Hi Ralph and Tony !
The ground glass of a view camera often rest on machined ridges (molded
and then machined to get the exact height/position required).
One can buy a simple and inexpensive fresnel on shops for people with
bad eyes. (A chemist of optometrist may have them) I've bought mine for
less than 15 € for an A4 size sheet. As it is thin plastic and
inexpensive (as opposed to Fresnels sold by Wista or the like), you can
cut it to size and ensure it rest not on the ridges for the GG but on
the bottom of the back. This way you have the Fresnel in the "better"
place and do not disturb the GG position. Remember to place the rings on
the GG side, this way you will only have ONE focusing surface and won't
do false focusing...
Bear in mind that a fresnel is a lens, and this will change the
resulting focal lenght of the taking lens... As you add a lens to the
optical system. But this is for purists ;-)
Have a nice day !
BTW, Kaiser and Hama had a few month ago test negatives in 125 and 6x6
size... so you may find some in German photo shops.
DarkroomMagic a écrit :
Tony
One variation in ground glass design is the addition of a Fresnel lens. Its purpose is to provide even illumination over the entire ground glass, making focusing, especially in image corners, significantly easier. The Fresnel lens is a flat piece of plastic, with one side built up from a series of thin concentric rings, which function like a lens. The rings are usually barely perceptible to the naked eye, but become obvious when viewed through a focus loupe.
A Fresnel lens will equalize image brightness when placed either in front of or behind the ground glass, but there are some pros and cons with either setup. When a Fresnel lens is added to an existing camera back, it is far simpler to place it behind the ground glass. The ground glass retains its position, and the alignment with existing film holders is maintained. However, in addition to image formation on the textured surface of the ground glass, it is possible to focus an image on the ridges of the concentric rings of the Fresnel lens. The image formation on two separate surfaces can make accurate focusing difficult.
Alternatively, the Fresnel lens can be added in front of the ground glass. This has the advantage of image formation only taking place on one surface, since the ridges are in contact with the textured surface of the ground glass. However, the disadvantage is that the ground glass, and the associated focus plane, is out of its original position. Consequently, the focus plane is no longer aligned with the film plane, and the camera back must be machined or adjusted to allow for the Fresnel lens thickness. In either setup, make sure that the textured surface of the ground glass faces the lens and is aligned with the film plane, and that the ridges of the Fresnel lens are facing the ground glass.
I advise against placing the Fresnel lens in front of the ground glass. If placed behind, no focus correction is required.
Regards
Ralph W. Lambrecht
http://www.darkroomagic.com
On 2005-11-24 18:06, "Tony Kilbane" <tonykilbane@xxxxxxxxxx> wrote:
Ralph,
I liked your simple method of testing whether the ground glass and
the film plane are within acceptable tolerance. When you have a
fresnel screen in front of the ground glass, a correction must be
made for it's depth. Have you any figure for the depth of the
average fresnel screen?
Regards,
Tony Kilbane.
*-----Original Message----- From: *DarkroomMagic <info@xxxxxxxxxxxxxxxx> *To: *PureSilverNew <pure-silver@xxxxxxxxxxxxx> *Date: *23 November 2005 16:12 *Subject: *[pure-silver] Re: [OT] Filmholders, Septums, Total-Disasters
Yes!
I designed a gage to check film holders (picture on request) and checked over a hundred holders. Many of them are outside of ISO tolerance, new or old, it doesn’t matter. Some brands are better than others. Richard wrote a while back on this list which brands were the worst.
With view cameras, the image is composed and focused on the ground glass. One surface of the ground glass is textured to provide a means for focusing the image. It is important that this textured surface faces the lens, because it is the image forming side. To take an exposure, the ground glass is replaced by the film holder. At this point, the film must be in the same plane as the ground glass was during focusing, so the negative is perfectly sharp. Camera backs and film holders must be machined to tight tolerances to ensure this condition.
A well-focused image and full utilization of the intended depth of field are achieved if these tolerances are close to zero. Small deviations can be tolerated, because the depth of focus for view cameras is relatively large (1 mm or 0.040 inch for a 4x5 inch film format at f/5.6), but even small tolerances will shift the focus and depth of field. It is, therefore, important to keep the ground glass in perfect alignment with the film plane. However, some of our cameras and film holders deviated enough from these standards to warrant a simple check.
In his May/June 1999 Photo Techniques magazine article, Jack East Jr. proposed a simple but effective alternate method to check whether the ground glass and the film plane are within acceptable tolerance.
Place a piece of film into a holder and insert it into the camera back. Remove the back from the camera, and lay it flat on a table. Rest the edge of a rigid ruler across the camera back. Hold a toothpick or cocktail stick vertically against the ruler, lower it until it touches the film and clamp or tape it to the ruler, thereby identifying the film plane location. After doing this with all film holders, leave the toothpick positioned for an average holder.
Now remove any film holder from the camera back, and compare the average film plane with the ground glass location. If the toothpick just touches the ground glass, then no adjustments are required. Knowing that a sheet of regular writing paper is about 0.1 mm (0.004 inch) thick provides a tool to quantify any offsets. If the toothpick touches before the ruler, then you could shim the ground glass with paper, but if there is an unacceptably large gap between toothpick and ground glass, then professional machining of the camera back is required.
With the toothpick still positioned to identify the average film plane location, measure all film holders for variation. According to the standard, a tolerance of ±0.007 inch, or two layers of paper, is acceptable for the 4x5 format. Discard or avoid film holders outside this tolerance.
Regards
Ralph W. Lambrecht
http://www.darkroomagic.com
On 2005-11-23 16:04, "Michael Healy" <emjayhealy@xxxxxxxxxxxxx> wrote:
Is there legitimacy to this guy's claim?
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