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What Is Focus Shift in Photography?
Focus shift is a focus error that leads to blurry images that are out of focus. Focus shift occurs when you attempt to focus on something at maximum aperture and then stop down the shooting aperture to take the actual picture. (Stopping down refers to increasing the f-stop number on camera, which reduces the amount of light entering the lens.)
This, combined with a spherical aberration in the lens itself, can cause the light rays being reflected through your lens to converge at different points on the focal plane, causing your image to be blurry.
What Causes Focus Shift in Photography?
Focus shift is something that photographers at every level have to contend with. Here are some factors that contribute to focus shift:
- Spherical aberration. Spherical aberration is a slight deformity in the structure of a lens that causes an optical problem where the light rays being reflected through the lens don’t converge at the same focus point. There are a couple of reasons why spherical aberration occurs in lenses, such as suboptimal lens design and poor quality of materials. Learn more about spherical aberration here.
- Fast aperture lenses. One of the primary causes of focus shift shooting is the use of fast aperture lenses. Fast lenses have a wide aperture and a very large surface area, which makes them more prone to focus shift problems. The wide surface area leads to light rays converging at different points, which leads to a blurry image. Lenses with smaller apertures will have fewer problems with focus shift. Learn more about lenses in our comprehensive guide here.
- Autofocus systems. Different autofocus systems affect shutter focus differently. Some cameras use what’s called phase detection autofocus, which uses two sensors to gauge focus position and lock focus at the camera’s maximum aperture. The camera lens has to move to a wider aperture because the sensors require a lot of light to work properly. Once the camera determines focus position, it will switch back to your original aperture. The issue is that the camera switches between different apertures which can also change the focal points, especially when there is also spherical aberration in play. Learn more about the advantages of manual focus in photography here.
5 Ways to Avoid Focus Shift in Photography
There are a few different methods that photographers use to avoid or correct focus shift in order to produce sharp images. The most popular include:
- Take photos at maximum aperture. If you want to continue to use your DSLR camera’s phase detection autofocus, you can always consider taking your photos at maximum aperture. This is not a very practical or common solution as large apertures greatly reduce your depth of field.
- Stop down. Stopping down a lens will decrease the probability of a focus shift. Decreasing the size of your aperture increases the depth of field and therefore avoids any focus shift issues.
- Use a contrast-detect autofocus mode. Contrast-detect autofocus doesn’t cause focus shift as frequently as phase detection does. However, contrast-detect only works to prevent focus shift if you can change the aperture in live view mode. The live view displays everything through the viewfinder and adjusts the focal length based on contrast.
- Use manual focus. Manual focus allows you to fine-tune the aperture and account for shutter aberration on your own without relying on your camera’s internal system. Using a focus ring can help you finesse your focus and overcome focus shift.
- Focus stacking. Focus stacking involves taking a large number of shots of the same subject with different focus areas and compositing them together digitally. This allows you to account for spherical aberrations and make sure the entirety of your subject is in focus.
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