Focus point lost when recomposing

[zwollenaar]

I was using spot metering mode, small f number (f/1.2- f/2) at night time for my pics. At first, the subject was good in focus but when I recomposed (reframing) the focus point (at middle point of camera LCD) moved to different location, therefore the subject was out of focus.

If the DoF is longer (e.g. f/8 or f/11) to make sure everything framed good and subject still on focus, I'm unable to use large apperture in low light condition and the subject is getting too small!


My question here is that if you have anyway to keep focusing on the subject when you are reframing the exposure so some details won't be cropped off.

[Dawn U]

Ok, stupid question, but are you holding the shutter button halfway when you recompose, or are you focusing, releasing the shutter, then recomposing? If you're releasing then recomposing, it's going to refocus every time you push the shutter button, unless you turn off the auto focus after focusing.

I think that made sense...

[zwollenaar]

Yes, half way while recomposing.

[Mark_Cohran]

With shallow depth of field, focus and recompose is more often than not is going to put your subject out of focus. Instead of using the center AF point, compose and manually select the AF point closest to your subject, use that to focus on your subject, then recompose. This will minimize the error in the recomposition.

[Hermeto]

Here is the longer version of Mark's explanation:

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

[zwollenaar]

With shallow depth of field, focus and recompose is more often than not is going to put your subject out of focus. Instead of using the center AF point, compose and manually select the AF point closest to your subject, use that to focus on your subject, then recompose. This will minimize the error in the recomposition.

Thanks Mark! My camera was setting with only one AF point (square) centered at the viewfinder's screen and spot metering mode. But for your advice, do I need to change my setting into more AF points to get more AF points in the same time and then recompose with an AF point closet to my subject, and use a better mode, evaluative metering for instance, insteads of spot metering?

[Mark_Cohran]

No, don't change to more AF points, but instead, manually select the AF point that best fits your composition, use that to focus on your subject, and then if you need to recompose it should be fairly minor. For info on how to manually select an AF point, look in your manual. The directions are pretty clear.

[zwollenaar]

Here is the longer version of Mark's explanation:

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

Thanks for the helpful link, Hermeto! I'm reading it...
One of the solution for focus-recompose problem is rotating the camera vertically and autofocusing. I haven't heard about that while I have lots of vertical shots with just only an idea of cutting back some unwanted detail on the photos.

What is your own advice?

[neil85]

did not read all the posts ... but do you have the camera set for "one shot" and not "ai servo" or "AI focus"

?

[Wilt]

Focus and recompose is generally not an issue if you keep the angular displacement under 20 degrees, even with a relatively fast lens f/2.8 aperture for the actual shot!

Simple high school-level trigonometry proves my claim...if the original distance is the Side of the right triangle, and the new distance is the Hypotenuse of the right triangle, Cosine 20 (side/hypotenuse) is 0.94, or the hypotenuse is roughly 6% longer; Cosecant 20 (hypotenuse/side) is 1.064). A 50mm lens at 10' shooting distance has DOF out to 10.056 at the far end. Of course the FL of the lens comes into play for the transition angle where DOF is not sufficient to compensate for focus plane shift.

[tkindler]

On my 350D I set it to use the "+" button on the back for auto focusing, instead of the shutter release button (custom setting #4, I believe). I set the focus, adjust the camera for the composition I want and the focus point does not change when I press the shutter release. Stays constant from shot to shot, too. Most SLR's should have this capability.