acceptable margin of error on aperture scale

[Songyun]

What is the acceptable margin of error on the aperture scale?

I have a prontor shutter that I like to caliber to one of my lens. The lens is currently in copal shutter with original aperture scale. I called S K grime this morning, got some instruction on how to do it myself. Basically as long as I match the opening area, it should be fine. My question is that what is the acceptable margin of error?

[Ron Marshall]

What type of film are you shooting? It is more critical for pos than neg.

Even if the scale is a bit off, you can shoot a few test sheets to calibrate it yourself, record how much the scale is under or over, and adjust accordingly each time you shoot.

[Bob Salomon]

The easiest way to accurately calibrate it yourself is to set up a grey card and evenly light it. Then put the lens on and focus on the grey card (a large one and one that fill most of the GG at a distance where you have no bellows factor. Meter the card with the lens wide open off the GG. See what the difference is from reading it directly. The difference is the amount of light your GG absorbs. Then put that factor in the meter and start to close the aperture. Mark every full stop as well as 1/2 or 1/4 stops. Now your lens is calibrated.

For best use you should use either a GG probe or a microscope adapter on your meter and block off all stray light. If you have a Linhof made since the late 1950s you could also do this with the Linhof Focus/Metering Bellows.

[Bob Salomon]

Or, since you are in NJ either Krimar Service or Danielle service could easily do this for you. They are both in N NJ.

[Songyun]

What type of film are you shooting? It is more critical for pos than neg.

Even if the scale is a bit off, you can shoot a few test sheets to calibrate it yourself, record how much the scale is under or over, and adjust accordingly each time you shoot.

I shoot slides and b&w. Is 1/6 stop acceptable?

[Songyun]

Thanks Bob, I was thinking using a caliper to measure the sides and then calculate the area, as long as I have the error of area within 7%, which is = 1/10 stop. The meter I have here has increament of 0.1 stop. 7% error in area shouldn't be too hard to beat.

[Leonard Evens]

A 6 percent error in the diameter will produce about one sixth stop error in the stop. That should not affect exposure materially.

Keep in mind that it is not the physical aperture that is relevant but rather the entrance and exit pupils which are the images of the aperture as seen through the front and and rear of the lens. This can be quite difficult to unders and well enough to measure properly, so you are better off using some method like that recommended by Bob Solomon.

[BradS]

Keep in mind that it is not the physical aperture that is relevant but rather the entrance and exit pupils which are the images of the aperture as seen through the front and and rear of the lens. This can be quite difficult to unders and well enough to measure properly, so you are better off using some method like that recommended by Bob Solomon.


but, as he states in th eop, he has the original shutter with the original aperture scale...would it no suffice to....

for each marked f-stop,
1) set the aperture to an f/stop,
2) measure the opening in the original shutter
3) adjust the opening in the new shutter to match the measured size.
4) mark the f-stop scale on the new shutter.
repeat

surely, this would be more accurate than fiddling around with a light meter reading off the GG...no?

[Glenn Thoreson]

but, as he states in th eop, he has the original shutter with the original aperture scale...would it no suffice to....

for each marked f-stop,
1) set the aperture to an f/stop,
2) measure the opening in the original shutter
3) adjust the opening in the new shutter to match the measured size.
4) mark the f-stop scale on the new shutter.
repeat

surely, this would be more accurate than fiddling around with a light meter reading off the GG...no?

That's the way I do it when I have the original shutter. Pretty hard to miss going it that way. Keep in mind that shutters can ofen be off by 1/6 to 1/3 stop even when in good shape. They also vary with temperature. Considering this, I would try to keep the aperture as accurate as possible. Even though it's expensive to do with LF slide film, after the initial test I would bracket important shots.

[Leonard Evens]

If you are measuring area, remember that the area is proportional to the square of the diameter. That means that a 12 percent error in the area will result in about a 6 percent error in the effective diameter, which in turns corresponds to about 1/6 stop. Tha t should be close enough for all practical purposes.

[BradS]

How would one measure area? (of a irregular shape like the opening of an aperture with five blades?)

Would it not suffice to get a pretty good estimate of some linear dimension? I'm thinking of someting roughly equivalent to the diameter....

[aduncanson]

If by "measuring the sides" you mean the exposed length of each aperture leaf, then you may want to consider that measuring the "diameter" directly might be easier to do accurately. The length of the exposed edges of the leaves can be very small and measurement error with calipers can become significant. Also, you might find that there is significant variation in length from leaf to leaf.

In measuring the diameter directly, if the shutter has an odd number of leaves then you will be measuring from the center of the exposed edge of one leaf to the apex where two join on the opposite side. You could probably use this as a good estimate of effective diameter. Alternately, if the leaves are straight it would be easy enough to calculate the area of the polygon from the "diameter" measurement.

In your situation, I would probably physically measure the diameters to set the new scale and use the light meter only to confirm that I did it all correctly.

[Bob Salomon]

"I would probably physically measure the diameters to set the new scale and use the light meter only to confirm that I did it all correctly."

Then it would be much easier to just use the meter in the first place. And, much faster!

[Songyun]

ok, measuring isn't the hardest part. The hard part is to find the area of copal shutter. The copal shutter has 7 leaves, however, it doesn't form a perfect heptagon (in between the heptagon and the circle that contains the heptagon). So the area of the opening is a bit off. The leaves in the prontor shutter forms a perfect pentagon, and area calculated is very accurate. Luckily, the lens I tried to mount has 5.6 at max opening, that saves me a lot of trouble.

[Songyun]

technically, I just need to establish the linear relation between the side(I use the long diagonal) of the pentagon and heptagon. I don't have to worry about the area. Anyway, it was a fun project.

Just a few thoughts, in practice, if we set the aperture at the left or the right side of the mark, maybe we can get 1/10 step off?