90mm Angulon Shutter Replacement - How does this affect Aperture?

[enrico__]

Ok this is what Ive done..:

 

I have a 90mm Angulon and the sutter died (dead, zip, gone,

finito..).

 

I needed a replacement shutter.

 

The front and rear glass of the 90mm Angulon fit exact into a

(chrome) series shutter from an 80mm Mamiya TLR lens. The focus is

perfect, Flash sync works, however Im concerned about how the

aperture has/is translated.

 

Maybe Im just confusing myself but Im wondering if there is any

translation between an 80mm lens designed for Medium Format, and a

90mm Lens designed for a Large/r Format when aperture/shutter is

interchanged between the two?

 

I thought the best way to find out would be to make a negative and

judge the exposure. However, I think the thing that confused me is

that I took a light reading and set the aperture/shutter according to

the reading and the exposure was fine. So Is the focal length,

regardless of lens coverage, the same as far as adption of a

different shutter/aperture goes? (Now I'll ask in a different

way..).. If I screw 90mm Angulon elements in a shutter designed for a

medium format 80mm lens then should, and How, would the aperture

translate?

[michael_briggs2]

The equation for f-number N is N = F/d, where F is focal length and d is the aperture diameter. So your aperture diameters are probably off by 13%, which probably doesn't matter for practical exposure accuracy. But I says "probably" because what really matters is the stop as viewed from the front of the lens. A lens can magnify the physical aperture. Either lens might have a pupil magnification, which would cause a deivation from the 13% figure.

 

You could measure the aperture diameters of the old shutter, adjust the control of the new shutter so that the diameters are the same, and scratch marks on the aperture scale to indicate the settings.

 

Or you could have a repair person examine the old shutter and see if it is repairable.

[mskovacs]

Since f/stop is related to the diameter of the aperture and the focal length, the scale is going underexpose 1/3 of a stop.

 

E.g. for a 5mm diameter, 80/5 = f/16, 90/5 = f/18

[nickc1]

The variables you have to consider is the diameter of the iris opening itself - which can vary between lenses/shutters of the same type I have found! and the focal length of the lens - BUT the exposure that the film receives will depend upon the T value, not the f-stop. The T(ransmission) value of the lens depends not only upon the aperture, but also the number of elements and their coating, and hence the light loss within a lens and also on the coverage of the lens. I have little experience in swapping lenses and irises, however when using an 'old' 35mm lens with f18 as a stop I can use the sunny f18 rule as easily as the sunny f16 one. Looking at the dials of a Weston and a Lunasix - the difference between the exposures given at f16 and f18 is well within the tolerance of a shutter's accuracy (the british standard is within 20%!) Focal lengths are also not necessarily what is printed on the lens. Curiosity made me look at the lenses I have for Kiev medium format cameras as I have both an 80mm and a 90mm - and I cannot see a difference in he size of the openings at the same setting!! - mind you that might just be FSU quality control.

 

Nick

[enrico__]

Hi all, thanks for the replies. I shot some more tests and the exposure seems just a wee little bit off (on film). It could do with a third stop more density.

 

Thanks again!