Focus problems with Maxwell screen

I've been having problems lately with poor sharpness and detail with my prints and I think I've got the problem narrowed down to a Maxwell focusing screen I had installed on my Wista 45DX-II about a year ago. I sent the back from my camera to Maxwell to have him install the screen to be sure the focal plane was accurate. Unfortunately I really didn't do any serious printing with negs exposed since having the Maxwell screen installed before tearing down my darkroom to move(stupid, stupid me!). Now, about a year later I'm finally getting back into some serious photography and my prints are definitely lacking in sharpness compared to the ones I made a year ago. After some troubleshooting I found putting some old negatives up in the enlarger and printing them has me about 95% convinced that the focusing screen is to blame - the old negatives print with the same degree of sharpness and detail as they did before I tore down my old darkroom. They are obviously sharper and show finer detail than the negs I've been shooting with the Maxwell screen.

Before I call Maxwell, anybody got any ideas about how I can strengthen my case with Maxwell? Because the screen is a combination fresnel / ground glass type of arrangement I'm not sure I can just measure the distance from the back of the camera to the screen and compare that to the measurement with a film holder in place. I also think the Maxwell screen is a one piece unit - could somebody verify this before I try to take the thing apart? Unfortunately I can't find my old focusing screen to use for comparison, it seems to have gotten lost in the move.

Anybody got any advice? I know it's my fault for not checking the camera thoroughly after having the Maxwell screen installed. Because I've heard nothing but good words about Maxwell I just assumed it would be ok, which was, admittedly, kind of stupid. I just hope I won't have to eat the cost of the screen.

Set up a test to check focus. The old classic test is a yard stick or ruler set up diagonally in front of the camera. Focus on some mark in the middle of the stick and make a picture, repeat a couple of times re-focusing each time on the same mark. Develop your film. See where on the ruler the sharp focus is. That will answer the question of exactly where your camera focuses. On a larger scale you could shoot a picket fence on a diagonal to get the same kind of test. Hang something on or mark the picket you choose to focus on. A yard stick is usually easier to find.

Even if your camera is not focusing correctly, some point within most photos will be in focus unless the error is pretty large. It could be closer or farther into the scene than you intend but something should be in focus.

Of course this does not include a picture of a near telephone pole against a distant vista. (for instance) If there is nothing in the picture but air where your camera is focused it could look like nothing is sharp - the nearby phone pole and the distant mountains might be unsharp with your camera focused in the empty space a foot past the pole.

So try the test and prove where your camera focuses.

Henry, I'm setting up a similar test right now. I printed out a grid of lines and text and stuck on a piece of mat board at a 45 degree angle. I sure hope this shows something. I have a hard time believing it could be the Maxwell screen but at this point I don't know what else to think. In the last couple of months since I've started shooting again I can't to save my life make a good, sharp negative. All my old negatives are printing fine. This is driving me crazy, it happens with all my lenses. The focus screen is the only variable I've changed.

Any other ideas would be much appreciated.

You shouldn't be approaching this as a "process of elimination" since you can test to see if the plane of sharp focus on the gg is the same as that on a negative. It is or it isn't, so find out by testing. Slightly more time consuming: Take a fast lens like a 210mm f:5.6, put a row of closely spaced but marked or otherwise distinct objects on the top rail of a fence, aim down the line, focus on one of them wide open, take the picture, develop the negative and see if that one is sharp versus the one in front or behind. Even faster technique if you still have the factory ground glass, put a magazine or something with reasonably large print down on a counter. Tilt the camera (not the lens) forward (looking down from above) and focus on just one row of print on the page. You will be focusing close and wide open and you can have just one line sharp this way. Does the Maxwell focus on the same line as the factory gg? If your screen passes either of these tests, I don't think it is the screen. I do question whether the problem is really the screen if you are perceiving a general lack of sharpness instead of noticing the on the negative that the plane of focus is not where you wanted it. I know how easily I can notice the Kalart needing adjustment when I shoot with a Crown Graphic, but then I am generally shooting wider open than I do with a tripod mounted camera. The fact that the plane of focus is in front of or behind where you wanted it is quite obvious in an exposure of something with some depth to it at a distance of 15 feet or so. Good luck.

I have a Maxwell screen, which I installed myself in my Toho FC-45X. The difference between the gg image plane and the film plane in my film holders is negligible. I verified this by testing in the manner Henry suggested. I taped fine print on a wood board about two feet long. I placed it between 10 and 20 times the focal length of my lens at a 45 degree angle to the lens axis. I focused several times on a reference line in the center of the target using a 7 X loupe and set the focus at an average position. In order to do this, you have to be able to mark your focus position accurately relative to the rail or bed. This is best done using the focusing knob, which is usually geared so that larger movements of the knob result in smaller movements along the rail or bed. I then inserted my film holder and made an exposure. I then repeated the whole procedure several more times and made other exposures.

Examining the negatives under high magnification on a light table, I was able to estimate how far off the true focus was from the aimed for focus. Here is how to do that. Measure the actual distance on the target source between the true focus and the aimed for focus. Take 70 percent of that to account for the 45 degree angle. Multiply that by the square of the magnification to obtain the focus shift of the image plane, which is what you are interested in. The magnfication is obtained by taking the distance between the target point and the lens, dividing by the focal length, subtracting one, and taking the reciprocal of the result.

Here is a worked out example. Suppose you are using a 150 mm lens. Suppose you measure a focus shift in the subject of 2 cm or 20 mm. Thake 70 percent of that to get 14 mm. Suppose the target is at 1.5 meters of 1500 mm away from the lens. 1500/150 = 10, so the magnification is the reciprocal of 10 - 1 = 9 or 1/9. The focus shift for the image is 14 x (1/9)^2 = 14/81 ~ .17 mm.

In judging whether the focus shift you measure is significant, keep in mind that you can never focus exactly, even with a loupe. When you look at the gg, there will be an inherent circle of confusion, and that will set a certain uncertainty in the image position. This will be generally twice the f-number times the size of the coc. My estimate for a 7 x loupe using f/5.6 is that the unavoidable focusing error is about 0.16 mm. You can reduce this further by focusing several times, marking your position carefully, and taking an average position. But in doing this, you are also limited in how accurately you can mark the position along the rail or bed. I think it is hopeless with any view camera to position either standard better than 0.1-0.2 mm. So if your measured focus shift is smaller than your inherent focusing error, it is probably not going to be signficant enough to matter.

Good info here, thanks a lot. I haven't printed the test negative yet but from looking it at it I'm now not so sure that the problem is the screen. The problem is not so much a lack of focus but overall impression of sharpness. One thing I'm finding is that because the Maxwell screen is so fine grained that I really must use a loupe to check focus. The old screen that came installed in my Wista (Beattie Intenscreen) was kind of crude, fairly coarse grained and the rings from the fresnel were quite noticeable. However the image would just 'pop' when the focus was on and I never needed to use a loupe. Still, when thinking about it, stopped down to f22-f32 a minor focus shift shouldn't destroy overall sharpness unless shooting at pretty close range. It seems there is some other factor at play here. I appreciate all your help though.

How do you know that your loupe is in focus??? If your loupe was not adjusted for the cover glass and thickness of the new gg then you would not be focusing correctly. Take a #6 pencil and make a very fine line on the inner surface of the gg. Then see if your loupe focuses on the line. If not then adjust the loupe and tape in place! Do not use a linen magnifier - they will not focus in the correct plane of the gg! They are meant to focus on the plane of the bottom frame.

Mike

I have only tried the Maxwell screen in my Rollei SL66 and it was a huge improvement over the Rollei screen. The image does snap into focus. I also think you should consider the comment about an issue with your loupe being properly focused. For years I used the basic Toyo loupe (either 3.6 or 4, I forget which, the basic black tube one with the rubber base) and it worked great. But then as I needed more correction, I found I couldn't focus well with it at all unless I held it off the ground glass, which was a pain. I got the Silvestri loupe, which focuses, but with my different cameras I have to make sure it is focused for the camera I am using. (Even moving between formats, from one Canham to another, I have to refocus it and tape it down.) If it is slightly off I just can't get a sharp picture with it, so I have marked the tube for the right setting for different cameras.

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Originally Posted by Michael Rosenberg

How do you know that your loupe is in focus??? If your loupe was not adjusted for the cover glass and thickness of the new gg then you would not be focusing correctly. Take a #6 pencil and make a very fine line on the inner surface of the gg. Then see if your loupe focuses on the line. If not then adjust the loupe and tape in place...

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I have a Maxwell screen (with gridlines) and am trying to understand the above. :)

What exactly does "inner surface of the gg" mean? Is it the outermost fresnal surface that faces towards the lens? Or do you mean the area between the "protection glass" and fresnal lens?

On the Maxwell, the gridlines are etched(?) on the fresnal facing towards the lens, and that is what I have used to focus the loupe. I wonder if I am focusing the loupe correctly - maybe I'm not and that might explain some of my soft prints.

Thanks

I

Quote:

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What exactly does "inner surface of the gg" mean? Is it the outermost fresnal surface that faces towards the lens? Or do you mean the area between the "protection glass" and fresnal lens?

On the Maxwell, the gridlines are etched(?) on the fresnal facing towards the lens, and that is what I have used to focus the loupe. I wonder if I am focusing the loupe correctly - maybe I'm not and that might explain some of my soft prints.

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Sorry I was not clear. No matter what the screen or gg, the loupe should always focus on the surface that the image forms - which will also be the plane of the film.

I hope that helps.

Mike