Question regarding fresnel lens...

[Al D]

I just took the plunge and received my Shen Hao HZX 45 II A yesterday.

Having heard that the stock ground glass is quite dim, I purchased a fresnel. The fresnel arrived as two pieces - the plastic fresnel and the glass with grid lines.

This is an improbably silly question but can somebody confirm that the plastic fresnel is the component that should be closer to the lens and the glass cover should lie on the outside?

Perhaps my stupidity is mitigated (if only slightly) by the fact that my lens has yet to arrive so I can't actually see an image on the ground glass or fresnel at this stage...

Thanks!

[Louie Powell]

Al -

I purchased a fresnel for my Zone VI - it was also plastic with a cover glass. I did quite a bit of research on the web and found this useful information:

http://www.intenscreen.com/pdf/INSTALLATION_GUIDE_FORVIEW_CAMERAS.pdf

Note that the fresnel is nearer the lens, while the cover glass is a protective layer that goes between the fresnel and the clumsey photographer. More importantly, note that whether the grooved side of the fresnel faces the lens or the camera back is brand-specific.

[Frank Petronio]

While I suspect the plastic is meant to be a protective cover, there seems to be no universial consensus on where to put the fresnel and experience proves different with different cameras. Until you hear from some one with your camera and your fresnel, stick with the stock ground glass to start with. You don't want to accidently introduce focal plane placement error before you learn how to use your camera. Once you are comfortable with your outfit, then you can make actual film tests to determine (or confirm) that your fresnel is placed optimally.

What make fresnel did you get? Maybe someone here has the same combination?

[Chris S]

My Shen-Hao HZX 45 II A has the fresnel cover glass towards the photographer and the fresnel itself towards the lens

Chris

[Phong]

There are 3 fresnel configurations I know of:

1. Some fresnels are meant to be used between the lens and the ground glass

2. Some are meant to be used behind the ground glass (lens, then ground glass, then fresnel)

3. Some are meant to use without a ground glass

In configuration 2 and 3, a protective glass is often used behind the fresnel. The standard 4x5 Tachihara uses a configuration 3 fresnel with a protective glass, as does a 8x10 fresnel I purchased recently from Bromwell Marketing. I have also used a 8x10 configuration 2 fresnel (Arca Swiss) with protective glass, (lens, ground glass, fresnel, protective glass) which makes for a relatively heavy combination. I have also uses a fresnel without ground glass or protective glass on a4x5 Gowland for a superlight outfit.

In any case, it's important to know what your particular fresnel is designed for to ensure image plane registration. In configuration 1 and 2 the image is meant to form on the ground glass; in configuration 3, the image is formed on the fresnel.

Hope that helps,

[Alan Davenport]

Short answer: unless your camera came from the factory with a fresnel on the inside, then it must always go on the outside. No exceptions.

There is one -- and only one -- thing that is critical for accurate focusing: the location in space of the frosted surface of the groundglass. IOW, the surface where the actual image that you use to focus, is formed. The correct location for the ground glass surface is in exactly the plane that will be occupied by the emulsion of the film when you load. If a fresnel is placed between the groundglass and the lens, then the manufacturer has to adjust the location of the GG to compensate for the focus shift induced by the thickness of the fresnel. I'm sure those manufacturers who chose to put their fresnels inside the GG, did so to protect the delicate surface of the fresnel, since there is very little technical justification for doing so and a lot of fuss and bother created as a result.

So.... if your camera came from the factory with a plain groundglass, then the fresnel MUST go behind the groundglass (away from the lens.) That way, the focusing surface will remain where it belongs. If you put the fresnel inside the camera, the groundglass surface will no longer be in the same plane as the film, and unless you remachine the camera back to compensate you will introduce focusing error. Putting a fresnel on the outside of the GG will not change the focusing of the camera in any way; the fresnel is only there to make the groundglass image brighter across the screen for you to compose.

BTW, the Tachihara (that's what I own) uses a plastic focusing screen with a frosted surface toward the lens, and the fresnel molded into the outside surface. Then a protective glass sheet outside that.

[Scott Davis]

Alan-

so you're saying that all the film I've shot on my Shen Hao over the past five years must be out of focus since I've got the fresnel on the inside?

Al D -

I have the same camera. Whose GG/Fresnel combo did you get for your Shen Hao? I got a Canham Fresnel/GG combo to replace the Shen's factory GG. The Shen screen is rather dark by comparison. What is most important with a fresnel is that the FLAT side of the fresnel go where the focusing plane should be. Therefore, the fresnel/gg sandwich should look like this:

fresnel(flat side toward lens, grooved side toward photographer):Ground Glass - frosted side toward fresnel, smooth side toward photographer. When in doubt, consult the manufacturer of your fresnel and ground glass set for installation instructions.

[Michael S. Briggs]

Alan's answer was excellent. Camera's can be configured to have the ground glass or the Fresnel closer to the lens, but the configuration cannot be trivially changed from one to the other without effecting the focus accuracy. In many cases the depth of field will cover up a focus problem.

As Alan said, the ground surface of the ground glass is the reference surface for focusing and should be in the same plane as the film. (There are some integral screens with combined Frensel and ground surface, or the Bosscreen, which uses wax.) The ground surface makes visible the focused image, scattering the rays, some of them towards the photographer's eye. If the image is not in focus at the position of the ground surface, the image will look soft and out of focus. Not all of the rays are scattered towards the photographer's eye -- the purpose of the Fresnel is to direct a greater fraction of the rays to the photographer's eye so that the image will appear brighter. The flat side of the Fresnel is not the reference surface for focusing.

If you have a camera with just a ground glass that focuses accurately, as we hope a camera from the manufacturer does, and you wish to add a Fresnel, you should add the Fresnel on the photographer's side. Adding the Fresnel on the lens side of the ground glass will cause two problems. The first is that it will normally mechanically shift the position of the ground glass, due to the manner that the screen rests on stops in most cameras. The second is that introducing a Fresnel in front of the ground glass optically shifts the focus by about 1/3 the thickness of the Fresnel.

It is possible to have accurate focus with a Fresnel in front of the ground glass, but these effects must be accounted. The ground surface of the ground glass won't be exactly at the position of the film. Some manufacturers design their cameras in this manner, probably to give greater protection to the plastic Fresnel lens. On such a camera removing the Fresnel would introduce a small focus shift unless the position of the ground glass is adjusted.