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-   -   What effects Depth of Field (https://www.dvinfo.net/forum/techniques-independent-production/2452-what-effects-depth-field.html)

Rob Lohman June 20th, 2002 06:48 AM

What effects Depth of Field
 
I have some questions regarding Depth of Field. I have been
experimenting with my XL1S and the standard 16x lens to shorten
the DOF (ie, forground subject in focus, background out of focus,
a bit blurry). I haven't had much luck.

Which parameters change the DOF? I know that the more the
lens is open the shorter the DOF (I was at f1.6 while testing).
Does shutter speed and where I am in the zoom range affect
any of this as well? Are there any other things?

Thanks!

Al Osmond June 20th, 2002 07:33 AM

Hi, Rob

I have always understood that depth of field is a function of the size of the lens aperture: NOT the aperture expressed as its relation to the focal length (f/2.8, f/16) but the physical size of the hole.

Thus, in the case of a 300mm f/2.8 lens, the aperture will be a certain size in millimetres. With a 10mm f/2.8 lens, the aperture is the same as far as exposure is concerned, but the actual hole will be considerably smaller.

A lens designed for use with a small target such as a 1/3" chip must have a much shorter focal length than a lens intended for 16mm or 35mm film if it is expected to cover the same field of view. The 16x Canon is 5.5mm focal length (?) at the wide end so the physical size of the aperture is miniscule compared to a lens covering a similar field of view on a 35mm camera.

The same problem (if problem it is) applies to 16mm and 35mm cameras, where lenses covering the same field of view on the two formats don't give you the same depth of field.

The shutter speed does come into the equation, but only because the shutter speed needs to be shortened to keep the exposure constant as the aperture is opened up. It is the aperture which affects the depth, though, not the shutter speed.

To reduce the depth of field to achieve a 'look' similar to that of a 35mm film camera, you're going to have to operate at the widest possible aperture (using ND as necessary) and the longest possible focal length. And the focal length is usually dictated by the size of the subject and the dimensions of the studio!

Realistically, it probably can't be done, although the film industry is populated by folks who believe they can change the laws of physics! :-)

Al

Margus Kivilaan June 20th, 2002 07:47 AM

>and the longest possible focal length.

is'nt it shortest?
Or maybe i'm messed up with terms (i don't feel very confident in English)

Margus

Al Osmond June 20th, 2002 07:52 AM

...better than I am in Estonian. :-)

If we are trying to reduce the depth of field we need to increase the focal length. Therefore, the longest possible focal length...

Al

Margus Kivilaan June 20th, 2002 08:20 AM

yeah Al you are probably right
i''ve used wide-angle lens for doing background smaller and thus less significant, but never looked for focus issues. By using wide-angle can happen bad distortion of linearity which is also no good

Estonian is a weird language and difficult to learn to anybody so 'dont try this at home'
hope to meet you at IBC in Netherlands this autumn (are'nt you coming?)

Margus

Al Osmond June 20th, 2002 08:22 AM

Hey, why not? I will take any excuse to visit the Netherlands...

Al

Margus Kivilaan June 20th, 2002 08:41 AM

done-deal
let's contact somewhere in the beginning of September
Amsterdam is a really good place to visit!

Margus

Jeff Donald June 20th, 2002 02:44 PM

Hi,

Speaking of Laws of Physics and depth of field, one is, that 1/3 of DOF is in front of the focused subject and 2/3 is behind the subject. Once you have focused on your subject, and established the DOF limits, lock your focus and move the subject to the near or far limit without refocusing. Sometimes I control depth of field by not focusing on my subject, but rather a very near object or very far object. Then the subject is placed at the near or far edge of the DOF. This doesn't work for all scenes, but occasionaly gets me out of a jam.

Jeff

Rob Lohman June 21st, 2002 04:43 AM

So Al, with the equipment that I've got, what would be the
most ideal settings to achieve this? Fully zoom in? Zoom out?
f-stop settings?

Since I live in The Netherlands I'll definitely be attending IBC this
year. Would be great to hook up with some people overthere.

Al Osmond June 21st, 2002 07:15 AM

To reduce the depth of field, you need:

the longest focal length you can (zoom in)
the widest aperture (f/1.6).

(Note that you won't get f/1.6 when you are zoomed right in.)

You can shorten the shutter speed to force the aperture wider, or you can add more ND using filters. A polarising filter may help by reducing the amount of light getting to the chips. You're possibly already using -5dB gain.

All of the above assumes your subject matter will allow it. If you're shooting in a small space under the blazing Dutch mid-summer sunshine, you may not have much control over things. :-)

I've marked the dates of IBC on my wall-planner...

Al

Rob Lohman June 21st, 2002 07:44 AM

Thanks Al! I think I understand it now. If time permits I'll run
some more tests this weekend. It is all about testing first. I shot
some beautiful snow shot last december when I just received
my XL1S. It had some gorgeous depth of field in it. Thinking back
I probably had zoomed in quite a bit and ofcourse the lens
was pretty open with an ND filter on it due to heavy sun with
snow light.....

Perhaps things fall into place now. The most times you want
some short DOF is when having a character in the frame. This
might make it a bit ackward with a heavy zoom. Guess I'll
have to see how it all goes.

How about using a wide angle adapter with my standard 16x
lens? Would this help too?

Thanks again.

Al Osmond June 21st, 2002 07:48 AM

The wide angle adapter will increase the depth of field by shortening the focal length of the lens.

You're right about testing. You never have enough time to do all the tests you need.

Al

Margus Kivilaan June 21st, 2002 08:25 AM

to be honest i've never tried to achieve film-like look with video camera by myself, i will try it for sure
i don't know maybe you've already have read it..
http://www.dv.com/features/features_item.jhtml?LookupId=/dv/xml/feature/2001/billups0901
Scott Billups writes how he used handycam camera for making playstation commercial (directed by David Lynch btw.) He used Sony PD150, but i think it is'nt big difference between PD150 and XL1. Not much about technical details, but maybe this helps

Margus

Margus Kivilaan June 26th, 2002 05:29 AM

hey
i found accidentaly from deepest internet corner a description of add-on for DV cameras, which makes filmlike depth of focus. Costs a hell of lot, but very clever solution
http://www.pstechnik.de/pstechnik.htm products>digital film>mini35digital

Margus

Rob Lohman June 26th, 2002 07:17 AM

Margus, that product is widely known (check www.mini35.com).
We even have one user here on this forum using this rig!


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