Depth of field and 1/3" chip cameras
 

I'm not getting what the relationship is between the two. Why is shallow depth of field so hard to attain with the lenses on todays camera's?

Got nothing to do with the lenses but rather how light is focused on a smaller/larger area. I would explain in more detail but without the use of diagrams to help you understand I'm not even going to try. :)

All other things being equal, there is no difference in available depth-of-field for a lens set at 50mm for the 1/3" (8mm approx) videocamera sensor and a 24mm wide film image camera gate.

There is another factor, the available field-of-view. The 1/3" video sensor sees a far narrower field-of-view than the film gate through the same 50mm focal length lens.

A 50mm lens at f1.8 aperture on a film camera enables a pleasing focal separation of an approximately television set sized subject from background when the subject is placed about 1.5metres from the camera and the background is about 4 or more metres distant.

A 50mm lens at f1.8 on a 1/3" videocamera will also enable this focal separation, but the whole television set sized object will not fit within the much smaller frame the videocamera sees.

To achieve the same field-of-view from the same camera viewpoint 1.5metres distant from the subject, the videocamera would have to wear an approx 16mm focal length lens or pull the zoom lens back to about 16mm.

A 16mm focal length lens has a much deeper depth-of-field so there goes your pleasing focal separation.

So, the next obvious choice is to move the videocamera back from the subject until the subject itself is framed identically with a 50mm lens or zoom setting. Then you have the available depth-of-field separation from the lens however, the subject is now furthur away and the separation of focus between it and the background is less, so you also lose.

A third kick in the ribs is that the perspective of the image composition also changes and appears un-natural.

The fourth boot into the guts is that there simply may not be enough space available for you to move back far enough from your subject to frame it at 50mm focal length lens setting.

Where the 1/3" sensor really shines is that you can run around all day among violently dynamic subjects with the lens set wide and achieve acceptably sharp images. With 35mm film or 35mm adaptors, you will be hard pressed to achieve the same acceptable results without an assistant.

I hope I have not been too pedantic or repeating stuff you already know in this response.

Everything you want to know (probably more) about depth of field, right here at DVinfo in Jeff Donald's article: http://dvinfo.net/articles/optics/dofskinny.php

thx to all for the reply's. Plenty to ponder now.

One measure of the depth of field is the hyperfocal distance. Everthing from the hyperfocal distance to infinity is in acceptable focus meaning the circle of confusion is appropriately tight for the given sensor size. As the sensor size decreases for a given level of sharpness the diameter of the allowable circle of confusion decreases by the same amount i.e. a sensor half the size of another sensor must be focused to a circle of confusion half the diameter of that of the larger sensor. At the same time to fill the sensor which is half the size of another sensor with an image of the same size of an object at the same distance requires a lens of half the focal length. The hyperfocal distance is approximately H = F*F/N*c where F is the focal length, c is the diameter of the circle of confusion and N is the aperture. Thus reducing the sensor size by 2 reduces H by 4 at the same c but c must be replaced by c/2 for equal sharpness thus H is directly proportional to F and F is directly proportional to sensor size. Thus depth of field is directly proportional to sensor size. Small sensors have large depth of field and conversely.

anyone know of a online dof calculator for DOF and hyper focal distances that has settings for 1/3" chips?

This one is great but doesn't have 1/3" chips:

Online Depth of Field Calculator

thanks.