G-Claron 270 aperture scale question

[friedemann_pistorius]

I mounted a G-Claron 270/9 into a new Copal #1 shutter and equipped it with a

proper f-stop scale (ordered from SK Grimes).

<p>To my (limited) knowledge and from what I've read in this forum, one can

calculate f-stops as "focal lenght/f-stop = diameter of the aperture". In this

case, the visible diameter of the aperture at f9, viewed through the front cell

at arm lenght, should be 30mm (270/9 = 30). This is exactly the case when I

close the f-stop lever to a point where the blades just begin to cut into the

lens area.

<p>With the new f-stop scales correctly mounted onto the shutter, at f9 the

diameter of the aperture is only 26mm, with the flats of all the iris blades

clearly showing. I asked Schneider/Kreuznach for an explanation, and they

confirmed that the diameter of the aperture at f9 should be 25.5mm.

<p>So, what's the truth now? Am I missing something? Of course I could run tests

with different lenses at similar f-stops and see if the G-Claron pictures are

off, but I would prefer to read your comments prior to wasting time and film :-)

<p>Thank you!<br>Friedemann

[rodeo_joe1]

You can't go by the visible "hole" diameter to accurately measure a complex lens's aperture. This only works for simple, i.e. single element lenses.

 

Most Schneider barrel lenses have the aperture arranged so that the blades are partly visible at "full" aperture. This is true of all their enlarging lenses, and also the G-Claron range, so I don't think there's anything wrong with the setting of your lens.

 

My own 210mm G-Claron has the points of the iris pentagon just touching the circle of the lens at f/9, with the flats cutting into the circle. In other words, it looks just like you've described yours.

[michael_briggs2]

I have a 270 mm G-Claron, factory mounted in a Copal shutter. Setting the aperture at f9 and sighting at a ruler, I measure about 28 or 29 mm.

 

The f9 setting has the aperture blades just cutting the aperture all around the lens. I disagree with the first sentence Rodeo wrote. The f-stop is based on the light rays that enter the lens -- these are the rays from the object that will reach the image. The diameter of this bundle of rays is defined by the size of the aperture hole as seen from the front from a distance. The aperture, as viewed through the lens, from the object focused on, is the entrance pupil. The diameter of the entrance pupil is used to calculate the f-stop. For a taking lens, ideally the measurement would be from a great distance.

[kelly_flanigan1]

If you are a real tooler you could calibrate the lens in T-stops; using another known lens as a reference. In Cine work one moves the aperture ring one way to bull out the slop; this removes the hysteresis of the iris petal pivots and rotating ring mechanism. In practice the slop is small.