hotspots and ground glass

[Darin Cozine]

I tried an acid-etched glass to replace my standard 4x5. While I was impressed with the clear screen and high contrast, I had a harder time focusing a wide-angle lens. This was due to an increased 'hot-spot'. So it became very hard to compose with my 90mm f8 super angulon.

I was thinking about trying a satin-snow glass. So I would like to know if anyone has opinions about the satin-snow product in context of the hot-spot. Thanks.

[KenM]

Darin, I have a SS GG, and while it's a well made GG, it suffers from the same problems as any plain GG - you get hot spots with wide angle lenses. Under normal lighting conditions I can focus my 75mm just fine, but I do have to move my head around to see anything in the corners. When it get's dim outside, then you're in a bit of trouble, but that's no different than with any other type of GG (exscluding fresnels, of course).

If you're using a loupe, I would suggest getting the Silverstri tilting loupe - this will let you see into the corners a bit better to focus.

[Darin Cozine]

I guess what I was getting at was that the hotspot is more pronounced with the acid-etched glass than with the standard gg. I can still focus with the acid-etched fine, but composing is a pain in the a$$.

[Josh Z.]

I actually just received a Satin Snow GG to replace/supplement the GG that came with the Shen Hao HZX45A. I haven't taken it out in the field yet (probably this weekend), but so far it looks like it has solved my composing problems that I had with a Schneider 90mm f/8 with the stock GG. While the hotspot is still visible, it is far less pronounced, and at least in testing at mid-day, it was very acceptable for my standards. There is a thread on this over at the Shen Hao forums at

http://www.phpbbforfree.com/forums/?mforum=shug

The wait that I had was at least 4 weeks, though for the price and the apparent performance so far, it can't be beat. I'd highly suggest at least trying it out, the other solutions can be expensive and or tricky. You probably should note though that they do not come with grid lines, but they do provide templates for making your own.

[Michael S. Briggs]

There is a tradeoff in the coarseness of the surface of a plain ground glass, whether made by grinding or acid etching. The coarser the surface, the greater the scattering of the light rays from the lens. With a coarse glass and large scattering, some light rays will be scattered from all portions of the ground glass to reach your eye -- so a reduced hot spot effect. The drawback is the large fraction of rays that are scattered in directions that don't reach your eye -- this reduces the overall brightness. A finer surface will have less scattering -- those portions from which rays reach your eye will be brighter -- but the hot spot effect with wide-angle lenses will be stronger.

The problem is worse for wide-angle lenses because the difference in angles between the rays to the center and corners of the ground glass is larger. Larger scattering is required so that some rays from all of these angles are scattered to reach the eye.

A Fresnel lens works by focusing the rays from the ground glass. This allows a finer surface to be used. But many , myself included, find the lines of the Fresnel to be distracting. I prefer a plain ground glass. With wide-angle lenses I have to move my head to concentrate on a particular portion of the image.

[Brian Ellis]

All Fresnels and plain ground glasses have problems with wide angle lenses (except Fresnels specifically designed for wide angle lenses). It isn't the glass that's at fault, as Michael indicated the problem is that light from the edges of any lens has to travel farther, and is at a more oblique angle, than light coming from the center of the lens (that's probably oversimplified but is basically accurate for photography purposes). With medium and long focal length lenses the difference isn't great enough to create the "hot spot" that you're getting. But with lenses of about 90mm and shorter it's very noticeable because the difference between the intensity of light travelling from the edges of the lens and the light from near the center is so great.

I've owned plain ground glass, Fresnels that were OE on Ebony and Tachihara cameras, a Beattie screen, a Linhof screen, three or four BosScreens, and finally a Maxwell screen. The Maxwell is the only one that is both very bright and that doesn't have a hot spot with a short lens (80mm in my case). The BosScreens eliminate the hot spot but they aren't any brighter than a plain ground glass and have some potential problems in extremely hot or cold weather because of the wax center (though for the $100 price difference between them and a Maxwell you might find the BosScreen perfectly adequate).

Ken - There was an article in View Camera magazine about a year or two ago that provided instructions on grinding your own glass.

[Eric Wagner]

Ken--I tried several different grit sizes after reading the article in the May/June 2003 issue of PhotoTechniques and concluded that I best like a glass ground with 25 micron size white aluminum oxide that I obtained from Willmann-Bell.