Depth of field

PhilPowers · July 12, 2004

Hi there again- I see nobody has been able to answer my last questions, so heres a much more simple one.

How can I achieve really narrow depth of fields for my MCUs like it was shot on 35mm when using Digibeta. Now I know all about ND so lets say im already wide open on a standard canon zoom and my backgrounds are still not blurred enough. Is it possible to use a fast film lens for instance (50mm T1?) and what are the back focus and mount implications?? Could I use some sort of split diopter the wrong way round?? Or Chromakey and blur my plates in post?? Whats the answer?

All the Best- Positive Phil (UK)

Michael Nash · July 12, 2004

Depth of field is what it is. Shoot wide open, go to a long focal length, pull your subject as close to the lens as possible (for the size framing you want) and as far from the background as possible.

How soft does the BG need to be? I shoot a billion talking heads on 2/3" video, and can usually manage to keep the BG soft enough in most situations. If I want it even softer, I shoot on a longer focal length from farther away (sometimes the camera is out in the hallway shooting through the door into a room, just to get enough distance.

FWIW, wide open (f1.6) on most video zooms softens the overall image quality too much for my taste, so I usually end up around a 2.0 or 2.5 for talking heads. A slight amount of lens diffusion will help create a slightly softer and filmic look to video, and make BG's go a little softer even though it's technically softening the subject as well. It's just that there's still some sharp detail on the person's face that creates more of a visual difference between the planes of focus.

The P+S Technik Pro35 adapter will let you mount 35mm PL mount lenses onto a B4 mount video camera, and preserve the 35mm dof by way of a relay system (oscillating ground glass).

You could get creative and try alternative techniques like green screen, diopters, and such. But usually these techniques introduce their own artifcats that may or may not look right to you. There's a device out there that's like a big clear frosted screen that you place behind the person's head, just for this purpose. You can also hang a screen of theatrical scrim material or Bobinette behind the person the same way. But these techniques are really only for talking head style interviews where the subject doesn't need to move, not really for drama.

Phil Rhodes · July 12, 2004

Hi,

> Is it possible to use a fast film lens for instance (50mm T1?) and what are the

> back focus and mount implications??

No. The mount implication is why you can't; there are also optical design implications based around the fact that a video lens has to land the RGB images on three separate sensors which makes it at least non-ideal to use film style optics.

Naturally you could use chromakey and blur the plates in post; someone also produced a slightly diffused (rather like a groundglass) perspex screen which you can put on a stand between your subject and the background, but that's got limited coverage.

The main way of doing this is the Pro35, which is a device designed to project the image from a film-type PL mount lens onto a groundglass and then rephotograph that with a macro lens into the video camera. This gets you the same depth of field characteristics as 35mm and allows you to use film-type lenses, but it isn't a cheap option and there are sometimes other imaging artifacts.

Phil

John Sprung · July 12, 2004

Hi,

> Is it possible to use a fast film lens for instance (50mm T1?) and what are the

> back focus and mount implications??

No. The mount implication is why you can't; there are also optical design implications based around the fact that a video lens has to land the RGB images on three separate sensors

Landing the image on three separate sensors is done by the prism block. The video lens could be put on a film camera and it would produce a single full color image. The problem with putting film lenses on a video camera is that often the back element of the film lens has to be where the optical block already is. The block also limits you to f/1.45. Put a faster lens on the camera, and wide open, you'd only get f/1.45. This is nothing new, three strip technicolor had the same limitation.

The pro-35 idea is probably your best all-around solution. Test it and see what you think.

-- J.S.

Luke Prendergast · July 12, 2004

Using the 'macro' ring, if your lens has it, can give very shallow depth of field at the expense of easy focus moves. It's not so user-friendly or ergonomic as an aperture ring of course

Phil Rhodes · July 12, 2004

Hi,

> Landing the image on three separate sensors is done by the prism block. The

> video lens could be put on a film camera and it would produce a single full color

> image.

I'm aware of that; I have been told that video lenses have additional considerations in the triple-element constructions that are normally used to create coincident images across the visible spectrum to accomodate the way the splitter works. This does not inherently stop you using a video lens for film, witness various 16mm conversions of TV zooms, but apparently there would be problems putting film lenses on video cameras.

Phil

Nate Yolles · July 14, 2004

I've heard of people using very thin diffusion - opal or hampshire - placed between the subject and the background to simulate a shallow depth of field. It sounds like it could be a clever idea in certain situations. I've never personally seen it done though.

Alvin Pingol · July 14, 2004

I've heard of people using very thin diffusion - opal or hampshire - placed between the subject and the background to simulate a shallow depth of field. It sounds like it could be a clever idea in certain situations. I've never personally seen it done though.

You mean, something like this? ;)

Michael Nash · July 20, 2004

Using the 'macro' ring, if your lens has it, can give very shallow depth of field at the expense of easy focus moves.

...provided your subject is close to the lens. If your MCU is on a person 7-10 ft. away (as is often the case), the macro doesn't do you any good. But if you can stage the action close to the lens, the macro does do a good job of buzzing the BG.