Depth of Field primer

[FaithfulPastor]

After taking a few shots where there were multiple subjects in the photo, and not everyone was in focus, I started reading more and more about depth of field.

Let's say that I'm shooting two people and one person is standing behind the other person, that means their noses are different distances from the camera. Further, let's say the two noses are 18 inches different in distance from the camera because of the thickness of the chair and what ever.

Of course, what I'd like to change is the 18 inches, but since this is a depth of field question, let's leave that alone.

Here's what I think I've learned, but I'd like to get a few pros to weigh in on this:

If I set my camera (Canon 5D full frame) to a focal length of 24mm and an fstop of 2.8, then I must stand at 5 feet away from the subjects so that both of them will be in focus. Any closer and somebody gets fuzzy.

If I change the focal length to 70mm, then I must stand about 14 feet away
If I change the focal length to 200mm, then I must stand about 40 feet away

Now, let me do this all over again, but use an f stop of 5.6
24mm means I must stand about 3 feet away
70mm means i must stand about 9 feet away
200mm means I must stand about 28 feet away

I used the dofmaster to do the calculations.
The distance was determined by the short and long dof being greater than 18 inches. For example: the near dof @5.6 for 70 mm is 8.24 feet. The far dof @5.6 for 70mm is 9.92 feet. The difference between those two distances is 1.68 feet or 20.16 inches.

Since my two subjects are within 18 inches of one another, that falls within the 20.16 inches so I'm okay.

Whew...

So here's question #1 - Is my math right? Am I understanding the dof calculations correctly?

#2 - How do you carry that information in your head so you don't butcher shots?

FP

[TCav]

Originally Posted by FaithfulPastor

... So here's question #1 - Is my math right? Am I understanding the dof calculations correctly?

Yes. Close enough.

Originally Posted by FaithfulPastor

... #2 - How do you carry that information in your head so you don't butcher shots?

Personaly, I don't. That's what I use the DOF Preview button for.

[VTphotog]

If it is the sort of shot you have time to sort out with DOF preview, then give it a try, but I have seldom had much success with it. The VF is too small to give me a really good idea, and when using f/5.6 or smaller, the light fallof makes it worse. My basic rule of thumb is to shoot at f/8 or smaller if I need lots of DOF.

Another method, if you are looking for a particular effect, is to use aperture bracketing. Some digitals (maybe most-I'm not familiar with that many) can do this automatically.

Otherwise, if you have the time, review your shots.

brian

[tjsnaps]

On my film camera’s I used my DOF preview all the time. ON my digital I don’t have the option. What I do have is my experience and my LCD (though I’d kill for a bigger LCD) What I suggest to you is that you practice, practice, practice. Soon it will be second nature.

[BillDrew]

I also have never found the DoF preview useful - the image is simply to dark at small aperatures (high f/number) to make out the subject very clearly. IMHO, tjsnaps' suggestion to practice, practice, practice, ... using the LCD and magnification is the best technique.

Also keep in mind that DoF is really a case of getting things good enough. Strictly speaking there is only one plane that is in focus - the DoF is the distance closer/further that is good enough for the size of the print, viewing distance, subject, ... Something that looks very good in a 4x6" print could look very out of focus in a 16x20" print. A shot of a foggy marsh might look in focus while a shot of a group of children at the same settings might not be clear enough.