Archived posting to the Leica Users Group, 2001/02/08
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At 7:51 AM -0800 2/8/01, Jim Brick wrote:
>The online DOF calculator that I used was obviously flawed. Look at any DOF
>chart or even your Leica lens and you will see that DOF extends farther
>behind than in front by roughly 1/3-2/3.
>
>To prove this, take a 50mm lens (mine is a Summilux) and set both 2 and 10
>meters on f/16. Then read what is opposite the center mark. It will be
>slightly over 3 meters. 3+ is not half way between 2 and 10.
>
>Another way is to use a SLR and put a tape measure in front of the lens,
>obviously going straight away. Focus on say five feet. Then using DOF
>preview, start stopping down and watch what comes into focus. 1/3 front,
>2/3 back.
>
>Or buy a good photo book like The Manual of Photography or The Hasselblad
>Manual, both of which describe this fact with ample drawings.
>
>I apologize for the apparent error in my example that you quoted below. I
>used one of those online DOF calculators and typed in the result without
>thinking about it other than the actual depth of the DOF in both cases.
>
>Sorry,
>
>Jim
>
>
>At 09:42 PM 2/7/01 -0800, Jacques Bilinski wrote:
>>> A 100mm lens at
>>> f/1.4 focused at 20 feet (exact same image size on the film as the 50mm
>>> example), the DOF is from 19'7" to 20'5". The depth of field is EXACTLY
>>the
>>> same. EXACTLY 10 INCHES IN BOTH CASES for the given COC.
>>>
>>
>>> From the exact point of focus, DOF extends 1/3 forward
>>> (toward the camera) and 2/3 back (away from the camera).
>>
>>> Jim
>>
>>If the second point (1/3, 2/3 etc) is true then why does the d.o.f extend
>>exactly 5" forwards and backwards (1/2, 1/2) in the example you gave? BTW
>>the image size of a 50mm lens at 10 feet is NOT the same (by quite a bit)
>>as 100mm lens at 20 feet for a 24mm by 36mm negative. And film size DOES
> >matter when comparing the coverage angles of a lens.
I'm glad that Jim is back to speak for himself!
On the other hand, the 1/3-2/3 thing on DOF behind and in front is an
approximation that only applies at greater distances, well short of
the hyperfocal distance. Obviously, at the hyperfocal distance or
greater, percentage wise the DOF behind the actual plane of focus is
100%, as the distance within the DOF behind extends to infinity by
definition, while in front it is finite. Conversely, at very short
distances (and accentuated by large apertures) the DOF front/back
percentage is asymptotic to 50/50. A short study of the theory will
corroborate this.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com