Ground Glass - clipped corners, or not?

[Sanjay Sen]

What is/are the benefit(s) of clipped corners on a ground glass? My Zone VI GG does not have clipped corners, but the Wehman does, and I did not notice the difference until I checked them.

So why do some ground glass have clipped corners, while others don't?


Thanks for any info.

[Mick Fagan]

I use the clipped corner to look through and see if the coverage of the lens covers the extreme corner.

It is a facility I use quite a lot, especially with my particular 90mm lens on anything other than straight alignment.

Mick.

[Vaughn]

I like them for the same reason as Mick -- as well as making sure one's lens hood is not blocking part of the image-forming light. One can also see how small an aperature one has to go down to avoid vignetting.

And lastly, for composing, I can see an aerial image in the corners which is much brighter than the image thrown on GG. I am very near-sighted, so I do not know if this (using the aerial image) is possible for those who are normal-sighted.

Vaughn

[Anscojohn]

What is/are the benefit(s) of clipped corners on a ground glass? My Zone VI GG does not have clipped corners, but the Wehman does, and I did not notice the difference until I checked them.

So why do some ground glass have clipped corners, while others don't?


Thanks for any info.

*******
It is my understanding the clipped corners allow rapid focussing without collapsing the bellows due to negative air pressure when moving the GG away from the lens.

[Ian Grant]

*******
It is my understanding the clipped corners allow rapid focussing without collapsing the bellows due to negative air pressure when moving the GG away from the lens.

I think that's an "Urban Myth" perpetuatedin many publications, because many old field cameras didn't use clipped corners, and non of my 5 German 9x12 folders and there's no problems with the bellows when opening/closing quickly.

A 3 of my Graphics (Speed/Crown) have unclipped corners.

The clipped corner was more essential for checking coverage with older lenses particularly EWA's (WA Protars etc) often with maximum apertures of f16.18, or less in the case of Hypergon. With no fresnel screens to help it was almost impossible to see the image at the edges & corners check coverage with movements.

Ian

[Sanjay Sen]

Sorry if I don't understand this correctly, but can I not check if the lens covers the extreme corners by checking the GG itself? For instance, last weekend I was out with the Zone VI (which does not have clipped corners) and a 90/8 lens, and I verified that I have corner coverage by looking at the GG corners (I'd used some front rise). I am probably missing something here, but not sure what...

[leicam5]

I think that's an "Urban Myth" perpetuatedin many publications, because many old field cameras didn't use clipped corners, and non of my 5 German 9x12 folders and there's no problems with the bellows when opening/closing quickly.

A 3 of my Graphics (Speed/Crown) have unclipped corners.

The clipped corner was more essential for checking coverage with older lenses particularly EWA's (WA Protars etc) often with maximum apertures of f16.18, or less in the case of Hypergon. With no fresnel screens to help it was almost impossible to see the image at the edges & corners check coverage with movements.

Ian

I do not consider this is an ‘Urban Myth’.
I have seen this happen in photo school, in the late ‘70, with a Linhof Cardan Colour 13 x 18 cm, with unclipped GG corners.
A fellow student, who apparently was in a hurry, pushed, after turning louse all the knobs on the monorail and the lens still mounted, in one movement the camera from full extension for a close up shot, back in to ‘ready to pack’ position. Not only the GG collapsed but the bellows were blown out too.
But, of course, the GG’s corners are clipped for checking the vignetting of the image circle and the compendium, not for playing accordion...

Philippe

[Ian Grant]

Of course you can check that way Sanjay, but some older cameras and lenses would not give as bright an image on the screen, try stopping your 90mm down to f16 and then focussing.

Many EWA's like the Protar's and Hypergon's were wider still and that makes the light fall off even worse, so clipped corners are then more useful. A modern equivalent would be a 65mm f8 Siper Angulon, these are hard enough to focus at the cntre of the sreeen and very dim at the corners and coverage is very tight, here the clipped corner helps.

Ian

[David A. Goldfarb]

Sorry if I don't understand this correctly, but can I not check if the lens covers the extreme corners by checking the GG itself? For instance, last weekend I was out with the Zone VI (which does not have clipped corners) and a 90/8 lens, and I verified that I have corner coverage by looking at the GG corners (I'd used some front rise). I am probably missing something here, but not sure what...

It's often hard to distinguish vignetting from falloff of illumination on the groundglass, particularly with wide lenses, but if you can see the aperture through the corners, you can tell more positively. Ideally, you want to be able to see the whole aperture from the corners. If you can see the lens hood or part of the lens barrel, then you have vignetting, which you may be able to correct by stopping down or using smaller camera movements. If the aperture looks like an American football, rather than a circle, then you will have falloff of illumination, and you may want to stop down, use a center filter, or increase exposure to be sure you have detail in the corners, so you can fix it in the darkroom. You can also catch situations where the bellows are obstructing the image more easily with cut corners.

I agree that the "air pressure" factor is not relevant in most cases, particularly with older wooden cameras that may be light tight, but certainly aren't airtight. One case where it is relevant, though, is with the Sinar shutter, where it is possible to damage the large shutter blades by expanding the camera too quickly. The Sinar groundglass is oversized, so it's a bit easier to see vignetting, and the corners of the groundglass are clipped enough to let air in, but not enough to make it particularly easy to see the aperture, though it's possible if you get close and squint.

If you want to check for vignetting directly and don't have a clipped groundglass, you can also sight in the reverse direction. Look through the lens and see if you have an unobstructed view of each corner of the groundglass frame.

[Thingy]

Of course you can check that way Sanjay, but some older cameras and lenses would not give as bright an image on the screen, try stopping your 90mm down to f16 and then focussing.

Many EWA's like the Protar's and Hypergon's were wider still and that makes the light fall off even worse, so clipped corners are then more useful. A modern equivalent would be a 65mm f8 Siper Angulon, these are hard enough to focus at the cntre of the sreeen and very dim at the corners and coverage is very tight, here the clipped corner helps.

Ian

Ian

You need a Maxwell HI-LUX Ultra Brilliant Matt 4.7 focussing screen. With most of my lenses I don't need to use the focussing cloth, and it gives a bright image in the centre even with my Schneider 120mm HM macro lens!

Steve