Changing shutters?

[Jim Andrada]

Interesting comments re shimming a few posts back. CNC machining hasn't obsoleted shims by any means. We manufacture complex mechanisms with micron level tolerances - would be a matter of testing a zillion of them to find a few in tolerance - very expensive. Shims are our salvation.

By the way, found the following

T-stops
F-stops are purely geometrical, the ratio of aperture to focal length, regardless of actual light transmitted. Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), f-numbers do not accurately correlate with light transmitted. F-numbers corrected to measure light transmission rather than aperture ratio, called T-stops (for Transmission-stops), are sometimes used instead of f-stops for determining exposure.[7] A real lens set to a particular T-stop will, by definition, transmit the same amount of light as an ideal lens with 100% transmission at the corresponding f-stop.

And the following

T-stops
In practice, even the best lenses exhibit light absorbance, effectively “stealing” some of the light going through them. This means that if you calculate the exposure based on the f-stop of the lens, you will end up underexposing the image, because less light is reaching the film plane than is expected in theory. T-stops are the f-stop of the lens corrected for its absorbance and reflectance. The T-stop is the true speed of the lens, calculated by compensating for its light absorbance and reflectance, and will result in accurate exposure.

[Leigh]

T-stops v. F-stops...

Irrelevant. The shutters in use, and the exposure meters, and the film speeds, all use f-stops.

The focus of this thread is to determine the accuracy of the f-stop scale on the new shutter v. the original shutter.

- Leigh

[Andrew]

this is great... I finally feel like I'm near a point that I can do something practical.

Leigh, please let me know if this is flawed before I invest the effort trying to put it into practice.

Ive got a technika, a minolta IV-f, some lenses that I trust and couple more that have been monkeyed with, either by myself or someone else

I can make a light source that can be pretty much anything diffused but basically something that'll act like a light box. Then I get a piece of MDF cut to sit in the universal back of the technika, drill a hole in the MDF to take the diffuser of the light meter and use some felt to make it all reasonably light tight when the meter is pushed into place. I think I can make the diffuser sit pretty much at the film plane.

Next step is to focus a trusted reference lens to infinity, stick it at a specified point in front of the light source and measure the light intensity in EV at the film plane as a de facto index of the aperture.

NOW... if I've got this right, when I get another lens of same focal length to my reference lens that is also focused to infinity, I should expect to get the same EV values for each lens at the same apertures? And if the dodgy lens is NOT correct, I use the EV values from reference lens to determine the correct apertures?

I hope this is correct too... if I get a lens of a *different* focal length to the reference lens that is also focused to infinity and then put it at the same point relative to the light source, I should get the same EV values as on the reference lens?

And this set up should get rid of any artifact arising from variation in flange to film distances for different lenses???

[Leigh]

I would recommend limiting the number of lenses involved in the comparison test to just the original shutter/lens and those same cells in the new shutter. There may be some difference in transmission between lenses of different makes and designs, particularly as regards coatings.

This is why I limit the number of makes and models in my lens inventory to the greatest extent possible.
All of my mid-range lenses are Rodenstock Apo-Sironar-S, while those 120mm and shorter, and 360mm and longer are Nikkors.

The methodology sounds like it will work fine.

You can certainly run the test with other lenses as a comparison, to see how uniform the performance really is.

- Leigh

[Steve Smith]

CNC machining hasn't obsoleted shims by any means.

Also no reason why CNC would be more accurate. A properly set up manual machine run by a competent operator can have the same level of accuracy.


Steve.

[Andrew]

The methodology sounds like it will work fine.

Thank Heaven I've finally got something right !!!

I'll agree that this ought to kept as limited in scope as possible but if you aren't hammering the logic that's all I need.... in practice, if I can test some of my older lenses and be reasonably sure of my apertures to within say 1/3 to 1/2 stops I'll be more than happy

thank you

[Leigh]

A properly set up manual machine run by a competent operator can have the same level of accuracy.

True, but not at the same cost of production (disregarding capital equipment costs).

I can hold sub-thou on my Bridgeport mill all day long, but I can't do it as fast as a CNC mill.

One major advantage of CNC milling is flood coolant, which allows much higher cutting speeds than a manual mill.

- Leigh

[Steve Smith]

If I was making a lot of shutters without the benefit of CNC I think I would cut and thread one side then mount the shutter part on a custom made threaded mount and machine the other side's depth relative to the first side. Probably on a turret lathe to perform many operations consecutively.

But I agree about the speed and other advantages of CNC (spoken as someone who programmes and runs CNC equipment as part of his job!).


Steve.

[Leigh]

But I agree about the speed and other advantages of CNC (spoken as someone who programmes and runs CNC equipment as part of his job!).

Yeah. I'm a journeyman tool & die maker, over 50 years now. When I got into the trade CNC didn't exist.

- Leigh

[Dan Fromm]

Dan,

Have you been drinking?

T-stops are just accurate apertures, used because they want extremely accurate exposures.

They DO NOT measure luminance at the film plane, as I've suggested for this application.

- Leigh

Leigh, I stopped drinking alcohol years ago.

I'm sorry, you're misinformed. Please see http://en.wikipedia.org/wiki/F-number