PDA

View Full Version : calculate converage of 360mm lens correct?


olk
12-Feb-2022, 12:09
Hello,
I've got an Rodenstock Eurygon Anastigmat 1 f4.5/360mm (from 1915).

Vademecum:
"Eurygon f4.5 300, 360, 420mm ... H&D gives 210-600mm, and confirms it is a triplet ... Coverage increases
from 48° to 60° or to 70° as it is stopped down..."

For portraits I did following calculation:

1/b = 1/f - 1/g
b: distance between lens and film, f: focal length, g: distance to subject

tan(alpha/2) = r/b
alpha: angle of view, r: radius of coverage, b: distance between lens and film

- with g = 2m -> b ~ 440mm

- f4.5 -> alpha = 70°
-> diameter of coverage ~ 614mm
-> 30x40cm film format should fit

- f25 -> alpha = 48°
-> diameter of coverage ~ 409mm
-> 9" x 12" film format should fit

Is my estimation correct?
Oliver
Bob Salomon
12-Feb-2022, 13:05
The easy way is to get a piece of cardboard, turn off any lights in yout room, pull the blinds down, point the lens out your window and point it at a distant object. Hold the cardboard behind the lens and move it until you see a sharp image. Now you see the coverage.
olk
12-Feb-2022, 13:21
Unfortunately I don't have the camera yet. I try to figure out what is possible and advised.
Bob Salomon
12-Feb-2022, 14:26
Unfortunately I don't have the camera yet. I try to figure out what is possible and advised.
A camera isn’t used. Just a lens and a piece of cardboard.
Kiwi7475
12-Feb-2022, 14:49
A camera isn’t used. Just a lens and a piece of cardboard.

And a room with at least one window ;-)
xkaes
12-Feb-2022, 14:55
No camera?

Try a camera OBSCURE - UH.
Dan Fromm
12-Feb-2022, 15:48
Is my estimation correct?
Oliver

No. The magic formula you want is, in pidgin Excel, diameter of circle covered =2*focal length*TAN(RADIANS(angle covered/2))

@ f/4.5 the angle covered is 48°, @ f/25 70°. You got that backwards.
olk
13-Feb-2022, 01:02
No. The magic formula you want is, in pidgin Excel, diameter of circle covered =2*focal length*TAN(RADIANS(angle covered/2))

I used this Formular, except that I used the distance between lens and film instead of focal length only.
I think this is more correct because for portraits I don't focus to infinity == focal lenght

Isn't this right.