On top of every other problem you’ll encounter in photography, there is one more. It goes by the name of focus shift.
In this article, you’ll find out what it is, what causes it and what you can do about it.
What Is Focus Shift?
When the lens operates ‘wide open’, incoming rays of light converge at different focal points along its optical axis.
When the lens’ aperture is ‘stopped down’, light rays no longer reach the edge of the lens. Only the ones close to the optical axis get through.
The result? Dense light rays now hit the same place. This creates a constant point of best focus.
As long as the focus isn’t readjusted after this aperture change, it will shift the sharpest focus away from the lens. It moves slightly behind the focus area. This is the ‘focus shift’.
The light converges at a differ to place to where you aim.
If you focus on an eye but find that you actually captured the nose without refocusing, your aperture is to blame.
Fast Aperture Lenses
The majority of lenses that are prone to focus shift problems have very fast maximum apertures. F/1, f/1.2 and f1.4 are the main culprits. The main reason is that a large surface area of these lenses transmits the light.
Very fast prime lenses with uncorrected spherical aberration will have constant focus shift problems. Any lens with spherical aberration problems will show a focus shift at different apertures.
Slower lenses with a maximum aperture of f/1.8 and smaller will show a less pronounced focal shift. This is even if they suffer from spherical aberrations.
Phase Detect AutoFocus Problem
This is something you might not know. When you use phase detection autofocus, your camera uses your lenses’ maximum aperture.
There is a reason for this. PD uses two small sensors to measure autofocus accuracy depending on the angle of light.
The lens stays open as the sensors need a lot of light to work in. The aperture then changes again when you press the shutter release button. It then returns after the exposure is complete.
Smaller apertures mean the light comes at a smaller angle.
This makes it harder for the sensors to spot the change in contrast. It results in autofocus accuracy issues. Some camera manufacturers claim that autofocus doesn’t work on lenses slower than f5.6-f/8, the above being the cause.
For phase detect AF systems have an optimal range, falling in the maximum apertures between f/2 and f/4. Large-aperture prime lenses with a maximum aperture larger than f/2 have a very small depth of field. They take longer to focus.
As we discussed above, the focus moves away from the lens when you change the aperture. Phase detect autofocus leaves the lens wide open when it focuses.
This means the sharpest focal point will shift when you capture an image with the lens stopped down.
This can cause a big problem. It means you might not be able to achieve a correct focus. This is until you stop the lens down enough to compensate the shift, which in turn increases the depth of field.
You may think that fine-tuning (found in Nikon’s D7000) your autofocus would solve the issue. It won’t help, as it does not allow for micro-adjustments at different apertures.
If you adjust the focus on an f/2.8 lens, it will shift at f/1.4, etc.
Phase Detect Vs. Contrast Detect
The contrast-detect autofocus system doesn’t pose the same problems. This is as long as you can change aperture in Live View mode.
Live View mode shows everything through the lens and adjusts the lens based on the scene’s contrast.
Changing aperture and then refocusing will provide a correct focus. This is because aperture will actually change on the lens while in contrast detect mode.
Some entry-level DSLRs do not provide the ability to change aperture in Live View mode. This would result in a focus shift when changing aperture.
Reducing Focal Shift
Focus shift is yet another challenge to overcome in the world of photography. By now, we are used to them. This is definitely something you need to think about when using fast aperture prime lenses.
There are things you can do to reduce or cut focus shift, but most of them aren’t practical. We all wish every lens would allow us to fine tune it’s autofocus at different apertures.
There is just no easy way to deal with this.
Lens manufacturers could do a few things to help us out. By using aspherical elements in the lens would help the issue but at a greater financial cost.
Perhaps focus shift data could come to use via firmware updates.
- Use Maximum Aperture – If you capture scenes at the lens’ maximum aperture, you won’t have to deal with this problem. This isn’t practical, especially if the depth of field of your scene is important.
- Use Autofocus Fine Tuning – If your lens or camera allows you to fine tune autofocus, set your lens to its optimal aperture. Fine tune from there. You will have to shoot at this aperture from then on, stopping down when need be. Be aware. You’ll encounter other problems when shooting with larger apertures with this calibration.
- Use Slower Lenses – To avoid focus shift problems, use slower apertures, such as f/1.8-f/2.8. Even better with lenses that don’t have focal shift issues.
- Stop Down the Lens – Usually, stopping the lenses down to f/2.8 will solve the focus shift issue. This is because of the increased depth of field. This is not practical for fast aperture lenses.
- Use Contrast Detect Autofocus – Contrast-detect autofocus is slow and requires locking the mirror in the up position. Not practical, but solves the issue.
- Use Manual Lenses With Aperture Rings – By manually focusing your scene, you control the lens’ aperture. You can stop it down before acquiring the focus. However, you’ll need to reacquire the focus each time you change the aperture.