How can i project a bigger image on the GG ?
 

Let me see if i can explain myself...

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I want to project a "bigger image" on the GG, so i don't have to GO "to close" to the GG or zooming too much, in order to avoid vignetting.
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I'm going well with very sharp images, but i think i'm going to deep in the GG.


Thank you very much

The size of the projected image is determined by the type of lens you use. The solution you are looking for is to use medium format lenses, which project a 70mm(?) sized image instead of standard 35mm lenses with 35mm image plane. This will allow you to use a bigger GG, however these lenses are considerably more expensive, slower (less bright), more difficult to find, and heavier.

I'm not sure why you think you're zooming in "too deep" into the GG. If you're zooming in just enough to avoid vignetting, you're right where you should be.

If you can't zoom in enough before the focus goes out, you just need a more powerful macro.

You can place a PCXL (plano-convex lens) right in front of the GG between the GG and the camera and that will almost eliminate vignetting and perhaps afford you a bigger image to focus on, however you will experience a tad bit of image distortion, which can be countered with a powerful enough macro, however chroma aberration will be an issue. Sharpness will take a hit too.

If the ground glass is bigger than the 35mm image plane (which it usually is) you'll undoubtedly see vignetting from any lens, since the lens format isn't constructed to project past that point. Regardless of whether you see it or not on the GG, zooming in past the vignetting will give you the correct image from the lens.

what is exactly the job of plano-convex ? --- how will this help me to avoid vignetting?

thanks a lot BEn.

I had a cool little drawing up here a while ago--not sure where it is now--but basically the plano-convex adapts the critical angle of light coming off the GG to a more suitable angle for your camera lens.

Here's a basic idea of how it works: Take your adapter off your camera, keeping the macro attached and an SLR lens attached to it, and look at the image on the GG. You'll notice as you move the adapter towards your eye, vignetting sets in, and as you pull away, the vignetting disappears. This is because when your eye is too close, the light hitting the GG at the edges is at too extreme of an angle to be refracted by the GG all the way back to your eyes. Back off a bit, and the angle required becomes much shallower. A plano-convex lens takes the light rays at the edges that are missing your camera lens and curves them more towards the center.

Consider this diagram:
http://www.molecularexpressions.com/...sesfigure3.jpg

Actually, once you consider it, it seems a lot of DIY'ers (and even the Letus35) are doing it wrong, facing the flat plane side of the PCXL towards the GG, when it should really be facing away.

Putting the ability to focus aside, if you were to attach the adapter to your camera at a greater length from the lens, you'd be doing the same thing as adding a PCXL, essentially.

Ben.

I think users of PCX/BCX elements in their adaptors started off emulating in full or in parts, the Movietube which has a close-coupled pair, one element either side of the groundglass.

With the Movietube, the flat faces almost touch with the groundglass layer, in this instance wax, residing in between the two.

Whatever distortion which might be introduced by one should be cancelled by the other, the brightness remains intact.

As I understand things, using just one is a bit of a compromise.