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I am not a professional photographer but shoot outdoor and indoor pictures, weddings, and portraits as a hobby. I use a Nikon D750 with a 70-200mm 2.8 VR lens and a 35mm 1.8 ED lens. I have been having focus problems with both lens while shooting in low light. The camera tries to focus but is unable to do so. It occurs more often with the 70-200mm lens and while I am using the camera's flash. What could be causing this problem?

  • How low is low? The AF system in the D750 is good down to -3 EV, which is not very much light and a whole stop better than pretty much everything else out there. (I've had trouble with my 750 intermittently not focusing my first-generation 70-200 f/2.8, but that's another discussion.) – Blrfl Oct 29 '15 at 10:18
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    Possible duplicate of Why can't my SLR autofocus on certain parts of a scene? – mattdm Oct 29 '15 at 10:39
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The problem is lack of enough light to create enough contrast between the darkest and brightest parts of the scene within the area(s) covered by your active focus point(s).

The reason you are experiencing the problem more with the 70-200mm f/2.8 lens than the 35mm f/1.8 lens is that the 35mm lens is a tad over one stop faster and will let in just over twice as much light as the 70-200mm lens. Remember that metering and AF are done with the lens wide open regardless of your aperture setting. The lens is only stopped down the instant before the shutter opens.

One thing that might help is to select only AF points that are more sensitive at wider apertures, are "cross type" AF points, or are both. I'm not very familiar with the AF system of the D750 and Nikon is notorious for keeping info about the AF points in their cameras close to the vest, but you might find the information somewhere if you look for it. It would be easy enough for a third party to deduce using comprehensive testing.

Note that the -3EV maximum sensitivity of the D750's AF system is almost certainly measured using only the center focus point. It's just the laws of physics. The center focus point will always enjoy the advantage of more light than any of the other focus points. The further away from center a focus point is, the less sensitive it can be due to the way the light from each side of the lens is split. So when in very low light and your camera is having trouble finding focus try using only the center point and see if that helps.

  • The D750 has 15 cross-type AF points, they are organized in the three vertical center rows. See them marked in this image: flickr.com/photos/dojoklo/15231370695 – kamuro Oct 29 '15 at 8:00
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    Yes, and it seems they are all sensitive at f/5.6, which means none have a long enough baseline to take additional advantage of faster lenses. – Michael C Oct 29 '15 at 10:01
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    +1 for: "Remember that metering and AF are done with the lens wide open regardless of your aperture setting. The lens is only stopped down the instant before the shutter opens." – Thomas Dekiere Oct 29 '15 at 10:04
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They always do that, it needs light/contrast in order to focus. Get more light, or get a lens that lets in more light. (lower f-stop, or more technically: t-stop.)

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