DIY extra zoom lens onto DVX100b? at

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Old October 9th, 2010, 08:12 AM   #1
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DIY extra zoom lens onto DVX100b?

Has anyone figured out how to increase the DVXb zoom? Like, add an additional lens onto the camera, like they do with the 35mm DOF hacks?
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Old October 12th, 2010, 02:29 AM   #2
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Nobody has replied so it looks like it is my lot to do so.

I have used a 35mm groundglass adaptor with the groundglass removed to confer the aerial image from a longer 35mm stills zoom lens on front of a Z1. This adaptor was a home-made AGUS35 with prism erector (flip system) I would not recommend non-flip with aerial image as it will be a nightmare to frame up, especially on dynamic subjects.

I don't think there is a significant benefit unless you are using lenses in the ballpark of 300mm-500mm.

There is considerable overlap of the zoom ranges of the normal camera lens and the 50mm - 500mm Sigma lens I added on front for chasing airplanes in flight. With two cameras, it was handy to able to aquire with the normal camera and then change over to the long camera. But it remains an awkard dog of a thing to use until get yourself well-practiced with it. In aerial-image arrangement, the camcorder zoom had to be all the way in. This loses a noticeable amount of light performance in the ballpark of 1.5 f-stops with the Sigma 50mm-500mm.

Here's some clips which inlude example of the differences, some on different groundglass devices, some with the groundglass still fitted soime with it removed.

Note: This last clip was shot groundglass removed. You will observe the vignette because I was careless and forgot not to close the iris on the Sigma lens.

The exit pupil on the back of the lens has to be in the ballpark of no less than about 20mm otherwise you get a vignette. There was no significant resolution penalty in keeping the groundglass and you lose the lens vignette but artifacts appear at apertures of f5.6 or higher numbers which are not apparent with aerial-image relay.

Going through all that glass including the achromatic dioptre in the relay loses you considerable sharpness.

You might be better served with a purposed accessory optic on front of your camera. I think they called them zoom or lens multipliers and were available for the DVX 72mm diameter filter thread mount I vaguely recall.

Dennis Wood's Cinevate Brevis flip adaptor might be the best of both worlds as his groundglasses can be swapped, therefore it likely can be operated with a groundglass removed.

Do-it-yourself without some machining skills and precision tools is not something I would recommend though of course not impossible, just plain hard work and about four protoypes before you get it right.

This clip shows what the inside of a home-made looks like.

The AGUS35 optical non-flip path front to back is :-

Nikon SLR 35mm lens mount > 46mm to groundglass or aerial-image plane > approximately 150mm to front rim of Century Optics 7+ power achromatic dioptre fixed to camcorder.

The prisms in the path of the flip or erecting version of the adaptor shorten the physical length between the groundglass and front of achromatic dioptre to about 75mm - 80mm. My design used a different prism layout to make it as short as possible whereas the Brevis, Letus, M2. SGPro flip version adaptors are longer with a different prism arrangement and use a lower power dioptre in the ballpark of 4+.

Tooperate in aerial-image mode you must first set your 35mm SLR telephoto or zoom lens on an infinity target, with zoom lens zoomed all the way to narrow view, focus it on infinity, then manually focus your camcorder until the image becomes sharp.

Provided the rear element of your SLR lens and the optics between it and the camera are clean, your autofocus will actually work whe shooting an aerial-image. It is best to only use autofocus in bursts to track a moving object. Anything closer which cuts in momentarily, like a bird or an insect, will cause the autofocus to crash to the closest object, the rear of the SLR lens if you leave the autofocus selsected "on".

There is about 10mm of wriggle room with the aerial-image focal plane however if you are trimming the front lens focus and using bursts of autofocus, eventually you will stray outside of the sweet zone of the aerial image plane and weird stuff will start to happen. Then you need to reset infinity focus again.

Good luck with your endeavours.

Last edited by Bob Hart; October 12th, 2010 at 03:35 AM. Reason: error
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Old November 22nd, 2010, 01:17 PM   #3
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I had a DVX with a cheap(ish) 500mm prime lens attached to a diy 35mm adapter and supported by indyFocus rails (the length of the set up made it look like a sniper rifle...).

I did like the effect as it gave me those very tight close ups I needed. It gave a very long zoom and a shallow depth of field, only problem was that because the zoom is so big, the slightest movement of the camera, even if its just to focus your lens, and the whole thing gets very shaky.

here is some sample footage with the 500mm lens (0:17 - 0:50), although those shots were with a HPX170, the principle is the same

YouTube - ProDrift HD test
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