AF issues

[SwissJon]

I've just spent a few hours with a very pretty blonde photographing her playing her Double Base. This was great fun, but what I've been finding is that my camera has started to refuse to focus properly unless the room is very bright. Normal room brightness reslts in the camera hunting for focus.

I've tried using my 85mm 1.4, my 50mm 1.4 and my 24-70 2.8 with the same result. The room was normally lit (I could have hand held without flash at about ISO 400) the subject was between 1 and 2 meters away. In order to get focus lock, I needed to light the subject with a halogen lamp, which kind of threw my creative lighting. It would lock on the subject if I focussed on her cheek, but not on her eyes, it just keeps hunting.

Has anyone had this kind of problem before? If so, is is solveable, and if it is, how?

TIA

[doctorjames]

What camera/body are you using? what autofocus system is it using?

[SwissJon]

Ooops.. Sorry..

Nikon D7000

AF-S (She was moving as she played. Although AF-C and AF-A wouldn't lock either thought)

I was using a single point which I set with the thumb dial, usually to one of the slightly off centre points.

[Niresangwa]

I usually find that knowing roughly what my DoF is, and then looking for a high-contrast edge is enough, then switching to MF... while I"m not familiar with the limitations of your gear, I do have to shoot in near blacked out conditions at 2.8 frequently (usually the couple's first dance) and this works well.

They key being the contrasting edges (grooms tux jacket against his shirt, for example) and knowing what my DoF is going to be.

[doctorjames]

I suspect that its an issue with the camera not being able to perform effective phase detection on that particular part of the image you have chosen (the eye)

Phase detection is a complex system to try and explain, so I wont as there are lots of (better than mine would be ) explainations out there.

another option is to use the AF assist light, or to use a flash with similar abilities for focusing help rather than exposure. I think the D7000 has an AF assist lamp, but you may have to turn it on.

[SwissJon]

Unfortunately, at 1-2m at F2.5 and 50mm the DoF is so shallow, if I focus on anything other than the eyes, then the eyes come slightly out of focus.. I'm looking for some very intimate portraits so the eyes have to be in focus.

Strangely the AF assist lamp seems to come on AFTER I've achieved focus lock..

Hmm.. Maybe it's time to invest in some proper studio lights.

[doctorjames]

Originally Posted by SwissJon

Strangely the AF assist lamp seems to come on AFTER I've achieved focus lock..

Hmm.. Maybe it's time to invest in some proper studio lights.

Thats a little odd...... unless its just confirming the focus, like a double check.....

have you had a flick through the manual to see if there is a way to set the AF assist?...



My route, which was the cheapskate strobist route, wouldnt have helped at all, thought studio lights would have. The portability of the strobist route is very handy though, feel free to pm me if you would like to know more..

[SwissJon]

I've looked through the menus and the af assist lamp only comes on when using the centre focus point in af-s mode and the subject is within 3m. Not particularly useful, since I rarely place a models eyes in the centre of the frame, and using that AF point then recomposing is going to throw my focus off uness I have the camera on a tripod, which I don't when photographing people.

Looks to me like the light I see is something else.

I'd take you up on the offer, but I think it's better to share whatever you know on site so everyone can benefit. I can't be the only one willing to learn.

[doctorjames]

cool, I didnt want to 'hijack' your thread.

Its pretty much bedtime here, and still need to get my 365 up before midnight, so will answer tomorrow if thats ok....