Focusing and distance

[SalveSlog]

A rather newbie question:
When using DOF tables or acessory/uncoupled rangefinders with old cameras; do I measure the distance to the film plane or to the lens?

 

[BrianShaw]

Film plane

 

[DannL.]

I measure from my eye's position, which is very close to the film plane. Whether it's right or wrong, it works well with my cameras (folders 6x9/6x6, 6x4.5). Focusing errors become evident when I choose to leave the rangefinder in the bag and rely on guessing. And especially when using larger apertures. So when I can, I try to use the rangefinder.

 

[RalphLambrecht]

A rather newbie question:
When using DOF tables or acessory/uncoupled rangefinders with old cameras; do I measure the distance to the film plane or to the lens?

Interesting responses. in all DOF calculations I've seen so far,the subject distance 'u' is measured from the subject plane to the nodal plane of the lens(roughly the centerbetween the front and rear of the lens.Have I done it wrong for decades?

 

[Hatchetman]

good question!

 

[removed account4]

i have never even thought of it, and never really thought it mattered much ... its just a couple of inches / cm's ...

 

[Ian C]

The standard optical formulas use the subject distance s = distance from object (subject) to first nodal point (node of admission).

Camera and lens focusing scales use the overall object-to-image distance. This exceeds the distance s by the sum of the nodal distance + the image distance, where the image distance is the distance from the second nodal point (node of emission) to the film plane.

In practice, this difference isn’t important, except for enlarging, copying, lithographer’s process, or macro photography in which it might matter.

Consider using a 150mm normal lens on a 4” x 5” camera at a typical distance in general photography. As an example, look at the 150/5.6 Schneider Super Symmar XL on a 4” x 5” camera at the hyperfocal distance H = 9783.5mm at f/22 and using circle of confusion diameter c = 0.1mm. [Note: actual focal length is f = 147.66mm].

The subject-to-film distance is H + nodal distance + image distance
= 9783.5mm + 24.76mm + 149.92mm = 9958.18m

The difference between s and the overall subject-to-film distance is 24.76mm + 149.92mm = 174.68mm.

Other than the close focusing situations such as given above, the difference can be ignored.

https://www.schneideroptics.com/pdfs/photo/datasheets/super-symmar_xl/super-symmar_xl_56_150_3.pdf

 

[BrianShaw]

Interesting responses. in all DOF calculations I've seen so far,the subject distance 'u' is measured from the subject plane to the nodal plane of the lens(roughly the centerbetween the front and rear of the lens.Have I done it wrong for decades?

Apparently I have... For decades!

 

[BrianShaw]

Thanks Ian!

 

[John Koehrer]

I'm very good at ignoring things.