Ground glass distance from film plane

I'm building a 4x5 wooden field camera and I need to know the

distance the ground glass is from the film holder film plane. Any

help would be appreciated. This is the last step in my project,

looking forward to getting out in the field with it.

In order to achieve correct focus at the film plane the ground glass needs to be in

registration with the film holder plane. Sometimes if a fresnel or in my case on a Graflex

Pacemaker Speed Graphic an ektalite screen, has been installed in the wrong order the

groundglass wont be in registration with the film holder/plane. This causes focus errors.

So to answer your question, the gg MUST be in registration with the film plane.

i.e. registration meaning same place, same plane, no distance between them. The film replaces the ground glass' position as the focusing surface. That is how things stay in focus. Presumably.

Greg:

 

As I understand it, you are asking for the distance that the film plane is from the plate in the camera body where the film holder rests.

 

This is the identical distance that the GG is to the camera body. Somewhere in the recesses of my mind I vaguely remember this distance to be 0.191". Best is to measure the distance from the outer surface of a film holder to the film location, i.e: insert a negative in a film holder and then measure the distance to the outer surface of the holder.

 

This distance is the same as the ground glass registration. The "frame" that holds the ground glass should contain the glass at this identical distance, otherwise good focus will not be achieved.

Does a frenel in front of the gg require a correction?

Yes, a Fresnel in front requires a correction, but the correction is smaller than the Fresnel thickness by a factor dependent on the index of the Fresnel's material. Unfortunately, I've forgotten my high school optics well enough I'd have to take a half hour or more to rederive the calculation, but for typical optical plastics used for Fresnel lenses I recall you need to use an offset of about 1/3 of the actual thickness.

 

This is because light rays entering an optical medium are bent "toward the normal" -- that is, the angle between the ray and a perpendicular to the surface at the point of contact is decreased by a factor dependent on the refractive index; this cause convergent rays to "focus longer" and moves the focal plane away from the lens by a fraction of the thickness of a flat plate. Add back the thickness, however, and the front surface of the Fresnel needs to be closer to the lens than a plain GG surface would.

<p>As Bruce said, the film plane and the ground glass need to be in the same plane. The relevant surface is the ground surface, which should be on the lens side. In answer to Ronald's question, my previous answer was for the case of a plain ground glass, or a ground glass + Fresnel with the Fresnel on the photographer's side of the ground glass. If the Fresnel is on the lens side, a correction is necessary because the optical thickness of the Fresnel lens is not the same as it mechanical thickness.</p>

 

<p>The word is that the ANSI standard for the depth of 4x5 filmholders is a depth of 0.197 +/- 0.007 inches (e.g., <a href="http://home.earthlink.net/~eahoo/page8/filmhold.html">http://home.earthlink.net/~eahoo/page8/filmhold.html</a>).

No doubt 0.197 inch was selected as 5.0 mm. What you really care about is the front of the film (or maybe a few thousands of an inch into the film), so allowing for this, the ground glass should be about 0.191 inches deep compared to the front of the film holder -- agreeing with the value that Diwan remembered.</p>

The distance from the surface of film holders to the film surface is exactly 5 mm. If you make the distance from the surface, where the holder sits, to the glass 5 mm, you will have no problems. The ground side must face the lens and watch out if you use a fresnel as they sometimes vary in thickness between 1.5 and 2 mm. The fresnel`s groovy side is placed against the ground glass`s ground side, but remember the distance to the glass must still be 5 mm!

Cheers.

I have a frenel on my Zone 6 and wanted that brought out to those asking the original question.

 

Thank you.

Does your Zone VI have the fresnel between the lens and the groundglass, or on your side of the GG?

Typically these days the fresnel is placed outside of the camera, with the grooved side facing towards the ground glass. In other words, it is put on the smooth side of the glass. The rough side of the glass must always face the inside of the camera.

 

If you have an old Graflex (Speed or Crown) the fresnel is usually applied to the inside -- ground side -- of the glass. The ground glass holding frame was machined to accommodate it this way, so remove the fresnel from one of these at your peril -- or shim the glass. (This seems to have happened only on the later models which came equipped with the Graflock back, but I might be wrong about this.) The Graflex cameras are the only ones I've seen that were configured this way.

 

I refer for this data to 1) Ansel Adams, "The Camera"; and 2) the ANSI specs, numerous other posts on this list, the Cameramakers List, and various other sites; as well as 3) my own experience working on and measuring many and various view cameras.

 

Hope this helps.

There are tons of oldie 4x5 woodies with great craftsmanship with complete backs on auctions if you wish to preserve the wood look and may even offer all the hardware to make it flip from horizontal to vertical.and 1 ruined session would cost you more?.

 

I customized a deardorff years ago for myself but when it comes to wood camera backs it is sort of like 35mm film cassettes, they are out there dirt cheap and standard . technical parts can go a long way so that while something is home made you can still maintain all the tech details without the effort and you are ensured they will be to acceptable professional standards.

 

you might even find that such oldie has many parts you can adapt to your project and enhance it dramatically not to mention movements and bellows etc. that way you can end up with a dream camera.

 

You might prefer to take on the entire challenge but alternatives are out there and with some of the earlier American cameras have great woods and the aging of the wood is sort of important as part of the answer you seek to ensure the film plane will not vary with the weather or over time.

 

No matter what you do if you make your own, make sure you use velvet type light traps on the film holder side , the lack of them was/is a problem , and I have bought some poorly made cameras that used foam rubber as light traps and had disintegrated from the friction with the film holder in a few uses.

 

You might be the Faberge of woodworking in which case you might make the best back but there are alternatives out there in case you wish that.

best of luck

<p>The ground surface of the ground glass tests for where the image. If both the ground glass case and the film case have only air between the lens and item, then it seems obvious that the ground surface and the film should be located at the same place. For the ground glass, this case implies that the ground surface faces the lenses and that there is no Fresnel.</p>

 

<p>I just measured a modern 4x5 Lisco holder and a sheet of 4x5 film. I got a holder depth of 0.196 inch, which is much closer to the nominal value (0.197) than required by the ANSI spec. This verifies that the 5 mm / 0.197 inch figure is for an empty holder. The piece of film that I measured was 0.007 inch thick. So you want the ground glass to be 0.191 inches behind the reference surface that will position the holders: 0.197 depth - 0.007 total thickness + 0.001 guessed to place the image in the emulsion.</p>

 

<p>Re a Fresnel lens in front of the ground glass: a simple derivation using Snell's law shows that a uniform thickness medium inserted into a converging beam shifts the focus point farther from the lens. Using the small angle approx, the focus shift for a medium of thickness t and index of refraction n is t (1 - 1/n). Guessing an index of refraction of 1.5, this becomes t/3.</p>

 

<p>For a thin enough Fresnel this shift might not cause problems, being lost in the depth-of-focus and in the accuracy to which a photographer can focus the camera. For a thick Fresnel, if the ground glass were not repositioned, there could be a significant focus error.</p>

 

<p>Some previous threads: <i>Ground Glass Test?</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005mjT">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=005mjT</a>,

<i>Calibrating the ground glass to the film plane</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=008cpD">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=008cpD</a> and <i>Cheapo fresnels and g.g. offset</i> at <a href="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00CIgP">http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00CIgP</a>.</p>

Mr. Klabouch:

 

After reading all the thread, it seems to me that your best solution is to:

 

Buy a Graflex pack film holder in your favorite auction site ( I got one for $.99), get a glass from a 5x7 picture frame, apply your fine sharpening stone and oil for a while ( one concert and a half on your stereo ) to grind the glass. This way you avoid buying grinding compound and making a mess with it.

 

Then, insert the glass inside the holder, the spring inside the Graflex pack will take care of holding it. Presto ! You have a GG device with the correct registration and with the advantage that it is Graflok back compatible.

 

I have done this. Drop me a line if you want more details.

 

Thank you.

Thankyou all for the input, I got the idea now, just needed some discussion on the topic to picture it more clearly-now to make a gg.