Prime vs. Zoom Lens DEPTH OF FIELD

[slubig]

The Quick and Dirty:

 

I am curious what, if any differences there are when comparing the depth of field between a Prime Lens vs. a Zoom Lens at identical focal lengths?

 

The Specifics:

 

I am shooting a short film with an HDW-730 and considering renting Canon DigiPrimes instead of using the Canon Zoom which is included with the camera. I am curious if there is a difference in the depth of field when comparing the two types of lenses. I am aware of the Pro35 and not keen on renting it and set of 35mm primes.

 

Recently I attened an HD conference where they demonstrated a digiprime attached to an F900. The depth of field was noticably shallower than a zoom at the equivilent focal length, which is leading me to research this topic!

 

Any help would be greatly appreciated, thank you!

[David Mullen ASC]

Digi-Primes are faster lenses, so you can open the iris more to reduce depth of field.

 

Otherwise, there should be no significant difference in depth of field between primes and zooms IF you match the f-stop, focal length, and distance focused. It's just that most of the HD primes open up at least a stop more than the HD zooms, and are sharper wide-open compared to the zooms. I generally try not to shoot most of the HD zooms any more open than a T/2.8, although I also try not to stop down any more than that either. But I feel fine shooting a Digi-Prime at T/2.0, if not even wider, all the way open.

 

T/2.0 on a 2/3" CCD HD camera is like shooting at a T/4-5.6 split in 35mm, in terms of the look of the depth of field. So if you wanted an even more shallow look than T/4 in 35mm, let's say, you'd have to start using the Pro-35 adaptor. Personally, I tend to try and shoot at T/4 in 35mm; I'm not such a fan of shooting features with a super shallow-focus -- it's too limiting on the actors, puts too much pressure on the focus-puller, etc. I don't like shooting an actor's CU and only being able to keep one eye in focus.

[Stephen Williams]

Otherwise, there should be no significant difference in depth of field between primes and zooms IF you match the f-stop, focal length, and distance focused. .

 

David,

 

One thing to bear in mind that the magnification may be different at the same measured distance when comparing a prime and a zoom! That is the main reason why many people including, myself have in the past, argued that the DOF of a zoom & prime may be different.

 

DOF is calculated from the front entrance pupil, on a long zoom this may be 10" or more further down the lens than the measure point. It took me some time to work out what was happening, I had a Cooke 20-100 & Cooke 100mm prime. By swapping the lenses the zoom was tighter shot. After backing off the camera to match the image sizes the DOF matched!

 

Cheers

 

Stephen

 

P:S

Page 187 of Blain Brown's book 'Cinematography & Practice' I quote:-

"Thus if you are shooting a close-up at the wide end of a zoom it's as if you were 10" closer to your subject matter. Being closer you of course have less depth-of-field. This is one of the reasons that zooms are seldom used in macro, table-top and other critical focus applications."

[David Mullen ASC]

Yes, but he was saying that he noticed LESS depth of field with the prime on the HD camera, not the zoom. This was because the prime is a faster lens. If the physical length of the zoom was a factor, he might have noticed MORE depth of field with the prime.

 

Sure, the length of a zoom will affect some issues when working very close to a subject. But for the most part, depth of field is controlled by the f-stop and distance-focused, and the distance is modified by the focal length chosen.