Clipped corners . . . What do I see?

[Steve Feldman]

Hi y'all,

I have a new (to me) 5x7 B & J wood view with a gg that has clipped corners. I understand that one reason for the clip is to relieve air pressure inside the bellows. The other reason is to see if your lens, when stopped down, is vignetting. My question: what does vignetting look like when looking through the back?

Thanks.

[Joseph O'Neil]

Monday morning cynical answer for you. This answer implies an obvious coffee/caffine shortage.

First, IMO, the real reason for clipped corners on ground glass is so the people assembling the camera do not cut themselves on sharp corners of the glass. It's also entirely possible, since making a nice, square corner is actually something that takes a few, living, functioning brian cells (not many mind you, just some), that any ground glass with clipped corners will allow for looser tollerances when assembling the camera.

A bit more seriously, on squares/sheets of glass, it's real easy to crack a corner. Also, I suppose once you use a micrometer to get the depth of the ground glass just perfect, it does help to have a bit of loose tollerance on other parts. Easier to manouver the glass overall during assembly.

The bit about air pressure (yes, heard that one myself too) is probally from some smart marketing type possibly too embarressed to tell the truth .

Vignetting - how do you mean - when you have reached the limit of your image circle? You can usually see that as a darkness on the corners of your ground glass. Ah, but wait a minute - if you don't have corners, how do you see the dark corners. A new conspiracy theory then - the purpose of clipped corners is to hide your vignetting, not let you see it.

joe

(of in search of another mug of coffee.....)

[John_4185]

Geeze, Joe, you were wrong on every count. You don't drive when you are decaffinated, do you?

[Eric Biggerstaff]

I don't think the clipped corners are there for air pressure.

To use them to check for vignetting, stop the lens down to your taking apreture. Once this is done, look through the clipped corners to the lens. If you can see out the of the lens, then you are not vignetting. Do this through each of the clips to make sure.

[David A. Goldfarb]

If you can see the entire aperture as a circle through the corners of the groundglass, then you shouldn't have any vignetting or falloff of illumination in the image. If the aperture of the lens is cut off by the lens barrel or shade or sagging bellows, or if it appears as an oval or (American) football shape, then you'll have vignetting or dark corners due to falloff of illumination.

[Paddy Quinn]

Wrong time of the month Joe?

"Vignetting - how do you mean - when you have reached the limit of your image circle? You can usually see that as a darkness on the corners of your ground glass."

point being it frequently doesn't show up as obviously darkened corners - so looking through the cut of corners is more helpful.

"The bit about air pressure (yes, heard that one myself too) is probally from some smart marketing type possibly too embarressed to tell the truth"

My camera came without cut-off corners on the GG, but is so nicely air tight when the back is closed and a lensboard in place that if I move the front standard in even moderately rapidly from say 450mm to 250mm or 210mm the bellows ballon out and the creases start to pop outwards. So that means I eather have to make sure I do that with the lenses off or let the new GG with it's cut-off corners do its thing

[Alan Davenport]

Clipped corners on the groundglass are NOT NECESSARY to check for vignetting, as this can be done by looking through the lens. Since cutting the corners requires an additional manufacturing step, there is no reason why it would have been done unless there were another reason, such as letting air move in and out. Ansel Adams said the corners were clipped to allow air to escape. I accept his statement as correct, and subsequent claims as revisionist.

When looking through the clipped corner, you are looking for the actual aperture of the lens. If you can see the entire aperture, and nothing in front of it but the desired scene, you'll have no problem. If you can see part of the lens shade in front, or if part of the aperture is blocked by the rear element structure inside the camera, you will have vignetting. The alternative is to look through the aperture from in front of the camera; if you can see the corners of the GG without them being blocked, you're OK.

[Jim Noel]

David Goldfarb gave you the correct answer.
If the opening appears like a cats eye, or american football, the corner is vignetted. If all of the diaphragm is visible and essentially round, you are in the clear.

[Steve Feldman]

Thank you all for your insitefull responses. Even Joe's.

Ok. Especially Joe's. Gotta lighten up - it's only exposed silver.

Now all I gotta do is get my head, a loop, an eyeball and my readers under the cloth, while the camera is only a foot of the ground and angled up. Make movements by feel. Upside down and backwards. Legs and feet cramping up all the while.

Easy - nothing to it.

I love is stuff.

~)

[Mark Sawyer]

Remember when checking the corners for illumination, on many lenses you could still be outside the "useful coverage" of the lens. The image can get fuzzy well before the light falls off. I usually only check one or two corners, as it's obvious where the vignetting might be.

"If the opening appears like a cats eye, or american football, the corner is vignetted."

I think this would qualify more as fall-off than actual vignetting. It might also contribute to the much-beloved "swirly bokeh" of some older lenses, but being a near-complete idiot, I'm unsure about this.

" Ansel Adams said the corners were clipped to allow air to escape."

It's a little known fact that Ansel Adams' crooked nose was broken when he folded up an 8x10 without clipped corners too quickly, and the sudden increase in air pressure caused the camera to explode in his face...