Stupid question about Fresnel glass

[norm the storm]

I'm a newbe to LF.

I have a stupid question about Fresnel.
Is this kind of glass should be put over the GG or replace the GG?

What's about the Beattie screen, it replace the GG like I did with a RB 67 in the past.Right?

What are the differences between these 2 types of glass?

Thank you

[darr]

Fresnel lens goes in front of ground glass and does not replace the ground glass. Depending upon your camera setup, removing/replacing the ground glass may interrupt the focusing position. Simply placing the fresnel lens on top of the ground glass should not affect your camera's original focusing alignment.

[Collas]

The Fresnel lens focusses the light onto the ground glass. Due to the pattern of concentric circles (or prisms) acting as a much bigger lens, it increases the light levels across the entire ground glass, making easier to see what coming through the lens. Have a look at an image of the lens used in a lighthouse - there isn't a single lens, but a lot of prisms - but remember that the lighthouse works the other way around, taking a small light source and converting it into a massively powerful beam.. Without it, the light forms a more tightly bunched beam of light (similar to a torch light) that follows the position of your head as you move it around the back of the camera, trying to see what is visible or in focus.

The fresnel is not essential for focussing on a ground glass, but it does make it easier to do as you get an overall view of what the lens is seeing.

Nick

[Sevo]

Fresnel lens goes in front of ground glass and does not replace the ground glass.

... or goes behind the ground glass (from the user perspective - where it affects focusing), or comes ready sandwiched with a ground glass and replaces the one already installed. YMMV - we'd really have to know which fresnel for which camera, as there have been many different products...

[Brian Ellis]

Beattie Intenscreens are Fresnel lenses. Beattie is just a brand name.

I used one in a 4x5 camera for a while. My recollection is that it replaced the ground glass and wasn't placed on top of it. But I could be wrong, it's been a long time since I used it.

There is a ton of information about various bright screens in this forum - how to install, what they do, advantages and disadvantages of using them. If you search you'll find more information than you'll ever need to know. There also is (or at least used to be) a short article in this forum by Q.T. Luong titled "Enhanced Focussing Screens" (the BosScreen discussed in that article is no longer made nor is the Satin Snow screen).

[rdenney]

I'm a newbe to LF.

I have a stupid question about Fresnel.
Is this kind of glass should be put over the GG or replace the GG?

What's about the Beattie screen, it replace the GG like I did with a RB 67 in the past.Right?

What are the differences between these 2 types of glass?

Thank you

Here's the Grand Unified Theory on viewing screens:

We focus on ground glass, always. And that surface has to be effectively in the same optical position as the film. The surface has to be rough enough so that we can see the projected image for focusing, instead of it just passing through. The rough surface may be made by grinding, acid etching (for the better ones), or by some secret process that may involve molding (but only in one case described below).

But, particularly with short and slow lenses, it's hard to see the image to compose using only the ground glass surface. We tend to see easily only the portion of the image that falls on the ground glass directly between our eye and the rear of the lens.

So, to make it easier to see the whole image, we need to gather the light from the periphery and aim it back to our eye. That's where accessory Fresnel lenses come into the story.

Fresnels are usually made of molded plastic and contain concentric rings of prisms that bend the light back to our eye. Depending on the design of the camera, they may be placed between the ground glass and the lens, or (much more commonly) between the ground glass and your eye. If you add one, use the latter method, or you'll have to modify the position of the ground glass to offset the optical effect of the Fresnel--not trivial. Many are detachable accessories, because the Fresnel prisms often make precision focusing using a high-power loupe more difficult, and their makers want you to be able to remove the Fresnel easily for fine focusing. Sinar works that way.

Fresnels alone may not be enough if the lens is short enough or slow enough. So, there have been products on the market designed to improve illumination across the focus screen as an aid for viewing and focusing. Beattie was one such. But there have been others that use a variety of techniques, even stuff as weird as two sheets of glass with a layer of translucent wax in between them.

Currently, only one of those products is readily available, and that is the Maxwell screen. He uses optical-grade plastic and micro-molding to create a screen that is frosted on one side (the focusing surface) and that contains a extremely fine micro-Fresnel on the other side. The Fresnel pattern is so sharp and so fine that it does not hinder focusing, even with a loupe. But because it is plastic, it is fragile, and it is installed with a sheet of clear cover glass behind it, to give you a safe surface on which to rest a loupe, for example. His screens are vastly superior to accessory Fresnels, in my experience, but they are expensive. I probably wouldn't buy one unless I routinely used short lenses (which I do, and thus it was worth it).

So, with most accessory Fresnel lenses, you put them over the ground glass unless the camera was designed to put them under the ground glass, in which case the Fresnel is probably already there. But there are products that incorporate both the Fresnel and the ground glass in single sheet, and one still available on the market.

Rick "hitting a 3-penny nail with a 10-pound hammer, as usual" Denney

[jcoldslabs]

Rick,

Very informative and useful response. I just learned a whole lot I never knew before. Keep swinging that hammer.

Jonathan

[ic-racer]

The Fresnel lens focusses the light onto the ground glass.

Not if it is mounted between your eye and the ground glass :)


- but remember that the lighthouse works the other way around, taking a small light source and converting it into a massively powerful beam.. k


Actually the path of light from a lighthouse bulb, through the plano-convex fresnel (flat side first) to your eye is the same direction as the path from view camera lens, through the plano-convex fresnel (flat side first) to your eye.

[Emmanuel BIGLER]

Stupid question..

Hi !

On this forum, be prepared, technical questions can never be stupid.
Answers, however .. :D

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I have added a small diagram as a tentative explanation on how a Fresnel lens works in addition to a ground glass.
Actually it is not the focal length of the taking lens which is important in properly choosing a Fresnel lens, but instead the actual distance between the exit pupil of the lens and the actual position of the ground glass.
For a view camera lens (of quasi-symmetrical design) focused at infinity, this distance between the exit pupil and the gound glass is extremely close to the focal length.

However for a retrofocus lens design (unknown in large format) the distance between the exit pupil and the focal plane can be significantly larger than the focal length. Hence users of wide-angle retrofocus lenses on a reflex camera hardly ever complain about corners of their ground glass being dark (at least, when they know what a ground glass is ;) )

For a view camera lens (of quasi-symmetrical design) focussed at the 1:1 magnification ratio, this distance is simply twice the focal length and the angle of rays in the corners is reduced significantly.

Hence various configurations are found in real photographic life where a Fresnel lens can be very uselful, or, on the contrary, is of marginal interest.

A Fresnel lens is .. a thin lens element and has a focal length, like any lens. Here we only need to speak about positive Fresnel lenses, but negative Fresnel lenses exist (for example as a wide-angle viewer that you stick on the rear window of a van, or for cashiers in supermarkets, checking that your cart is actually empty).

If you look at an image through an eyepiece, like in a good ol' reflex camera, where the observers's eye HAS to be located a a certain fixed point just behind the eyepiece, the best configuration for the Fresnel lens should consist in re-focusing rays emitted by the exit pupil exactly on the observer's pupil. Optical engineers speak about "conjugation of pupils" a very important additional focusing condition which is generally ignored by amateurs who try to stack several optical systems one upon another ... and do not understand why their field of view on exit is simply: nil.

However in large format we demand to examine the image on all the surface of the gournd glass, hence a compromise which is usually chosen is to take a focal length for the Fresnel that bends the rays in order that they are more or less parallel on exit. This corresponds to a Fresnel's focal length equal to the distance between the exit pupil and the ground glass, hence for a view camera lens of quasi-symmetrical design, a Fresnel's focal lens equal to the focal length of the taking lens.
If the taking lens is a so-called "standard" lens, its focal length by definition is equal to the diagonal of the format, hence the Fresnel should have a focal lenght equal to the diagonal of the format : in terms or relative aperture, this is a f/1 lens.

This extremely wide relative aperture of f/1 is common for Fresnel lenses, even those that you buy for $5, but do not ask them to make good images as a taking lens ! I have such a f/1 A4-size Fresnel lens, focal length = 310 mm, perfect as an add-on for a 8x10 camera (so I' m ready for the day when I'll own a 8x10" camera, but still need the camera ;) )

Relative apertures as high as f/0.7 can be found on catalogues for Fresnel lenses, in principle this is what should be chosen to properly help viewing behind a wide-angle view camera lens.
In fact, the choice of the Fresnel's focal length is a trade-off, and except special "very short" Fresnel lenses (in the very last versions of the optional "film plane" ground glass for the 38 mm Zeiss Biogon, Hasselblad had chosen a Fresnel lens of focal length much shorter than the standard one for 6x6, 80 mm)
Usually the f/1 relative aperture is chosen, i.e. a Fresnel focal length equal to the diagonal of the ground glass (equal to the diagonal of the image format, within a few millimetres)
This is also the kind of setup chosen in overhead transparency projectors (who remembers them today ? ;) )

[EdSawyer]

The Canham fresnels are made by the same place that makes Maxwell's screens, btw.