the WHY of Depth of Field

honey

honey

Active member
Why does changing the apature affect depth of field?

Why does zooming in affect depth of field?

Why does the size of the sensor affect depth of field?

I think most people here understand what depth of field is, what it does to your picture, and how to manipulate it and now we only need to understand why it exists and why do the tools we use to manipulate it have the effects that they have.
 
It all boils down to light, basically.

How much or little is available to the iris affects how wide or small it opens, which affects the depth of field. Zooming affects this, filters affect this, and I believe the size of the sensor affects the ability to read light, which affects the depth of field as well.
 
Using shallow or deep focus is merely another technique in which to tell our stories. Both can be completely the right choice
 
It's the same reason why things look sharper to most people in full daylight, and why squinting makes distant things easier to read for most people. Not a scientific explanation, but an easy parallel.
The Wikipedia article is remarkably detailed if you want a scientific and mathematical understanding.
http://en.wikipedia.org/wiki/Depth_of_field
 
Download and watch the full version video tutorial on this link and you will have a great understanding of the answers to your questions :)

http://snodart.com/tutorials.php

There isn't anything explained about sensor size in the video, but the link to the sticky up above helps to explain how the sensor size can impact DOF

Still .... this video does a great job of explaining DOF. You will be glad you watched it.
 
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I think it sounds like the person wants to know the scientific reasons for the depth of field.... not how to get it and such.

The Snodart tutorial is a great one for knowing DoF, but not necessarily the physics and science of WHY. That all has to do with light waves and how they are processed through optics... something I dare not try to explain.
 
=Prodigy= said:
I think it sounds like the person wants to know the scientific reasons for the depth of field.... not how to get it and such.

The Snodart tutorial is a great one for knowing DoF, but not necessarily the physics and science of WHY. That all has to do with light waves and how they are processed through optics... something I dare not try to explain.
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Did you watch the snodart tutorials? I guess not. At least the Apperture part is explained really good and detailed.
 
I agree that part on apperture was very well explained. I really like being shown visually like they did with apature. It's fitting because we're learning about a visual subject. Does anyone know of any visual explanations like they gave on apperture, only for focal length and size of focusing plane? I think a visual aid on focal length would be especially helpful.
 
As I understand it focal length has no direct bearing on the effective DOF, it does have a bearing on apparent DOF to some degree or another.

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)

from Walter Graff
http://www.bluesky-web.com/dofmyth.htm
 
ya .... I think where focal length gives the perception of changed depth of field is in the changed perspective ratio between the subject and the background.

With a long focal length, a much smaller area of the background is filling the entire screen and the perspective is much more like that of a short DOF in the way it looks in that the background becomes more "blown out".
 
As I understand it focal length has no direct bearing on the effective DOF, it does have a bearing on apparent DOF to some degree or another.
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No comprendo.

www.ic.sunysb.edu/clubs/sbutv/glossary.htm said:
Depth of Field
- The area in which all objects, located at different distances from the camera, appear in focus. The depth of field depends primarily on the focal length of the lens, its f-stop, and the distance from the camera to the object.
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tradingideas.orientaltrading.com/trading_ideas/2006/05/scrapbooking_gl.html said:
Focal Length
- The distance between the film and the optical center of the lens when the lens is focused on infinity.
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So if I focus on infinity with my widest, then zoom in, the DoF gets smaller, and objects that were in the foreground are no longer in focus. What am I missing?
 
They are in focus in relation to the background, think of it as a big giant mural, and you are zooming in to a tiny portion. If you step back and take a look at it all, it all seems to be in focus... I'm probably not explaining this very well...

Take a look at:

http://en.wikipedia.org/wiki/Circle_of_confusion

as jenjerdan said...
 
Or check out the link I posted above from Walter Graff he does a great job demonstrating this principal.

So if I focus on infinity with my widest, then zoom in, the DoF gets smaller, and objects that were in the foreground are no longer in focus. What am I missing?
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Whats happening here is that you are seeing what was in actuality out of focus to begin with in the wide shot, your shot was just so wide you couldn't see it. Try this zoom in set your focus for your subject and then come back wide, if all other factors stay the same the DOF should stay the same and your subjects focus in fact shouldn't change either, the focus plane remains at the same absolute distance from the focal plane to the subject regardless of focal length throughout the zoom.
 
LighthouseMedia said:
Whats happening here is that you are seeing what was in actuality out of focus to begin with in the wide shot, your shot was just so wide you couldn't see it.
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Exactly ...... this is the same priciple as to why focusing in HD is so much more critical than focusing in SD. In HD you can see critical changes that would not even become apparent in SD.

And by the same flip of that coin, you can easily accept what appears to be sharp from a distance to be perfectly focused, when it is actually slightly out of focus when you zoom in and see the detail up close.
 
Sure, the foreground subject won't be any more or less 'fuzzy' per inch of detail on the subject but it will be more or less 'fuzzy' on the print/screen. We measure DoF as the range of the maximum possible circle of confusion on the near and the far end of depth on the sensor, film, print, etc., creating an "in focus" zone that shrinks in depth as the lens gets longer. Now it's not reducing any clarity of details but it's definitely less focused.

Wiki quote:
An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged.
 
honey said:
Why does changing the apature affect depth of field?

Why does zooming in affect depth of field?

Why does the size of the sensor affect depth of field?

I think most people here understand what depth of field is, what it does to your picture, and how to manipulate it and now we only need to understand why it exists and why do the tools we use to manipulate it have the effects that they have.
Click to expand...

I was thinking that I covered sensor size in the DOF video, but I guess I didn't (at least not in a way that directly answers your third question).

Here is an old post that should help explain it:
(http://www.dvxuser.com/V6/showthread.php?t=71373)

snodart said:
The reason that 1/3” CCD cameras are limited to a long DOF:

The target size is the size of the physical element that is capturing and recording the image in a given camera. In a motion-picture film camera, it relates to the size of the film. With a DV camera, the target size is the size of the CCD(s) that are recording the image and converting that information into a digital signal. In the case of most miniDV cameras, including the Panasonic dvx-100, the CCD is 1/3". 1/3” does not reflect the actual physical size of the sensor however. The actual width of a 1/3” CCD is somewhere around 5mm wide. Likewise, most 35mm film does not actually have a frame size of 35mm since some of the space is taken up by the perforations in the film and by soundtrack information. The actual frame width of 35mm is often closer to 22mm. So from this, we see that the target size of the 1/3” CCD camera is about 4 ½ times smaller than the actual frame size of 35mm film.

What does this mean exactly? Let’s say we are using a lens with a focal length of 50mm for a 35mm film camera and for a 1/3” CCD camera; both of which are at the same distance from the subject.. The images below show how much of the subject each would see.

ccd1.gif



So, in order for the 1/3" CCD to achieve the same subject size in relation to the frame, we would need to do one of two things… either move the camera way back until the subject size is the same in relation to the frame (as compared to the 35mm frame), or use a lens with a shorter focal length. Always shooting from a great distance is not a very practical solution (and technically it lengthens the DOF anyway), so the better option is to use a shorter focal length lens. This would increase our field of view and allow us to obtain the same subject size within the frame... from the same distance as the 35mm camera (or a comparable distance anyway)..

ccd2.gif


This is why prosumer DV cameras have considerably short focal lengths. The Panasonic DVX100B for example has a zoom lens that ranges in focal length from 4.5mm to 45mm. Having a shorter focal length essentially compensates for the very small target size. Since a short focal length inherently creates a long DOF, 1/3” DV cams are limited to a long DOF.

You can always back the camera up and zoom all the way in so that the focal length of the DVX is at 45mm. Though technically the DOF would be the same, visually it appears to be much shorter. The reason for this is an whole other can of worms...
:)
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