Wrong aperture blades on Grandagon 90mm

[Aender Brepsom]

Hello,

the aperture blades on the Copal 1 shutter of my recently aquired Grandagon-N 4.5/90mm don't match. (I hadn't noticed it before as I did not have the time to use the lens since I have received it.) They go from 5.6 to 64, while they should go from 4.5 to 45.
As 4.5 to 5.6 is 2/3 of a stop, will I get correct exposures if I simply set the lever to 22 2/3 if I wish to have an aperture of 22? As I am curently on a trip where I would like to use the lens, I cannot run test shots first.
Your help would be much appreciated. Cheers

Aender

[Bob Salomon]

The aperture blades are correct, as we discussed earlier, you need the correct aperture scale.

[Aender Brepsom]

Of course you are right, Bob, sorry for my mistake. Of course, the blades are correct. I meant the scales. But since I could not get an answer to this precise question from you, I thought maybe someone here could help me out and make the lens usable until the scales issue can be solved. I just want my exposures to be correct in the next few days.

[Bob Salomon]

Of course you are right, Bob, sorry for my mistake. Of course, the blades are correct. I meant the scales. But since I could not get an answer to this precise question from you, I thought maybe someone here could help me out and make the lens usable until the scales issue can be solved. I just want my exposures to be correct in the next few days.

Aender,

I couldn't be any more specific. I gave you the full contact information for the Rodenstock distributor in the Benelux. They can solve your problem.

[Michael Mutmansky]

In a word, No.

If you are lucky, the 5.6 spot on the scale is the same as the 5.6 spot on the correct scale, but there is no easy way to check that without having the correct scale handy.

However, in a pinch, if you have any way to accurately measure the scale, you can set it to a point on the scale that is marked (say f11 or something like that) and then measure the aperture.

In an idealized lens (no real lens is ideal), the aperture at f11 for a 90mm lens will be 8.1mm. It will actually be a little different on a real lens, due to the location of the aperture relative to the center of the lens. However, if you can determine what the diameter is at f11, then you can figure out an adjustment factor to apply until you get home.

Take the focal length and divide by the 'stated' aperture, and you should get the diameter of the opening. Suppose you set it to f11 and then measure and you get an aperture diameter of 12.25 instead of 8.1. That would be about the proper aperture for a 153mm lens rather than a 90mm lens (12.25/8.1)*90=135.

So when the stated aperture is f11, you have an effective aperture of f7.3 (90/12.25=7.3), which is about 1-1/3 stops more than the scale is telling you. That will then give you the adjustment factor to be applying. In this case, if your meter says 1/250 @ f11, you need to stop down the aperture 1-1/3 stop to about f18 to be putting the proper amount of light on the film.

So, the formula is:

Focal length divided by the 'stated' aperture will be close to equalling the diameter of the aperture if the scale is correct.

If not, take the focal length of the lens and divide by the actual diameter to get the effective f-stop. Compare that to the 'stated' aperture and determine the difference (in stops). Apply that on the aperture scale to all shots, and you'll be pretty close. If you calculate a larger aperture, then you need to stop the lens down further (higher f-stop value). If you calculate a smaller aperture (not likely, as that means the scale was from a shorter focal length), then you open up the lens (lower f-stop value).


---Michael

[erie patsellis]

In a word, No.
In an idealized lens (no real lens is ideal), the aperture at f11 for a 90mm lens will be 8.1mm. It will actually be a little different on a real lens, due to the location of the aperture relative to the center of the lens. However, if you can determine what the diameter is at f11, then you can figure out an adjustment factor to apply until you get home.

Focal length divided by the 'stated' aperture will be close to equalling the diameter of the aperture if the scale is correct.

If not, take the focal length of the lens and divide by the actual diameter to get the effective f-stop. Compare that to the 'stated' aperture and determine the difference (in stops). Apply that on the aperture scale to all shots, and you'll be pretty close. If you calculate a larger aperture, then you need to stop the lens down further (higher f-stop value). If you calculate a smaller aperture (not likely, as that means the scale was from a shorter focal length), then you open up the lens (lower f-stop value).


---Michael

The easiest way is to calculate what the aperture should be, measure it, holding the lens at arms length (which will very closely approximate the entry pupil diameter, the f stop of a lens is the focal length divided by the entry pupil diameter) then see how far off it is, you may get lucky and it may be very close, but it only takes a min or two to do with a caliper and you'll know for sure how much correction you will need to apply. You should only need to check one point on the scale, as the error would be the same throughout the scale, but checking another as a sanity check would be prudent.

erie

[Aender Brepsom]

Hello Micheal,

thank you very much for your detailed answer. I'll try to do as you wrote. Many thanks.

Aender

[Michael Mutmansky]

Aender,

One more note, if the scale is for another 90mm lens, (the 5.6 version), it may be pretty close. If it was for a longer focal length, it will be larger, and require you to stop down, and if it was for a shorter lens, it will be smaller, and require you to open up.


---Michael

[Aender Brepsom]

Erie,

thank you for your help and please forgive me for being ignorant, but is it correct that I should try to measure the aperture with the lens cells fitted to the shutter, not directly on the shutter blades (which would be easier).
I'll have to buy a caliper tomorrow morning (it's late in the evening here now) to get it right.

Thank you very much.

Aender

[Dan Fromm]

Aender, I have a heap, well, three, shutters whose scales are wrong for the lenses I've put in them. What I did was hold the shutter up with the aperture wide open and then slowly close the aperture until the blades' edges were just visible through the lens' front cell. The aperture setting at which that happens corresponds to the lens' maximum aperture. Since the aperture scale goes in one stop steps, and this doesn't depend on the lens at all, once I found the true wide open setting (for one f/5.6 lens, true f/5.6 = marked f/8, and so on) I was set.

This is less expensive than having the shutter rescaled, but requires more thinking.

Cheers,

Dan