 Senior Member
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Join Date: Aug 2008
Location: Portsmouth, Hants, England
Posts: 622
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Quote:
Originally Posted by SteveL
The laws of optics is that the DOF extends from 1/3rd in front of the point focused on, to 2/3rds behind it. In other words, you have twice as much DOF behind your point of focus than in front of it. Therefore there is sharpness in front of the focus point as well as behind it.
If using a DSLR you should use the depth of focus preview button on the camera. A much forgotten but very useful tool.
Steve
If using a DSLR you should use the depth of focus preview button on the camera. A much forgotten but very useful tool.
Steve
For example, if I used a 50mm lens to shoot an object 10feet away at F16 I get:
Near limit 7.33 ft
Far limit 15.7 ft
Total 8.41 ft
In front of subject 2.67 ft (32%)
Behind subject 5.74 ft (68%)
But at F2.0
Depth of field
Near limit 9.56 ft
Far limit 10.5 ft
Total 0.91 ft
In front of subject 0.44 ft (48%)
Behind subject 0.48 ft (52%)
Now the DoF front/back is 50/50 (ish)
Actually, I am not sure the Focal length does make any difference, but the aperture certainly does.
Alan.
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Senior Member
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Join Date: Sep 2007
Location: east sussex
Posts: 3,484
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What others havn't mentioned is how close you are to a subject. Depth of field is relative to distance, so you have to take that into consideration as well.
Bazza
Bazza
 Senior Member
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Join Date: Aug 2008
Location: Leicestershire, UK
Posts: 2,133
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Quote:
Originally Posted by Bazza
What others havn't mentioned is how close you are to a subject. Depth of field is relative to distance, so you have to take that into consideration as well.
Bazza
Bazza
The thirds rule was a rule of thumb I was taught years ago, always seems to look about right in practice in general photographic situations even if it isn't arrived at mathematically. I guess what is meant by the focal length is just that you get a lot more latitude on focussing with short focal lengths than the more critical focus of a telephoto. Thanks for pointing out the anomoly though Alan. Something I perhaps should have mentioned as do a fair amount of work at relative close ups.