Aperture/F-Stops

Joined: 02 Apr 2006 Posts: 12 Location: Pahrump, NV

Photography has been a hobby of mine for about a year now and I enjoy it more than just about anything. I think the hardest part for me to comeprehend has been the setting of the aperture (f-stops).

I've read and seen diagrams that show how certain f-stops allow a subject to be in focus at a certain distance while another is not. This rule being called depth of field. It seems (after looking at sample pictures) that this rule is inconsistant. (Don't worry, I know the problem is me and not the rule)

Example: I've seen a landscape photo taken at an aperature of f-11 and than another portrait shot taken at the same f-stop of 11. If the depth of field rule applies, how can the same aperture setting work when in one photo the subject is 4 feet from the camera and the other photo the subject is 20 feet from the camera?

It's extremely hard to be in the moment of taking the shot and have to perform math equations of whip out a book to figure out what f-stop to use. Does this get any easier with experience or am I just impatient? I've got exposure down pat but this never seems to get any easier.

Joined: 18 Nov 2006 Posts: 288

Yes, experience always helps. The good thing about digital is that you can see the results of your exposure immediately in the LCD screen. Learn to zoom it. If you have the wrong aperture for the depth of field you want, then adjust and shoot again.

Some cameras have dof preview that allows you to view through the stopped down lens. That does take some practice because you are viewing through a darker viewfinder.

Your lenses probably have dof scales on them. They aren't critically accurate but they can get you in the ball park.

Finally, there is dof bracketing. If you're not sure you nailed the dof you wanted, take a few exposures at different apertures and look at them later.

I think using a calculator or dof chart goes beyond the call of duty.

Joined: 03 Feb 2007 Posts: 122

Although it's certainly personal choice but you would not normally take a portrait shot at f-11 as you DON't want the background in focus, just the subject. That general rule does not apply if you are at some scenic location and you actually want the person and background in focus to show where they are.

Posted: Sun Feb 04, 2007 9:50 am

I think you are slightly confusing the two related but separate issues of Depth of Field and Focus.

Focus controls the distance from the camera at which the image is sharp

Aperture controls depth of field, which is the margin for error in focus.

If I am focussed at 8 feet and have a DOF of 2 feet, then anything between 7 feet and 9 feet away from me will be more or less in focus.

Posted: Sun Feb 04, 2007 1:30 pm

Quote:

Example: I've seen a landscape photo taken at an aperature of f-11 and than another portrait shot taken at the same f-stop of 11. If the depth of field rule applies, how can the same aperture setting work when in one photo the subject is 4 feet from the camera and the other photo the subject is 20 feet from the camera?

Agreed- you got focus point, DOF, and hyperfocal distance all mixed up. You're trying to separate them, but they're all interrelated.

You can use any f-stop for any picture just as long as you also pick the proper shutter speed for the correct exposure.

You can use any f-stop and focus anywhere, near or far.

However:
1- The more wide open the lens the shallower the DOF (and vice versa).
2- The closer you focus the shallower the DOF (and vice versa).
3- The longer the focal length of the lens the shallower the DOF (and vice versa).
4- Any combination of these will make the DOF even more shallow.

So, how can you use f11 in both kinds of shots? See #2. Focus close on a person and the background far away is blurred. Focus far away (ie, a landscape) and everything is in focus.

Now, do you need to check your Palm Pilot with a Hyperfocal Distance Calculator Program on it before you take a shot to see what is/isn't going to be in focus? Naw. Just remember the 4 rules above. After a bit you'll know. (But if it's a shot, usually a closeup, where certain things must be in focus and others not, the chart or program is invaluable.)

Joined: 18 Nov 2006 Posts: 288

Pokinatcha wrote:

Example: I've seen a landscape photo taken at an aperature of f-11 and than another portrait shot taken at the same f-stop of 11. If the depth of field rule applies, how can the same aperture setting work when in one photo the subject is 4 feet from the camera and the other photo the subject is 20 feet from the camera?

Because aperture is only one of the three factors involved in depth of field. The other two are focal length of the lens and subject to camera distance.