Fresnel Focusing screen

[hankchinaski]

Hi,

I know what a ground glass focusing screen is,
I know a focusing screen with micro prisms, I know a split image focusing screen.

What about a fresnel? Is it in addition to the above? You put a fresnel on top of the above? Instead of? Can you get both (ground glass and fresnel for example) in one glass? How does the focusing work?

Thanks for clarifying the issue for me.

 

[Alan9940]

For LF cameras, a fresnel typically sits on top (behind) a standard ground glass screen. This would be a DIY install. There used to be a few makers of bright screens for LF cameras (Maxwell comes to mind and may still be available), but no clue how exactly they're made. You focus on the ground glass combo like you would if the fresnel wasn't there. The downside to a fresnel is that the screen becomes very dark as you move your viewing angle off-axis.

 

[hankchinaski]

For LF cameras, a fresnel typically sits on top (behind) a standard ground glass screen. This would be a DIY install. There used to be a few makers of bright screens for LF cameras (Maxwell comes to mind and may still be available), but no clue how exactly they're made. You focus on the ground glass combo like you would if the fresnel wasn't there. The downside to a fresnel is that the screen becomes very dark as you move your viewing angle off-axis.

Thanks - but what’s the advantage?

 

[AgX]

In general the idea is to get as much light from the groundscreen into the eye of the photographer, whether he looks directly at the screen (LF) or via an eyepiece (SLR). To achieve this an added lens is needed. It is generally called a field lens. This lens is best calculated for the respective focal lens of the taking lens, the courseness of the screen, but also for the circumstances the screen is looked at.
And for mere practical reasons (thickness), most often not a bi-convex lens is used, and no plain plano-convex lens, but a plano-convex fresnel lens.

 

[hankchinaski]

Thanks!

 

[koraks]

Thanks - but what’s the advantage?

Simply put: more light especially towards the edges and corners without the need to view the ground glass in a very awkward angle. It's easier to see the entire image all at once. Especially with wide-angle lenses, a fresnel is very convenient. And on anything larger than 4x5" too.

 

[hankchinaski]

Simply put: more light especially towards the edges and corners without the need to view the ground glass in a very awkward angle. It's easier to see the entire image all at once. Especially with wide-angle lenses, a fresnel is very convenient. And on anything larger than 4x5" too.

Thanks for the explanation.

 

[AgX]

At SLR's various constellations were applied.

A classic solution was to use a plain (thus thick) plano-convex lens which plano side was ground. This approach shrunk so to say by using a fresnel lens instead into the thin plastic screen, with at one side the ground area and the focusing aids and on the other side the fresnel lens.

 

[nosmok]

I had an 8x10 with a fresnel screen and I couldn't hang with it, didn't like the off axis thing at all. Then I talked myself around in circles about how would the focal distance shift if I just took out the fresnel (as opposed to swapping in a new back), and it made me so agitated I sold the camera. Have not regretted that in the slightest-- not all technology is good for me, as it happens. A better dark cloth, a slightly faster lens (hard to do sometimes, I know) solved the problem, without viewing axis problems and nagging worries about where exactly the focal plane is.

 

[Doremus Scudder]

Fresnel screens are collapsed flat-convex lenses; the convex surface reduced to rings with the same curvature, but without the intervening depth. https://en.wikipedia.org/wiki/Fresnel_lens

For ground-glass viewing purposes, they can re-direct the collimated light from the lens so it hits the eye at a better angle for viewing. The problem is that Fresnel lenses is that they have a focal length, which, ideally, would be matched to the focal length of the taking lens for optimal viewing. Most OEM Fresnel lenses on view cameras are optimized for "normal" lenses, so viewing can be more difficult with very short and very long lenses.

That said, I love the OEM Fresnel screens on my Wistas and my Graphic View cameras. I have other cameras (Horseman, Alpina, Zone VI) with just the frosted ground glass. That works fine too as long as you move around with the hot spot and have a loupe you can tilt to view in the corners.

The main problem with Fresnel screens that aren't supplied by the manufacturer is the risk of focus shift after installing them. After-market Fresnel screens should always go between the ground glass and the photographer's eye to avoid this.

Best,

Doremus

 

[AgX]

Fresnel screens are collapsed flat-convex lenses; the convex surface reduced to rings with the same curvature, but without the intervening depth.

Not quite.
There are flat-concave Fresnel lenses too.

And bi-sided Fresnel lenses are imaginable at least, but I do not know of such being offered.

 

[Doremus Scudder]

Not quite.
There are flat-concave Fresnel lenses too.

And bi-sided Fresnel lenses are imaginable at least, but I do not know of such being offered.

Of course. I just don't think that they are used for viewing-screen applications.

Doremus

 

[AgX]

You are right. My point was just that you made a generalizing statement. One tends to reduce fresnel lenses to one certain, most common, type. There even are fresnel mirrors.

 

[Terryro]

I have a screen for my 1956 Rolleiflex. It's labeled Rolleigrid and used to brighten the image at the edges of the screen. It works nicely for me. I have old eyes and trifocal glasses so any improvement is greatly appreciated.
I guess it's a user preference.

 

[DREW WILEY]

My favorite fresnel screen for large format use is NONE. Hate those things with their distracting lines. And sometimes in damp cold weather, condensation can occur between them and the ground glass itself, hard to clear up. But they can be useful for very wide angle lens usage to even out the central "hot spot" during viewing.

 

[ronw]

@DREW WILEY thanks for sharing your experience, the lf camera i just got has one and wondered if it was a good idea or not

 

[xkaes]

Some people love them and some people don't. Do you prefer blondes or brunettes? I would not get rid of my intenscreen. Just my preference. I do a lot of wide-angle stuff.

 

[ronw]

@xkaes thank. guess i'll try with the 90/4.5 once i get it and see. cheers.

 

[xkaes]

Try it with and without. Then decide if "Gentlemen prefer blondes".