Mastering the Art of Lens Testing – 5. Focus issues

Welcome to lesson 5, devoted to 2 issues related to focusing. We’ll start with Focus Shift then take a look at Focus Breathing. Since both do little else than change focus, there’s not a lot I can show other than sharp pictures and fuzzy pictures 😉


Out of focus

Out of focus


Focus Shift: What it is

Focus shift is more of a defect than an aberration. It happens when the plane of best focus (plane on which the image is the sharpest) changes as you change the lens aperture.


What it does to the image

Focus shift can cause unsharpness when you take your photograph at an aperture that’s not the one you used to focus. Since autofocus lenses focus wide open and close down for the shot, they are often well corrected for this aberration. But for best results, high-end DSLRs let you enter fine-tuning offsets for each lens at each aperture. Except in very low light conditions, live view often makes focusing at the final shot aperture the most accurate solution.




It’s most problematic with rangefinder cameras and in some cases with mirrorless or DSLRs using live view: imagine focusing a portrait in low-light and using the lens’ max aperture (say f/2) to focus on a bright image in your live view. You nail focus, everything is perfectly sharp on your model’s eyes. You close down to f/5.6 for more depth of field and click. When you inspect the photograph, you realize your model’s hair is sharp but the eyes are completely out of focus. You blame the fidgety model, but it’s your lens that have moves focus back 2 inches as you closed from f/2 to f/5.6


How to test for it

For this, it’s best to use a stationary subject, preferably one with some depth. A menu on a restaurant table for instance. Hey, this is one of these super rare cases when a test chart can even be useful 😀

Use a tripod for your camera or place it on some object where you will not move it when changing aperture.

Make a series of exposures from max aperture to f/11, in one stop steps.

If the point of sharpness on all your photographs is centered on the same spot, all is well. If it moves about, the lens has focus shift.

Before you worry, this is only an issue if you are going to focus at one aperture and shoot at another. With the high-sensitivity sensors in today’s cameras, this is very rarely necessary. But for night landscapes, for instance, you’ll want to use a lens with no focus shift. And anyone using a rangefinder camera will avoid focus shift like the plague.


How to fix it

There’s no easy way of fixing an out of focus photograph. It’s best to know your lens before using it and nail focus each time, particularly in portraits where the eyes should always be in focus.



Almost in focus


You shouldn’t shy away from a good lens simply because it has focus shift. Many excellent lenses do. You only need to know that it does to avoid changing aperture between focusing and capture.

If you do fluff focus on a face, as above, it helps to increase its brightness and clarity to maintain an illusion of sharpness. This is what I did in the photograph above, which I stole in a split second using an 85/1.4 lens at full aperture.



Focus Breathing : What it is

Sometimes, lenses have a different focal lengths at different focusing distances. A lens could be a 35mm at 50 cm and 37mm at infinity.


What it does to the image

Not much in photography. But videographers dislike this because the (fixed-focal) lens appears to be zooming when the operator switches focus from a close subject to a distant one. Download the two images below and switch rapidly between them in your photo viewer. You will see the mannequins jump back and forth every time.



Longer focal at close range


Shorter focal length at longer focus distance



How to test for it

Place your camera on a tripod or be sure not to move as you make the two exposures. Focus on a subject near to you (close to the min focusing distance of the lens) make your first picture. Then focus on the background or at infinity and make a second one. When you switch between the two, you should see no change in position in the frame.




How to fix it

You can’t. But it’s only an issue in some video applications.




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  • Marc Baillargeon says:

    Great job with this series! Without getting dragged into incredibly technical stuff, you cover all the bases in the various topics.

    Carry on!!



    • pascaljappy says:

      Thanks a lot, Marc. Much appreciated. It’s a fine line. Some will regret that I do not cover the finer technical details but I really don’t think they help anyone become a better photographer. So I’m really happy you find the balance right !

  • Yeahhh says:

    Thank you for pointing me to focus shift. I never heard of it and I always focus wide open and than stop down. No I test all my lenses against it and luckily I own none with focus shift. But I definitely have to check it with future purchases because it would change my workflow.

    • pascaljappy says:

      Glad it could help. With modern mirrorless cameras, ykou can usually focus directly at your intended aperture, but with rangefinders or less sensitive sensors, it’s important to keep focus shift in mind. Cheers.

  • Miko says:

    Thanks for the work you put into this Pascal !

    I have just one question : you said autofocus lenses are not concerned by focus shift, but usually on DSLR (Nikon in my case) the focus is set at the fastest aperture and only when the picture is taken the lens stop down to the dialed aperture. (I think the A7 series can behave like that to, depending on the configuration in the menus, but i’m not sure i remember correctly since I have switched to M mount glass (Zeiss C-Sonnar, a wonder) after a few months with the Zeiss 55 f/1.8)
    So could it not suffer from focus shifting ?

    Regards, Mikaël

  • Micheal says:

    The definition helps me describe what a lens is doing accurately. I know I confused the terms before. Since I rarely video anything I can use the lens properly.



  • Dave says:

    Thank you for the great series on lens testing

  • Scintilla says:

    “Oviously, autofocus lenses are not concerned by this issue.”

    I’m confused — assuming we are talking about DSLRs, how can they not be affected? Native-mount lenses always autofocus wide open when shooting through the OVF and only stop down right when the exposure is taken.

    Thank you for writing this series of articles; it is good to get more ideas on how to test for these things than just my own amateurish attempts.

    • pascaljappy says:

      Hi Scintilla, thanks, I hope you find it useful. You are right, my sentence is much too vague. I should have written that most AF lenses are corrected for focus shift as part of their design brief. That’s not entirely true either and for best results, high end cameras let you enter focus offsets for specific lenses at specific apertures. I’ll correct this (and the missing “b”). All the best, Pascal

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