Some DOF and Focus Questions

[QTip]

Hi. I'm just getting started messing with photography and got a used 40D. I've read several photography books now and things are starting to come together. However, I'm a bit confused about DOF (which seems to be "distance settings" in some camera books).

I understand the shallow DOF with lower apertures and then opposite with higher apertures. However, here are a couple questions I have.

Let's say I'm talking a picture of 2 kids and one is a bit closer to me than the other but I want them both sharp. I've noticed the A-DEP setting on my camera, but getting the items I want on focus points seems like it happens rarely. I've not seen where I can manually select two focus points. That would be handy if they can do that. I know I can increase the aperture; however, what if I want the background a bit blurry and the background is fairly close?

Also, I've noticed that when I'm closer to the subject, the DEP is really small. Like is only a few inches deep. Now, let's say I'm doing a full-body portrait shot and the subject is standing in front of a wall. What can help me get their whole body clear but the wall really blurry. Would an extension tube create a type of macro-type DOF and accomplish that?

Thanks for any help.

[bjyoder]

You kinda can't (realistically) get the wall "really blurry" if the subject is standing near - or at - a wall. Your best bet is to get some distance between your subject and the background.

As for the two people in one shot, sometimes there, your only choice is to use the smaller aperture. In general, a good rule of thumb is that the DOF is distributed roughly 1/3 in front of the focal point and 2/3 behind. Use this to your advantage, and try focusing on the front subject to be able to blur the background a bit better.

[gjl711]

Let's say I'm talking a picture of 2 kids and one is a bit closer to me than the other but I want them both sharp. I've noticed the A-DEP setting on my camera, but getting the items I want on focus points seems like it happens rarely. I've not seen where I can manually select two focus points. That would be handy if they can do that. I know I can increase the aperture; however, what if I want the background a bit blurry and the background is fairly close? Unfortunately, you can not select two separate focus points. Best bet is to select the center point, focus on the target you want most in focus and recompose the shot placing both where you want them. To get both in focus you will need to close the aperture. How much depends on how far apart they are. Unfortunatly, as DOF increases so will the clarity in the background. It's unavoidable.

However, you can get fancier and manually set focus at some point between both subjects. That way the aperture can be as open as possible, both subjects are in focus, and the background is as blury as you can get it for that aperture.

Also, I've noticed that when I'm closer to the subject, the DEP is really small./quote] Yup, DOF is a function of how close you are focused. If you are focused on something a foot away, your DOF will be very small indeed, maybe an inch or so depending on the aperture. If your focused on something 100 feet away it will be much larger, maybe 10 feet or something. In reality there is only one distance where the camera is in perfect focus, the focal plane. Say it's focused at 100 feet. THe image at 100 feet is perfect. but the change from in focus to out of focus is gradual. as you move closer, 99 feet, 97 feet, the image is slowly going out of focus and when it becomes large enough, you notice it. There is a great tutorial discussing everything you ever want to know about DOF here:
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

[quote=QTip;8465472]Now, let's say I'm doing a full-body portrait shot and the subject is standing in front of a wall. What can help me get their whole body clear but the wall really blurry. The only way you can do that is by moving the subject away from the wall, or opening the aperture so large that the DOF becomes razor thin. Here is a very nice little DOF calculator. Just plug in you f/stop, distance from the subject, and it will tell you what the DOF is.

[QTip]

Thank you both for the help.

[yogestee]

QTip,,take your camera out of A-Dep and use Av mode.. It will give you more control when needing just the right amount of DOF.. On the left hand side of your camera near the lens mount is a depth of field preview button which allows you to check the DOF at any given aperture.. A very useful tool..

[QTip]

QTip,,take your camera out of A-Dep and use Av mode.. It will give you more control when needing just the right amount of DOF.. On the left hand side of your camera near the lens mount is a depth of field preview button which allows you to check the DOF at any given aperture.. A very useful tool..

Thx. I've been messing with that, but it's really difficult to see well in the viewfinder at the higher apertures since it gets so dark.

Seeing the DOF calculator has really cleared up this whole topic for me. I appreciate the responses.

[kjonnnn]

On the Canon 10S you could select to points of focus and tell the camera to make those two points be within the area of depth of field. The A-Dep feature on my 20D works similarly and is pretty consisent. Check you manual and make sure you're using A-dep correctly. They may have changed it in your camera.