fresnel + wide angle

[false_Aesthetic]

Do wide angle lenses and fresnel GG do weird things?

I threw a 90mm on my 5x7 MQC earlier today for kicks and saw some weird distortion/unsharpness. Supposedly the image circle is big enough (216mm). I didn't have film with me to take a pict (just playing around) so I don't have proof of what I saw. I was just a bit surprised.

T

[IanG]

There are fresnels which are optimised/designed for WA lenses so it's possible. I haven't had any problems with the fresnels on my own cameras (5x4 & 10x8) but it's possible your resnel isn't flat.

Ian

[rdenney]

In my experience, Fresnels are usually designed for lenses in the normal range--something like focal lengths longer than 3/4 of the diameter of the film. My Sinar Fresnels show ghosting, double images, and cut-off with lenses shorter than 90 on 4x5.

There are two approaches to solving this problem. One is to take off the Fresnel and use a tilting loupe. I have one labeled "Silvestri" that actually works quite well, once I removed the foot that got in the way of my ground-glass frame on my Sinar.

I have a Horizon loupe (not-so-bad Russian cheapie) with a square base, and I despise the square base. It's fine for viewing on a light table, but the base is ALWAYS in the way when I tried to use it with a camera. I ended up holding it backwards and using the square as an eye cup--not at all satisfactory.

Rick "who solved the Fresnel problems with a Maxwell screen" Denney

[E. von Hoegh]

Do wide angle lenses and fresnel GG do weird things?

I threw a 90mm on my 5x7 MQC earlier today for kicks and saw some weird distortion/unsharpness. Supposedly the image circle is big enough (216mm). I didn't have film with me to take a pict (just playing around) so I don't have proof of what I saw. I was just a bit surprised.

T

It may not be the Fresnel, 90 is damn wide on 5x7 and there will be some inevitable "distortion" at the edges. The unsharpness could also be due to a non-flat Fresnel as has been pointed out. Just what sort of Fresnel do you have?

E. "who solved his Fresnel problems by not using one" von Hoegh.

[false_Aesthetic]

Hey,

1) I'm using whatever fresnel Keith Canham puts on his MQC 5x7s. It was installed when I purchased it and the type isn't listed on his site (at least, I didn't see the brand mentioned during my 25 seconds perusing his site).

2) Rick, how does using a tilting loupe help me out exactly?

3) Ok this is all just hypothetical but if the weird distortion is from the fresnel/lens combo I guess that I'd get that distortion no matter what the image circle is, right?

[rdenney]

Hey,

1) I'm using whatever fresnel Keith Canham puts on his MQC 5x7s. It was installed when I purchased it and the type isn't listed on his site (at least, I didn't see the brand mentioned during my 25 seconds perusing his site).

2) Rick, how does using a tilting loupe help me out exactly?

3) Ok this is all just hypothetical but if the weird distortion is from the fresnel/lens combo I guess that I'd get that distortion no matter what the image circle is, right?

I have no experience with the Canham line, but I'll assume two things: 1.) it's good quality, and 2.) it is not optimized for very short lenses. Is it installed permanently or can you pop it out at will as with the Sinar?

When you view a very short lens on a ground glass without a Fresnel, you see a bright spot between your eye and the back of the lens and not much else. The shorter the lens, the smaller the spot. To get any brightness, your sightline has to be aimed at the back of the lens, and so the shorter the lens, the shallower that angle has to be. A tilting loupe allows you to view at a shallow angle through the ground glass so you can aim at the rear of the lens in a way that puts you in the bright spot.

A Fresnel redirects those angled light rays back to your eye has to be designed for the angle, which means a Fresnel will be optimal for a range of lens focal lengths. When the focal length is too short, they behave strangely. In my experience, that means heavy blur, ghosting, and double images.

And you're right that the image circle does not affect "distortion" with modern lenses at least, though with lenses of inadequate coverage you might not be able to tell what the limits of that coverage are because the screen is so dark outside that bright spot.

Rick "who uses lenses down to 47mm" Denney

[false_Aesthetic]

Thanks Rick