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Old April 2nd, 2007, 11:16 AM   #1
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What decides DOF?

Hi,

I'm just wondering what decides DOF on a camera. On our small HVX-200's the DOF is very great, but on our large 2/3" shouldercams it's very shallow.

So what affects the DOF on a camera?

Simple question. Simple answer? ;-)
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Old April 2nd, 2007, 11:35 AM   #2
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Mainly, what afects DOF is light. The more light get's into the camera, less DOF it will show.
In a 2/3" ccd , the pixels are bigger than in normal DV cameras, so the ccd receive more light.
You can do a simple test, point the camera to something, then try different diaphragm positions. You'll see that open and near open, DOF will be smaller.
Small DOF is a characteristic of film cameras, while bigger DOF is characteristic of video cameras.
This is a very simple answer, you can find more complete explanations.

Regards

Bruno
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Old April 2nd, 2007, 11:50 AM   #3
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The aperture size.

The small the aperture, the greater the depth of field but the less light.

The larger the aperture, the shallower the depth of field.

The focal length of the lens also determines the depth of field.

To achieve a given light level, the two adjustable factors are the aperture and the shutter speed. If you want a shallow DOF, you need to open the aperture and use a faster shutter speed.

Another factor is the CCD size. The bigger, the more light it can collect, so the faster shutter/smaller aperture you can use compared to smaller CCDs. A crude analogy would be the ASA speed of film.

See http://en.wikipedia.org/wiki/Depth_of_field
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Old April 2nd, 2007, 11:53 AM   #4
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Light is not the actual determining factor, though it is related.

Image size coupled with lens aperature opening and focal length is what is primarily involved in determining depth of field.

For instance, if the imager is the size of 35mm negative, you will note that with aperatures of 1.4 and a 50 mm lens, you will get a very shallow depth of field. A 135 mm lens at 3.5 or 5.6 will also have shallow depth of field with your 35mm size image.

Now, with video imagers, you have smaller output images. The smaller the imager, the "deeper" the depth of field will be. A 2/3 inch chip will have a much shallower depth of field than a 1/3 inch chip.

To minimize depth of field in a small chip camera, then, you try to open the lens as wide as possible aperature. Alternatively, in a large chip camera, if you want to extend the depth of field, you try to close down the aperature.
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Old April 2nd, 2007, 11:54 AM   #5
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This is a frequently discussed topic on DV Info Net -- please search for a number of existing discussions about it. See also our DOF article, located at http://www.dvinfo.net/articles/optics/dofskinny.php
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Old April 2nd, 2007, 11:55 AM   #6
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For a detailed technical discussion, see Jeff Donald's "Ultimate Depth of Field Skinny" in our articles section: http://dvinfo.net/articles/optics/dofskinny.php
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Old April 2nd, 2007, 12:13 PM   #7
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Well, it's true that aperture size and shutter speed affect DOF, but, what do these settings directly influence ?? It's the amount of light that reaches the CCD, that is why i told in first place that light is the most important factor in DOF.
The other explanations are by far more accurate than mine, but Thomaz wanted a simple answer...

Regards

Bruno

Last edited by Bruno Vaz; April 2nd, 2007 at 07:10 PM.
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Old April 2nd, 2007, 12:46 PM   #8
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I believe that aperture is the ONLY factor that effects DOF.

Light levels, shutter speeds and even lens length are secondary results of this.

A long lens actually requires a wider aperture than a short lens in order to achieve the same f stop. This is because f stops are a function of lens length. Eg, f5.6 on a 300mm lens is a much larger aperture than f5.6 on a 28mm lens. However they both allow the same amount of light through because the longer lens is collecting a narrower beam of light and spreading it out across the imaging device. This is also one reason why it is hard to make a fast, long lens and also why such lenses are so large.

Imaging device size is only relevant because a larger imager requires a longer lens in order to achieve the same angle of view. This results in a wider aperture (all else being constant) and therefore a narrower depth of field.

Shutter speeds and ND filters can be used to control DOF only by the secondary effect of dictating aperture.
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Old April 2nd, 2007, 07:25 PM   #9
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Quote:
Originally Posted by Thomas Hartz-Olsson View Post
Hi,

I'm just wondering what decides DOF on a camera. On our small HVX-200's the DOF is very great, but on our large 2/3" shouldercams it's very shallow.

So what affects the DOF on a camera?

Simple question. Simple answer? ;-)
Because any image in any camera is two dimensional, there can only be ONE PLANE OF ABSOLUTE CLARITY OF FOCUS. Anything in front of or behind that plane of focus will be out of focus to some degree. However, by reducing the aperture (higher number value), one can create the illusion of focus both in front of and behind the actual subject. This is depth-of-field. With 35mm film, the total depth of field extended 1/3 in front of the actual plane of focus and 2/3 behind. All standard film formats have a variation to this 1/3-2/3 rule of thumb. Add the vagaries of digital sensors and one has a very thick forest to negotiate before one can see the trees, where the important information is located. There was a time when dept of field scales were stamped on all lenses. I wonder which MBA idiot identifed that as a cost saving venture? Experiment. Find out what works. Know the limitations of your camera, because the manufacurers do not really care beyond the point of sale.
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Old April 3rd, 2007, 01:13 AM   #10
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Aperture and capture plane (ccd or cmos) size are the only two considerations...Technically. Realistically, focal length will accentuate the DoF and distance of the objects relative to the focal plane.

Aperture/Iris: The wider the iris (smaller f-stop numbers), the shallower the DoF

Capture plane: The larger the capture plane, the shallower the DoF...DV (my XL1s is a 7mm chip) is comparable to 8mm...2/3" chips are about 16mm.

Distance to capture plane: As the point of focus moves away from the camera, the acceptable focus are lenghtens...i.e. focussing on something close to the camera will give a shallower depth of field than something far away.

--VooDoo based on "Circles of Confusion"--

Focal Length: Longer focal lengths magnify the background in relation to the foreground accentualting the out of focus areas in the shot.

Distance between objects: Putting more space between your subject and their background/forground will put the bg/fg elements farther out of the acceptable focal range...longer focal lengths will accentuate the bg/fg element's natural bluriness.

References:
http://www.mediachance.com/dvdlab/dof/index.htm
http://www.luminous-landscape.com/tutorials/dof2.shtml
http://www.waynesthisandthat.com/dof.html
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Old April 3rd, 2007, 01:59 PM   #11
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Thomas,

Focal length and distance have nothing to do with depth of field (DoF): they only affect background blur, which is not the same as DoF. There are only three things that affect DoF: framing, focal ratio, and capture medium size.

For example, if you frame the exact same headshot with a wide-angle lens and a super-telephoto lens, both at f/2.0, you'll find the depth of field is the exact same. However, the super-telephoto shot will appear to have more background blur. What may not be obvious is that the blur is actually the same as the wide-angle shot: all that has changed is that a smaller area of the shot is seen, but at much higher magnification. (Perspective distortion and bokeh are separate issues.)

To see a demonstration of the background blur effect, read the "Background Blur Comparison" on this page:

http://the-digital-picture.com/Revie...ns-Review.aspx

You can actually see how all the focal lengths have the same amount of blur, but different magnifications.

By the way, if you were curious about what causes background blur, it's the same as DoF, with one addition: focal plane to background distance.
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Old April 3rd, 2007, 02:33 PM   #12
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My Kelly wheel tells me that DOF is determined by three things: F-stop (physical aperture size), distance to subject and focal length. I consider those three things factors because each one of them must be included in the calculation to determine DOF.

I've read discussions where focal length as a factor has been debunked, the argument being something like the fact that the same framing with difft focal lengths means you're changing the distance to subject and, therefore, focal length isn't really a determining factor (or something like that)

I guess you could also argue that CoC influences DOF or, rather, that your decision about what size CoC is 'in focus' and what size CoC is 'out of focus' would also determine DOF. If I decided that a 0.03mm CoC is just barely acceptable enough to be called "in focus" and you decide that even a 0.04mm CoC is 'in focus' then you'd have more DOF than I would.
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Old April 3rd, 2007, 03:07 PM   #13
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Quote:
Originally Posted by David Garvin View Post
My Kelly wheel tells me that DOF is determined by three things: F-stop (physical aperture size), distance to subject and focal length. I consider those three things factors because each one of them must be included in the calculation to determine DOF.
Well, "framing" is obviously just a short way to say "focal length and subject distance". It's just too bad that so many people mistakenly believe long focal lengths have a thinner DOF than short focal lengths.

Quote:
Originally Posted by David Garvin View Post
I've read discussions where focal length as a factor has been debunked, the argument being something like the fact that the same framing with difft focal lengths means you're changing the distance to subject and, therefore, focal length isn't really a determining factor (or something like that)
That argument is correct.
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Old April 3rd, 2007, 03:15 PM   #14
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Quote:
Originally Posted by Daniel Browning View Post
Well, "framing" is obviously just a short way to say "focal length and subject distance".
Right. The portion of the calculation that the Kelly wheel is doing with distance and focal length is, basically, a calculation of your image size (or a 'relative calculation' to basically compare the image size at one focal length compared to the image size at another focal length where the distances would be different to achieve the same image size)

That being said, it makes my head hurt to things like "focal length doesn't have anything to do with DOF", but I understand (or I think I understand) the reasoning behind that arguement. The reasoning being (afaiunderstand it) that focal length is a factor only in so much as it determines the framing of a shot with one lens at one distance as compared to a different lens at a different distance.

But since those three things are all taken into consideration to make the final calculation of DOF, I would still say that those three things are involved, but not in the sense that a longer lens has less DOF at the same framing as a shorter lens. Just in the sense that I use those three things to determine what my DOF is going to be.
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Old April 3rd, 2007, 04:06 PM   #15
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Keep in mind that the "...doesn't have anything to do with DoF" is a technicality...the reality is that they are voodoo pieces that increase the size of the Circles of Confusion through magnification. So technically it doesn't play in if you were dealing simpley with the straight physics problem for a class...bet we're making pretty pictures so realistically distance and focal length and atmospheric fog and multitudes of other things play here, but technically in a pure environment, the only things that make a difference to the photons are the imaging size and the aperature size...nothing else. The rest is all mechanics and voodoo :) But mechanics and voodoo make pretty pictures. This forum tends to run very technical at times...take everything here with a grain of salt, we're trying to make art in each of our specialities...don't let the laws of physics stand in your way.
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