ground glass corners how to use please

[Richard K.]

Oblate spheroid...that sounds like it hurts!

It DOES !! I had one once but had it removed!

[Paul Fitzgerald]

"I thought the purpose of cut corners was to allow air to travel in and out of the interior as bellows are drawn."

Old wives tale, total claptrap. The clipped corner are to allow dust to enter and contaminate your static charged film, plain and simple.

[Sheldon N]

This link has a good diagram as to why you want to see a full circle aperture from the corner of the glass... look at the images of the 50mm lens and it makes a little more sense.

http://www.vanwalree.com/optics/vignetting.html

[Vaughn]

Take the old Deardorff or whatever and with a fine woodworking chisel clean out the curved corners, refinish and then put in a full coverage ground glass. That way you won't be surprised by something intruding into the corner of the negative. Something you would have seen if you had the full ground glass but don't see when the corner is cut off.
When closing the view camera, do it gently and there is not problem. They aren't air tight.

Actually, it is possible to see the image all the way to the corner even with cut corners on the GG...I believe it is called an aerial image -- and it is brighter than that thrown on the GG.

I might have the advantage of being very near-sighted and can focus the aerial image easily without my glasses. I have been tempted to clip my full 8x10 GG, but don't wish to risk breaking it.

Vaughn

[Capocheny]

Actually, it is possible to see the image all the way to the corner even with cut corners on the GG...I believe it is called an aerial image -- and it is brighter than that thrown on the GG.

I might have the advantage of being very near-sighted and can focus the aerial image easily without my glasses. I have been tempted to clip my full 8x10 GG, but don't wish to risk breaking it.

Vaughn

Hi Vaughn,

If you take the glass to a glass shop... they'll just grind/sand the corners down to whatever size you require.

I had mine done at a cost of $5.00 and it just took them minutes to do.

Cheers

[Vaughn]

Thanks C -- but just the idea of removing the GG from my 8x10 camera makes me nervous. Which is totally silly since I have to replace the GG on our 4x5's when our students go "Opps!" on a semi-regular basis.

Perhaps if I have a replacement GG on hand, I would feel more comfortable letting someone handle my nice one!

Vaughn

[Kuzano]

Oblate spheroid...that sounds like it hurts!

I think it's a procedure known only to Proctologists, and their patients, and not discussed much outside those environs.

[aduncanson]

With a 75mm lens on 4x5" film, you will have light fall-off. If it's a modern "wasp-waist" WA lens, light fall-off will follow cos^4; if an older it will follow cos^4 (or thereabout).

Ole,

Did you mean cos^3 in the case of a modern wasp waist lens?

Thanks

[Ole Tjugen]

Yes of course. Thanks for pointing it out.

Modern "wasp-waist" wide angle lenses have a fall-off approaching cos^3(theta), while the older compact WA lenses (Angulon, Protar V/f:18 and the like) have fall-off approaching cos^4(theta).

The only way to reduce this fall-off is by making a retrofocus WA lens; and as far as I know there are none for LF.

[aduncanson]

The only way to reduce this fall-off is by making a retrofocus WA lens; and as far as I know there are none for LF.

Not to be difficult but just for your information, the Komura 90mm/6.3 Super W is said to be a retrofocus lens. I just focused mine at a light across the street (about 30m) and found the lensboard to ground glass distance to be about 130mm. By comparison, Schneider gives the "Flange Focal Distance" for the 90mm/5.6 Super-Angulon as 102.7 mm.

Long ago I bought a recessed lens board to use this lens on a Calumet CC-400 and decided that the recessed lens board was neither necessary nor worth the hassle.