No aperture scale

[Milton Tierney]

Help!
I was given a 5.6 150mm Rondenstock Technika Sironar lens w/ a Copal no.0 shutter without an aperture scale. Is there a way I can determine the f/stops without sending it to a tech/lab?

Milton

[Bob Salomon]

Yes. You can measure the effective size of the aperture at various points and divide that into the focal length and calibrate an aperture scale. Or you can send it to a technician and have them calibrate it and check the spacings since someone switched the shutter and you may need shims installed or removed for the lens to perform properly.

[Peter K]

There http://www.largeformatphotography.info/forum/showthread.php?t=32332 #2 is a measuring method to determine the f/stops described by Emmanuel BIGLER. This methode was invented by August von Steinheil (1801 - 1870) but it works very well also today.

[Scott Kathe]

To check one of the older lenses I thought I was having an aperture problem with I used Adobe Illustrator and created black circles with diameters that corresponded to the f stops I wanted to test. I printed it on normal paper, put the paper on my light box and turned on the light. I set my lens/shutter on top and then adjusted to each f-stop and checked it against the corresponding circle on the paper with my eye directly over the aperture. Worked great, the apertures were fine.

Scott

[Peter K]

Scott, the f-stop is the diameter of the diaphragm projected in the entrance pupil and not the mechanical diameter of the diaphragm. To use black circles one has to know the magnification factor of the front and rear part of the lens.

[seawolf66]

Bob Salomon - HP Marketing
Is there any difference between this lens,]5.6 150mm Rondenstock Technika Sironar lens w/ a Copal no.0 shutter [ and the one I have Rodenstock Sironar F-5:6 if not , I shall foto it at one to one and send him the scale :

[Scott Kathe]

Scott, the f-stop is the diameter of the diaphragm projected in the entrance pupil and not the mechanical diameter of the diaphragm. To use black circles one has to know the magnification factor of the front and rear part of the lens.

Peter,

I was concerned about this so that is why I left the front and rear elements on but I see your point. So let me see if I can understand how to do this. The critical thing is to light the lens from behind and measure the diameter of the 'circle' of light coming out the front of the lens. Now does this light source have to be at the focal length of the lens or just far enough back so that the light rays coming through the lens are parallel. If they just need to be parallel couldn't I set the lens on my light box since the light source is diffused and not a point source?

Scott

[Bob Salomon]

Bob Salomon - HP Marketing
Is there any difference between this lens,]5.6 150mm Rondenstock Technika Sironar lens w/ a Copal no.0 shutter [ and the one I have Rodenstock Sironar F-5:6 if not , I shall foto it at one to one and send him the scale :

Yes and no. The scale you have would be approx. correct but not actually correct as he has a different shutter and any two 150mm lenses at random are not the same exact focal length. That is why most modern aperture scales have a very small number on them that identify the specific scale.

It is more important that he find out if the spacing is correct as that will directly impact the performance of the lens.

[Peter K]

Scott,

the f-stop number noted on the lens/shutter is only valid for a lens focussed at infinty. This is the reason macro-lenses are marked with different f-stop numbers. So the iluminated diaphragm to measure the f-stop must be in the same plane as the lens is focussed at infinity.

For measuring the f-stops I've a ground-glass with a clear spot in the center. This ground-glass is normaly equipped with a fresnel-lens between ground-glass and the eye. I've made a black plate with a white LED in the centre in the same size as the fresnel-lens. The lens to check for f-stop diameter is mounted in the camera and will be focussed at infinity. Than the plate with the LED will be mounted behind the ground-glass and the diameter of the diaphragm can be measured with a semi-transparent paper and a ruler layed over the front of the lens.

Peter K

[Scott Kathe]

Thanks Peter,

I'll file this away for future reference.

Scott

[Milton Tierney]

I tried the measuring method to determine the f/stops described by Emmanuel BIGLER. This seems to be the easiest without sending the lens to a tech/lab. I’ll run some test to see if the scale is correct.

Milton