ILUV767
Topic Author
Posts: 3035
Joined: Mon May 29, 2000 2:21 pm

Help! Depth Of Field

Tue Oct 02, 2001 1:49 pm

Hi everyone,

I am in a begining photo class at my school, and I was wondering if one of you could give me a solid definition for depth of field? What factors affect it? I have very little understanding of it, and I have a test on it tomorrow.

Your help is greatly apreaciated.

I L U V 7 6 7
 
PUnmuth@VIE
Posts: 3119
Joined: Thu Aug 24, 2000 9:31 pm

RE: Help! Depth Of Field

Tue Oct 02, 2001 2:00 pm

Depth of field is mainly influenced by the aperture you are using. The higher the number the more depth of field the lower the number the less you get.
Depth of filed describes which parts of the picture are in focus. Low depth of field: The main subject you are focussing on is sharp and the fore and background are unsharp. Higher depth main object you are focusing on and parts of the fore- and background are sharp.
Peter
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Jan Mogren
Posts: 2014
Joined: Tue Dec 26, 2000 2:47 am

RE: Help! Depth Of Field

Tue Oct 02, 2001 2:26 pm

Factors affecting depth of field are:
1. Aperture (Peter described this)
2. Focal length (The longer the lens, the shorter depth of field)
3. Focusing distance (The closer the object, the shorter the depth of field)

/JM
AeroPresentation - Airline DVD's filmed in High Definition
 
ckw
Posts: 4586
Joined: Fri Aug 27, 2010 12:26 am

RE: Help! Depth Of Field

Tue Oct 02, 2001 5:08 pm

One final detail - depth of field extends further beyond the point of critical focus than it does ahead of it.

Cheers,

Colin
Colin K. Work, Pixstel
 
cfalk
Posts: 10221
Joined: Mon Dec 04, 2000 6:38 pm

RE: Help! Depth Of Field

Tue Oct 02, 2001 11:10 pm

Depth of field (or DOF) is, as stated above, a measure of how much of your picture will be in focus.

No lens will have everything in focus, from a section of chain-link fence touching the lens right out to infinity. when you focus on an object, say 10 feet away, you may have, for example, everything between 8 and 15 feet away in focus, but the horizon and stuff in the forground less than 8 feet away is blurry. In this case, you depth of focus is 8 to 15 feet. Most old SLRs and some new ones have a function which will actually let you "preview" the depth of field.

Depth of field depends on 2 things: 1) the design of the lens, and 2) the aperture that you use. (Shutter speed has in itself no relation at all to depth of field.) The first one, you can't do anything about (other than buy another lens). But the second you have control over. The larger the aperture (which is equal to low f/stop numbers, like f/2.8), the smaller the depth of field will be. Portrait photographers often use lenses with very wide apertures, as it allows them to have the subject in focus, and all the backgroud be totally blurred, which prevents background feature from distracting attention from the subject.

A small aperture (equal to a high f/stop number like f/16) will give you a much larger depth of field.

If you focused on a specific object at 10 feet away, at f/2.8 your depth of field may be in the range of 9 feet (inner-limit DOF) to 12 feet (outer limit DOF). At f/22, your depth of field maybe from 2 feet to 60 feet.

Note when you look at the focusing distances on your lens (any lens) are not linear, but logorithmic. This results in that as your depth of field changes by changing the focusing point (keeping the f/stop constant) the distance in absolute terms of your outer limit DOF changes faster than your inner-limit DOF. Theoretically, the inner and outer limit DOF will intersect right in front of the lens, 0 inches away.

One concept that very few people use because of the popularity of rotating ring-type zoom lenses is hyperfocal distance, which is immensely useful, as it eliminates any calculation or trial and error needed to get the correct depth of field. Tell me if you want a description of that. But you won't be using that if you use most zoom or AF lenses.

Cheers,

Charles
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