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I know what it means for a camera to be able to autofocus at EV 0. As the answer to this question says, EV0 is the light level to achieve correct exposure with 1 second of exposure at F1 and ISO 100.

But what does it mean for a camera to be able to autofocus at EV 0 at ISO 100 with F2.8 lens attached? This comes from the specifications for my camera, the Sony NEX-5R.

What does F2.8 mean in this context? Since there's three stops of difference between F1 and F2.8, should we convert "EV 0 with an F2.8 lens" to -3 EV, as conventionally defined (i.e., with an F1 lens)?

Or can I ignore the F2.8 and just assume that the 5R can focus at EV 0 as conventionally defined (with an F1 lens). In that case, why is F2.8 even mentioned in the autofocus spec?

I have seen this F2.8 thing elsewhere, for example in this review, and I am at a loss as to how to interpret it.

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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.

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  • When you say that "at EV0 [...] , your camera will let in enough light through an f/2.8 lens to focus", that means the AF performance of the camera is indeed better than EV0, because a camera rated EV0 would require an F1 lens to focus under those circumstances. I had asked this in the question, too. Please also see my comment to Michael's answer. Would you agree with the last sentence there? – Vaddadi Kartick Jun 11 '14 at 1:14
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    EV0 isn't a measure of af performance. It is a measure of light. If you had a faster lens then the af could function with less than EV0. With a slower lens it will take more. Your last sentence on the comment on Michaels post is incorrect. With an f/2.8 lens, the camera requiring an f/1.8 will fail to focus as the 2.8 lens doesn't let in sufficient light. – AJ Henderson Jun 11 '14 at 2:37
  • Understood. So, since the NEX-5R is rated at EV 0 at F2.8, it means that it can autofocus at the same light level that a camera rated as EV -3 at F1 can focus, assuming both cameras are equipped with F2.8 lenses. Correct? – Vaddadi Kartick Jun 11 '14 at 6:44
  • @KartickVaddadi - yes, the same amount of light will reach the sensor at EV -3 with a f/1 lens as will reach the sensor in EV 0 conditions with an f/2.8 lens. – AJ Henderson Jun 11 '14 at 13:34
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    EVO is not, strictly speaking, a luminance value. It is a combination of sensitivity, shutter speed, and aperture and nothing more, regardless of the amount of light present in a scene. If you are in a totally dark room but use a set of exposure values that equals EV10, you shot a totally dark exposure at EV 10. In common practice many manufacturers assume it is the amount of light needed for a proper exposure at a specific combination of ISO, Tv, and Av. Please see the notes to this answer: photo.stackexchange.com/a/45738/15871 – Michael C Jun 12 '14 at 0:51
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With conventional Phase Detection AF systems found in SLRs the width of the entrance pupil of the lens is one of the factors that determines the baseline: how far apart the rays from each side of the lens are from each other. The wider the aperture, the wider the baseline. The wider the baseline the more difference there will be between out of focus light from each side of the lens. This allows the camera to focus more accurately and with less light if the focus array is designed to take advantage of the wide baseline provided by a large aperture lens. But if the PDAF focus array is only designed to take advantage of f/2.8 lenses, putting an f/1 lens on the camera would make no difference in that regard because the rays closer to each edge of the lens than those admitted by an f/2.8 lens would not be falling on the PDAF sensor.

As a result specifications for focusing performance in low light have traditionally included the aperture required to allow the camera to perform at a certain EV level. But with conventional mirrorless cameras there is no comparison between the rays coming from one side of the lens to the rays coming from the other. Instead, contrast detection AF is used. The maximum aperture of the lens is still important, however, because the amount of light admitted by an f/2.8 lens is more than the amount of light admitted by, for example, and f/4 lens. Twice as much light, in fact. So if your camera can focus in conditions requiring EV 0 to expose correctly with an f/2.8 lens, it would need conditions of EV 1 to focus with an f/4 lens, EV 2 to focus with an f/5.6 lens, and so on.

Newer mirrorless camera models often include hybrid CDAF. This is the case for the Sony A600 reviewed at the link in your question. Some of the sensor's pixels are masked in such a way that they only collect light rays from one side of the lens or the other. The data from these special pixels is used to do PDAF. How wide the baseline is will affect the performance of such systems to one degree or another.

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  • Assuming that I'm not using a wider aperture than F2.8, is the conclusion that I can ignore the F2.8 part, and assume that the camera can autofocus at 0 EV, rather than -3 EV? Put differently, if I take two cameras that are rated to autofocus at F1.8 and at F2.8, both at ISO 100 and 1 sec exposure, as long as I use an F2.8 lens, I should not see a difference in the ability of the two cameras to autofocus under low light. Correct? – Vaddadi Kartick Jun 10 '14 at 6:40
  • I depends. The baseline in a PDAF system determines the widest aperture that makes any difference. If a system is designed to be just wide enough for an F/2.8 lens, putting an f/1 lens on the camera will make no difference, because the PDAF sensor isn't wide enough to take advantage of the wider baseline between each side of the lens. Likewise, if a system has some PDAF points that are wide enough for f/2.8 and others that are only wide enough for f/8, putting an f/5.6 or f/4 lens on the camera will not make the f/8 points perform any better than they would with an f/8 lens. – Michael C Jun 11 '14 at 3:49
  • At wider apertures the limiting factor is the width of the focus sensor. At narrower apertures the limiting factor is the amount of light needed to overcome the noise floor in the sensor elements. – Michael C Jun 11 '14 at 3:50
  • You'll almost always see a difference in the ability of two different focus systems to work in low light, even if they are rated at the same EV. There's much more to AF performance than that. – Michael C Jun 11 '14 at 3:53

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