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Range of Focal lengths for an RB Auto

 
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Sirius Glass



Joined: 06 Jun 2010
Posts: 123
Location: Southern California & Virginia

PostPosted: Sun Jun 13, 2010 11:13 pm    Post subject: Range of Focal lengths for an RB Auto Reply with quote

Now that I found a source for Graflex A boards, I am wondering what is the range of focal lengths for an RB Auto Graflex? I have a 7 1/2 inch lens [190.5 mm] and I wonder if I could use
    a 90mm lens with the drop front,
    a 127mm,
    a 135mm.

I am concerned that a short focal length lens could interfere with or damage the mirror.

Which are the longest lens practical for this camera, while holding the camera.

Steve
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45PSS



Joined: 28 Sep 2001
Posts: 3231
Location: Mid Peninsula, Ca.

PostPosted: Mon Jun 14, 2010 9:37 pm    Post subject: Reply with quote

from the Graflex Instruction Manual:
Quote:
The "normal" lens for a given size of negative has a focal length approximately equal to the diagonal of the negative. The Revolving Back Graflex and other reflex cameras require focal lengths slightly greater than "normal" to allow clearance for the mirror when it swings up.



For a 3 1/4 x 4 1/4 camera the diagonal of the negative in millimeters is:
3.25*25.4=82.55*82.55=6814.5025;
4.25*25.4=107.95*107.95=11653.2025;
adding the squared amounts=18467.705;
taking the square root of that=135.8959 rounded to 4 decimal places;
so 136mm is normal for 3x4.

Take an empty lens board and mount to the front standard and place the front standard at the rear most point of the focusing track with the focusing fully retracted. Attach a film holder with a sheet of scrap film or unimportant negative inserted to the camera back. With the mirror up measure from the surface of the film to the back edge lens board. Next slowly close the mirror and determine where in its travel it is closest to the back edge of the lens board and measure that distance. Add the two distances and that will be the distance from the film plane that the rear edge of the lens barrel must be when focused at infinity for the camera to operate.

To find the longest lens you can use move the front standard to the front edge of the of the focusing tract and extend the focus to maximum. Measure from the back edge of the lens board to the film plane. This is the longest focal distance that will just make infinity.

Depending on the design of the lens determines where the rear nodal point of the lens is. A general rule of thumb is the aperture blades, but it will only get one close for a starting point.
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Les



Joined: 09 May 2001
Posts: 2682
Location: Detroit, MI

PostPosted: Tue Jun 15, 2010 12:38 am    Post subject: Reply with quote

According to 11th edition Graphic Graflex Photography, the normal lens for a 3x4 RB Auto is 7.5" They don't say what the minimum focal length is, but I doubt it's much shorter than 7.5. I know a 5" lens on my 4x5 RB Auto will not come close to infinity and at best focuses to about 6ft or closer. The standard lens for a 4x5 RB auto is 10" or about twice the focal length of a 4x5 Speed or Crown.
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C. Henry



Joined: 13 Dec 2005
Posts: 358
Location: North East Georgia, USA

PostPosted: Tue Jun 15, 2010 11:58 am    Post subject: Reply with quote

Charles;

In theory your math is correct, however (as Dan Fromm pointed out to me a while back) the nominal dimensions of sheet film are somewhat larger than the actual image size. I do not know exactly what the image size is on a 3 1/4 x 4 1/4 camera but the film gate on a 2 1/4 x 3 1/4 Grafmatic measures 2 1/16" x 3 3/32", therefore I would assume that the dimensions on a 3 1/4 x 4 1/4 would be at least 1/8" less than nominal.

C. Henry
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45PSS



Joined: 28 Sep 2001
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Location: Mid Peninsula, Ca.

PostPosted: Tue Jun 15, 2010 9:10 pm    Post subject: Reply with quote

And the manual says "approximately" and the actual film is about 1/16 shorter in each dimension but that still leaves the format measurements in the approximate ballpark.
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Sirius Glass



Joined: 06 Jun 2010
Posts: 123
Location: Southern California & Virginia

PostPosted: Sat Jun 19, 2010 9:39 pm    Post subject: Reply with quote

C. Henry wrote:
Charles;

In theory your math is correct, however (as Dan Fromm pointed out to me a while back) the nominal dimensions of sheet film are somewhat larger than the actual image size. I do not know exactly what the image size is on a 3 1/4 x 4 1/4 camera but the film gate on a 2 1/4 x 3 1/4 Grafmatic measures 2 1/16" x 3 3/32", therefore I would assume that the dimensions on a 3 1/4 x 4 1/4 would be at least 1/8" less than nominal.

C. Henry


3^2 = 9
4^2 = 16
square root of (9+16 = 25) = 5" ==> 127mm

Much shorter that I think will fit on this camera.

Steve
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