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-   -   Letus Mod - replace macro lens (https://www.dvinfo.net/forum/alternative-imaging-methods/76995-letus-mod-replace-macro-lens.html)

David Delaney October 6th, 2006 05:45 PM

Letus Mod - replace macro lens
 
I have recently taken apart my buddy's Letus35A (he doesn't seem to care about it anymore, so why not).
I has a pretty weak Macro lens that is closest to the DV. I have noticed problem with vignetting because of this. I wish I could get another one with that size ration to replace it. Has anyone else attempted this?
I am leaving the Achromat that is hot-glued in the cylinder, it seems to do a decent job for now...

Bob Hart October 6th, 2006 09:56 PM

The "weak macro lens" you describe and the "hot glued" macro may be a companion pair in a complementary function.

The power of this lens combination is also tied to a practical need for the camcorder to be able to focus on the groundglass whilst the mirror flip module can be fitted in between. Removal of any one and a substitution may spoil things.

I think you may find that the "hot glued" macro faces the camcorder and may be functioning as a condensor or distance corrector for the "macro lens" closest to the camcorder.

As for vignetting or hotspot/edge or corner brightness falloff, many 1/3" camcorders have to be zoomed almost all the way in with many adaptors, in the ballpark of 54mm - 70mm.

Users are tempted to try for the widest possible frame closer to the 36mm x 24mm still-camera image they are accustomed to when using SLR lenses. The motion picture camera image is 24mm x 18mm, which confers a narrower field of view.

If they want to more faithfully replicate the motion picture film image then zoom inside the vignette or hotspot and they will get there.

Backed off, there will be the edge defects and they may also pick up the chroma defects you describe which would otherwise fall outside of a more tightly framed groundglass image.

It seems rather a pity your friend seems to have abandoned his Letus35A. Most of these adaptors require a lot of practice and new camera skills t be developed before their benefits can be predictably realised.

David Delaney October 6th, 2006 10:22 PM

Bob,

The main problem I am having is that my camera cannot zoom in more than 2X before it starts blurring. Using just the regular macro lens gives me vignetting. So, I took it out and placed a condenser/achromat right in front of my DV lens and found this zooms me in plenty. I am still looking for a better way ,but at least it is something.

I am also getting a lens' flare now that I have changed the Macro - is this from the inside of the cylinder being silver? How should I get rid of it?

Ben Winter October 10th, 2006 09:45 AM

I don't understand.

Are you saying there are two macro lenses glued in the Letus? That shouldn't be the setup. And I doubt the Letus has a macro as high quality as an achromat. Achromats are different than just plain macros; they're two piece optics glued together to have the same function as macros but with limited chroma aberration. They are much more expensive.

There is no use for two macros in a row. What you are probably seeing inside the Letus is a condenser PCXL. This helps reduce vignetting the closer it is positioned to the ground glass. The farther away,the more distortion and chroma aberration but these are way more fixable in post than vignetting. You would see best to replace the macro with an achromat(ic doublet) as you have done and that has fixed your problems. Vignetting can be solved by increasing the distance between the camera and the focusing plane however you will probably need a higher diopter macro if you do so.

Bob Hart October 10th, 2006 04:57 PM

Ben.

What I have found when mining around in a broken XL version to repair it, was what appeared to be an unmounted doublet, attached to the relay lens.

As an interference fit in the front of the flip module, about 18mm behind the focal plane (GG), reversed in its orientation, was a single element, mounted, close-up lens (macro) which in this role appears to serve as a BCX condenser.

If the arrangement in the Letus35A is similar, except delete the relay lens and substitute camcorder's own inbuilt lens, then the sequence of optics from GG to camcorder is valid.

Unless the power of Quyen's achromat is known, changing it for another may yield inferior results and may require lengthening and shortening the GG to camcorder lens distance.

Ben Winter October 10th, 2006 08:24 PM

Oooh. Didn't know it was a relay unit. Apologies. What you say makes good sense.


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