Copal 3 and max aperture question

[genotypewriter]

So, Copal #3 shutters have a maximum iris opening of 45mm. Then how is it possible for lenses like, for example, 300mm f/5.6 designs that (should) have an aperture of 53mm come installed on a Copal #3 shutter and still be f/5.6?

Sticking to the same example, a 45mm aperture on a 300mm lens means a f-number of f/6.7. Another way to say it would be, the longest f/5.6 lens with a 45mm aperture can be at most a 255mm.

I'm surely missing something here... can someone care to explain?

Thanks!

Guy

PS: I tried searching but couldn't find a direct answer...

[Oren Grad]

Strictly speaking, the definition of the f-stop is the ratio of the focal length to the diameter of the entrance pupil, which is the optical image of the iris opening as seen through the front of the lens, rather than the diameter of the iris opening itself. The two can differ, depending on the magnifying power of the front cell.

[ic-racer]

The entrance pupil on my Fuji 300mm f5.6 in a Copal #3 is 46mm, so there is about one millimeter of obvious magnification by the front cell. The indicated aperture may be optimistic.

[Leigh]

Measuring the diameter of the entrance pupil accurately can be a challenge.

Manufacturers use lens design programs to calculate that value with a high degree of accuracy.

I think you'll find the markings on any modern lens to be correct, unless the shutter has been replaced.

- Leigh

[genotypewriter]

Thanks for the insights, everyone.

I'm guessing the entrance pupil diameter should be measured from infinity to get the most accurate number. I can see how this can be difficult!

Also this raises another question for me: If we can see the entrance pupil, which is the shape of the iris (at max aperture), does this mean by using an even bigger iris an even larger max f-number can be achieved?

I'm thinking, the maximum entrance pupil size would be an iris that is only "just" outside of being visible from the front of the lens.

Any thoughts?

[Leigh]

Also this raises another question for me: If we can see the entrance pupil, which is the shape of the iris (at max aperture), does this mean by using an even bigger iris an even larger max f-number can be achieved?

Yes, under certain circumstances and within design limits.

A good example is the Hasselblad lenses made by Zeiss for the lens-shutter cameras and for the focal plane shutter cameras.

For lenses that exist in both series, i.e. one with a shutter and one without, those without a shutter are a full stop faster.
The difference is the aperture restriction imposed by the shutter.

However, the maximum aperture that can be achieved can never exceed the design-maximum for a particular optic.

- Leigh

[Emmanuel BIGLER]

Measuring the diameter of the entrance pupil accurately can be a challenge.

... a challenge for a XVIII-th century scientist but not for a XIX-th century photogapher
This is how the procedure was described in old textbooks : Place a piece of thin metal in the focal plane, drill a small hole in the centre and place a candle behind, so that the light will illuminate the lens backwards from a point source located at the focal point. Place a piece of photographic paper in front of the lens, expose, develop. You'll get a black circle on your paper, the diameter of this circle is the diameter of the entrance pupil.

In our XXI-st century with all the available technology, including this forum, plus I-phones that you can transform into spirit levels, stop watches, exposure meters etc.. you'll simply use a small flashlight located at the focal plane, illuminate the lens backwards and place a piece of paper (translucent paper, if you wish, or a ground glass, but plain ordinary paper will do the job) in front of the lens, as close as you can to the lens. The illuminated circle is the parallel projection of the entrance pupil. Measure its diameter with not more than one millimeter in accuracy, and you'll be all set.
You can, of course, take a picture of this illuminated circle with your I-phone, instead of looking at it with your eyes and measuring it with a ruler

[Dan Fromm]

A good example is the Hasselblad lenses made by Zeiss for the lens-shutter cameras and for the focal plane shutter cameras.

For lenses that exist in both series, i.e. one with a shutter and one without, those without a shutter are a full stop faster.
The difference is the aperture restriction imposed by the shutter.

So one might think. But see http://www.mir.com.my/rb/photography...lens_entry.htm and compare lens cross-sections. Focal length for focal length, the faster lens differs from the slower.

This is sometimes the case, although not, it seems, with Zeiss lenses for Hasselblad. Eric Beltrando, who has some of Boyer's archives, tells me that the f/7.7 Beryl S and f/6.8 Beryl were made to the same prescription.

[genotypewriter]

... a challenge for a XVIII-th century scientist but not for a XIX-th century photogapher
This is how the procedure was described in old textbooks
... Place a piece of photographic paper in front of the lens, expose, develop. ...

I can think of many ways how that could be difficult

[Cor]

I take the liberty a quoting a post by Richard Knoppow, which describes a procedure which is easy and works well:
..............................................................................
Here is how to determine the focal length and size of the
entrance pupil of any lens.
First, determine the focal length by autocollimating. This
requires a mirror capable of covering the front of the lens.
It can be done using a view camera. A first surface mirror
is ideal but a standard makeup or shaving mirror will do.
Just be sure to use the plane side, not the magnifying side.
Place the mirror over the lens. Distance does not matter
but the mirror should be parallel.
Place a small light, a pencil flashlight is ideal, against
the ground glass, near but quite at the center. The mirror
will reflect an image of the light back to the ground glass.
For easier focusing you can draw a small cross on the ground
side with a pencil (will come off again). Focus the image as
sharply as you can. This focuses the lens _exactly_ at
infinity. Mark this distance at some convenient point on the
camera.
Now, set the camera up for an exact 1:1 image of some
object. A small ruler is handy since you can tape a similar
one to the ground glass to match it. When adjusted for 1:1
the diference between the focus point and the infinity focus
point will be _exactly_ the focal length of the lens.
To measure the _effective_ size of the stop set up a
pinhole source at the exact infinity focal plane (which you
determined in the first measurement). Place a sheet of some
translucent material over the front of the lens, and record
the diameter of the circle of light. This is the effective
size of the aperture. Devide this into the focal length to
get the f number. The size of the effective aperture may not
be much different than the physical aperture in many lenses
but the difference will be significant for some. This method
takes into account the magnification of the stop by the
lens.
If you make a test stop of known size you can determine
the difference between the physical size and effective size
and use that to calculate the size needed by the actual
stops.

Since you now know the focal length you can determine the
locations of the principle points by focusing the lens
exactly on infinity and measuring back one focal length
toward the lens from the focal plane. That is the location
of the principle point for that end of the lens. When the
lens is in its "normal" position this is the second or rear
principle point. To get the first or front principle point
turn the lens around and refocus it. Knowing the principle
points is sometimes useful.
The internal structure of lenses which are not too
complicated can often be determined by shining a small light
into each end of the lens and counting the reflections.
Easier if you can examine the cells separately. Glass/air
surfaces, even when coated, are bright, cemented surfaces
are dim. Sometimes its possible to determine the power, or
rather the sign, of the surface this way, but it can be
tricky.
My guess is that this is probably a Petzval portrait lens.

---
Richard Knoppow
Los Angeles, CA, USA