Depth of field is the range of acceptable focus in front of and behind the primary focus setting. It is a function of not only the specific lens used but also of the distance from lens to primary focal plane and of the chosen aperture.

Larger apertures will narrow the depth of field smaller apertures will increase it ie f1.8 has much less depth of field than f 8.

The aperture is controled by operating a mechanical iris. This is a ring of blades that open and close. The greater the number of blades the more circular the aperture and more circular and natural looking the out of focus highlights. We have become used to hexagonal out of focus highlights in video as most video lenses use 5 bladed iris

Fujis new super zooms introduced a 11 bladed iris.

A wide angle lens inherently has more depth of field than telephoto lens.

A lens with a large minimum aperture is refered to as a fast lens.

In land of HD anything faster than f2 is considered fast.

In 35mm still photography fast lenses begin at f1.2

To create a fast zoom lens the front element has to be rather large.

The fastest HD zooms have front elements that are 200mm in diameter. As a guide compare the diameter of front elements in the list. The 10x Fuji super zoom is 200mm with a minimum aperture of f1.6 whereas the 21x zoom has a front element diameter of 130mm at f2.1

Most primes are fast f1.6. It would be possible to make a f1.0 prime but within the 2/3 inch chip HD cameras their is a prism that causes a kind of optical bottle neck and f1.4 is the limit.

The bottle neck is eliminated with the use of single chip HD imagers. There is much talk of the size of future HD imagers. In a ideal world using 2/3 inch imager for low light feature and documenrty work would be ideal. Swapping the imager for a larger one would give some control of background for other feature work.

So generally speaking in respect of zooms the larger the front element the less depth of field.

The 2/3 inch image size of HD is less than half that of 35mm motion picture film. As a result a wider lens is required on HD to match a given angle of view on 35mm.. Wider lenses inheritly have a greater depth of field, this greater depth of field is both a help and a hindrance.

Great for low light work. Also good for bluescreen work where the subject should be sharp, but the more light one puts on the subject the more needs to be put on the bluescreen, so HD requires less light for bluescreen than 35mm.

However 2/3 inch imager is not so good for controlling the sharpness of backgrounds

For a given angle of view depth of field for a T4 on 35mm compares to T1.6 on 2/3 inch chip.

We have become accustomed to out of focus backgrounds in movies in modern times and associate sharper backgrounds with TV news and soaps. Interestingly sharp backgrounds were in vogue in the 40s, for example look at Gregg Toland's work. The introduction of less sensitive colour films stocks meant a change in practice to using lenses with wider apertures, which meant that backgrounds became softer.

This electo plasm looking background enables distracting backgrounds to be easily eliminated. The viewers attention is focused on the subject more easily. The subject "pops" from the backgound. It may be that this look is fashionable, albeit fashionable for 50 years! One wonders what the impact of consumer cameras with great depth of field will have on the the taste of the cinema going public. Will there come a day when a out of focus background in itself be distracting?

Summing up HD requires less light than 35mm film for a given depth of field.

For a given angle of view and aperture HD has more depth of field.

35mm film lenses have less depth of field for a given focal length and aperture.