Help with focus! Ahh!

[aliciaqw]

I've been reading a lot of forums and have seen the suggestion about back button focusing. I tried that a couple of days ago and again yesterday. I seemed to have great success with it intially, buuuut I feel like I took a step backward yesterday.

I took a couple hundred shots of DH and his dirtbike (posed, not action) and I had a difficult time focusing on his face. After looking at the pics on my computer I noticed often times the focus fell on his chest or forearms. How is that happening when I lock focus on his face and he's not moving? Do I need to hold the back button DOWN until I take the picture? Do you have any more tips? I pressed focus, recomposed, and then snapped. I think about 3/4 of the pictures were not focused on his face.

I'm growing so frustrated. HELP!?!

I am using a Canon XSi, 50mm 1.4 and am using option 1 in the command. I have my focus mode in "one shot" (not AI servo, etc). Where am I going wrong??

 

[Big Mike]

Where you shooting at F1.4 or close to it?

It could be that your DOF was very thin, which requires very accurate focusing. Sometimes, even just recomposing the shot is enough to change the distance enough that what you focused on, won't be in focus anymore.

Also, it's possible that your camera/lens is front or back focusing, and with a shallow DOF, you really need to have accurate focus.

 

[Geaux]

Post up some example shots. That'll help us figure out or try to figure out the problem.

Could be Aperture settings.

 

[KmH]

Using to big an aperture opening is the likely cause of your problems.

Large lens openings can make for very shallow depth-of-field (DOF).

Subject to image sensor distance, subject to background distance and the focal length of the lens all contribute to DOF.

Visit www.dofmaster.com

An Xsi with a 50 mm at f/1.4 and a subject distance of 10 feet gives a DOF of:

In front of subject - 0.31 ft (48%) (about 4 inches)
Behind subject - 0.33 ft (52%) (about 4 inches

Your total DOF would be only 8 inches.

At f/2.8

In front of subject - 0.61 ft (47%) (about 7 inches)
Behind subject - 0.69 ft (53%) (about 7 inches)

 

[aliciaqw]

OK, here are two pictures I took (isn't he a good sport? LOL...). Please please ignore the crap color and the ridiculous ISO. I fixed it after a couple of pictures...not realizing ISO was in Auto from when my hubby was playing around with it. Anyways, there are a lot of issues with these pictures, I know, but focus is what is killing me in my head. I'm agonizing over it, really-- which is pathetic because I have so many other things I should be worrying about...haha. That's another story, though

f/5 (looks like focus is on his forearms?)

f/4 (I focused on his face and now I don't know where it is...his arms again?? grrr!)

 

[fokker]

Using focus-recompose is not recommended for situations where the subject is somewhat close and/or using a shallow depth of field.

 

[Big Mike]

I don't think there is a focus issue here.
He face looks just as in-focus as anything, from what I can tell.

How close are you zooming in when you are seeing these problems? With digital photo software, it's very easy to zoom in to 200%, 300% etc...and at those levels, it's not going to look perfectly sharp.

Have you tried applying some sharpening? Many people find that a little bit of sharpening during post production, can really go a long way.
Also, shooting at ISO 800 isn't helping.

 

[LoveAlwaysJami]

I agree... doesnt look like a focusing issue to me. Possibly a zooming.

 

[aliciaqw]

I know ISO 800 was a dumb move, but it was a mistake that I later corrected. I still noticed better focus on his beater and tattoos in most of the shots. Maybe I was zooming in too much in Adobe Camera Raw? I was at 100% as I was applying sharpness, etc. I zoomed in also to check the focus since that was my goal of the "shoot". Maybe my expectations are too high for this camera body? I'm saving for an upgrade. Geez...what an expensive hobby this is going to be...haha.

Am I using the backbutton focusing correctly, though? I have 4 options on the XSi. 0) AF/AE lock (default) 1) AE lock/AF 2) AF/AF lock, no AE lock 3) AE/AF, no AE lock.

I am using #1 since I like to be able to lock the AE using a half push of the shutter. My * button is my focus button. The manual says to press it to focus, then release, recompose and then shoot. That's what I'm doing. Should I be using a different option? I know that if I want to use AI Servo for my baby for instance, I should use #3, right? I'll deal with that more once I am satisfied with my ability to photograph someone standing still for me...

 

[Big Mike]

It sounds like you are doing it right. But just to be sure, do you get different results when using the shutter button for focus?

Another thing to try is just switching the focus point so that you don't have to recompose so much. If you get better/different results with those techniques, then you should be onto the problem.

 

[aliciaqw]

It sounds like you are doing it right. But just to be sure, do you get different results when using the shutter button for focus?

Another thing to try is just switching the focus point so that you don't have to recompose so much. If you get better/different results with those techniques, then you should be onto the problem.

I switched to BBF because I was having a hard time focusing normally, using the shutter. And I have yet to try changing the focus point. I'm a little neurotic when it comes to that little red dot. I feel a pull in my gut if the center one is not lit up. Weird I know...

So I feel like a dingbat, but what would the problem be if I do in fact see better results with changing the focus point? That the focus is somehow shifting when I recompose? Typing that makes me feel even dumber because it makes sense...haha... In theory, it shouldn't shift, though, right?

Thanks again for all the help/tips/suggestions!

 

[fokker]

The focus does indeed shift when you recompose. Think of it like this: Say you focus at a point 10 feet from the camera. Everything in a flat plane that passes through that one point and is parallel with the camera sensor plane will be in focus. When you focus on that one point, then reorient the camera, you shift that plane of focus behind the point you first focused on because when you shift the camera you do so in a spherical manner... that doesn't make sense I know but I can't really explain it very well, but I can visualize it in my head. Do some googling and you will see what I mean.

 

[fokker]

This does a slightly job explaining than me: Focus and Recompose

 

[aliciaqw]

I totally get what you're saying. The angles change ever so slightly. I'll check out the link, too. I guess the slight focus shift is slight but obvious to me when I'm zooming in on the photo.

So if I'm going to recompose after focusing, my best bet would to instead use a different focus point? Maybe one that allows me to recompose just a tad rather than shift the whole plane...?

 

[Big Mike]

That can indeed make a difference, but you also need to consider the DOF that you are using. In this case, with an aperture of F5 and I'd guess a distance of 10 feet or more, the ever-so-slight change in camera angle shouldn't put your subject out of focus because the DOF should be a couple feet think at least.

It would make a much bigger difference, if you were shooting at F1.4 and only two feet away from your subject.

So I don't think that the focus & recompose is the problem here.