ground glass hot spots or fresnel?
Got a question for you guys with big ground glass.
With a straight ground glass, do you have pretty good view of the overall composition?
I am still in the process of putting my 11x14 together. For my ground glass, I went down to my local plastic place, and picked up a piece of frosted plastic. It shows a good image for focusing, but the hot spot, or perhaps I should say bright spot is not very large.
I went down to Staples and picked up one of those 8x10 plastic magnifying fresnel page reader things. It was amazing what it did for the overall view. Its like it bulb was turned on. The combination was 100% usable, but its too small.
So, do I dump the plastic, and get a piece of glass, or look for a big fresnel?
I don't have a fresnel on my 8x20. I think you need to have a dark cloth that permits you to be far from the glass so you can see the whole image at one time easily. Then you move in close and focus with a loupe after the composition is settled. YMMV.
Thanks John, I'll try moving farther back. Yesterday I was perhaps 2 ft back, and I was still having a pretty major bright spot. I'm starting to think that my easy plastic gg is a little bogus.
I'm just curious, how far do you have to get back to see the whole 20" picture?
Hmmm.. 2 feet should be far enough. You know, I've been doing this for so long that there's a possibility I don't see the hotspot any more. I need to check.
For what its worth, this weekend I went down the local rock store and got some grinding grit, and picked up some glass. Time to do a little testing.
I did a small test piece of glass with 200 grit and a piece with a grit that was marked as 400-600f. To be honest, there wasn't much difference in the glass, but there was a huge difference in the size of the grinding compound. It did seem like the 400-600 was a little sharper, maybe a little brighter, but the 200 seemed a little less prone to the hot spot.
So, I thought, what the heck lets rub some 200 on the plastic and see what happens. It was magic. In ten minutes time I had a usable ground plastic-glass. At 18 to 24 inches I could easily see the entire 16" screen for composition.
I then did a 200 grit on real glass. It was sharper, clearer, and maybe a tick brighter. But not enough better in my mind to make up for the "break factor" of glass.
I may try a 400-600 and see if its better. But for now my 200 grit plastic looks pretty good.
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I think what John said in post #2 is the best advice. To me, a fresnel is wonderful for composing, but I find that it is harder to focus critically, even the Maxwell that came in my 8x10. And the coarser the GG is, the easier it is to focus but the harder it is to compose because of the "hot spot". The plastic "glass" I had in my Wehman popped into focus like nothing else, but it was just too dim for me. So it's a balancing act that may not have the same solution for everyone.
I was chatting with Don Dudenbostel, who has been a commercial and fine art photographer since the 1960s, and he said he never knew of a professional that used a fresnel in a large format camera.
The secret is to double cut the screen, grind with #400 first and then #600 afterwards that give a screen that's slightly less prone to hot spots. It's a case of finding the right balance.
I think I will try that.
Originally Posted by Ian Grant
Looking at grinding my own ground glass for a recently acquired 8x20. No sense starting too small...
The existing ground glass is quite dark, and so looking to do something brighter.
Given the effort involved, is it worth the additional cost of using borosilicate glass rather than soda lime glass?
I have read the various articles include Ian's on this site and the instructions on the Hopf web site. Any other insights or cautions?
I did pose this question on another site that focuses upon LF photography, as well.
I've found that a quick re-grind usually brings new life to an old dim screen and it'll be as god as a new screen.
You'll be surprised just how easy it is to make a good glass screen that will match any commercial screen, Hopf included, in terms of brightness and usability.
Cost wise I think I'd rather make a couple of spare screens and just go for the normal glass unless you can get boro-silicate glass shheets at a reasonable cost. Optically it's additional brightness will be barely noticable/measurable and it can still get broken