Focusing screens

[seany65]

Hello all, me yet again. With another thread I'm not sure where to put. I was going t put it in the box camera thread which was on the old forum, but I can't find it here.

Anyway, I have just received a cheap, after-market plain 6x9 focusing screen which I bought to carry out an experiment with on a broken Gevaert Gevabox 6x9 version 1 (1a? 1b?). I wanted to find out exactly at what distances the three red dots on the front of the lens were focused. I'd expect the one between 5 and 10 to be at 7.5ft, with 1/3rd of the dof in the front and 2/3rds behind, giving 5ft to 12.5ft with the lens set at f8. I was told on the previous forum that the third red dot is probably at 50ft for 23ft to infinity, and I'm unsure where the red dot is for 13ft to 20ft. I keep vacillating between where I think the last two red dots are actually focused, so I decided to saw the back off the broken gevabox and put a focusing screen where the film would be on the film insert thing.

I'd also wanted to add a few focusing points just to get better focus.

I had to take the shutter mechanism out as it wouldn't stay open and unfortunately the aperture mechanism was damaged, so I had to take that out as well.

I've put the screen in place, and I'd expected that I'd have to be pretty close to it to be able to check the focusing with a magnifier after focusing the lens.

I am puzzled as to how I have to be about 30cm away from the screen for the image to seem to be in focus, even when the lens is set between 23ft and infinity and I'm pointing at "infinity".

Has removing the aperture mechanism affected anything? Is the fact that the screen is much thicker than the film affecting anything, with my side of the screen not being at exactly the film plane, even though the ground glass is at the film plane?

Any help would be much appreciated.

Yes, I realise that the exercise is probably pointless because my working gevabox operates at f8 and smaller, so specific focusing marks aren't necassary, but I've got the idea bouncing about between my two braincells to do this experiment anyway.

 

[Mos6502]

I am puzzled as to how I have to be about 30cm away from the screen for the image to seem to be in focus, even when the lens is set between 23ft and infinity and I'm pointing at "infinity".

I'm not sure what you're trying to get across here. You mean that your eyes have to be 30cm away from the screen? If this is what you mean, then yes, because your eyes have a very hard time focusing and staying focused on things only a few centimeters away from them. That's why if you look at old SLRs like the Graflex, they have that tall viewing hood. Yes it keeps out the light, but it also puts your eyes far away enough from the ground glass that they can focus easily on it. If you use a magnifier you will be able to view the ground glass from a much closer distance. This has nothing to do with the focal range of the lens. If this is not what you mean, then please reword your question to make it clear.

 

[seany65]

I'm not sure what you're trying to get across here. You mean that your eyes have to be 30cm away from the screen? If this is what you mean, then yes, because your eyes have a very hard time focusing and staying focused on things only a few centimeters away from them. That's why if you look at old SLRs like the Graflex, they have that tall viewing hood. Yes it keeps out the light, but it also puts your eyes far away enough from the ground glass that they can focus easily on it. If you use a magnifier you will be able to view the ground glass from a much closer distance. This has nothing to do with the focal range of the lens. If this is not what you mean, then please reword your question to make it clear.

Thanks for the info. At the time, I was thinking about users of 10x8 View cameras and so on as they have to get close up to the focusing screen and often use a loupe to check the focus, I'd assumed that's what I'd have to do when viewing the focusing screen I'd put in the film plane of the box camera. If my head is 30cm away from the back of the camera, it would be hard for my hand to be able to reach the lens to focus it. I also assumed that the lens would focus the image onto the film plane as that is where the film is, and so my eyes being 30cm away I wouldn't be able to tell if the image is in focus or not.

 

[Mos6502]

So are you using a loupe to view the ground glass or not? You should not have any trouble viewing the ground glass with a loupe. If you are, then you've made a mistake somewhere. If you're trying to view the ground glass with your naked eye, then yes, you will not be able to see it sharply from a very close distance. It's why waist level viewfinders have magnifiers, if you just hold the ground glass up to your eye you (your eyes) won't be able to focus on the ground glass, but when you use the magnifier, your eye will focus just fine from a distance of 4cm or so.

Maybe you should post a picture of what you're trying to do. Or maybe you should just try running a roll of film though your camera? Which would make far more sense. Simply get out a measuring tape, match the distance from the camera to the subject with the distance on the lens, then check the resulting photos for accuracy. It's going to be much more accurate than fussing about with a different, broken camera.

 

[Rob-F]

As I understand it, the problem is with viewing the groundglass, and not whether the camera is in focus, which is another matter, one you won't address until you can first see the groundglass clearly. With normal vision, about ten inches, 30CM, is normal close viewing distance. Nothing wrong there. So let's talk about use of the loupe or magnifier. I have more than one kind of loupe. One is an eye loupe, which I use by holding in my eye socket, and moving in very close, maybe two inches, to the object being inspected.
The other kind of "loupe" is a magnifier that has a focusing mount, usually plastic. This type is placed directly on--usually--a negative or transparency being inspected on the light box. If that is what you have, then for your present purpose you would hold it right on the groundglass, and adjust its focusing mount until you see the grain pattern of the groundglass sharply. With that accomplished, you can then turn your attention to the image on the groundglass and determine if it's in focus.

No worries about the thickness of the groundglass. With the frosted side toward the camera lens, it's in the same plane as the film. That your loupe is on the back side of the glass is corrected for when you adjust the loupe focus to get the groundglass's "grain" to appear sharp; or if using an eye loupe, you hold your eye at that distance.

Hope I understood correctly!

 

[seany65]

Thank you Rob and Mos for the new replies and info. I've not actually got a loupe as yet (Thanks for letting me know there are two types), I was going to use my 50mm nikon lens to see if I could use that to check the focusing, then get a loupe if it proved to be not a good idea.

I've got two Gevaert gevabox 6x9 (type 1?) cameras, one will be the user on which I will have marked extra distance indicators on the front of the lens barrel (Probably utterly pointless as the lens is an f8, but I've got the idea in my head to do it, so...), the other one is a rusty broken one of which I have taken the back off, but it still has the film loading section which is attached to the left panel and that whole thing slides out to allow loading of the film onto it. After cutting off the back of that camera I slid the whole left side back into the camera and will be taping the screen to the rails of the film plane. I will then lay out a long tape measure from either the front of the lens or the film plane and putting a focusing target at the distances where I think the marked distances on the front of the lens barrel are (I think probably 7 1/2 feet (the red dot between the written distances of 5ft and 10ft), 16 feet (red dot between the written distances of 13ft and 23ft), 50ft (which is what I've been led to believe in a post in the box camera thread on the previous version of this forum) could be the distance of the red dot between the written distances of 23ft and infinity. Once I've found them, (to check if my method is good enough) I'll be getting a couple of distances between each of the red dots and marking them on the front of it's lens barrel and then transferring them to the user camera by measuring how far around the lens barrel each new mark is.

All this because the lens barrel can be turned and left at any distance from the minimum to the maximum distances.

And because it's one of MY ideas and thus totally mad, lol.