DOF Problems

[Richard DeYoung]

I am having problems getting DOF with my Canon 40D and a 17-50 lens. No problem using my 70-200 on the same camera. What I am trying to do is to throw everything out of focus when taking a portrait of a person out doors at about 10 feet away. I have used everything from f4 to f22 and there doesn't seem to be any difference. I have tried manual and auto focus and it doesn't seem to make any differece.
Any thyoughts/suggestions?

[venchka]

Hmmmmmmmmm...Aperture Priority at f/4.0 didn't do the trick? Was the lens set at 50mm? Get closer.

Here are the things that affect DOF, or in your case, eliminate DOF:

Focal length: Longer
Aperture: Bigger (wide open)
Subject distance: Closer

If you do all of that, you should end up with very little DOF. BTW, how much DOF are you trying to get? Your 70-200 may be the answer. I know that my 70-200 at 200mm and f/2.8 and up close and personal has very shallow DOF.

[m_mphotography]

Is your subject too close to the background?? try moving him/her up a few feet and see if that doesn't do the trick. F4 should blur thing out in the background, but NOT if they are immediately behind your subject.

[Paulo]

Richard, can you post some of the images you feel you had trouble obtaining the desired DOF? I agree with the previous posts. Aperture chosen plus distance between camera to subject to background can affect the outcome.

[Richard DeYoung]

Unfortunately I blow away all test photos, so I have no pics. However here is what I am doing. Mount the camera and lens on a tripod, Set the lens at 50, set the apeture at 4 or 5, lens is on auto focus on a subject 12 feet away. Take photo: everything as far as I can see in the background is in focus from 20 to 100 feet. I am taking these photo from my patio overlooking the golf course I live on. I want to have a sharp image for my subject and as much of the back ground as possible out of focus.
As I mentioned in my original statement this seams to work well with my 70/200 but not my 17/50 lens.
Thanks for your input and help

[brad]

At 12 feet away, even f/4 is going to be a pretty deep field.. measured in feet.

[Teddington]

I rarely know the answer but I can Google pretty good. There are lots of DOF charts out there. Here's one specifically for 50MM and 40D. Brad was right on that DOF at 12' is measured in feet and the transition is much more gradual than if the DOF was narrower. You're looking for DOF in inches to blur out the BG.

http://www.dofmaster.com/doftable.html

[Paulo]

Great link, Terry! Here's another one but pretty much does the same thing...

http://bobatkins.com/photography/technical/dofcalc.html

[brad]

At the distances you are from subject, for a really narrow DOF, you're gonna have to be at 2.8 or lower.. maybe even 1.8.

[Richard DeYoung]

Thanks to everyone that responded. I got hold of a chart that gives me the depths of field for my 40D and it will help me alot. Thanks to all.

[brad]

Just remember, too, that DOF is not an immediate cutoff point.. even if the chart says "1 foot," that doesn't mean that at the one foot point, things are magically out of focus - beautiful and creamy smooth. DOF falls off gradually from the focus point.. the 1 foot number in the example is "acceptable focus" .. but the stuff at the edge of that one foot is not as sharp as the stuff at the point of focus.

Also, the OOF area's appearance - creaminess or bokeh is influenced by a number of factors, not the least of which is how many blades make up the aperture and how round of a hole is created by those blades.

[Richard DeYoung]

Using a dof chart and settings I was able to produce the bokeh I was looking for in some photos I took this morning. Amazing what education can do.; Again thanks for evryones help. This is what makes this forum so special.

[venchka]

The DOF Table probably says this , but in case it doesn't, DOF extends roughly 1/3 in front of and 2/3 behind the plane of focus. Make your autofocus lenses focus on your subject's eyes. That way you won't be wasting any DOF. Besides, folks and critters look better if their eyes are in focus.