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.
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 detect autofocus is used. The reason for this is that phase detect requires 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. Ever wondered why the center focus point is the most accurate one in any DSLR camera? That’s because it receives the most amount of light from the center of the lens. This is especially true on slow f/5.6 lenses or lenses coupled with teleconverters. Due to the massive amount of light loss, the outer regions of the lens do not transmit enough light to the phase detect sensors, often causing the dreaded “focus hunting”. This is the main reason why camera manufacturers claim that autofocus does not work on lenses slower than f/5.6 – the amount of light is simply insufficient for the phase detect system. Now imagine what would happen if lenses focused while aperture was closed down. 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 low 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.
About Nasim Mansurov
Very well written! Thanks!
This is a problem that spoils a lot of images but many people don’t know about it and blame the lenses for this.
I have 50/1.2 Ai-s that has a sever focus shift at close focusing distances and pictures should be taken very carefully using Live View.
The viewfinder of the D7000 reacts on brightness changes up to about f/2.5. I suppose that the electronic confirmation dot has a similar “max. aperture” and cannot guess the right focus. There is suppose that the focus shift also messes up but I am not sure.
Nasim, do you have an idea about the electronic confirmation dot light sensitivity? Is it really that limited?
Marian, as far as I know, the electronic confirmation dot you are referring to uses phase detect to determine focus in normal mode. In live view mode, you should only see a red/green square. Since you are using a manual focus lens, you should rely on sharpness you see on the LCD only.
Excellent. Great illustrations, too. Will read a few times.
One thing, I’ve always been told there is a “sweet spot” in a lens, usually midpoint on the aperture scale. Also, if you stop down too much, like f/16 or f/22, you will lose sharpness. Now focus shift. There seems to be a mess of overlapping lens issues which can get confusing. Con you enlighten me?
Also, all this seems to be a good reason to” buy the best lens you can afford.”
Peter, yes, every lens has its own sweet spot. For some it is f/4-f/5.6 and others are sharpest at f/8-f/11. And yes, stopping down to very small apertures like f/16 will decrease image quality due to diffraction (on any lens) – I will write an article on diffraction soon. And buying the most expensive lenses won’t help, since even the most expensive lenses have issues with chromatic aberration, distortion, etc. You just need to know which lens has which problems and try to work around those :) More details to come!
Hi Nasim,
Thanks for an very informative article, I didn’t know this problem exists!
I would explain some of my ‘not so sharp’ images and wondering what went wrong!!
Regards,
Rod
Ah… thanks for the explanation. I think this explains why my AF-S DX 35mm f/1.8 Nikkor suffers from focus shift and severe chromatic aberration in its widest aperture.
I wonder if any AF-S DX 35mm owner has the same experience.
Regards,
iqbal
Iqbal, yes, the 35mm f/1.8G DX is known to have issues with heavy CA, especially in the corners.
Thank you, well written. It seems that being able to autofocus while DOF preview is active would help with this. Let’s say you have a fast lens, stop it down to around 2.8, use DOF preview, focus while it is still active, then expose. Unfortunately, at least my D7000 does not autofocus while the DOF preview button is pressed.
Ondrej, it would be difficult to focus accurately while looking into the viewfinder…but I will give it a shot with the DOF preview button – that seems to be a good idea.
Thank You. It was very interesting. :) I mostly use 3 buttons on my d700. First one I press is the function button closing the aperture to desired stop. Next one is AF-on to start af in continous mode and try to keep focus on the corner of the af field if I’m composing this way. Then the shuter relese button to measure light and release the shuter. Have You tested focus shift in this config? I believe aperture is set this way to it’s position before AF start to mess things up the way You described it. I believe this function was designed for testing bookeh or subject isolation before taking a picture, but still there…. it seems You have Your problem solved?
Or am I mistaken?
I do so because at some point I noticed that having focus set in the center of the frame and after blocking AF and recomposing to avoid focus change and taking the picture…. still ends up in a bit out of focus subjects in the corners. ;) Then I developed this way and I can see AF working while I’m recomposing so there must be a difference between focus in the corners measured by AF and focus taken in center and then recomposed with AF blocked.
Many guys recomend this way of making AF more user friendly but it’s just not enough.
Ohhh I use mostly 50mm 1.4g version (a bit soft but not a bad lens), 24-70 2.8, 70-200 2.8 on d700.
What do You think?
Pawel, interesting, I have never tried to focus in a depth of field preview mode (I am assuming that’s the button you are talking about). I should try it out and see if it works.
Yet another well written article.. I always had trouble
Understanding focus shift, until i read this..
Though i don’t reply much, i ve been following your
Site for over a year.. It’s so helpful and thanks for providing
The entire artice on feeds.. Am a surgeon & i read
Most articles in my android feed reader, during my breaks :)
Great article Nasim, a lot of valuable info there for any DSLR shooter. I’m glad there’s a post regarding this on such a well known Nikon/DSLR/Photography site.
Cheers,
Steve
Hi Nasim,
This article has really helped me understand the problem regarding focus shift, as I own the Canon 50mm 1.4 and it isn’t great at Af, wether focus shift is the problem or just poor Af. I would love to know if there are any fast canon lenses out there that have had their software inside tweaked to combat the problem.
I hear sigma are making a devise for the public to buy that enables us to fine tune the lens to the body,will this tool be able to program in anti focus shift issues on the lens?
Regards,
Andy.
thanks for article Just found my 50mm 1.2 ais exibits focus shift :(
Nasim,
I bought a Nikon 24 1.4G recently. I found the focus at 1.4 is
inaccurate and exhibit shift which required fine tuning in my d600
body (~ -12).
I bring this to the Nikon cs to fix it, after checking, both of my d
600 and 24 1.4G have shift in focus.
The CS suggested to tune my 24 1.4G at 1.4 (if i am using 1.4
frequenetly), but he says this tune would affect across all aperture
as step down (ie may have shift when step down from 1.4). The CS says
it is a “normal” optical phenomenon for a fast lens to exhibit “focus
shift” when step down from big aperture like 1.4. I asked them whether
it is a defect of my lens, he insist not a defect but rather a
“normal” optical phenomenon.
As I read through your article on focus shift, focus shift is due to
spherical aberration. So, I want to ask:
1) will lens like 24 1.4G (with aspherical element) will also suffer
from “irreversible” or “intrinsic” foucs shift that cannot be amended?
(for which they called it normal optical phenomenon)
2) So, for me, is a tuning of the lens at 1.4 for my 24 1.4G is a good
solution to me? Or should I insisted on changing the lens if this is a
real defect?
Thanks for your expert advice
Lam