Clipped corners on ground glass - why?

[Dazzer123]

Hi folks,

I'm ordering a new (gridless) ground glass for my Sinar P2 to use as an alternative to the heavily gridded one i already have.

The ground glass maker asks me if i want clipped corners.

Can anyone tell me?: what is the reason the corners are clipped and are they any advantages / disadvantages?

I'll be using this gridless GG purely for making aesthetic decisions in composition and lighting (via the reflex viewer) and will swap back to the gridded GG for more technical aspects.

Thanks!

 

[Nitroplait]

I am very green in LF, but I heard there is one reason related to focusing - the details now forgotten because I didn't really understand - but someone here will pitch in I'm sure.
Another reason is to allow air to escape when compressing the bellows with the lens/bord attached.

My Intrepid camera came with clipped corners.

 

[Photomultiplier]

They are used to check the setup for vignetting.
By the shape of the aperture seen through the corners, you can determine if there is vignetting or not.

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[Dazzer123]

Thanks!

I really don't understand about the vignetting thing.

I'll take a look at my P2 tomorrow to see if i can work out what that means!

 

[DMS206]

It also lets air in when you extend the bellows to keep them from collapsing

 

[Dazzer123]

To show what a complete noob i am (didn't take a pic yet with the P2):

Are the clips extending into the image area? If not, i see no reason not to have them clipped.

 

[BrianShaw]

Get clipped corners… better to have them clipped and later learn how to use them than not clipped and later wanting them clipped.

 

[Nitroplait]

To show what a complete noob i am (didn't take a pic yet with the P2):

Are the clips extending into the image area? If not, i see no reason not to have them clipped.

Yes - that is the point - See what @Photomultiplier said and linked to.
His post made my old brain recall a few things. The corners are very dark and it is typically difficult to see if vignetting occurs - peaking through the cutouts will let you see if they are covered by the image circle or not.
Never practiced that myself as I don't use much movement.

 

[BMbikerider]

Hi folks,

I'm ordering a new (gridless) ground glass for my Sinar P2 to use as an alternative to the heavily gridded one i already have.

The ground glass maker asks me if i want clipped corners.

Can anyone tell me?: what is the reason the corners are clipped and are they any advantages / disadvantages?

I'll be using this gridless GG purely for making aesthetic decisions in composition and lighting (via the reflex viewer) and will swap back to the gridded GG for more technical aspects.

Thanks!

The clipped corners are quite often used in cutting glass for use in picture frames. In UK this is known as 'arissing'. What this does is help to eliminate stress in the glass because when it is cut off the main sheet it will not always break evenly away from the piece it has come off. and leave sharp edges. All that is done to the glass is to run the edge/corner over a sanding belt to relieve it from stresses and sharp edges..

It is very much the same reason when making mechanical components such as connecting rods for auto-engines, especially in high performance engines, polishing the rods will relieve stress and the chance of breaking under high loads.

 

[FotoD]

It's a good way to check for mechanical vignetting when you've set up your composition.

If you can see the entire oval shape of the aperture through the hole you're good.

If the shape is uneven you have vignetting, either internal or external from something like a hood or filters in front of the lens. In that case you stop down the aperture, focus closer, move the camera or change the lens.

 

[Dazzer123]

Nice, i think i understand, will play with the P2 tomorrow to fully get my head around it!

 

[xkaes]

From Stroebels' book -- which you should get:

 

[nosmok]

Another, more prosaic reason for clipped corners: some older cameras had cut channels for the GG that DO NOT extend to the corners. I had a full-plate (or 8x10, can't remember which) camera where it appeared that the cutting tool lifted up over the last 1/4 -1/2" toward all 4. The replacement GG that I bought for it didn't have clipped corners, so it absolutely would not go in. Good times.

 

[Tel]

Another, more prosaic reason for clipped corners: some older cameras had cut channels for the GG that DO NOT extend to the corners. I had a full-plate (or 8x10, can't remember which) camera where it appeared that the cutting tool lifted up over the last 1/4 -1/2" toward all 4. The replacement GG that I bought for it didn't have clipped corners, so it absolutely would not go in. Good times.

Yup. Kinda like this (old Gundlach Korona)....

 

[eli griggs]

The medium format, Hasselblad Flexbody, ground glass has a black plastic back cover designed to let the user peek through four tiny holes at the corners to check for any vignetting of its tiny bellows and the instructions on this feature in the user manual, shows at least four examples of what that should and should not look like, including full aperture and, a long, complete oval, with no intrusion or interference.

It's a neat check for the proper framing of your shot, in whatever format and, should be used where available, IMO.

 

[Doremus Scudder]

Reasons for clipped corners:

1. Because the original camera back doesn't have allowance for a full sheet
2. Because they allow air to escape when the camera is collapsed/folded up (many camera are quite air-tight).
3. So you can look through them to the aperture to check for vignetting,
4. So you can focus on the aerial image (see here: https://en.wikipedia.org/wiki/Aerial_image)
5. To take stress of the glass sheet in the mount.

That said, I think none of the camera I currently own have clipped corners on the ground glass/Fresnel screen.

The alternate way to check for vignetting if your ground glass doesn't have clipped corners is to look back through the lens (at taking aperture and with all filters attached) at the corners of the ground glass. If you can see the corners, everything is fine. If not, you're vignetting.

Focusing on the aerial image is nice; almost magic. If you've never experienced this, you should.

Best,

Doremus

 

[eli griggs]

I think that not hanging in there and practicing with your camera, at picking up details, good or bad in your images, causes a lot of people to have view cameras on hand that rarely, if ever use over their photo taking life.

That's a real shame to, as the view camera, like waste level SLR's, from small to large formats and medium format TLR cameras, all give the attentive photographer a huge composing edge by forcing the brain to see differently and more clearly, spreading up the overall process of using slow, deliberate photography to make good images.

IMO.

 

[ic-racer]

When using lenses with 'close' coverage, like the Fujinon 180mm with 305mm coverage, I find the best way to center the lens on the camera is by peeking through the clipped corners.

In fact anyone that uses off-center Linhof boards could use that method to see where the front standard needs to be to center the image circle.

 

[Vaughn]

Interesting concept, ic-racer, I'll check that out. Usually my indents and camera body marking take care of that, but not always.

I like clipped corners and often use them to check for mechanical vignetting, and they let me see exactly how closed down I need to go to avoid it. I have no problem seeing the aerial image, which is quite bright and I can see exactly what is going on in the corners.

Some folks check for vignetting thru the lens from the front of the camera, but I prefer the back, and I can not always get to the front of the camera.

My Eastman View No.2 (5x7) will also not take a full sheet of glass and requires the GG to have clipped corners.

 

[Dazzer123]

Thanks guys, i get it!

I'll normally be shooting at pretty small apertures, so less chance of the vignetting problem i guess.

 

[DREW WILEY]

I don't see why it would have anything to do with stress on the glass when a properly sized ground glass is always slightly smaller than the cutout to allow for expansion/contraction differential between materials to begin with. And I have never ever heard of "arissing" in relation to picture frame glass; and I've got quite a framing setup myself and multiple wholesale framing catalogs. It must be some arcane Victorian type of thing that never crossed the Pond.

The main reason for cut corners is to allow viewing of the lens aperture to check for image vignetting. One of the primary advantages of view cameras is the plane of focus and perspective controls they provide by using movements. But too much tilt or rise might cut off portions of the image; so by looking through the corners you can check for that. The full aperture should be fully visible at the final f-stop setting you intend to use.

 

[eli griggs]

Thanks guys, i get it!

I'll normally be shooting at pretty small apertures, so less chance of the vignetting problem i guess.

It's not for your "normal picture taking" that you need to be prepared for, but those occasional 'prize' low light shots that do pop-up, pops-up!

Get the clipped corners and rest easy some future opportunities are not unprintable because of a bad bellows intrusion.

IMO.

 

[_T_]

It’s important to understand that the trick of looking through the clipped corners is not a hard and fast physical property of reality but only a helpful procedure that can be useful in certain circumstances.

It is possible to have no vignetting at wide apertures even though you can’t see the entire aperture from the clipped corners, and it’s possible to get vignetting when you can see the entire aperture through the clipped corners. There are a number of lens and camera design factors that influence this and while it’s helpful to be able to rely on this rule in uncertain situations, it’s no substitute for learning and knowing the limitations of your system.

 

[Dazzer123]

 

[guangong]

My thoughts are that as BMbikerider explained, corners are clipped to remove stress, by glazers whose expertise is glass. Later, LF users discovered that the open corner spaces could be used to check exposure at corners. My LF camera is light tight, but I doubt that it is so air tight as to cause bellows collapse.