View Full Version : Focus on ground glass?
Under the heading Large Format Photography primer: equipment regarding loupes: "The other requirements are that /.../it is focussable so that you can make sure it is focussed on the ground glass..."
Would somebody be kind to explain what this means in a real situation? Should I focus on the ground glass, i.e the grain of it or should I just try to get the p icture sharp? I am somewhat confused in this matter...
Bob Salomon - HP Marketing
A ground glass, and perhaps with a frenel, have thickness.
The image is formed on the grain side of the ground glass.
If you place a loupe on the top of the ground glass and do not focus it on the grain side of the ground glass every shot is out of focus by the thicness of the ground glass.
If a frenel is on top of the ground glass then an unfocused loupe can make you out of focus by as much as the thickness of both the ground glass and the fresnel.
Focusing a loupe on the grain of the ground glass is very easy.
1: Take the lens off the camera 2: Point the camera at a light 3: Place the loupe on the ground glass and/or fresnel and focus the loupe till the grain of the ground glass is sharp.
That's all it takes.
A correction to what Bob Solomon wrote: An unfocussed loupe WILL make the groundglass image look blurry, and therefore difficult to focus. It WILL NOT under any circumstances cause the apparent position of sharpest focus to be shifted (unless you somehow manage to form a secondary image on the near, unground surface of the groundglass, but that's well-nigh impossible so don't worry about it).
The notion that an improperly corrected viewfinder or loupe can cause focus shift is the Energizer Bunny of optical falsehoods: No matter how many people try to kill it, it just keeps going, and going, and going...
Thank you for your info on this.
Now for an follow-up thought regarding the focusing on the grain on the GG: Is this the procedure I have to do every time whenever I make a picture? (Take off lens, etc?) Isn't there any way to make a lupe which is taking this thickness in account? By the way, how the **** did they do this in the olden times?
Could a normal camera lens do this job?
Is this the way all you old fellows do this? ;)
A focussable loupe is nice, but not essential. A loupe is just a magnifying glass with a skirt that hold it a fixed distance from the thing you are looking at. A focussable loupe is one with a variable skirt depth (or indeed, no skirt at all). A skirt is useful: it means you can just slap the loupe on the GGS and look through it. Without the skirt, you need a steady hand (which is OK in the studio, but harder on a wet and windy hillside). No, there's no need to go through the procedure of taking off the camera lens, etc, every time. All you need to do is to check, once, that the skirt depth is such that when you hold it on the smooth side of the GGS, your eyes can focus on the ground side.
These posts may make it seem harder than it really is. You need a dark cloth because the image on the ground part of the ground glass screen is dark. You need a magnifying glass because you want the image to be really sharp, and your eyes can't focus close enough without it.
Once your loupe is focused on the ground glass it will also focus on the image projected there. Focus the loupe once and just leave it that way. In the old days we had to take the sweat from an angry grizzly bear and apply it to the GG with a cloth made of marmot hide, before every exposure. Somehow, modern technology has made this unnecessary ;-)
Sean Billy Bob Boy yates
Hell, When I was a boy, we didn't even have film, cameras neither. We just held a picture frame up in front of our faces.
Bob Salomon - HP Marketing
"Now for an follow-up thought regarding the focusing on the grain on the GG: Is this the procedure I have to do every time whenever I make a picture? (Take off lens, etc?)"
No that isn't really what I said.
I was describing how to focus the loupe to the grain side of the ground glass. Once you do that you do not have to do it again as long as you leave the loupe at that setting. Or as long as you don't change the ground glass/fresnel screen. Or as long as your eyes don't change.
But there is no reason to do this in the field once the proper focus is set.
As to anyone who doesn't think that focusing on the grain side helps why not try it and see if you can see a difference.
A very simple loupe is the focus hood from my waistlevel finder on my Hasselblad. Works beautifully.
I don't use the skirt on my loupe. The reason why is that with movements, especially with wide lenses, if you view the GG from the angle at which the rays of light are exiting the lens, the image is much brighter. You can only do this without a skirt (otherwise, the image looks too dark at the edges). Next time you have lots of movement dialed in, try looking at the GG from an angle...you'll be surprised at how much brighter the image is compared to placing the skirt flat against the GG everytime (bright enough so I can use a lightweight, navy windbreaker as the darkcloth rather than buy/carry a dedicated one..not as hot under the sun, plus you can wear it, too).
James Chow's suggestion of looking at an angle to the GG (when looking near the edges of the frame) with wide angle lenses is very important. As James suggests, you can use a loupe without a skirt so it can be held at an angle, or you can flip your skirted loupe over, and use it backwards. I use the Peak Anastigmatic 4x loupe for focusing most of the time. This is a rather large loupe with a fairly flat and wide field of view. To a limited extent while viewing near the edges, I can hold the loupe flat against the GG, and view though the loupe with my eye at an angle to the loupe. This has a similar effect as holding the loupe at an angle, but the whole GG area stays in focus. I guess what I'm saying is that you have to experiment and find what works best for you.
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