Why does more Light increase DOF?

[Andrew McCarrick]

Why does adding more light into a scene increase DOF?

[Luis Caffesse]

Depth of Field is affected by three factors - aperture, focal length, and distance to the subject.

Increased light would force you to shoot at a higher aperture, hence increasing your depth of field.

[Andrew McCarrick]

According to this post,

http://www.cinematography.com/forum2...dpost&p=238039

it says equivalent DoF, so I took it to mean the same DoF as 35mm in 65mm under the same stop on the lens. So am I wrong to take it that way?

[Luis Caffesse]

Ah - well if you're talking about different formats then it's a different story.
To get the same shot with different formats you're either going to have to change the focal length of the lens or the distance to the subject (in order to get the same field of view).

Doing either of those is going to change the depth of field (you will wind up with a narrower depth of field in 65mm than 35mm).

By increasing the amount of light (and thus forcing yourself to shoot at a higher aperture) you will negate the difference and wind up wiht the same equivalent DOF.

Make sense?

[Chris Gearhart]

Luis said it.

The DOF characteristics are not so much a factor of "more light" per se, but the size of the aperture--the size of the hole.

FreshDV.com presented a very helpful tutorial on the whole subject and is worth a few minutes' look.

http://provideocoalition.com/index.p...epth_of_field/

[Tabula Rasa]

Depth of Field is affected by three factors - aperture, focal length, and distance to the subject.

Increased light would force you to shoot at a higher aperture, hence increasing your depth of field.

Newb question: Does focal length affect the DOF, technically speaking? I know the out of focus-areas becomes more blurry the longer the lens is, but what's in focus (DoF) is the same on a 50 mm and a 100 mm with the same aperture. Or am I wrong?

[J. Eric Camp]

Four things WILL change your depth of field.

1. Distance to Subject
2. Aperture
3. Focal Length
4. Image plane size (corresponding with the lens design)

oh and the fifth

5. Jim Jannard with his mind.

Optics is geometry. DOF is about tolerances.

[Conrad Hunziker]

Newb question: Does focal length affect the DOF, technically speaking? I know the out of focus-areas becomes more blurry the longer the lens is, but what's in focus (DoF) is the same on a 50 mm and a 100 mm with the same aperture. Or am I wrong?

Nope- The DoF at the same distance is different when using either lens. For example, shooting 35mm film, 2.40:1 with a 50mm lens at f/4 focused at 10' yields a DoF of 2'6". A 100mm lens with the same specs yields a DoF of 7".

Exactly why a good focus puller is essential.

By the way, the resolving power of the imaging medium - i.e. circle of confusion - also figures into the depth of field.

[Nick Gardner]

3. Focal Length

No it doesn't. This has been covered to death, and yet for some reason it's the biggest misconception out there.

Look at it this way, if you did a wide shot on an 18mm at an f 5.6, almost everything would be in focus. Now if you took a 300mm at the same stop, and backed way the hell up to get the same frame, surprise surprise, same depth of field.

Nick

[Daniel Browning]

Four things WILL change your depth of field.

1. Distance to Subject
2. Aperture
3. Focal Length
4. Image plane size (corresponding with the lens design)

If you ignore the bellows factor and post-processing, there are really only two factors:

1. Focus distance
2. Aperture

Note that aperture does not mean "focal ratio". Aperture is the focal length divided by focal ratio.

The image plane and focal length only affect the field of view, not the depth of field. So if you change the image plane and focal length by the same amount, the field of view stays the same, and if the aperture stays the same, then DOF will also be the same.

All have the following have the same 16.4-foot focus distance, 40 degree AOV, and 6.7 feet DOF:

Scarlet 2/3": 14mm f/0.9
4/3" DSLR: * *24mm f/1.6
Super35: * * *34mm f/2.2
Still FF35: * 49mm f/3.2
645: * * * * *76mm f/5.0
617: * * * * 220mm f/14

EDIT: Notice how they also have the same aperture: 15mm.