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Old June 17th, 2006, 10:47 PM   #1
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XL2 + Adapter - Focal Length question

Hi all,

I'm looking to kill 2 birds with 1 stone - I'm wanting to be able control my dof and also widen the focal length of my XL2. I've read that when using a 35mm adapter it's necessary to zoom the stock lens onto the imager...would you still get (for eg) 28mm focal length from a 28mm lens when doing this?

I'm shooting in very tight spaces soon, and need plenty of width. I have the option of getting the 3x wide angle lens (26.6mm equiv) but want to get away from the video look.

I've been doing plenty of reading but can't get this straight in my head. Any help would be appreciated...


Steve Madsen is offline   Reply With Quote
Old June 18th, 2006, 01:59 AM   #2
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As I understand things, two methods of relaying into the XL series of Canon cameras have been used for home-made devices. Relay via a close-up attachment into the stock on-camera lens the Canon uses, or a direct method which replaces the stock on-camera lens with a small fixed relay lens.

The last method is the one used by P+S Technik and once set up properly is the more predictable and reliable. Quyen Le's Letus for the XL camera family also now uses a direct relay lens and he is also beta testing one for the JVC HD100.

I understand and maybe incorrectly that the P+S Technik Mini35 uses a smaller image off the groundglass than the standard 35mm motionpicture camera frame. This frame is 24mm x 18mm. P+S Technik apparently use a smaller frame describe as 21mm, corner to corner.

This method should save a lot of the problems associated with home-made adaptors and makes for a very compact appliance.

There is one other issue and that is, some relay systems convey the image into the camera upside-down and this must be erected or "flipped" in post or the camera itself operated upside-down, which is not a convenient arrangement.

Compared to the 35mm still-camera frame of 36mm x 24mm, the 35mm motion picture frame is about half the size.

I have seen reported here elsewhere that a 28mm lens on a motion picture frame presents a similar field of view as the 50mm lens on a still-camera frame.

The depth of field for a 28mm lens does not change between formats for a given aperture. The available field of view or the angle of view for a given available narrow depth of field does. That is what concerns people who build or fit the adaptors.

If you are expecting the field of view you get with your still-camera, you likely won't get as much with a 35mm adaptor. The field of view will be more consistent with that of a modern smaller format digital still-camera. If you are familiar with those, then the effect from the adaptor will be closer to your expectations.

In practical terms, the 12mm end of a 12mm - 24mm Nikon lens I have used onto a 24mm x 18mm motion camera sized frame in a modified AGUS35 design, presents about the same usable field of view as the wide view of the in-camera lens of the Sony FX1 or PD150. The wide end of the FX1 zoom is 4.5mm.

The available depth of field at a wide aperture is shallower. I can't speak for the XL cameras.

The softer image comes at a price. The apparent resolution via any groundglass relay system takes a negative hit. Good glass in front of the groundglass is very important. The diffusion effect of the groundglass tends to amplify any softness in a 35mm still-camera lens image.

If you are shooting handheld agile portable, I would favour going with the available wide-angle direct-to-camera options for the XL camera family not a 35mm groundglass relay. Relay adaptors require more precision in focusing and do not lend themselves to handheld work as conveniently.

In the case of the XL camera family, relay systems fitted to the front of the standard on-camera lens make this camera very - very nose heavy and awkward to use and will cripple the operator if handheld for long periods. The direct relay systems are a much better solution for balance and useability.

In the case of the detachable lens style cameras like the XL family and JVC HD100, direct relay systems which flip the image upright, behave more like a normal lens system in terms of operator comfort.

Don't take too much notice of my comments as I am neither industry professional nor engineer.
Bob Hart is offline   Reply With Quote
Old June 18th, 2006, 03:00 AM   #3
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Originally Posted by Bob Hart
I have seen reported here elsewhere that a 28mm lens on a motion picture frame presents a similar field of view as the 50mm lens on a still-camera frame.
DV camcorder records picture 3/4. Which means we can have for instance following image formats:
18x24 (diagonal 30mm)
24x32 (diagonal 40mm)
27x36 (diagonal 45mm)

We can't actually record 24x36 because of distorted ratio. So if You use 18x24 image plane with 35mm optics and You use central area of projected image(windowing) then You loose some degrees at border areas. You can calculate it via diagonal size.

Ratio of Your image plane and actual will be 40/30 (40 is because it's closest to 24x36 - original 35mm image plane) =1.333. If You use 28mm lens then multiple 28 with 1.333. It's 37.33 the new focal lenght of Your 28mm lens. But sometimes You zoom even more than to 18x24 because of vignetting, which appears very well because of difference optical principles between GG and film - first diffuses second absorbs light rays.

So You get from 28mm lens 50mm.
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Old June 18th, 2006, 03:22 AM   #4
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Thanks Bob, that's very thorough. Though I think I'll bow out of this discussion gracefully...my head's hurting :)

One thing that did make sense was the fact that I'd need a pretty wide dslr lens to get wider than than the stock xl2 lens, which has a 5.4mm fov (42.3 35mm equiv). Your comment about the 12mm end of your Nikon being simliar to your fx1 was a bit of a shock. 12mm, just to get a 35mm equiv of 35mm (4.5 x 7.8, if that's the right math)...wow!

And thanks Frank, the picture's getting clearer...
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