I've recently bought an older Crown Graphic. The Camera seems to be in OK shape but the ground glass is really dark. Using a 90mm 6.8 Linholf Schneider lens w ide open I just can't seem to get enough light to focus, even using a dark cloth . Can you get a "bright screen" for a Crown? I'm using this as a field camera so I'm shooing oftentimes in full sunlight. Thanks.