Focus and Recompose Problem

[justinisfilming]

Hey guys! My name is Justin, I'm new here. I don't know if this is the right place for this question (if it's not, please tell me the right place), but I'm gonna give it a shot.

So I just got the Canon 50mm 1.8 for my T3i and it's wonderful. The large aperture is great. The only problem is, whenever I focus on the subject, then recompose, my subject becomes blurry. Is it because I'm recomposing my shot? The shallow DOF is great but with a slight movement, your subject goes out of focus. Any tips? Should I change my AF points? I currently have it on the center. What I do is I light the center dot to the subject's eye then recompose. Should I use a different AF point? Or should I use a different AF mode? I have it on ONE SHOT right now so that the focus won't change when I recompose.

Any help would be greatly appreciated!! =))

Thanks!

[TPF Sponsor]

[usayit]

* Yes... there is an "error" when you focus on the subject in the middle of the frame and recompose. Diagram below



Plug in some numbers to figure out how large or small the error really is. Often it still falls within the DOF which is a function of focal length, aperture and distance to subject (and format)

* Yes... it would be nice if you can select a different AF point. The T3i should be capable of this... refer to manual.

* Often, lenses are their sharpest towards the center of frame. This is especially true with fast lenses.


I often shoot with a 50mm f/1 and try to compensate by leaning back a little for a shot. Practice to get a feel for it. Otherwise, use the nice feature of that camera and move the AF point.

[SGorman]

If you are recomposing, it is entirely possible that you are moving just that much to affect the image. With a shallow DOF, you have a very narrow plane. If you move forward even a quarter inch, you push the focus off. You might not even realize you're doing it. In instances where you can, I'd switch to manual focus.

*edit -- ... and someone posts a friggin' hand-drawn formula. Whaddaya know ... those trig classes CAN be used for something! I was an art major dammit!

[cgipson1]

When using a large aperture.. the DOF can be razor thin. Any movement on the part of the camera or the subject can throw it OOF. For shooting like this, use your focus points in the viewfinder... and specify which focus point you want the camera to use, rather than moving the camera.

I usually use single point focus (spot) and move the focus point where I want it to be. Don't know if your Canon will allow that or not... but it should do something workable. My advice would be to stop down a couple of stops... unless you just really need that razor thin DOF.

[usayit]

*edit -- ... and someone posts a friggin' hand-drawn formula. Whaddaya know ... those trig classes CAN be used for something! I was an art major dammit!

LOL... came up for discussion a year or so ago. I had numbers crunched shooting a 50mm f/1 at various subject distances to show how small and controllable the focus shift can be. Personally, I wish I was more artistic. :-)

[unpopular]

Is there a 50/1 lens? Isn't it usually 50/0.9?

[Forkie]

I hope this doesn't sound patronising, but have you got your camera set to continuous focus instead of "One Shot"? I've done that a few times and missed shots by forgetting I have it on continuous and vice versa.

[usayit]

Is there a 50/1 lens? Isn't it usually 50/0.9?

The max aperture of a lens is by design and not limited by some sort of physical law. There are versions of my lens in f/1.2, f/1, and f/0.95.

[Edsport]

When you focus on your subject by pressing the shutter button half way, you have to keep the button pressed half way while you are recomposing and take the shot by pressing the shutter button the other half.

[usayit]

When you focus on your subject by pressing the shutter button half way, you have to keep the button pressed half way while you are recomposing and take the shot by pressing the shutter button the other half.

Half shutter button (as you describe) does not correct for focus error due to recomposing the photo.

[MReid]

For those razor thin depth of field shots, use Continuous focus and choose your single point focus point so it can be right on your subjects eye area, where exactly depends on the subjects face angle in relation to your lens. With practice this is faster than recomposing.

Focus recompose will probably work outside of 10 feet or so but the closer you get to your subject the more it will fail.

[Edsport]

When you focus on your subject by pressing the shutter button half way, you have to keep the button pressed half way while you are recomposing and take the shot by pressing the shutter button the other half.

Half shutter button (as you describe) does not correct for focus error due to recomposing the photo.

True but i'm wondering if the OP was focusing then letting go of the shutter button and recomposing to take the shot...

[usayit]

That is not how I read it.... either way... focus error is still present.

[Steve01]

If you're shooting wide open you may want to enable all your focus points and use the one closest to your subject.
As already mentioned at f1.8 the DOF can be virtually non existent if your really close to the subject.

[MTVision]

I shoot wide open fairly often due to low light but I never focus and recompose at 1.4/1.8. The DOF is just too shallow. The slightest focusing error will cause the photo to come out OOF and that's without recomposing. Is this only happening when you are shooting with a large aperture or all the time? Use another focus point or close the lens down to a smaller aperture.