Relative aperture scale

[dj_judas21]

I recently bought a Schneider-Kreuznach Symmar-S 150mm lens. It came to me mounted in a shutter that is marked f=90mm. As the typical f-stop aperture notation is relative to the focal length, I assume this means the aperture scale on my lens will be off by almost one stop (as 150mm is nearly double 90mm)?

e.g. the f/11 setting on the shutter actually corresponds to about f/8 when used with this 150mm lens?

 

[Steve Smith]

I think it's the other way round.

at f8, the aperture on a 90mm lens would be 90/8 = 11.25mm diameter.

For a 150mm lens, the f No. for an 11.25mm diameter opening is 150/11.25 = f13.33

So about half way between f11 and f16.

So f11 and f16 go either side of your marked f8 position.


Steve.

 

[dj_judas21]

Ah yes, got it back to front. So the aperture scale on my shutter will read about 1-point-something stops fast for my lens.

 

[Doremus Scudder]

There are many threads about making an aperture scale (or having one made) for lenses mounted in different shutters. Do some searching.

If you let us know the size and make of shutter you have, someone here may even be able to supply a photo or template for your combination.

Maximum aperture on your 150 should be f/5.6; you can extrapolate from there, but a dedicated scale would be the best solution IMO.

Best,

Doremus

 

[dj_judas21]

There are many threads about making an aperture scale (or having one made) for lenses mounted in different shutters. Do some searching.

Thanks, I hadn't realised it was a popular topic. I'll do some reading.

If you let us know the size and make of shutter you have, someone here may even be able to supply a photo or template for your combination.

Maximum aperture on your 150 should be f/5.6; you can extrapolate from there, but a dedicated scale would be the best solution IMO.

OK. My father-in-law builds model live steam engines and has all sorts of cool precision metalworking tools. If I can get hold of a specification he can easily make it.

I'm at work and I don't have a picture of the shutter, but it is a Copal #0. I guess this isn't sufficient information.

 

[RalphLambrecht]

d=f/N;use this equation to solve for the new focal lengthand see how the Ns compare;hope this helps a little

 

[Ian Grant]

There are many threads about making an aperture scale (or having one made) for lenses mounted in different shutters. Do some searching.

If you let us know the size and make of shutter you have, someone here may even be able to supply a photo or template for your combination.

Maximum aperture on your 150 should be f/5.6; you can extrapolate from there, but a dedicated scale would be the best solution IMO.

Best,

Doremus

You can't always extrapolate from the Maximum aperture some lenses don't use the full aperture scale, the 90mm f6.8 Angulon for instance, and in a Copal #0 the 150mm f9 G-Claron.

However there's plenty of images of the Copal scales online as a quick Google search shows. I need to make a scale for a 210 lens (Copal 1) the shutter came with just a front cell for a 300mm f9 Apo Ronar and has the 300mm scale

Ian.

 

[Jim Jones]

d=f/N;use this equation to solve for the new focal lengthand see how the Ns compare;hope this helps a little

The d in this formula is the apparent diameter of the aperture as viewed through the front elements of the lens, not the actual diameter of the aperture. When measuring this diameter, correct for parallax by moving the eye from side to side by about the diameter of the aperture.

 

[dj_judas21]

The d in this formula is the apparent diameter of the aperture as viewed through the front elements of the lens, not the actual diameter of the aperture. When measuring this diameter, correct for parallax by moving the eye from side to side by about the diameter of the aperture.

Oh really! I didn't know that. I suppose it makes sense. What's the best way of measuring this then - hold a ruler across the front element of the lens?

 

[Ian Grant]

There was a tool made to measure apertures an it measures the actual diameter, it could be set to give a specific aperture for a Focal length. I nearly bought one on Ebay about 2 or 3 months ago, it was quite old.

The formula Ralph gave works perfectly in practice.

Ian

 

[dj_judas21]

I took some fairly rough estimates of the apparent pupil size when the aperture was set to certain values on the shutter. Can confirm that the aperture scale appears to read about one stop fast. Simply knowing that is probably sufficient, and I can bear this in mind when making exposures. Working to "about one stop fast" is probably accurate enough given that the leaf shutters in old lenses can vary by quite a bit each way, too.