Depth Of Field and Ramping?
 

Hey, sorry, should be racking not ramping. :/

I'm looking for some tips on working with DOF on the VX2100, specifically racking focus between a near subject and one a few feet behind.

The higher the exposure, the more shallow the depth of field meaning that there is a less in-focus area in front of and behind the subject than with a lower exposure, when there is a deeper and greater depth of field, right?

The is the focal length the distance between the lens and the subject or the lens and the film plane?

The focal length of a lens is the distance from the rear element of the lens to the film / CCD plane.
As for the relation of lens aperture to depth of field, the wider the aperture of the lens (eg. f2.8) the less DOF there will be, compared to a small aperture (eg. f8) - where you'll have a greater DOF: things will be sharper over a given range of distance.

Robin

So would one adjust the focal length with the focus ring or the zoom ring? My camerawork notes say this:

"The subject may be at differing distances from the lens and to compensate for this the image must brought into focus. This is done by moving the lens backwards and forwards in relation to the film plane, away from the plane to accomdodate a closer object and towards the film plane for a more distant object"

Does this refer to zoom? (accomodating an object by it's distance)...Does one zoom in by moving the lens towards the plane? Therefore moving the lens away from the plane zooms out? Or does a zoom-in occur when the lens is moved closer to the subject? I feel a bit queasy when I look at the formulae shown on Wikipedia trying to explain the concept...Does zooming move the lens farther away from the CCD/film plane to a large degree while focusing only shifts the lens farther or closer slightly?

When racking in focus, would you use the focus ring or the zoom ring?

The zoom ring controls the focal length. The focus ring controls focus.

In these lenses, internal elements move to change focus and focal length, not the lens itself relative to the CCD. Look inside the lens while rotating the rings and you will see 'stuff' moving.

I'd ignore the how and focus (urp) on the use of the lens controls. Experimenting is the best way to try everything out.

Rack focusing with the controls on these cameras is fairly difficult as the controls are not directly connected to the lens elements, they are only signals to the servo-motors that move the lens elements. Therefore, speed as well as degrees of rotation have an effect.

Thanks Mike...Are my notes wrong though? Surely to accomodate a close object, you'd want to zoom out...So wouldn't you want to move the lens towards the film plate (decrease the focal length) to do that?

Also, a more specific VX2100 question, we've been taught that when manually focusing, we should zoom in as far as we can go onto our subject and focus that way, than zoom out to the desired level...However when I zoom in far with the VX, the "Subject too close to focus" warning pops up.

Lenses will have a minimum distance at which they'll be able to focus - with a wideangle, you'll be able to focus almost up to the lens surface, but with longer focal lengths (especially as you've zoomed right in to focus) you'll be able to focus only on objects more about three feet away. However, if you zoom back out, you'll find that objects closer than that will become sharp again. You'll just have to learn to focus on a wider setting - or use a closeup lens which will change the range of the minimum distance.

Welcome to the Wonderful World of Optics...

Robin

Robin, you say: ''The focal length of a lens is the distance from the rear element of the lens to the film / CCD plane''. This is quite right when you talk about single element lenses (think magnifying glass being used to burn a newspaper in sunlight) but is not so when we talk about multi-element lenses and especially optically complex zoom lenses.

The focal length of the VX2100 fitted with a 0.5x wide-angle converter is a miniscule 3 mm (1/8"), and the rear element is some way from the three beam-splitting prisms, let alone the chips.

tom.

Yes Tom, you're quite right - we're now going to get into rear nodal points which could be anywhere amongst the various lens elements :-)
At the moment, I don't want to have to go there...

Robin