Cheap Plastic Fresnel "Page Magnifiers" on Ground Glass - Try it, you'll like it!

[holmburgers]

The topic of using cheap Fresnel lenses on one's ground glass comes up often and here is some photographic proof that it does in fact WORK!

All I had to do was cut it to size and put it between the g.g. & the lens, ridges facing the lens. However, ridges facing the g.g. didn't make any noticable difference.

As you can see it makes a big difference, and it would be much more apparent if I had used a more evenly lit subject. Notice that a lot of the apparent vignetting is actually due to the subject.

The biggest improvement was not in brightness per se, but in off-axis viewing.

No special mounting or adhesive was necessary, the flimsy page magnifier is rigid enough to sit nicely in place and not buckle. Also, I haven't measured it with a pair of calipers but the thickness is no more than a credit card, probably considerably less. Let's say it's between 10 and 30 mils; do the math and see if that would affect your focus plane appreciably.

For less than $5 you can considerably improve your ground glass viewing. My next goal is to find a cheap method to add an anti-reflective film to reduce reflections from the viewing side. Any ideas?

 

[L Gebhardt]

It also has a focal length, so it will affect your focus. Put it behind the ground glass and you will get the same benefits, but no focus shift.

My Chamonix came with the fresnel incorrectly mounted. It wasn't readily apparent that the focus was off for most shots since I had stopped down to f/22 or more, but all shots were softer than they should have been upon very close examination.

 

[holmburgers]

On other fresnel screen installation instructions that I've seen some say behind, some say in front, depending on the camera. Why would that be?

I didn't notice any focus shift at f/4.7 and close range with this setup however. But thanks for the heads up; it's definitely worth inspecting closer.

 

[Toffle]

I picked up a couple of these sheets last year, but have held off for the time being. There was quite a long (and sometimes bitter) debate on LFinfo regarding the supposed focusing problems with the Chamonix fresnel. Have you noticed any difference in the sharpness of your images?

(While I was typing, my question was asked and answered... oh well... )

Cheers,

 

[scheimfluger_77]

putting the magnifier in front of the ground glass will change the focus point 'in theory'. however remember that as you stop down the "depth of focus" at the film plane also increases, so that should make up for the slight distance change. Beyond that, any degradation of the ground glass image would probably be due to the optical quality of the magnifier. At the prices of these sheet magnifiers you can count on marginal optical quality, at least as it relates to photographic optics.

 

[ic-racer]

Behind the GG for me. This one is pretty thick plastic. Acts as a GG protector also

 

[wildbill]

I've been using them on my cameras for several years. They work wonders. There's a guy that's been selling these as 8x10 fresnels on ebay for quite some time. Cut to fit your camera for about $60 more than the version you get at staples. Quite a racket.

 

[L Gebhardt]

On other fresnel screen installation instructions that I've seen some say behind, some say in front, depending on the camera. Why would that be?

I didn't notice any focus shift at f/4.7 and close range with this setup however. But thanks for the heads up; it's definitely worth inspecting closer.

If the camera is designed for a fresnel it will have the GG in a different plane than one designed to not use one. So the directions will depend on the camera design.

When i was testing my Chamonix I first tested close up and didn't notice a shift with a few casual shots. However further experimentation proved I was wrong.

If you do move the fresnel to behind the glass you may want to protect it from scratches. I use a sheet of mylar and hold it and the fresnel on with scotch tape. Looks sloppy, but works fine and is as bright as when it was in front of the glass.

 

[rknewcomb]

Ok, so the answer is....? Put in between your eyeball and the groundglass side nearest your eyeball for no focus shift?
thanks!
Robert N.

 

[holmburgers]

That's what L Gebhardt is saying and it seems like that would be a safe bet. But I'll say, in my limited testing, the difference appears to be nominal.

I'd really like to tackle the calculation, but I'm not sure if I'm up to it at the moment... still sipping my coffee.

Let's imagine we have a 135mm lens at f/4.7 (Raptar, Optar, Graftar) and our subject is only 2 feet from the lens (a fairly critical circumstance I would suspect). What difference in the subject distance would be necessitated by a .003" change at the plane of focus; again, 3 mils being a liberal estimate of the fresnel thickness.

 

[Ken Nadvornick]

I gave this a try today. Went to Staples (an office supply place) and got one of these for only $3.99:

Returned home and set up the 8x10 in the yard and pointed it at a mixed sun/shade scene. Took a careful look at the normal ground glass image, then overlayed the fresnel screen on the back of the GG. (Fresnel placed between my eyes and the non-frosted back side of the GG, concentric grooves touching the GG and facing forward towards the lens.)

What a remarkable difference!

As expected, the center spot was no brighter. But the off-axis periphery areas just lit up. It was so nice that I went and found my 4x5 Crown that has a newly installed OEM fresnel (between the GG and the lens, per the original design spec) and visually compared the same scene. The two looked essentially identical.

The Crown fresnel has much finer ridges spaced closer together than the cheap fresnel. But when I tried focusing the cheap fresnel with a small loupe, it wasn't bad. I figure if I can come up with a way to easily pop the screen on and off the GG, then I can use it just for compositional purposes and remove it for final fine focusing.

Ken

 

[Joe VanCleave]

I used one of these fresnel magnifiers as the view screen in my DIY 8x10 nested box camera. I took the flat side of the fresnel and sanded it with 400-grit emory paper using a random orbital sander. The results are a ground glass on the front side and a fresnel on the rear side, all using one sheet of plastic.

~Joe

 

[Roger Cole]

That's what L Gebhardt is saying and it seems like that would be a safe bet. But I'll say, in my limited testing, the difference appears to be nominal.

I'd really like to tackle the calculation, but I'm not sure if I'm up to it at the moment... still sipping my coffee.

Let's imagine we have a 135mm lens at f/4.7 (Raptar, Optar, Graftar) and our subject is only 2 feet from the lens (a fairly critical circumstance I would suspect). What difference in the subject distance would be necessitated by a .003" change at the plane of focus; again, 3 mils being a liberal estimate of the fresnel thickness.

I don't know the answer or exactly how much difference it makes, but I do know that the thickness of the fresnel is not the issue, or at least not the only issue. The fresnel is a magnifying lens in its own right that adds its own focal length into the optical mix.

 

[holmburgers]

Ken, I'm so glad that you gave it a try and moreover that it worked so well.

There's too much theorizing and optical-hypothesizing about these Fresnels (though I'm sure some considerations are valid); in reality you can stick one on your ground glass and it'll make a marked difference.

 

[Mustafa Umut Sarac]

Chris ,

I looking some optic design books and all I understand that , if you put a lens distant and back of a lens , second lens focus the first inside of it , not in front or backside of the second lens. If your fresnel is thin enough you will lose sharpness approximetaly half of the thickness of fresnel.

Umut

 

[Jim Jones]

My sheet magnifiers measure about 10 inches in focal length and about .022" thick. If placed between the ground glass and lens, this thickness would certainly make a difference in accurate focusing at large lens apertures. The pitch of most of the sheet magnifiers was 50 lines/inch; painfully coarse. One measured 100 lines/inch, and was slightly dimmer. A Speed Graphic fresnel measured about 200 lines/inch. Not all fresnels, even of identical pitch and focal length, are created equal. Some will work better than others with a ground glass.

 

[Steve Smith]

If you know the focal length of the fresnel (e.g. use the 10" Jim suggests) and the focal length of the lens, you can convert them to dioptres, add them together and convert back to focal length and compare it to the focal length of just the lens to find the difference in ground glass position:

Assuming a 150mm lens, this is 0.15m in focal length or 1/0.15 Dioptres. A 10" lens is 0.254m or 1/0.254 Dioptres.

So 6.6667D + 3.937D = 10.604D or 0.094m or 94mm so 56mmm closer. Or in other words, my theory is nonsense!

(I couldn't not post it after going through all the maths!).


Steve.