Re: [Leica] 1/3-2/3 (Jacques Bilinski)

[John Collier <jbcollier@home.com>]

Another odd Illusionary Depth Of  Field (IDOF) quirk is that with high
magnification photography, it changes to being equidistant from the plane of
focus.

John Collier

> From: Jim Brick <jim_brick@agilent.com>
> 
> 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.
>