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 😉
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.
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
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.
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.
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.
Sometimes, lenses have a different focal lengths at different focusing distances. A lens could be a 35mm at 50 cm and 37mm at infinity.
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.
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.
You can’t. But it’s only an issue in some video applications.
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