Newbee ?'s About Focusing & Zooming

[Bob Hart]

I am going to get chastised for this but here goes.

I have found - and a DP here in Perth also found likewise with the Mini35-400, that sometimes, focus on the groundglass texture of the stationary groundglass puts you in a very precise "ballpark" of where the relay (camcorder) focus should be and that you can then cheat just a little extra sharpness by focussing on the actual projected image which is made using a Siemens star or similar large target normally used for setting backfocus on ENG cameras.

This requires you to mess with both the focus of the SLR lens on front and your relay focus until you get it spot on. There is a sting in the tail of doing this with home-mades. If your groundglass is passing aerial image, your focus by this method can sometimes drift way off.

With the Sony HVR Z1P, I have found that through the +7 Century Optics dioptre I have used, also the +4, a ten second burst of the camcorder autofocus works as well as the manual focus/human eyeball to small LCD screens and faster at getting to the optimum relay sharpness point, leaving you only to get the SLR focus on the Siemens star at its sharpest.

The same sting in the tail applies. The autofocus will sometimes get enough aerial image to chase a totally way off setting and you have to bring it back manually.

A bonus is that you don't have to set the SLR lens iris at f11 or tighter to provoke the groundglass texture to show itself.

Because my disk motor has a small amount of end-float and stabilises after getting up to speed, I give the autofocus a touch to trim the relay focus after starting the disk motor, then lock it off by switching back to manual. I have the manual focus ring taped to avoid bumping it off. This also helps to maintain a fair average optimum if your disk is running out.

[Peter Moretti]

Originally Posted by Paul Cascio

...
I have an SG35, which I am just beginning to use more often. I'm wondering if I can use my digital SLR lenses without any negative impact, or if I need to stick to 35mm SLR lenses. I have a Nikon mount, but I imagine the answer is the same for Canon too.
...

[Deleted Original Post] Paul, my original reply was not accurate. Here's a link from the Brevis FAQ's:

http://www.cinevate.com/phpBB2/viewt...digital+sensor

Essentially, the answer ranges between maybe to probably not.

[Rich Hibner]

I can't imagine why a DSLR lens would be any different. Nobody I know has actually said otherwise. And I believe you mentioned focusing with it spinning or it not, Wayne has suggested you focus on your image while it's stopped and then turn it on.......I believe. I think he said that a long time ago.

[Bob Hart]

Focusing on the groundglass when it is stationary is the recommended method for all moving groundglass adaptors.

DSLR lenses most likely will have a narrower available field-of-view. Depth-of-field for a given aperture setting and focal length should be the same. You may even get a slight sharpness bonus depending on the quality of the lens.

A Nikon 12mm - 24mm f4 zoom for digital cameras will give you an adequate image at the 24mm end but becomes vignetted at 12mm on the full 36mm wide still-image frame. For a 24mm wide motion picture image frame, it is fine but being a rectilinear lens tends to be a bit stretchy into the corners.

[Paul Cascio]

I'm thinking the DX lens will produce a small image on the GG. Is that correct?

What is the impact of having to zoom in on a smaller image?

[Dennis Wood]

Peter, the Brevis imaging element will actually allow a FOV wider than 36mm with fast 35mm film SLR lenses. Several rental shops use our PL mount (the new one), which is designed for lenses projecting the much smaller 35mm cinema film image size. The Brevis adapter is designed to accomodate the full range of frame sizes from 16x22 to 24x36. I use and test with a set of LOMO cinema lenses designed for the Konvas 35mm motion film camera. Every video camera and 35mm lens has their own unique issues, but we've pretty much sorted them out at this point.

The sensor size of a nikon DX is 15.7mm x 23.7mm if memory serves correct. This would suggest that if it's a fast lense and edge fall off is not an issue, it would be similar to using the 35mm motion film lens...and would be worth testing.

Most people are shooting 16:9 format either SD or HD. This video frame is basically superimposed onto the lens' projected image...and you would zoom in as appropriate. The trick is keeping edge falloff to a minimum so that you can use, or exceed in our case, the full image size possible with a given format lens. This is why the Brevis is configured with different imaging elements depdending on the type of cam being used.

[Bob Hart]

Paul.

http://www.youtube.com/watch?v=WokhLQv7fQE

This clip includes some Nikon digital 12mm - 24mm f4 zoom lens still lens images.

Fall-off along the left edge is due to confinement of the prism path and the centre of the lens mount is a bit low as viewed (high on the adaptor).

The clip was sent up as a DivX so might take a while to convert.