Field Camera ground glass design

[Songyun]

Since the other thread is so hot. I am just curious what are the design used in different brand cameras.
For what I know,
1. Shen Hao (old model 45, I haven't played with any new models) offer only ground glass with a slot for fresnel (no adjustment)
2. Tachihara fresnel is between glass and lens (I don't know if any adjustment was made)
3. Canham use the screen has fresnel on one side and ground glass on the other side and a cover glass.

[Gem Singer]

Tachihara and Shen Hao designed their backs with a recessed cutout so that a Fresnel can be mounted between the lens and the ground glass. These are factory adjusted for proper depth.

Tachi includes a Fresnel, frosted on one side, grooved on the other side, with their cameras. Shen Hao does not.

The 4x5 and 5x7 Canham Traditional backs allow an accessory Fresnel and cover glass to be used instead of the original ground glass. The Fresnel is mounted between the frosted surface and the photographer's eye. No adjustment needed.

The later 8X10 Canham backs have a recessed cut out that allows an accessory Fresnel to be mounted between the lens and the ground glass. No adjustment needed.

I have owned all four of these cameras with Fresnels. Still own the Canhams. No problems with focus.

[Mike1234]

I've decided to buy an acid etched focus panel (no grain and no melting parafin) and have at least three fine quality fresnels of differering FL's used on the back side (viewing side) with quick/easy temporary clips to hold them in place and swapped as needed to match the FL of taking lens. It seems like the best of all scenarios.

[Songyun]

Tachihara and Shen Hao designed their backs with a recessed cutout so that a Fresnel can be mounted between the lens and the ground glass. These are factory adjusted for proper depth.

Gem, I just have a question about ShenHao. Since it doesn't offer fresnel as standard purchase. Does the recessed cutout is shallower than the thickness of fresnel to push the ground glass back, or it equals the fresnel thickness?

[Gem Singer]

The Fresnels that I use are made out of plastic and have a frosted surface on one side and Fresnel grooves on the other side.

The Tachihara, Maxwell and the Canham Fresnel's are made that way.

As long as the frosted surface of the Fresnel is placed in the same position as the original ground side of the ground glass, the total thickness doesn't matter.

A cover glass is used to prevent the focusing loupe from scratching the softer plastic screen. Available in plain or gridded.

Using a ground glass as a cover glass will cut down transmission of the light.

Recessed cut outs are usually milled to high tolerances to accept a factory furnished Fresnel, that is placed between the ground glass and the lens, in order for it to focus correctly.

[Tim Povlick]

The 4x5 and 5x7 Canham Traditional backs allow an accessory Fresnel and cover glass to be used instead of the original ground glass. The Fresnel is mounted between the frosted surface and the photographer's eye. No adjustment needed.

The later 8X10 Canham backs have a recessed cut out that allows an accessory Fresnel to be mounted between the lens and the ground glass. No adjustment needed.

Hi Gem,

I have a Canham 8x10 Traditional (recent model) that I placed the Fresnel in the recessed cut out as you mention. I assume the late models one still uses the ground glass, is this correct? Everything seems to work fine, focus / brightness.

It cracks me up when folks look under my dark cloth at this screen and comment that's it's like a computer LCD only better.

_ .. --
Tim

[Gem Singer]

Correct Tim.

The Canham Fresnel for the 8X10 has a clear back instead of a frosted back.

Therefore, the light doesn't pass through two frosted surfaces.

[RichardRitter]

The most common way is to put the Fresnel lens (clear) on the ground glass closes to the viewer eye. This way the image is focused on the ground surface. The problem with putting the Fresnel between the lens and the focusing area is with very short and very long lenses focusing starts to shift on to the ridges of the Fresnel lens. Causing a focusing problem. Problems also crop up when using swings and tilts with the Fresnel lens on the inside of the camera. Cameras that have the Fresnel on the inside of the camera often have very little in the way of movements and are limited to the type of lens used on the camera.

Fresnel lenses can be made to different focusing lengths. The length used for 4 x 5 camera covers lenses from 90 mm to 270 mm lenses with little to no problems. Some times problems accrue with a 90 mm or a 270 mm lens when tilt and swing are used. Having the Fresnel lens on the outside of the camera makes it easier to remove. Sinar camera backs are made so the Fresnel lens can be removed very quickly.

Best thing to do is keep the ground glass clean and use a very dark dark cloth big enough to wrap around the camera back and seal out all the light.

I took the Fresnel off may camera 15 years ago and have not missed it at all. I was always having problems with it when using a 305 mm lens with swings and tilts.

[RichardRitter]

There is on more thing that gets miss informer from time to time that putting a Fresnel lens on the camera can produce up to a stop more light to see the image with. This is false. The lens is only projecting “x” amount of light onto the focusing area. This does not change when you replace the ground glass with a Fresnel. The only thing that changes is that the Fresnel lens makes the image on the ground glass look evenly lighted.

The reported increase of viewing light out put by others can be for the following reason.
1 Did the testing wrong.
2 Ground glass was dirty, poorly made, made on the wrong type of glass or some other problem and replace with a Fresnel that was a ground glass and Fresnel lens in one.

Fresnel are made by cutting the lens into optical grade plastic. This type of plastic is what they make glasses that ever one wears. The optical transmission is about 2% more then what is used to make the standard ground glass. But most people offset this 2% gain by putting the wrong type of cover glass on the Fresnel. The only way to save the gain is to buy an optical grade cover glass.

Dirty ground glass can cut the see-able light by up to 15% or more.

[Mike1234]

Richard... is your ULTIMATE solution to use a non-frosted fresnel on the focusing side of a super-fine ground or acid etched screen? Furthermore, if one has a vast range of lenses, one should have at least three fresnels from which to choose... one for 38-75mm lenses, another for 90-200mm lenses and yet another for 300-600mm lenses? I'm just trying to get this straight in my feeble mind.