Flange focal distance / DOF question

[Don Parrish]

Everyone seems to like that razor sharp DOF on flies, faces and all kinds of things. Personally it bugs me like fingernails on a chalkboard does to others ( my usual high school classroom entrance ). I was trying to learn stacked focus merge on adobe when a thought came to me, has there ever been a camera that had an adjustable flange distance. I realize some lenses may be pushed beyong their infinity ablilities but it would be very useful for those of us that like more than a persons *boogers in focus. If a cameras use TTL focus and exposure comp then flange distance would not matter, even AF would work.

Anyway, just a thought.

* Disclaimer ; "boogers" is strictly a term used by the OP and is not to reflect on DV info.

[Brian Drysdale]

This may be what you're looking for:

Picture Gallery | Lytro

[Don Parrish]

That was unique but I was talking about the ability to move the entire lens, mount and all, in and out to change DOF.

[Chris Medico]

That is called backfocus or flange back adjustment and many cameras have this adjustment.

[Don Parrish]

Is this a user friendly easy access active feature or a maintenance adjustment ?

[Chris Medico]

Its only a maintenance feature.

The issue you will run into is that when you move outside the backfocus distance the lens was designed for you loose the ability to achieve focus towards infinity or you will increase the minimum focus distance depending on which way you move the lens relative to its designed backfocus position.

Such a feature might be useful if lenses focused WAY beyond infinity but most only focus slightly beyond infinity to make up for focus shift at different temperatures.

[Brian Drysdale]

Many cameras have an adjustable flange distance, film cameras use shims and the RED uses an adjustment screw. However, this doesn't affect your DOF, but has mentioned it may allow you to focus closer. In reality it's something you tend to set up and leave alone otherwise you can land yourself with a load of problems if you play around with it during every shot. If you want to change the DOF you have to stop the lens down.

I'd describe the back focus adjustment on many video lenses as being slightly different in that this occurs within the lens rather than changing the flange distance in the camera itself, although the net effect is the same .

[Bruce Watson]

Originally Posted by Don Parrish

...has there ever been a camera that had an adjustable flange distance.

This is the raison d'etre for view cameras. It's what they do -- you bring the scene into focus by moving the entire lens assembly -- moving the flange if you will. That's why these cameras have a bellows -- to facilitate the movements you can make when you decouple the film stage from the lens stage. Lots of very useful creative potential when you do that.

That said, my guess is you really mean movie/video camera when you say "camera". In that case, I can't think of any that make adjustment of the flange focal distance a creative choice. Maintenance/setup yes. Creative, none come to mind. Sorry.

[Alastair Traill]

I used to use 16 mm cameras underwater. As these were very bulky and hence very heavy I tried Nikon’s approach with the waterproof Nikonos. In other words I made a waterproof camera. To do this I removed the movement from an Arriflex camera and built it into a waterproof body of my own construction. I soon learnt just how vital an accurate lens flange to film distance was if you wanted to use a zoom lens and have it hold focus during a zoom. If my setting was 0.0005” out I was in trouble holding focus. Shorter focal length lenses were even more critical. Furthermore the adjustment could not be made using the camera viewfinder as a guide, instead some form of collimator was required.

[Don Parrish]

Maybe a lever that had 2 positions, that way accuracy could be achieved. one with greater DOF one with less. My XL1S had much less DOF than other cameras, as it had 1/3" chips. I am not sure what the FFD was for the XL1S was vs a 2/3" inch camera. I was thinking that a small movement could provide a usable feature to increase DOF. I cringe when I see a nose in focus and eyes that are not. My 2 position flange idea was to help with DOF keeping the camera op from being forced to change aperture, and will aperture even be enough on these things ?.

What about lens adapters, I see that the PL adapters for the AG AF100 do not have glass in them, what happens to the flange distance there ?

http://www.bhphotovideo.com/c/product/752784-REG/SOLID_CAMERA_500009_PL_Adapter_for_Panasonic.html

[Brian Drysdale]

The PL adapters are precisely machined so that when fitted to an AF 100 the PL lens mount will have the correct flange distance. They don't need any optics in the adapter to be used on this camera, since it has a single sensor, which is pretty much the same has having film. The only difference will be that your angle of view will be narrower than when the same PL mount lens is used on a 35mm camera.

The flange distance has nothing to do with the DOF, other than smaller sized sensors and film formats often have smaller flange distances (although not always - you can have PL mount or C mount on 16mm cameras for example, but the DOF remains the same for a particular focal length and f stop), it's just a mechanical distance.that a particular lens mounts need to have, so that the lens is correctly aligned..

Depth of field - Wikipedia, the free encyclopedia

You can increase the DOF by using a tilting lens, rather similar to what you can do with a bellows camera, but this is different to the flange distance.

[Don Parrish]

Brian, not trying to argue with you but flange distance I thought has always had a great effect on DOF.

Flange focal distance - Wikipedia, the free encyclopedia

"For an interchangeable lens camera, the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focal length (FEL), or register, depending on the usage and source) of a lens mount system is the distance from the mounting flange (the metal ring on the camera and the rear of the lens) to the film plane. This value is different for different camera systems.

The range of this distance which will render an image clearly in focus within all focal lengths is usually measured in hundredths of millimeters and is known as the depth of focus."

[Brian Drysdale]

That refers to the depth of focus that you have to form the image at the focal plane, which is different to the depth of field (or depth of acceptable sharpness) that you have for your subject matter at the front of the lens.

The depth of focus is your margin of error (hundredths of millimetres) that you have for mounting your lens onto the camera, which explains why lens adapters & mounts have to be manufactured so precisely.

The method used by the Lytro may be the closest you'll get to the effect you're talking about.
Light Field camera | Lytro

[Don Parrish]

Bare with me Brian, I am not trying to prove you wrong I am trying to learn, I may not be right.

I remember the XL1S as not having great bokeh because it had 1/3 inch chips. This placed the flange distance closer to the senor correct ?? Larger chips require the lens be farther away in order to cover the sensor correct? Such as the 7D vs the 5D Mk II. Doesn't a larger sensor automatically force a longer flange distance ? What creates the razor sharp DOF on the F3, 5D MK II etc. Wiki says this about depth of focus

While the phrase depth of focus was historically used, and is sometimes still used, to mean depth of field, in modern times it is more often reserved for the image-side depth.

""Depth of field is the range of distances in object space for which object points are imaged with acceptable sharpness with a fixed position of the image plane (the plane of the film or electronic sensor). Depth of focus can have two slightly different meanings. The first is the distance over which the image plane can be displaced while a single object plane remains in acceptably sharp focus;[1][2] the second is the image-side conjugate of depth of field.[2] With the first meaning, the depth of focus is symmetrical about the image plane; with the second, the depth of focus is greater on the far side of the image plane, though in most cases the distances are approximately equal.""

Determining factorsIn small-format cameras, the smaller circle of confusion limit yields a proportionately smaller depth of focus. In motion picture cameras, different lens mount and camera gate combinations have exact flange focal depth measurements to which lenses are calibrated.

[Brian Drysdale]

One factor that decides how close a lenses has to be is the mirror. There are lenses which get physically closer to the focal plane because they don't need to clear a mirror shutter, so you can have a 35mm sensor with the lens having very different flange distances, but the same depth of field

The number of iris leaves in the aperture can affect how the bokeh looks.

It's the image size that is an important factor in the depth of field, if it remains the same the DOF is tmore or less the same, but the angle of view with a larger sensor increases compared to a smaller sensor. That means that for the same f stop a 50mm lens on a 35mm motion picture camera has more or less the same DOF as the 50mm on a 16mm camera (There's actaully a bit less on the 16mm, but there's not much in it). .