Could the Letus EX/Mini Canon FD mount cause vignetting?

T

Toenis

Well-known member
After a long wait and messing with import papers I received the Letus FD mount today. The first thing I noticed is that it`s inner diameter isn`t as wide as the 35mm image circle (Vistavision and SLR full frame) it`s like c-mount. Actually it is 36mm in diameter as opposed to at least 43mm circle projected on the 35mm frame the lens were designed for.
Could that smaller inner diameter cause vignetting? It does in my mind
Seriously I´m paranoid here and before I´ll try to modify the mount I just want to be sure.
By the way this is the new Letus FD mount.


Thanks in advance,
T
 
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there's a lot of LEX users happy with the LEX, myself included and im quite confindent to say it is not your mount causing the vignetting.

You need to be zoomed well in (im zoomed in to z80 on my EX1) to the ground glass before you get the vignette free image and also make sure you have your canon lens at fastest aperture, which should be something around f1.2 to f2.8.
 
Even at full camcorder zoom (about 400mm) there is some vignetting present and I`m talking about fast FD lens that cost as much as Lex itself if not more. Overall falloff is about a half stop at f2.8 but it doesn`t get worse when I close down. Not as bad as cheap f3,5 $65 lens but I haven`t experienced it on still camera maybe because I wasn`t paranoid then and I`wasn`t producing moving images.

T
 
I`m using 14mm 2.8 on Letus Mini with Canon HV20. I did some initial tests and this the worst case in my almost three years of DOF adaptor experience.
Plus I`ve never experienced vignette that bad in my still photography. It`s like using the cheapest wide converters on HD camcorder lens as edge softness and vignette are that bad - try to crop that out.
vignette_and_softness.jpg

It is already cropped image by the way. And if I crop the edges so to the width of the building it will be the field of view equivalent to 22mm lens.

Cheesh, I don`t know what am I doing wrong here.

T
 
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you should contact Letus and ask their opinion on this. Please be sure to mention all the relevant info.

I have one lens, Canon FD 35mm f2.8 that shows more vignetting then my other lenses when I use it with a miniDV cam. But on my Sony EX1 its as should be.

Sami
 
I noticed that the back focus on the mount is off so it needs some corrections to be able to focus to infinity about where the infinity mark on the lens is and hopefully this will correct something at least. My lens will go out of hyper focal infinity focus past the 1m mark already and it is so with all my lenses.
[edit] Seems that the back focus issue solved the vignetting almost + proper exposure of course and the photo function on that camera just uses another area to take the photo, much wider than the camcorder lens (and I`m not talking about usual overscan here)

Thanks,
T
 
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hi,

i just read your post about vignetting and backfocus with your letus mini.

i am experiencing the same thing, i have to zoom in quite a lot to loose the vignetting with both canon 1.4 50mm and 2.8 28mm lenses.
how did you adjust the backfocus resulting in less vignetting?

thanks very much in advance!
 
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