What does “autofocus at EV 0 with an f/2.8 lens” mean?
Asked 6/10/2014
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My camera’s autofocus spec says it can focus down to EV 0 at ISO 100 with an f/2.8 lens attached. I understand EV 0 in the usual exposure sense, but I’m confused about why f/2.8 is mentioned. Does that mean I should convert the AF sensitivity to a different EV compared with the usual EV scale based on f/1, or does it simply mean the camera can autofocus in EV 0 light as long as the lens is at least f/2.8? Why do manufacturers include the f/2.8 lens condition in AF specifications?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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It means the same thing as it normally would. A lens can not let in more light than the size of its widest aperture though. Focus is achieved with the aperture all the way open.
In other words, at EV0 (enough light for an f/1 image at ISO 100 for 1 second exposure), your camera will let in enough light through an f/2.8 lens to focus. If, however, you have an f/4 lens on the camera, then less light will be able to reach the sensor and it won't have enough light to autofocus at EV0.
Originally by user11392. Source · Licensed CC BY-SA 4.0
user11392
12y ago
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Generated from our catalog & community — verify before relying on it.
You should not reinterpret EV 0 as a different EV value here. It still refers to the scene light level in the normal way. The f/2.8 note tells you the lens speed used for the AF sensitivity rating.
AF is performed with the lens wide open, so the amount of light available to the AF system depends on the lens’s maximum aperture. If the camera is rated to autofocus at EV 0 with an f/2.8 lens, that means it can still focus in EV 0 light when the attached lens can open to f/2.8.
A slower lens, such as f/4, passes less light to the AF system, so AF may no longer work as well at that same low light level. A faster lens can help, but depending on the AF system design, the camera may only be specified to take advantage of up to an f/2.8 baseline, so going faster than f/2.8 may not improve the published AF sensitivity spec.
So: keep EV 0 as EV 0. The f/2.8 part is a condition of the autofocus test, not a conversion of the EV scale.
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