Wista Ground Glass Spot

[geyes30]

Hi,

I just got an Omega View 45E, and it came with an additional Wista Spot ground glass. I was wondering, what is the purpose of the circle in the middle of the glass? I tried looking for some clues online, but can't seem to get a straight answer. I checked my medium format cameras and realized that most of them also had a similar circle, but I didn't notice them before. Thanks!

 

[Ian Grant]

Are you sure it's not a Wista combo screen/fresnel, there's a center circular area of plain screen surrounded by the fresnel rings.

A fresnel acts to bend the light from the screen back to wards the viewer and isn't needed in the central area, with a plain screen you get a bright center hot spot and adding a fresnel brings the edge & corner brightness closer to this.

Ian

 

[geyes30]

Thanks! I think it is a combo screen/fresnel. That's exactly the sort of answer I need. Now everything makes sense!

 

[Ian Grant]

The Wista combination fresnel/screen is very bright and the apparent brightness compared to a plain screen can be in the order of 3 stops which makes composition and particularly focussing very much easier. Most of the time I don't need to use a dark-cloth.

Ian

 

[j_landecker]

The circular area is meant for fine focusing - there are no fresnel lines over that portion of the ground glass, which makes focusing easier.

 

[geyes30]

The circular area is meant for fine focusing - there are no fresnel lines over that portion of the ground glass, which makes focusing easier.

Hi landecker,

If that is true, I wonder if that might be a little limiting composition-wise. Does that mean that if we want the flexibility of fine focus across the screen, we should be using plain ground glass?

 

[Ian Grant]

If that is true, I wonder if that might be a little limiting composition-wise. Does that mean that if we want the flexibility of fine focus across the screen, we should be using plain ground glass?

It's swings and roundabouts, both have their adavantages and disadvantages. However on balance the greatly increased brightness across the screen makes life as a whole so much easier composing and focussing with a fresnel. I use 5x4 & 10x8 cameras with & without fresnels but the three main cameras I use all have a fresnel and I don't need to use a loupe for fine focus with them, however that centre plain area is there if you want to use one.

Ian