Depth of FieldComplications Ensue
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Thursday, April 12, 2007

Q. What does ccd size have to do with dof? dof is a function of the lens, no?
The lens focuses light onto the image plane. A smaller CCD gives you a smaller image plane, which gives you the same effect as a smaller aperture.
Q. Check this out.
Or maybe I'm misinformed. Can someone straighten us out?



Depth-of-field is a function of aperture width to image-surface width (that is, the width of the lens opening to the size of the ccd sensor). The length of your lens does not change dof, but it makes it more visible in the final image (because you are, in essense, zooming in on the same image as you zoom your lens).
The reason small ccd cameras exhibit less dof is because they have roughly the same width lens as the larger ccd cameras--that is, for a given lens/aperture width, increasing the size of your ccd increases the perceived dof. This is the same reasoning as the reverse--for the same ccd, increasing the aperture (f-stop) will increase the dof.
Lastly, the Redrock M2 works by using standard 35mm lenses (even still camera lenses) and projecting them onto a frosted glass plate of 35mm size. Your video camera is just taking a picture of that glass, not of anything out in the real world, so to speak.
Clear as mud?
By the way, I just finished "Crafty TV Writing" and loved it!

By Blogger rjreimer, at 9:01 AM  

Thanks, Bob!

Actually that's quite clear.

And a clever work-around the Redrock folks have come up with.

By Blogger Alex Epstein, at 9:13 AM  

Disclaimer: as my son has smartaleced (sp?) back to me: "If you bigger, you actually mean smaller, well, then yes."
To the point: I am using depth-of-field ( = depth of focus) in an opposite kind of way. A "lot of dof" means the picture is sharp from here to infinity. I am using the same term to actually mean "clearly visible issues with focus, as in some parts are sharp, so are blurry."
So, yes.
The explaination stands -- it's my use of the term that was a bit reversed. Mea culpa.

By Blogger rjreimer, at 10:50 AM  

A couple further notes:

A shallow depth of field (part of the image in focus, but part not) is associated with a "filmic look" and you can really isolate your subject matter, make it pop from the background.

The bigger the CCD or CMOS, the shallower your depth of field. Since you can't make your CCDs or CMOSs bigger, the only conventional way to affect your depth of field to make it shallower is to open you Iris, or aperture up. Plan your lighting for this. If it's already pretty bright and you can't control it, you can cheat and snap your Neutral Density Filter into place, or even throw a polarizer on.

Changing your Zoom WILL NOT affect your depth of field, but it does make it look like it. Back your Cam up, and Zoom in. Also, work with the space of your Subject in relation to background. If your background is really far away, it will blur...or try to move your subject closer to the background, but move your camera farther away. Your mileage may vary.

35 mm adaptors like the REDROCK or Go35 are cool, but they add hassles ranging from really having to watch your focus, to your image being upside down.

Not all shots need a shallow dof, (though it is awesome in most contexts). Remember, on Citizen Cane they went out of their way to ensure that everything was in focus.

I know some of these points were re-iterated, but I wanted this post to be as un-cryptic as possible.

BTW: I just started on Crafty TV writing. digging it so far.

By Blogger Tim, at 5:17 AM  

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