Nikon D90 autofocus

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Fossegordon

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hi all. I have a D90 that i am struggling to autofocus in fairly dim but not especially dark late. Any ideas why this may be?? I can't see the AF illuminator working, although it is set to auto in the settings. I wonder if I have a problem with it? I am using this with a sigma 70mm - 200mm 2.8m lens.

I am also using a Nikon SB40 flash unit which I thought was supposed to also help the autofocus in dim light through use of the infra-red but not sure if this is working either???
 
My first thoughts are the lens your using for the SB40 may have too long a mm length. Have you tried it with a shorter lens under the same conditions? If the SB40 is set as TTL this could be the problem.

Realspeed
 
thanks, i'll try that. certainly the SB-40 only shows as going to 85mm so this may be a problem. The pics when it takes them look fine but it just struggles to auto focus with or without the flash unit.
 
Nikon doesn't make an SB-40 flashgun; do you mean the SB-400? If so, it doesn't have AF illumination or zoom so won't help focussing and isn't limited to 85mm (and that's just a zoom limitation on the better flashes anyway; they don't stop working if the lens goes beyond that).

Just what are you having trouble focussing on? The camera needs something with contrast to be able to focus so check that there's some sort of pattern or lines, etc. where you're trying to focus.

Jonathan
 
As you also posted this question in the D90 owners thread but don't appear to have read it (or just ignored it):

Have you checked your focus mode? The AF assist light only comes on in certain modes - page 174 of the manual.

It will also not work with certain lenses - page 231. This only lists Nikon lenses, but they are mostly of the longer focal lengths, including a 70-200mm.

Do you mean SB400? This doesn't have the AF assist illuminator.
 
hi yes - sorry - SB400, i also have an sb-600. i'll have a read through those manual pages as suggested, very helpful, thanks.
 
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