If you take a shot at f/1.2, the mirror lifts, the shutter opens and the image will be nice and sharp. What happens when you fully press the shutter button depends on the selected aperture. The appropriate lens elements are moved so the image on the sensor is as sharp as it can be at the selected focus point*. When you trigger an AF operation (typically by half-pressing the shutter button), the camera will first make sure the lens is wide open, and will then trigger the autofocus operation. So, how does Focus Shift manifest itself? Let’s take the Canon 50mm f/1.2L lens as an example, as this is well known to shift focus on stopping down. Theoretically, contrast detect autofocus can work at any aperture, but as you stop down the lens, less light reaches the image sensor and the resulting image becomes very noisy leading to either intolerably slow or inaccurate autofocus. Due to the way phase-detect autofocus operates, stopping down will significantly affect the performance of the af system and around f/8 is the limit of the phase-detect autofocus system in even the best cameras. The logic behind this is that when a lens is wide open, the depth of field will be narrowest and therefore the camera will most easily be able to distinguish between in-focus and out-of-focus conditions.
This applies to both phase-detect (“Quick”) and contrast-detect (“Live”) autofocus.
Why does Focus Shift matter?įirst, it’s important to understand that when a DSLR perfoms autofocus, it always runs the operation with the lens wide open. This post takes a look at why that matters, and some results from development testing of Reikan FoCal which allows the effect of focus shift to be quantified. Focus Shift is a phenomenon which affects certain lenses and causes the point of focus to shift with a change in aperture.
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