Groundglass novice

Sorry for what is probably a dumb question but whats the purpose of 'clipped corners' on my sinar groundglass ?
Is it to allow air escape from the bellows or I've read something about checking the lens coverage by peering through the cut off ......?

I'm thinking of replacing it as I'm finding it impossible to focus on large areas of even tone that don't have much fine detail in ie: sea and sky etc. Also I keep (carelessly) getting unwanted intrusions in the corners of my developed negs.
I'm wondering aswell, if using a 7x loupe is too powerful, as it seems to magnify the 'grain' in the glass too much ?

Cheers
Bill

Both cases are true for the clipped corners.

I've been using a 5x loupe.

And there are a few different brands of glass out there that have less pronounced "grain" to them. Off the top of my head I can't say which but in about another 20 minutes, this thread will be full of brand suggestions...

I'm wondering if you have a factory groundglass in your Sinar or not. I had one in my A-1 (Alpina to the rest of the world) with the factory fresnel, and it was bright, sharp, and very even across the entire glass, even with fairly wide lenses. Double-check that your glass is the factory unit. If you are still dis-satisfied with it, there's a guy selling custom-made groundglass on ebay whose Ebay ID I can't look up right now (I'm at work and they block Ebay at the firewall). Just search Ebay on ground glass. His glass is bright and smooth, and fairly inexpensive (at least as cheap if not cheaper than SatinSnow, and quicker turnaround too!). I have two groundglass screens from him in 5x7 and 11x14 and I'm very happy with them.

The stock Sinar groundglass is quite fine, so you will see a hotspot with wide lenses, but a fresnel lens will even out the illumination of the glass.

If 7x is showing too much of the groundglass grain, try 4x. I prefer 4x, but I've used various loupes from 3.3x to 6x.

The corners are mainly so you can check for vignetting. If you can't see light coming through all of the round aperture in the corners, or if the aperture appears oval shaped, you will have falloff of illumination. If you can't see any light through the aperture when sighting through the corners, then you'll have vignetting. If you can see the lens shade through the aperture, it's vignetting.

Thanks for the advice everyone.
So much to cope with in this LF game.
Bloody good fun though !