Help with a fresnel?

[jwarren116]

So, I'm still relatively new to large format. I've had my B&J 5x7 for a while, and I've been using it with a 4x5 back. I got the camera used. I just ordered a Shen-Hao to replace it (it was too heavy and bulky). I was recently introduced to the GG enhancements, like fresnels. Now, more to the point... on my B&J, there was a plastic GG cover. I just thought it was a protector, but after researching fresnels noticed it was no ordinary protector. It has consecutive circular grooves on one side. For all the research I can do it seems to be a fresnel, and not one of the other GG enhancers. Is there any way to find out what focal length it is optimized for? I know on my B&J it made my 90mm almost impossible to see anything unless I had my loupe in the exact middle of the GG. And if fresnels are "optimized" at a certain focal length, how come the only Shen-Hao fresnels I can find don't specify a length?

I read the page about the fresnels, but for the life of me I'm still utterly confused. Can anyone recommend a "fresnels and other GG stuff for dummies" page?

[Tracy Storer]

This will get you started:

http://www.wisner.com/viewing.htm

Generally, wider focal length lenses will require a shorter focal length fresnel, almost any fresnel will be better than none with a wide lens, unless it's installed backwards, then the "hot-spot" problem will be worse. Most fresnels should be installed with the grooves against the plain side of the ground glass, and the ground side of the ground glass towards the shooting lens.

[Alan Davenport]

The idea that the focal length of a fresnel lens needs to be matched to the focal length of the camera lens, is a fallacy. The ideal viewing distance will vary slightly but it's not going to be a problem.

It makes no difference which direction you mount a fresnel; it will work either way. The reason for mounting the fresnel with the grooves against the ground glass is to protect the delicate grooved surface from damage that would be caused by loupes and fingers.

[Tracy Storer]

Sorry Alan,
I beg to differ with you, but many fresnels need to be installed one way or the other, it depends on the the fresnels conjugates. (equal/1 short, 1 infinity, etc.) The fresnels I've been using for big cameras all have one conjugate set to infinity, so if you put the lens in the wrong way, it worsens rather than helps the hot-spot issue. Apparently the fresnels you have experience with have equal conjugates allowing installation either way.
Best regards,Tracy

[Bill Kumpf]

All thought it does not answer your question directly, the Anchor Optic website has interesting reading on lens.

http://www.anchoroptics.com/documents/

[Alan Davenport]

...many fresnels need to be installed one way or the other, it depends on the the fresnels conjugates. ... if you put the lens in the wrong way, it worsens rather than helps the hot-spot issue.

If I'm hearing you correctly, you're saying that a fresnel can be a positive lens if turned one way, and a negative lens if turned the other way? A fresnel lens is simply a positive lens; it's going to converge the light rays regardless of orientation. I repeat, conjugates notwithstanding: a fresnel will do the job if turned either direction.

[Tracy Storer]

I said it above, not all fresnels are the same. Many have an infinity conjugate one side and the short focus on the other side. They do not change from positive to negative, but if installed the wrong way, the hot spot problem is exaggerated. Fact.

[Tracy Storer]

Since I observed the above mentioned problem first hand as fact, I set it aside without much further thought. As I sit here thinking about it now, the issue is probably that the image light at the corners of the ground glass exceeds the transmission angle of the fresnels "prism" faces. It'd be like putting the condenser lenses in your enlarger in backwards....hot-spot-city.

[Alan Davenport]

Hmmm. I'm going to ponder this for a bit...