Shutter Aperture not matching theoretical size

[Max Grew]

Hello,

I will admit I have probably missed something obvious but I have noticed that the actual diameter of the aperture blade opening on many of my lenses/shutters does not match the size it theoretically should be

I am using the Equation Focal Length ÷ F-Stop = Diameter of Opening

for a 210mm f/5.6 lens the diameter of the aperture at f/5.6 should be 37.5mm (210/5.6=37.5)

but I am measuring it at 32mm, its the same for several of my other LF lenses, all coming in a few mm under the theoretical dimension.

does anyone know why this could be?

[Jim Jones]

The cited formula should be Focal Length / F-stop = apparent diameter of opening as viewed through the front lens group.

[Mark Sawyer]

What Jim said. The front element will be a positive, meaning it will magnify the size of the aperture.

[Max Grew]

The cited formula should be Focal Length / F-stop = apparent diameter of opening as viewed through the front lens group.

Ah that makes sense, thank you.

Is there a way to work out the diameter of the aperture without the lens in hand, using the lens data sheet perhaps?

[Dan Fromm]

Is there a way to work out the diameter of the aperture without the lens in hand, using the lens data sheet perhaps?

No. Why do you ask?

[Max Grew]

No. Why do you ask?

We are working on a project that uses a motor controlled aperture that can be used with many different lenses. We want the user to be able to input some info into the controller i.e maximum aperture for lens, focal length etc and the motor move the blades into the correct position for the various aperture sizes i.e 5.6, 8, 11 etc etc.

Trying to work out a way to keep it simple so that the user doesn't have to take a load of measurements

[Doremus Scudder]

We are working on a project that uses a motor controlled aperture that can be used with many different lenses. We want the user to be able to input some info into the controller i.e maximum aperture for lens, focal length etc and the motor move the blades into the correct position for the various aperture sizes i.e 5.6, 8, 11 etc etc.

Trying to work out a way to keep it simple so that the user doesn't have to take a load of measurements

Sounds like an interesting project. Based on lens design, you may be able to extrapolate from one lens to others of the same design. I.e., measure aperture sizes for a Plasmat of a given focal length at all f-stops and then use the factor derived from the difference in focal length to generate aperture sizes for other Plasmats of different focal lengths. This should work, if (and it's a big if) the magnification factor of the aperture by the front element is the same throughout the series. The same principle should apply to other lens designs, but the problem may be ensuring that a group of lenses, like Gauss or Super-Angulon or whatever were actually the same design.

It may come down to your getting your hands on a large selection of LF lenses, removing the front element, and simply measuring the aperture at all f-stops, thus generating a data base for your software. You might be able to enlist the help of forum members, who undoubtedly own a huge percentage of all the available lenses for LF. A dedicated thread with a request and some basic instructions on measuring the aperture might generate a huge amount of data for you.

Best,

Doremus

[Mark J]

I think Doremus is correct, that you can get a fairly accurate estimate of the aperture from assessing lenses in a group ( plasmats ) and then you can apply a constant factor to go from focal length to the aperture. The other lens types will need a different factor, and there is less commonality of designs when you go to double gausses for instance. I'm not even sure that the ratio for f/5.6 super-angulons compared to f/6.8 or f/8 super-angulon types would even be quite the same, though. Could get a bit complicated.