the WHY of Depth of Field

a little more.....

Keep in mind that focal length DOES alter the actual DOF.

If I focus on a subject at 25 ft with a 50mm lens and then focus on the same subject from 25 ft with a 100mm lens, the actual DOF will be very different. At f/5 the DOF with the 50mm lens would be about 29 ft. At f/5 with the 100mm lens the DOF would be about 5.7 ft.

The problem with this of course is that the subject size within the frame is much different between the 50mm and 100mm lenses. With the 50mm we might have a medium shot while the 100mm might give us an extreme close-up.

Now if I place the 100mm lens 55 ft from the subject so that the subject size is comparable to the medium shot that we have with the 50mm lens, then my DOF is 29.6 ft (about the same DOF as the 50mm at 25ft).

In this case the DOF is technically close to being the same, but the apparent DOF in the final images will look very different. This difference in the apparent DOF between the two images is due to the fact that the degree of detail blur varies with the lens focal length. With the 50mm lens, that fact that the background is out-of-focus is less noticeable. With the 100mm lens though, we are seeing a magnified portion of the background... so the fact that it is out-of-focus is amplified.

fov_dof.jpg


So in the image above, the actual DOF might be very similar, but because the telephoto lens is only seeing a small portion of the background... it appears as though the telephoto shot has a much shorter DOF.

Focal length and camera-to-subject distance have a sort of balancing act going with each other when it comes to the actual DOF. As a general rule, if the f/stop and the subject's size within the frame remain constant, changes in focal length and camera-to-subject distance will not alter the DOF (at least not by very much). Focal length and camera-to-subject distance sort of cancel each other out (IF the subject framing remains the same). But again, the telephoto will give the appearance of a shorter DOF.

A telephoto can come in handy when you need the appearance of a shallow DOF, but don't want to risk loosing focus on the subject.
 
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Thanks for the explanation, Snodart. Why is it that if one shoots the same thing with the same subject size and f/stop on the DVX and on 35mm film that the DOF is different? Is it because technically the subject's size in the dvx's footage isn't as large as it is in the 35mm but it appears as such because of the crop factor.

That is to say you shoot a close-up of a person's face on 35mm film and the DVX. On the 35mm you use a 75mm lens while on the DVX the zoom is at about 10mm. With a 10mm focal length the size of the person's face would be quite small compared to how it would look with the 75mm at the same focal length. But since the DVX's imaging chip has 7x magnification factor it appears the same size to us. Would this explain why the DOF is different even though the outputed size of the subject in the frame and f/stop are the same?

davide
 
fov_dof.jpg


This shot is an exact illustration of why Samuelson's statement is 100% wrong. The statement was "There is no point in changing to a shorter focal length lens and moving closer, because if the image size remains the same so will the depth of field." The implication of that statement is that choosing a shorter lens and moving closer will yield identical results; obviously that is not true. Therefore there most certainly *is* a point in changing the focal length and keeping the subject size the same. While technically the actual DOF may be the same, the observed difference in the image is huge. The perception of DOF most definitely does change.

In what is considered the bible of cinematography, "A Hands-On Manual for Cinematographers", David Samuelson states at the conclusion of his section on depth of field : "Depth of field remains the same, regardless of lens focal length, so long as the image size (and f-stop) is the same. There is no point in changing to a shorter focal length lens and moving closer, because if the image size remains the same so will the depth of field." (Focal Press, London, second edition, 1998, p.218)
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Thank you Barry!
Snodart, do you own that image? I'd like to use it for teaching.
Davide, it sounds like you have a correct understanding.
 
davide said:
Why is it that if one shoots the same thing with the same subject size and f/stop on the DVX and on 35mm film that the DOF is different?
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Specifically because the focal length is different.

The smaller your sensor size, the wider-angle lens you need to use to get an equivalent field of view. The wider your focal length, the deeper the apparent depth of field.
 
Barry_Green said:
Specifically because the focal length is different.

The smaller your sensor size, the wider-angle lens you need to use to get an equivalent field of view. The wider your focal length, the deeper the apparent depth of field.
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Okay, that makes all of this easier to understand. Thanks.
 
ProfessorU said:
Snodart, do you own that image? I'd like to use it for teaching.
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Yes, it's mine. Feel free to use it. I uploaded the photoshop file for the image to my site if that helps:

Photoshop version (probably have to right click and save)

ESTEBEVERDE said:
Is that you in your video or an actor?

Great Job!
Click to expand...

Thanks. The woman in the video is an actor. I'm the one behind the camera (and behind my cool DIY teleprompter screen) :thumbup:

Barry, great info as usual.
 
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