Clipped corners on ground glass?

[Patrick Dixon]

What are they for?

Do I need them?

I've cracked my gg and need to get another - but budget doesn't stretch to Maxwell I'm afraid. The existing one has them, but I haven't ever 'used' them 'cos I don't know how.

[PAUL O]

Hi Patrick
Clipped corners prevent the build up of "pressure" in the bellows - and also allow viewing to see if aperture in use will vignette on film.

[Patrick Dixon]

Thanks Paul. The pressure thing sounds sensible, especially with the rapid speed at which I zero in on optimum focus and the fast altitude changes my camera experiences, but I don't understand how you check for vignetting with the clipped corners.

[IanG]

Clipped corners are purely for checking lens coverage, a great many cameras never used them at all.

Ian

[BarryWilkinson]

Patrick,

Once you have composed your image with any necessary shift, look through the cut corners to make sure you can see the whole image without vignetting.

[PAUL O]

The pressure Issue does sound risky - I always thought it a myth. However I have seen a bellows pop on a rail camera that had been extended and then the standards collapsed quickly to pack it down!

[Joanna Carter]

Certainly, the corners allow air pressure to equalise but, as others have said, the most useful function is to allow you to check for vignetting.

Some folks look from the back of the camera, through the corner, towards the lens, which should be closed down to shooting aperture. I have found it easier to look from in front of the stopped down lens, towards the back of the camera. What you are looking for is a complete circle of light, through which you can only see ground glass. If you see anything other, then you are either seeing the bellows or the edge of the ground glass screen frame.