Finding the f-stop values of a vintage camera
f-stop values for a vintage camera
Many vintage cameras do not document the aperture sizes. However, if you want to use them successfully with modern film, you will need to know the f-stop. Knowing the f-stop value will ensure successful exposure.
Definition
The f-stop for a particular aperture is found by dividing the focal length by the diameter of the entrance pupil. The entrance pupil is the image of the aperture as seen when looking into the front of the lens. This may be simply the size of the hole through which the light passes but for more complex lens systems, the aperture may be magnified by the lens in front of it.
Measurement
First ascertain the focal length of the camera lens. If you can't find any reliable documentary evidence of the focal length then use the methods outlined on the following page:- Find the focal length of a lens
Now, open the back of the camera and hold it against a bright background - e.g. computer screen. Put the camera in 'T' or 'B' mode and hold the shutter open. Look through the front of the lens and measure the diameter of the bright entrance pupil.
The f-number of the lens is found by dividing the focal length by the diameter of the entrance pupil.
Easier method of measuring entrance pupil
It is often difficult to hold the camera whilst measuring the entrance pupil. Also direct measurement of the entrance pupil is often difficult due to parallax issues.
View of entrance pupil
An easier way is to use a digital camera to take a picture of the entrance pupil. Be as far away from the vintage camera as possible when you take the picture. Then, view the image on the digital camera, or better still on a computer, and measure the entrance pupil(B) and another distance that would be easy to measure on the vintage camera(A). Expand the image to make it easier to measure accurately. Measure the equivalent distance to 'A' on the actual vintage camera(C)
The size of the camera's entrance pupil is calculated as BxC/A.
Example
The focal length of a Beau Brownie No.2 is 85mm. I took an image of the entrance pupil and viewed it on my computer. 'A' was 79mm, 'B' was 21mm and the equivalent distance to A on the actual camera(C) was 30mm. The actual entrance pupil size was therefore 21x30/79 = 7.97mm. The f-number is focal length divided by the entrance pupil size which is 85/7.97 = 10.7. This can be rounded to f/11.