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Fresnel or not, how to tell?

 
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Fri Feb 01, 2002 5:10 am    Post subject: Reply with quote

I'm asking here because this is the back of a GVII, spring back. I finally received my GVII and am just thrilled with it even though it doesn't have a Graflok. But I also noticed it doesn't have a fresnel in front of the GG. Now, I know that they made models without fresnels, but is there a way to tell if one is supposed to be there? There's no model numbers on the back and it does look right, no extra space. But I'm just wondering because it does seem a bit darker than my Speed or Crown, both with fresnels. I know about modifying to put one in, but this just got me thinking that if there was supposed to be one there in the first place, the focus would be off. So how can I tell if it's right without taking a dozen pics? Must be a way to measure or see by design? Thanks for any info...

Rich...
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45PSS



Joined: 28 Sep 2001
Posts: 3219
Location: Mid Peninsula, Ca.

PostPosted: Fri Feb 01, 2002 5:49 am    Post subject: Reply with quote



[ This Message was edited by: 45PSS on 2005-12-24 20:03 ]
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Fri Feb 01, 2002 2:57 pm    Post subject: Reply with quote

Ohhh...Duhhhhh.....
My mind got trapped with thinking of measuring from the lensboard. A bit more difficult... Didn't even think of removing the back! Sure, and with a metal bar and digital caliper, an exact measurement would be easy...
Thanks!

Now to add to this... I've heard of placing a fresnel behing the GG. Is this possible _and_ practical? If this back is designed for no frasnel and I could place one behind, the focus would remain correct and it would be brighter. But would it really work?

Rich...
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Les



Joined: 09 May 2001
Posts: 2682
Location: Detroit, MI

PostPosted: Fri Feb 01, 2002 3:14 pm    Post subject: Reply with quote

In practice all fresnel lenses go on the lens side of the GG.
Now if the camera was not designed to have one there is argument that the fresnel has a focal lenght and must be compensated for, other people who have done field studies say no. Beatie and others say no too. You'll hve to mount the fresnel without moving the GG somehow.

I'm not sure how much of an effect a fresnel lens would be on the person side of the GG since the idea is to disperse the image before it gets to the GG not after. The easiest way find out is to go to an office supply store and pick up a fresnel " pocket page magnifier" They're only $5-6. See what happens and report back!
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Fri Feb 01, 2002 5:40 pm    Post subject: Reply with quote

Quote:

On 2002-02-01 07:14, Les wrote:
In practice all fresnel lenses go on the lens side of the GG.
Now if the camera was not designed to have one there is argument that the fresnel has a focal lenght and must be compensated for, other people who have done field studies say no. Beatie and others say no too. You'll hve to mount the fresnel without moving the GG somehow.

I'm not sure how much of an effect a fresnel lens would be on the person side of the GG since the idea is to disperse the image before it gets to the GG not after. The easiest way find out is to go to an office supply store and pick up a fresnel " pocket page magnifier" They're only $5-6. See what happens and report back!

I wondered about the fresnel redirecting the light after the GG. Makes sense to me because it's already dim & scattered. I personally can't see how the fresnel can't have a focal length? Unless they take the lensing (a word?) of the fresnel into account and that somehow makes up for the change in distance?
Testing with a cheapo is a great idea. I've seen 8x11 'magnifiers' for a couple of bucks. I've already thought about trimming a screen and attaching it to the GG on the inside in some non-permanent way. Of course the back could be trimmed the thickness of the screen, but it would add cost and who could I trust it to? Backs are a bit hard to come by. I'm sure with a dark cloth and shooting under decent lighting conditions this will be perfectly useable, as they were for a hundred years. But if I can find a good used fresnel, it'll be another project

Thanks
Rich...
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45PSS



Joined: 28 Sep 2001
Posts: 3219
Location: Mid Peninsula, Ca.

PostPosted: Sat Feb 02, 2002 1:06 am    Post subject: Reply with quote



[ This Message was edited by: 45PSS on 2005-12-24 20:04 ]
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RichS



Joined: 18 Oct 2001
Posts: 1467
Location: South of Rochester, NY

PostPosted: Tue Feb 05, 2002 6:02 pm    Post subject: Reply with quote

Quote:

On 2002-02-01 17:06, 45PSS wrote:
Double check, I have a Lisco Regal that the film surface is right on on one side and 1 1/2 the film thickness deeper on the other!

Charles



Thanks. I haven't yet checked the back, but I did impulsively take a couple of polaroids with it and so far it works fine. But now that you've brought up the point of film holders, I may just go through and check everything just to make sure. Overall it doesn't seem too critical until I do extreme close-ups.

And as far as 'needing' a freznel. Well, with decent light the plain GG does just fine. It's just indoors with the 90mm f6.8 that's awful dark. So there's no rush to install a fresnel and I'll wait to pick one up at a good price (or a graflok back!).

Rich...
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