Autofocus problems in a dark room

[whitworth photography]

I want to start by saying that I've taken a few thousand shots using

my Rebel XT with my 420EX on a flash bracket that puts the flash

mount 4-6 inches above the camera hot-show mount. Also, I am using

the OCSC-2. 95% of them were taken outside and came out in perfect,

or at least acceptably good focus. I just want to establish that

neither I nor the camera have any focus problems under good lighting

conditions.

 

Having said that, I took some pictures at a party this past week in

a very dark room and quite a few of the pictures were not in good

focus.

 

I used the center focus point and had the AF mode set to "One

Shot". I also noticed the "infrared" cross-hatch a couple times

which leads me to believe that was coming on and working correctly

through the 420EX. All of the shots I took were at F2.8 and I did

do some focus/recompose.

 

I've read through the EOS Flash Bible and done some searches on

photo.net, but couldn't really find anything that seemed to help.

If you guys have any guidance it would be much appreciated. I'm

sure focus/recompose didn't do me any favors, and that was worsened

by shooting at F2.8, but should I have just used a different focus

point on the top edge? Maybe it's that simple, but I'd like to hear

what other thoughts you guys might have.

 

Thanks,

 

Kirk

[rodney_gold1]

It could be that the cross hatch pattern does not co-incide with the AF point you are using due to the repositioning of the flash on the custom bracket. Focus and recompose with 2.8 is fraught with problems anyway and this is also likely to have contributed. Focus and recompose is also MORE of a nightmare with ettl II and correct flash exposure unless one has done FEL as the flash will meter whats centre frame and not the actual subject if its out the centre. Perhaps a 430/580 ex is better in regard to the AF assist than a 420 with a wider spread or more lines or more co incidence with AF points?

[henrik.ploug]

For party pictures in low light with a flash I would put the camera in manual mode, set the aperture at f5,6, the shutter speed at 1/200 seconds and let the flash do the rest. At f5,6 you should be able to focus and recompose, except at really close distances.

[rob_murray]

I would have stopped down to about F8 or F11 and let the flash do its thing. This will cover most focus errors. Your flash was not aligned

to the AF sensor you were using.

[mark u]

The 420EX should be fine as regards AF point coverage - provided you have it aligned to project the AF patterns where you focus . If you can adjust your flash bracket a little for tilt, you can make sure that the patterns coincide with the AF points at typical shooting distances (test against a wall). The extra coverage provided by the 420 EX is actually an advantage in this situation, as even with a slight tilt of the bracket, the flash beam will still provide full coverage of the subject. When shooting wide open it's best to minimise the amount of recomposing you do, so select the closest focus point to the part of the image you want to be in sharpest focus.