View Full Version : Have you checked your back? (& plane of focus)

29-Mar-2008, 22:22
I've only been in LF for about five years so I'm still learning every day. Today my lesson was about the ground glass and plane (pain?) of focus.

I just received a beautiful new 4x5 ground glass from Steve Hopf (hopfsteve@gmail.com). I ordered it with no marks - no grid or crop lines, so I can see a clean, bright image without distractions. Installation should be a piece of cake, right? Just swap the new ground glass for the old.

I selected one of the four Graflok backs for my Toyo cameras (45AII & 45G) and removed the six screws which hold in place the two retaining clamp strips. To my surprise, what came out was not just the old ground glass but a sandwich of Toyo factory ground glass plus a Fresnel screen on the film side of the glass. No wonder my image was hard to focus! It was bright, but confused by Fresnel rings (a standard side effect of Fresnel lenses).

Some extended searching on the Web and a little reflection on Newtonian optics lead me to believe that the Fresnel lens has a focal length of its own, effectively modifying the plane of focus as it falls on my ground glass. General advice is that you mount the Fresnel on the side of the glass away from the lens, and that furthermore you really ought to match the Fresnel to the specific lens focal length. Toyo probably understands this and accounted for the change of focus in their construction of this ground glass-Fresnel sandwich. No matter; I donít want a Fresnel so I can ignore all that.

I inserted my new ground glass and screwed things back together. Then I started to think. (Isnít that backwards? I frequently forget to think before I act, instead of after). Will the film-facing side of my new ground glass be at the proper plane of focus? Oh, boy, now Iíve messed up. Iíd already disassembled all four of my Graflok backs to see what kind of glass each contained. How do I re-establish a standard?

Now that I was thinking, that didnít seem so hard. Recently Iíve moved on from Readyloads to sheet film. I bought a bunch of film holders on eBay and then got hold of a very nice set of Grafmatics holders, which are all I use at the moment (and theyíre great.) So, to determine the designed focus plane of the film, I loaded up some discard film sheets, cocked the dark slide, and started measuring. My digital micrometer soon gave the answer. Or answers: 4.75mm, 4,79mm, 4,85mm, 4.91mm, Ö from the contact face of the Grafmatics to the actual emulsion face of the film.

Back to lab methods. I ran six test sheets through the Grafmatics a couple of times, measuring each sheet twice at all four corners. The best I can determine is that this particular Grafmatics is aimed at about 4.8mm depth from contact face to film emulsion. Then I repeated the test on a couple of random Elite film holders, which are much less precise since they are flexible plastic compared to the stiff metal of the Grafmatics. Nonetheless, the measurements indicate about 4.5mm-4.9mm.

With this target in mind, I thought it would be easy to check the position of the ground glass in my Toyo back. I selected one of my discarded glasses and clamped it carefully into the Toyo back, resting on the ground metal edges provided for the glass. Inverting the back to expose theside facing the lens, I again applied my digital micrometer to measure from the ground contact surfaces on the camera back to the lens-facing side of the ground glass.

Wow! 5.05mm, 5.10, 5.08mm, 5.15mm. Hmm. I better check another Toyo back. 4.65, 4.73, 4.69, 4,80. After measuring all four backs twice, carefully remounting the ground glass each time, I discovered that each is different in both its average and its consistency from corner to corner. When I take the trouble to focus carefully with my 4x loupe after swings and tilts, a .5mm change of focus is significant. But whatís the point if the film is not going to be on the same plane as that to which I focused?

So I mounted the Hopf glass into the camera back which offered the most consistent measurements just under 5.00mm. I marked another back as usable and the remaining two as suspect for fine focus. Now Iíll order another ground glass from Steve Hopf. And Iíd love to hear your comments. Is all this unnecessary? Did I do something stupid or miss something obvious? Or is it worth the trouble I took?

29-Mar-2008, 22:54
I bought my Zone VI 8x10 used. It came with a fresnel that was mounted between the GG and the lens. I don't like fresnels so I took it out before I even exposed my first sheet of film...thus moving the GG closer to the lens by the amount of the thickness of the removed fresnel. Several years later I got to thinking -- is my GG still on the same plane as the film? I occasionally have an odd focusing mishap, but I always blaimed it on operator error -- not a misalignment.

So I measured and all seems to be well -- at least by the accuracy of my non-digital ruler. Perhaps that is why the camera was on the used market (Midwest Photo) -- the previous owner might of had focusing problems when he/she mounted the fresnel on the wrong side of the GG.


Steve Barber
30-Mar-2008, 00:07
First, I think you have too many variables and eliminating as many as possible would be the way to begin. Also, I learned the hard way that the manufacturer determines the required distance from the camera reference point to the ground glass, based on the way they assemble the sandwich when they use a Fresnel lens. If the Fresnel, or any spacers associated with it, are moved or eliminated, the ground glass will have to be moved to compensate for it. To start, since you do not know what the original measurements were and you do not want to use the Fresnel, I would take one of the Grafmatics, the one that has the most consistent measurements from the film plane to the contact surface of the camera back, and use it to take a couple of test images. This will establish what the distance of the ground glass from that reference point will need to be.

The image needs to have a number of references, lines, like sighting down a ruler running lengthwise away from the camera or down along the top of a picket fence. There are some good examples of doing this on-line and your searching or googling for them would be worth your time. Anyway, the idea is that, having a number of identifiable reference points, each spaced exactly the same distance apart, you focus on one in the middle of the range, and mark the one you focused on and take the picture. Start by using the distance to the ground glass frosted side, with that side toward the lens, the same as the distance of the film plane to the same reference point of the camera. When developed, the image should show the reference point of focus to be in sharp focus. If it isnít, the distance you have to change the focus of the camera will be the distance that you need to move the ground glass to get it the proper distance from the camera point of reference. When the image on the film corresponds to the focused image on the ground glass, you will have the ground glass the correct distance from the camera back point of reference. Having established that distance, you will be able to determine which of your back/holder combinations are usable. Also, although Iím sure you know to do so, I would suggest that you be sure that your camera standards and shutter are square to your film plane before you focus the image for the test.

30-Mar-2008, 02:48
You have a good point, Duncan. There is a real problem here. I will tell you what I did when I discovered the possible differences. I took a sheet film holder, made a hole through it so that I would be able to see the film in it from behind and was then checking the point of the sharp focus with a loupe right on the film (transparent, developed). You can also scotch a fine tape on the film so that you see the image easier. Like that I discovered that my film holders give me the correct focus but you can also check on any discrepancies. Worked well for me. (I then covered the hole and I still use the film holder.)

30-Mar-2008, 05:15
I just found an interesting article on this subject at http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=005XQp

"The conclusion to be drawn is that the GG ought to be at a distance of 4.8mm from the mating face of the camera back."

Ted Harris
30-Mar-2008, 05:31
If you search the archives here you will find numerous threads discussing this issue.

30-Mar-2008, 05:39
I just found an interesting article on this subject at http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=005XQp

"The conclusion to be drawn is that the GG ought to be at a distance of 4.8mm from the mating face of the camera back."

Duncan, it's not that easy, I'm afraid. There are several norms when it comes to the film holder dimensions and then there are those that are out of any norm and then there are gg+Fresnel composites that make things more complicated, as you have learned. The biodiversity of film holder beasts is significant - depending also on your format. Unfortunately.

30-Mar-2008, 07:46
After measuring I usually double check by doing some wide open shots of "infinity," which, for me, is usually is some very distant landscape. I focus on the ground glass and also 'bracket' the focus on either side of the ground glass (& keep notes on which frame is which).

Leonard Evens
30-Mar-2008, 09:22
It seems to me that what counts in practice is what is recorded on film. To check that you should photograph some target with fine detail mounted on a flat surface making a 45 degree angle with the camera. By examining the image, you can estimate how far the gg image was from where the film was.

Here is how you do it. first, you multiply the distance subject distance appears to be off by .707 to account for the 45 degree angle. You then calculate the magnification by dividing the subject distance by the focal length (using common units such as mm) and taking the reciprocal. If you mulitply the apparent subject error (corrected for angle) by the square of the magnification, you get the error in the image plane.

But there is one thing that should be kept under consideration, the so-called depth of focus. This is the total variation in image distance which will not yield a visible effect. This is usually give as plus or minus the f-number times the maximum acceptable circle of confusion or coc. What you choose for coc depends on your criterior for sharpness in a final print viewed in some designated way. Viewing the gg at 2 X magnification, i.e., form about 6 inches, is equivalent to viewing an 8 x 10 print at 12 inches, a fairly normal distance. (It is not quite the same because of differences between a gg image and a print.) Using a loupe to view the gg at 4 X would be equivalent to reducing the coc by a factor of two. In order to see the same thing in a print, you would have to use a 2X magnifier. Alternately, it would be the same thing as viewing a 16 x 20 at 12 inches.

In practice, it is important to have the film placed close to where the gg image is formed, but there will always be s residual error. Working too hard to reduce that error can be counterproductive, since you may ignore other more important factors affecting sharpness, as well as a waste of time.

Doremus Scudder
31-Mar-2008, 02:48
Low-tech version of the above (trial-and-error method). Photograph a ruler at an angle to the camera close-up and wide open with a relatively long lens (i.e. minimizing depth-of-field). I lay the ruler on a tabletop and point down at it with the camera. Focus, say, on the 6-inch mark. Develop the neg and check under the 10x loupe. If the 6-inch mark is ok, then you are good to go.

If, however, another part of the ruler is more in focus (or the "center" of focus is elsewhere), then you need to adjust the gg. If the gg is farther forward (i.e. closer to the lens) than the film, the point of sharp focus will be farther out (more distant) than the 6-inch mark you focused on. The reverse if the gg is farther back.

Make very small adjustments by adding or removing shims, etc. (this can be the most time-consuming part...). If you are just a little off, a very thin shim (less than a mm) might work. Install or remove a shim and check again as above. Fine tune as needed.

This eliminates measuring and calculating, and can be just as fast if not faster. I usually test my cameras once a year while I am printing and have a tray of print developer handy. I shoot a few sheets and simply hand-develop them in the print developer for about 4 minutes. Then it is stop, fix and inspect.

Best and good luck

Doremus Scudder

31-Mar-2008, 11:48
Not to introduce thread creep, but why did you move from Readyloads to sheet film? Is the film you want to use not available in Readyload? Is it the higher cost?

I've just recently started to do LF photography, and I'm still working things out. I brought sheet film, readyloads and Polaroids out and shot some of each. The readyloads just seem so convenient.