View Full Version : Correct F stops ?

John Dorio
26-Oct-1998, 16:13
I have a 150 Symmar- S lens f/5.6. I know its a good lens but there seems to be a problem with the f/stops. I measured the diaphram hole after removing the rear element and checked the aperture at the different f/stops. Here is what I found :

f diameter(mm) new f ?

5.6 20.7 7.25 8 15 10 11 10 15 16 7 21.4 22 5.1 29.4 32 3.75 40 45 2.75 54

min hole 1.5 100

rear element stop 21.5 7

The lens shows almost a stop too slow for its setting. The max aperture can also only be f/7 since the rear element lens opening is 21.5 mm. What f/numbers are valid for this lens ? I also have a Componon 150 5.6 that shows the same apertur e sizes. Does the lens design compensate for the smaller holes ? Doesn't f/ratio = F.L./diameter always without exception ? Why does't this lens follow this gol den rule ?

Ron Shaw
26-Oct-1998, 17:20
The actual diameter of the aperture may be different from what the textbook number says, depending on the optical formula of the lens. I would think it is probably correct. Take a lens you know is ok, and do a test. Using a brightly lit wall, set the known lens on some f number, and take a reflective reading off the ground glass with your light meter. Replace the known lens with the lens in question, set it at the same f number of the control lens, and see if your light meter gives the same reading. This method is also used to calibrate a replacement shutter that has a blank aperture scale.

Sean Donnelly
27-Oct-1998, 23:34
"The conventional method of calibrating the diaphragm opening is in terms of f-numbers (or relative apertures), which are calculated by dividing the focal length of the lens by the effective aperture. The effective aperture, in turn, is the diameter of the beam of light entering the lens that will just fill the opening in the diaphragm. If the diaphragm is located in front of the lens, the effective aperture and the diaphragm opening will be the same, but since the diaphragm is normally located between the elements of the lens, entering light is refracted before passing through the diaphragm opening."

-- Leslie Stroebel, _View_Camera_Technique_

In other words, f-number cannot be correctly calculated from the actual diameter of the diaphragm opening.

Alan Gibson
28-Oct-1998, 07:53
To add to Sean's point, you can measure the aperture diameter by laying a ruler across the front of the lens (without dismantling it), and measuring the diameter of the apparent hole. For a 150/5.6 lens, this should be about 27mm.

Tony Brent
5-Nov-1998, 00:34
Have you taken some photographs with this lens?

Assuming the shutter speeds are good, f/8 should be f/8 regardless of the physic al size of the aperture (not necessarily the size you get by working the formula , as pointed out in the other replies) and should result in the same exposure as f/8 on any other lens.

If it exposes the film correctly, it's probably fine. Go ahead and use it, and d ont worry too much about the math.