[pure-silver] Re: [OT] Filmholders, Septums, Total-Disasters

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <pure-silver@xxxxxxxxxxxxx>
  • Date: Fri, 25 Nov 2005 01:22:13 -0800


----- Original Message ----- From: "Michael Briggs" <MichaelBriggs@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, November 24, 2005 1:34 PM
Subject: [pure-silver] Re: [OT] Filmholders, Septums, Total-Disasters



On 24-Nov-2005 DarkroomMagic wrote:

...

Alternatively, the Fresnel lens can be added in front of the ground glass.
This has the advantage of image formation only taking place on one surface,
since the ridges are in contact with the textured surface of the ground
glass. However, the disadvantage is that the ground glass, and the
associated focus plane, is out of its original position. Consequently, the
focus plane is no longer aligned with the film plane, and the camera back
must be machined or adjusted to allow for the Fresnel lens thickness. In
either setup, make sure that the textured surface of the ground glass faces
the lens and is aligned with the film plane, and that the ridges of the
Fresnel lens are facing the ground glass.

I advise against placing the Fresnel lens in front of the ground glass. If
placed behind, no focus correction is required.

A agree that placing a Fresnel lens in front of the ground glass is undesirable
because of the focus shift, but some manufacturers ship cameras this way. They
probably make this choice to have the ground glass protect the Fresnel lens.
I think owners altering their screens, such as incorrectly altering Fresnel
lens setups, is probably a more common cause of mis-focusing than bad holders.

The above description of the focus shift from placing a Fresnel lens in front
of a ground glass is correct mechnically but omits an optical effect.

The optical effect is that a Fresnel in front of the ground glass shifts the
location of the focus plane. When the screen is removed to insert the holder,
the focus plane returns to the original location. So in the configuration
of a Fresnel in front of the ground glass, you do not want the textured surface
of the ground glass aligned with the film plane, as stated above.

This optical effect occurs when any plane-parallel plate is inserted into a
converging optical beam. Assuming that the index of refraction of the plate
(Fresnel lens) is 1.5, the shift in focus is 1/3 the thickness of the plate
farther from the lens.

For the original question from Tony about how to make a physical measurement to
verify focus: I think he will have to be able to accurately measure the
thickness of his Fresnel in order to be able to calculate the correction
between the film and ground glass positions. The simple almost-no-tools method
won't work. The ground glass should be 1/3 the thickness of the Fresnel behind
the correct film plane. So the front surface of the Fresnel should be 2/3 of
its thickness in front of the film plane. So the depth to the front of the
Fresnel should be 0.190 inch - 2/3 the thickness of the Fresnel. (0.190 is the
film holder depth less the thickness of film.)

It might be easier to make a photographic test of focus.

--Michael

Ideally, the prism side of a Fresnel lens should be in contact with the diffused side of the ground glass in order to minimize internal refections.
The effect on focal position if the Fresnel is in the image forming path will depend on its material and focal length. Generally, a Fresnel for viewing should be of sufficient focal length to cause the light striking the ground glass surface to be parallel or somewhat converging. This means its focal length should be about equal to the lens although a bit longer will also work.
The combination of focal lengths with the taking lens is minimal. Assuming the Fresnel has a focal length equal to the taking lens it will have no effect when the sytem is focused at infinity.

The formula for combined lenses (in air) is:

1/F = 1/f1 + 1/f2 - (d/F1*f2)

Where:

F  = The combined focal length
f1 = The FL of the first lens
f2 = The FL of the second lens
d  = The distance between them.

Focus shift due to a plane-parallel surface.
The shift is proportional to the ratio of the index of refraction of the sheet to that of air (assuming its in air). The average index for glass is around 1.5 so the difference is 0.5 and the ratio is 1/3.
The shift due to the thickness of a Fresnel lens will depend on what it is made of. Some plastics have an index equal, or even higher, than the average for glass, but it can also be lower. Looking at the range of indices for plastics the 1/3rd rule is probably safe to use for them.
About the only way to find out the actual effect on focus is to have a sample of the lens that can be used in the image path and measure the difference. In backs like the Graflok back, found on late Speed and Crown Graphics, the reference surface for the screen locates the combination rather than the ground surface of the ground glass. If the ground glass is used alone its surface will be in the wrong place unless shimmed. Because of the optical effects of the Fresnel lens the shimming will not be equal to the thickness of the Fresnel lens but, rather, some greater distance.
Using a Fresnel on the viewing side of the ground glass is quite satisfactory and has none of these placement problems. There can be somewhat more internal reflection but its not usually troublesome. On a view camera a removable, outside, Fresnel can be useful because it can be easily removed when its presence causes problems. For instance, unless the focal length of the Fresnel is changed it exaggerate the darkening of the corners when a wide angle lens is used. Also, the optical path through the Fresnel is a straight line perpendicular to the light path of from the lens so, when camera movements are used it can also cause exaggerated darkening. The would probably not be so if the ground glass were a perfect Lambertian diffuser but its far from that so much of the light that strikes it continues along the same path it took from the lens.
I've tried cheap desk magnifiers from the stationary store. The tend to be too large in pitch but will work to even out the illumination. The pitch of the one I got to experiment with is too coarse for 4x5 but would be OK for _composing_ on 5x7, provided you focused with it removed. I don't remember what I paid for this but probably less than $2 US at Staples or Office Max. Finer pitch Fresnels used to be available from Edmund's but I don't know if either of the current Edmunds still carries them.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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