Canon 40D focus question

[Fireball]

I've had a Rebel XTi for a few years and upgraded to the 40D a few weeks ago and am having problems with the AF lock mode.

I know that you focus on your subject, hold the shutter button partway down to lock it, and while holding it down you recompose the shot, but when I do that, my lens starts focusing on something else and NOT what I locked it on. Very frustrating when you're doing a photo shoot! I usually shoot in AV mode with my lens on auto focus.

Do I need to set my lens to manual focus instead? I didn't have to do that with my Rebel.

So much more to learn and keep straight on this camera! :(

[Codebreaker]

Check some of the focus modes, there is usually a choice of three...



One Shot
AI Focus
AI Servo

The first one will lock in the focus point when you push the button. The other two will continue to track a moving subject - or you if you move.

Colin

[Fireball]

Thanks Colin. I'll check it out and see if that's what I'm doing wrong.

[megorin]

Colin - maybe you can help on this question as well:
I have a 40d also, using manual shooting mode, AI Serv focus, and I am trying to get 2 babies in focus that are not on the same parallel. Right now one of them is in crisp focus and the other is a bit fuzzy. Maybe I need to increase the aperature??

Thanks,
Michelle

[Codebreaker]

Michelle....

Your on the correct track but heading in the wrong direction :)

If you have two subjects in differing planes you need to have a larger depth of field than what you may have already to get them both in focus. To do this you need to decrease the Aperture.

What's confusing is that a large aperture is a smaller f - number e.g f2.8 and smaller apertures are larger f - numbers e.g f22

Small f - numbers > Large aperture > Small Depth of Field e.g f2.8
Large f - number > Small aperture > Larger Depth of Field e.g f22

Remember also that as you decrease the Aperture your Shutter Speed will need to be slower - risk is camera shake. You can boost the ISO setting to allow an increase in the Shutter Speed for the same Aperture - risk is noise in image.

Colin

[Chuck S.]

Colin - maybe you can help on this question as well:
I have a 40d also, using manual shooting mode, AI Serv focus, and I am trying to get 2 babies in focus that are not on the same parallel. Right now one of them is in crisp focus and the other is a bit fuzzy. Maybe I need to increase the aperature??

Thanks,
Michelle

Michelle, you may also want to try the A-DEP mode, one click away from M on the mode dial (see page 91 of the manual for details). It will adjust aperture to achieve focus depth, then will select shutter speed to go along with that aperture to achieve correct exposure. It's not Manual, but it could be used for a test shot before switching to Manual.

[Codebreaker]

Chuck....

I was tempted to mention the A-DEP mode but I've never used it - have you?

Doesn't it rely on having the subjects covered by the Auto Focus points?

Colin

[Chuck S.]

Colin, I've never used it either....:o It does sound like an interesting feature, though; I may have to give it a go! Yes on the auto focus points; not sure how that works but we'll see.

[ljameso1]

I've used A-dep and it works well. Set the dial to this and point/click the main focus point at the object closest you want in focus, then repeat with the object that is furthest that you want in focus. The 3rd shot will choose a depth of field to encompass both provided it is physically possible with the lens you are using.

[Chuck S.]

Linda, thanks for explaining how A-DEP works. Now I just need to remember how to point-click the focus points....:o

[Derry]

interesting feature,,

does the camera have a preview button to shut the lens down so you can view at the actual f stop you will have set,,?? of course lighting may not be adequate to allow good seeing to determine what all is in focus,,

Derry

[Inspeqtor]

Linda,

Your knowledge of photography still astounds me! Thank you for explaining A-DEP as the XSi also has this feature and I like many others have never used it.

[ljameso1]

Glad to be of help. You guys are going to give me a swelled head!! The 40D does have a depth of field preview button.

[Inspeqtor]

The 40D does have a depth of field preview button.

As does the XSi

[megorin]

Thank you so much Colin, Chuck, and Linda - I will try decreasing my aperature first, and also give A-DEP mode a try.

[Codebreaker]

I've found the stop down button that gives a DOF preview to be of limited use. As it actually stops the lens down it reduces the brightness in the viewfinder to a degree that makes it unusable in my opinion. Maybe Live View fares better.

Its better to learn roughly what DOF you'll get for a given aperture and distance, in my opinion.

Colin

[Chuck S.]

Colin, I've had DOF preview buttons/levers on cameras all the way back to my earliest film SLR's. I never could make any of them work for my eyes - glad to hear it's not just me!

There are some good depth of field calculators available online. The one with which I'm familiar is found at:

DOFMaster (http://www.dofmaster.com/)

[ljameso1]

I've always had excellent vision in low light so am actually able to use DOF. Must have been all those carrots I ate growing up- the neighborhood kids wouldn't play after dark tag games with me, cause I could always see them coming:D