and stop down when needed. Using larger apertures will result in focus errors after this type of calibration.
- Use a slower lens – if you want to avoid focus shift problems, use slower f/1.8-f/2.8 lenses that have much less issues with focus shift.
- Stop down the lens – usually stopping down the lens to apertures smaller than f/2.8 will take care of the focus shift problem due to increased depth of field. Not very practical for fast aperture lenses, but will certainly take care of the problem.
- Use Contrast Detect AF – not practical for most situations, because Contrast Detect AF is slow and requires the mirror to be raised up, blocking the viewfinder.
- Use Manual Focus Lenses with Aperture Rings – a manual focus lens with an aperture ring will allow you to control the aperture from the lens, so you can stop it down before manually acquiring focus. You will have to reacquire focus every time you change aperture though.
If you are interested in reading more, below is the list of articles on other types of aberrations and issues that we have previously published on Photography Life:
interesting explanation about focus shift.i imagine the effect is partly offset by the increased depth of field that one gains as the lens is stopped down, but i could not speculate which effect dominates.
Dear Nasim and team,
I’m happy I’ve recently found you page! I like the reviews of lenses and cameras from the photographer’s point of view!
Concerning the focus shift, so far I’ve encountered this problem with my Pentax fa 50 f1.7. I ended up using this lens on f2, for which I set the af microadjustment. I find this feature of fast lenses very frustrating as I practically get a “fixed aperture” lens.
Currently I’m close to purchasing a new camera as I’d like to capture my ACTIVE little son :) I need a device with very good AF including low light performance. I’m thinking about Nikon D500. I’ve ruled out the D750 due to poorer build quality (as reported by many in discussions and reviews), although it’d be nice to have FF for that price it’s being sold now. I also like Fuji, mainly for it’s 56 f1.2 but the AF performance of the x-t2 seems not to be on par with that of D500. May be the next generation… :)
I’d like to ask you and other experienced users for kindly sharing your experience (or opinion) with some Nikon lenses, especially in relation with focus shift.
I want to have a 16-24mm lens, a 35mm lens and a 50 or 85 mm lens for portraits.
I’ve never been a fan of zooms but the 16-80 f2.8-4 seems to be a good purchase as a kit lens. Probably no focus shift? Having the 16-80 I’d be fine with additional 50 or 85.
What do you think about these (in relation with focus shift)?
– Nikon 50mm f/1.8G AF-S
– Nikon 85mm f/1.8G AF-S
If I don’t get the 16-80, I’ll need at least a 35mm lens. The AF-S Nikkor 35mm f/1.8G DX is probably not for me (see the Muhammad Iqbal’s comment). What lens would you recommend instead? Maybe the FX version (used on D500)?
What glass would you suggest getting in the 16-24 ish area? Here, the focus shift wouldn’t probably bother me a lot as I’d use this for landscapes around f8 anyway.
Thank you for your help!
I also have to mention that the autofocus speed of the lens has to be considered. According to the review here on Photographylife the 16-80 is not among the best in this area :(
What about the new 35/50/85 1,8G lenses?
I got here after having purchased a nikkor 50mm 1.8G for my Nikon D5500 and seeing most of my photos out of focus, actually back-focused by some centimetres when shooting at wide aperture and close to the subject.
I understand that there is no solution to this, and it’s extremely disappointing.
Would a Mirrorless camera suffer the same issue or not?
You probably aren’t experiencing spherical aberration. Spherical aberration would show up if you were using a smaller aperture. You may want to send your camera and lens to Nikon to get things calibrated. Some bodies have a focus fine tuning (called “AF fine tune” in Nikon parlance), but I think you need to jump to the D7xxx series. If you use live view, chances are that your images will focus properly. Also be sure you are using single point focus if shooting wide open.
thanks. after 30+ years of shooting I never had this problem until recently. I’d open lens to T1.5, focus with 2X on EVF, then stop down to the shooting iris of T4. watching the 4K video I saw my focus had shifted several inches back ( 6-7′ to subject ) and subject was slightly soft. I guessed this was the problem and thanks for the confirm, since everything is going 4k->1080 it’ll still look perfectly fine but now I feel like getting my manual glass collimated so I can use a tape measure instead when working at T4-8
Nasim, great advice on how to work around focus shift: “…set your lens to its optimal aperture that you will be primarily using, then fine tune autofocus. You will then have to shoot at this optimized aperture….” My 85mm f/1.4G, after many attempts to focus adjust @ maximum aperture would not focus accurately at f/5.6 (even beyond minimal focus distance) so it went went back to the store. Now I have a D800e so I might try another copy and resort to your suggestion if focus shift should occur.
Great article, thank you. Can anyone shed any light on the fact that with Nikon D800 and 50mm 1.4G the focus shift is around 10 cm behind at 1 meter but with Nikon D3 and same lens it is pin sharp ie. no focus shift at all?
I would really like to hear from any other D3 owners.
On my 80-200mm f/2.8 Nikkor lens, close is around 3 to 6 metres for focus shift to be an issue, depending on zoom setting.
To get around this, use live view as the aperture is set to the desired value when using live view.
nasim…thanks for the article…i am considering the purchase of a lens with well-known
documentation of focus shift…i have read many articles on mitigation of the focus
shift problem and many articles point to “close-focusing” distances as a contributing
factor…no article has yet really said what is meant by “close-focusing” distances…do
you think subject distance from the film plane matters…if so what distances are most
responsible… head and shoulders vs. 3/4 body vs. min focus distance of the lens…
nasim, thanks for the article. you’ve seen many good questions about focus shift, but no one asked about distance.
as i unread, wide aperture at close distance leads to noticeable phenomenon of focus shift. so i want to know what
is meant by “close distance”. does it mean anything closer than infinity, or one meter from the closest focus or one-
half meter or three meters from the closest focusing capability of the lens. is focus shift a concern only at the closest
Sadly modern DSLRs are completly useless at allowing accurate manual focus of large aperture lenses. One giant step backwards for camera design.