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01/24/2011 08:46:15 AM · #1 |
Could someone with a better understanding of photography than I please explain a phenomenon I see often while taking pics.
If I set my camera to a specific aperture (say f/5.6) and shutterspeed and ISO, and focus on something 5 feet away, most of the frame is still in focus. In otherwords, a fairly deep DoF.
However, if I move up to that same object and take a picture from maybe 2 feet away with the same settings, the DoF is much much shorter with there only being a short area in focus.
Why is it that getting closer to something causes the DoF to get shorter, even when settings stay the same?
Message edited by author 2011-01-24 09:55:02. |
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01/24/2011 09:06:07 AM · #2 |
Depth of field is also a function of distance. I recall a diagram on a blog somewhere that helped to visualize it. Let me see if I can find it for you.
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01/24/2011 09:10:25 AM · #3 |
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01/24/2011 09:15:51 AM · #4 |
Can't find the diagram I was looking for, but if you read up on hyperfocal distance, it might provide some explanation that helps. The article that tholmir found looks good as well.
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01/24/2011 09:19:11 AM · #5 |
Originally posted by gcoulson:
If I set my camera to a specific aperture (say f/5.6) and shutterspeed and ISO, and focus on something 5 feet away, most of the frame is still in focus. In otherwords, a fairly deep DoF.
However, if I move up to that same object and take a picture from maybe 2 feet away with the same settings, the DoF is much much shorter with there only being a short are in focus. |
When the subject is 5 feet away, it falls within the hyperfocal range, but when you bring it closer, it is not within that range and you cannot get everything in focus at once.
Not sure if that helped. There's a lot of physics of light involved. I can visualize the concept myself, but I'm having a hard time explaining it.
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01/24/2011 09:20:54 AM · #6 |
Excellent reference.
I use DOF Master on my iPhone. It's a very useful app. |
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01/24/2011 09:36:31 AM · #7 |
A couple of points:
1. DOF is a function of the physical size of the aperture, not the f/stop itself. F/stop is a ratio between the size of the aperture and the focal length of the lens: on a 50mm lens, a 25mm opening is f/2.0, on a 100mm lens the same opening is f/4.0, and on a 200mm lens it is f/8.0....
2. Until you get into extreme closeup work, you can roughly visualize DOF as extending 1/3 of the way in front of, and 2/3 of the way behind, your point of focus. That is, if with a given lens at a given aperture you had 12 feet of available DOF when focused at, say, 20 feet, 4 feet of that would be in front of the point of focus and 8 feet behind it: the DOF, the zone of acceptable focus, would extend from 16 feet to 28 feet. Roughly...
3. DOF is not a precise measure, it is dependent on many factors. You've probably noticed how the DOF looks fine on your camera's 3-inch screen when you review the shot, but it turns out to be woefully inadequate on the computer screen? DOF is dependent on the relationship between the size of reproduction and the viewing distance. Sharpness, in general, is the same way. A 16x20 print that looks great from 5 feet away may not look so good when viewed from 12 inches away. So all this is subjective, to a certain extent.
R. |
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01/24/2011 10:01:06 AM · #8 |
Thanks for all the explanations folks! That depth of field calculator is very nice:
For instance, regarding my original scenario:
At a focal length of 55mm, with an aperture of f/5.6 and subject 5ft away, I have an effective DoF of 0.52 feet.
If bring the subject to 2ft away, that DoF drops drastically to 0.08ft. |
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01/24/2011 10:19:17 AM · #9 |
| I have a tip to offer. Avoid stopping down to f/22 just automatically in order to maximize DOF. It is true that your smallest aperture (biggest f/number) WILL maximize DOF, but at the expense of sharpness and clarity. On most lenses, f/22 is a tiny pinhole. Rays of light passing through that tiny hole will suffer diffraction. In my earlier days of landscape photography, I made the mistake of stopping down too far. I could have achieved almost all of the DOF with f/16. Turns out f/16 was the honey spot of my Canon 24-105mm f/4 IS USM lens. All of a sudden, an adequately stopped down aperture gave me more clarity and sharper detail. It's because I was avoiding diffraction from the f/22 pinhole. |
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