Camera fresnels?

[Ed Richards]

Back or front of the gg? My Sinar fresnel clips on and off the back, so thickness does not mattet. Isn't the Horseman back the same as a Sinar? Wonder if a Sinar would clip in yours?

[Struan Gray]

In general, book magnifiers (which I use) are floppier, use a less transparent plastic, have coarser grooves, and have to be cut to size. They also only come in a limited range of focal lengths.

Edmund Optics sells quality Fresnels, and for single items you don't save much over a photographic one. They are a good option if you want an oddball focal length for a particular lens, but for regular use I don't see an advantage over the camera-branded Fresnels.

[Brian Ellis]

In my experience there's certainly been a difference among different Fresnel lenses in terms of ease of focus and light fall-off. I've used the Beattie Intenscreen, the Maxwell screen, and OE Fresnels on Ebony, Tachihara, Deardorff, Chamonix, and probably other cameras I no longer remember. The Maxwell screen was by far the best - almost no difference in focusing between it and a plain ground glass, virtually no light fall-off even at the edges with a lens as wide as 80mm. The Ebony screens were the worst of the OE screens, harder to focus, quite a bit of light fall-off around the edges as the lenses got shorter. The others were somewhere in between. Unfortunately the Maxwell screens are very expensive.

[CP Goerz]

Second for Brian on the Maxwell, great screens that give a nice smooth image.

[Ari]

I like the fresnel/gg combos, i.e. just one screen, usually made of acrylic.
They are very bright and contrasty, require only a plain glass or acrylic overlay, and don't break easily.
Wista makes the best one by far, but they are $300+ new; the Tachihara screen might still be reasonably priced.

[rdenney]

In my experience, there are three attributes of a Fresnel for viewing: Focal length, spatial frequency, and clarity. The first two are set by design, but clarity seems to be an outcome of molding method and material. The finer the frequency, the greater the clarity needs to be.

Most accessory Fresnels seem to be finer than book magnifiers. I've compared the Sinar to the Ektalite to the no-name brands sold on ebay to the general-purpose Fresnels sold by Edmund Scientific. They all look pretty similar to me, though they vary in focal length somewhat. These seem to be focused to provide even illumination for a central eye position using normal lenses. But most people want to use them to make it possible to see the corners clearly when using short lenses, not necessarily with a central eye position. My Sinar Fresnel is too long--it causes ghosting and double-images in the corners with lenses of 65mm and shorter.

I've never inspected a Beattie screen, but I gather it claims superiority on the basis of a finer frequency--the rings of the Fresnel are closer together.

I do own a Maxwell screen, and I can confirm that its frequency is MUCH finer than my other Fresnels. And the molding is so precise that the lines normally seen, especially at smaller apertures, tend to disappear altogether. I have no problem whatsoever focusing through it with any lens, even using a 6x or 10x loupe. That confirms Ari's experience--the Maxwell is a single molding--focus screen facing forward and micro-Fresnel facing the user, with a sheet of clear cover glass for protection. But I think I'd have to inspect the Wista and Ari would have to inspect the Maxwell (which is about the same price) before either of us could make the assertion that either one is "by far the best".

Some focus screen improve evenness using higher dispersion rather than a Fresnel. The Satin Snow (I think) screens were made with two sheets of glass surrounding a highly dispersive wax layer. The evenness was better, but at the expense of overall brightness. The Maxwell gets the evenness without giving up the increase in brightness provided by the Fresnel.

My experience is also that the finer the Fresnel, and the more finely made it is, the broader the lens focal lengths that will be served by it. My regular Maxwell screen works well even down to 47mm with no ghosting or other artifacts, despite that Maxwell also has a special version intended for very short lenses.

Rick "all of whose SLR cameras--in a range of formats--have Fresnels with no apparent effect on focusing accuracy" Denney

[Ari]

Well said, as usual, Rick; I do prefer the Wista to the Maxwell (just for the slight gain in contrast), having used both.
I should also clarify that the Wista is the best screen I have ever used or tried out, but there may be a better screen out there that I haven't yet tried.

[Brian C. Miller]

Rick "all of whose SLR cameras--in a range of formats--have Fresnels with no apparent effect on focusing accuracy" Denney

With the SLR cameras, the focusing screen design is integral to its manufacture. The problem with LF cameras is how all of it was designed in the first place.

For my Super Graphic, the design was to place the Fresnel lens between the GG and the camera frame. What happens is that when a non-Graflex replacement is used, the focal plane of the GG may not be anywhere near the actual film plane. This was the case with my camera, because at some point the GG was replaced with just a generic sheet of GG. This threw off critical focus for everything. Beings that it's large format and it was my first LF camera, I didn't know. But I found out.

The Brightscreen Fresnel and ground glass combo put the focal plane over 10 mils forward of where it should have been, due to the thinner plastic used. It took some measuring with a dial caliper to figure out what had to be done and several sheets of film. Final verification was conducted using Techpan.

[rdenney]

For my Super Graphic, the design was to place the Fresnel lens between the GG and the camera frame. What happens is that when a non-Graflex replacement is used, the focal plane of the GG may not be anywhere near the actual film plane. This was the case with my camera, because at some point the GG was replaced with just a generic sheet of GG. This threw off critical focus for everything. Beings that it's large format and it was my first LF camera, I didn't know. But I found out.

No argument--but I think that Graflex was the only commonly available large-format manufacturer to put the Fresnel in front of the focus screen. At least I haven't seen it that way on the other cameras I've owned or handled. I'm also not sure that's the relevant issue for Kirk, or what drove him to pose the question he did.

Rick "focus surface positioning is critical no matter what" Denney

[Arne Croell]

Some focus screen improve evenness using higher dispersion rather than a Fresnel. The Satin Snow (I think) screens were made with two sheets of glass surrounding a highly dispersive wax layer. The evenness was better, but at the expense of overall brightness.

Rick, that wax layer one was the Bosscreen, made by a Dutch company, not Satin Snow (which was really a "ground" glass).