How do you focus with shallow DOF

[Baadil]

Hi,

This questions is for wedding photogs but I guess it applies to anyone shooting in low light.

I have two fo the most recommended lenses for weddings, 17-55mm 2.8 IS, and 70-200mm 2.8 IS. I mostly use single center point for focus and recompose as needed. I used dof master's calculator to calculate some of the extreme points for these lenses for my XTi.

1. 17mm, f2.8, 8 feet - 9.04 ft
2. 55mm, f2.8, 8 feet - 0.68 ft
3. 70mm, f2.8, 8 feet - 0.42 ft
4. 200mm, f2.8, 8 feet - 0.05 ft
5. 200mm, f4.0, 12 feet - 0.11 ft

I can understand being able to focus easily for #1 and #2. But if you are focusing and then recomposing, how do you keep your focus for other three with such shallow DOF?

Thanks.

[picturecrazy]

The best way to focus is to select the focus point closest to your focusing target. Sometimes, centre point is way off the mark and you end up putting your subject out of the DOF. So always use the closest point when you can.

The situation where I do not use the outer focus points is when it's really dark. In that case usually the centre point is the only one that really works well. If that's the case, then do some focus recomposing practice. When I'm shooting at 200mm in the dark (usually for stuff like speeches) I focus with centre point, and depending on how much and which direction you move your lens, you can actually move the camera forward or back to make up for it. After you do it for a long time you get used to it. It's not perfect but I have found the results are better than a straight recompose. Practice with a high contrast object in your home and get the feel for it.

[Philco]

I use the outer points a lot (probably from shooting vertical most of the time) It's rare that I have to rely on center only - using a 580 focus assist beam makes a huge difference in low light.Like Lloyd said - use what's closest to the target. I will often use the outer point to focus on the eye, and then I can recompose slightly so there's not too much top-space for the composition- which is quicker and easier than making a huge recomposition with the center point. With the exception of group-grip-n-grins at receptions, I'm always at F2.8 or wider and this works for me just fine. A lot of it is timing - hitting the shutter when the subject isn't swaying forward or backwards. My favorite reception lens is the 85mm - when you can open up to F2 or more, it does let a little more light in- you might want to give that a try.

200mm at 8ft? On an XTi that seems really, really tight.

Phil

[Baadil]

Thanks guys.
When you guys talk about outer focus points, are you still talking about a single focus point or simply 'all' focus points turned on...

Phil,
200mm @ 8ft on XTi was one of the extremes I was calculating. I think with these settings anda lot of luck I might be able to focus on a single eyelash and not the eye :-)

[Philco]

I use the joystick to push the focus point where I want it - some people prefer the wheel. - so the focus point is always deliberately chosen.

[picturecrazy]

We mean selecting an individual focus point.

I (and most others) never turn on ALL focus points. That's just asking for trouble imo. Fine for vacation snaps, but not for any kind of wide aperture professional work.

[Baadil]

Sounds good. Thanks.

[KennethPhotography]

Just a side note here on center-point focus accuracy with fast lenses (2.8 or faster for the XTi). You don't have to use an fstop of 2.8 (or faster) to benefit from the improved center-point focus when shooting. The lens is always wide open when you are composing a shot (it only stops down when you take the photo or use the DOF preview) so even if you use f/11 for your shot with the 70-200 f/2.8 IS L the camera still benefits from the fast glass and improved center point accuracy.

Most people probably knew this but some may not.

[Baadil]

Thank you Kenneth. :-) Most probably knew, but I didn't so it's good information.

[KennethPhotography]

Glad I could help.

For me the biggest rule of thumb is the closer the subject is to the lens the more careful you have to be re-composing the shot (or biting the bullet and simply using another focus point so you DON'T have to re-compose).

[Baadil]

Thanks Kenneth. I will keep that in mind.

It just seems that if I stop to change focus point I might miss some 'moment' in the wedding environment.

[Wedding Shooter]

Practice changing focus points on the fly with the camera to your face. One reason to move up from the xti is for easier adjustment of settings on the fly.

[fmyers711]

I am pretty old-school as well as just old, but I find myself falling back on my sports photography standby in exceptionally low light - manual focus. It takes some practice, but is very helpful in fast moving low light situations. The other thing I do is focus to a point at the proper distance, turn off the AF and wait for the subject(s) to move into the focal plane area that I have framed...

[kenwood33]

You can not. I switch focusing points all the time when shooting at 2.8 or wider.

[Mark_Cohran]

Lot's go good answers here - I guess mine would be....very carefully. :)

Realistically, when using a shallow depth of field, I choose the best AF point for the composition as to require minimal recomposition.

Mark