Redrock M2 design Q for homemade solutions
 

Hi all
I've recently been building my own spinning CD style 35mm adaptor and I've been wondering about the M2's rear lens element.

If it's a Pcx lens, then I'd guess that it's there for two reasons that I can think of -

a) to provide a condenser-like function to spread the light out to the edges a little more;

b) to help get the overall grain size down. You blow up the image on the GG slightly, then zoom out on your camcorder a little more. Overall focal length and DOF effect is similar, but since you're sampling more of the GG image now, when you zoom out to fit, the GG grain size is diminished.

Can anyone comment either way? I'm looking for ways to improve on my design.


[edit]
Doh, another reason that just occurred to me is that the M2 has a multiple lens mount design, right? So the additional glass may most probably be there simply to compensate for varying back focus. Again, just a thought.

Hi,

They use a DCX lens between 35mm lens and GG, its purpose is to spread the light to eliminate vignetting.

Doing this actually creates a smaller projection onto the GG, not bigger, so grain size is bigger in relation to the frame. So for example, if you removed the DCX from the M2, you would need to zoom the camcorder out slightly to get the same FOV.

Thanks Wayne.
I guess the GG movement makes up for the smaller frame, right?
Cheers
Chris

Hi,

Yes once the GG is spinning, grain is not an issue anyway.

However, I would not recommend this setup. Placing a DCX lens between the 35mm lens and the GG means that the 35mm lens can not project an edge to edge sharp image into the GG. It is worse with the lens aperture wide open.

Chris:

I made my own Redrock type DIY adapter without anything other than an achromat. In my case, with FX1, I used a 72mm Cinevate achromat mounted on FX1, and had 5 inches of spacer between camera and GG. Gave me a lot of zoom play.

Letus35a has following: 35mm lens, GG, a condenser lens, achromat, then Camera. This does work pretty well, but leaves little zoom play, and results in vignetting on some lenses. I ve added my Cinevate achromat to that set up to supplement, and it seems to give me extra zoom room to fight vignette, but haven't test the results yet.

Wayne, that's what I would have thought too. Thanks for your input.

All:
I've set up a DOFbox with a 72mm diameter 10x achromat doublet on the front of the camcorder, going direct to a spinning CD style GG. Nikon lenses.
Distance between the front of the achromat to the GG is around 3" / 75mm, and I'm roughly a third zoomed in from full wide on the camcorder to get good coverage.

On set up, I find that I have to zoom/crop the projected image slightly tighter than the corresponding 24x36 image on a stills camera to avoid corner fall off.

How much tighter? Maybe 2 or 3 horizontal degrees tighter on a 50mm/1.2 lens than the same lens on a still camera, kind of thing, not much more than that,though.

I've got a 16:9 frame on the camcorder so maybe that's a trade-off right there, but I would have thought that I'd just be cropping top and bottom to compensate.

So now I'm thinking, do I need to add a light spreader lens between the GG image and the achromat?

Or do I simply try to get the grain finer on the GG, leading to more light dispersion off the GG itself?

There's only a couple of degrees in it at the moment, I know, but it's going to get worse when I swap out my 50mm for something wider.

Chris:

One thing I've found out is that just the rear element of the 35mm lens makes a difference on the vignetting. I have a Nikon 35mm F 2 which has much less vignetting issues than my Canon 24mm F 2.8. Rear element on the 35mm is much larger. The same thing applied to the simple Nikon 100 mm E Series, it has a larger rear element that seems to keep edge vignetting/fall off to a minimum. Don't ask me anything about the optical principles, just an observation.

Chris,
Thanks for your input.

I've got a set of (mainly Nikkor) highspeeds, 24/2, 28/2, 35/1.4, 50/1.2, 85/1.4, 105/2.5, 180/2.8, and it's big rear elements all around.

Also I have two 105 macros (an f.4 Nikkor and the incredible 2.5 Vivitar Series 1) which should prove interesting, as well as a 17 and a 3.5mm in the "speciality lens" department. The 180, FYI, is a re-multicoated example of one of the original 2.8 Olympia Sonnars, which has an immense circle of coverage.

Actually I'm not so sure if I'll ever use my Lomos on this box, since they're slightly slower (not the high speed set), and the Nikkors seem sharp enough at this stage. However, I've not blown any of my video up to more than 40" or so, so who knows?

Dealing with the 1.77 HD format (16:9) means that you've effectively got a 20.25x36mm "gate" on your GG. This leads to a screen diagonal of roughly 41.3mm to the classic stills camera's diagonal of 43.2mm, or a difference of roughly 5% in the comparative angle of view. I know that's not a lot, but it may account for the slightly different frame that I've been getting.

Chris,

I recommend placing a PCX or DCX lens between GG and achromat, close to the GG. Which camcorder are you using?

Wayne

It's a Sony HVR-FX7.

What diameters/focal lengths would you start at?

I have a set of 52mm PCxs of varying FLs I could use to start tweaking with, but there are also 75mm PCXs with a either a 185 or a 125mm FL available immediately. Which length to try first?



Thanks!
Cheers
Chris

Chris,

Try the 75mm dia 125mm FL one!

Done.
Thanks again!

Actually they were both only a few bucks each so I got both of them.

That light baffle with a rectangular cutout that I was thinking of including in the box on the camcorder side would be a good spot to add the condenser, right?

BTW when I'm done with this I'll have a good grab bag of loose lenses, mainly PCx, DCx, 52mm, but one of those 75mm condensers also, plus a set of 72mm close up filters (+1, +2, +3, Macro +10),
and a couple of very pretty 50mm achromats that should go on to the next fellow who wants to try this for him or herself.
Also a 5x7x2 alu box, still in wrapping, since I decided I'd go with a 3" deep box in the end.

All for a good price, obviously (probably less than half of what I paid, plus shipping), but preferably to a deserving tinkerer who will undertake to contribute his or her findings to this thread.

Think of it as handing on a torch. Maybe you will get other stuff for your build, find out you don't need most of it, then hand it on again.


Here's my rig, temporarily set up, BTW.
I'm planning a feature length dramatic piece with it soon after it's done.


From the front:
- 4x4 Chrosziel matte box (off my Aaton system)
- Cavision 15mm support system
- Nikkor 85mm 1.4 on it at the moment for testing
- DOF Box mark 1
- 72mm Redrock HD x10 Achromat
- 62/72mm step up ring
- FX7
- everything's sitting on a Beachtek DXA-8 (but not for long, I can just see the whole lot tipping over)
- Varizoom 7" with unflip and unflop
- Bogen cine tripod setup.