the so-called ‘normal’ lens is the measured diameter of the image…
i.e. on a 35 mm still camera its 24mmx35mm= 50 mm diagonal distance across the image..
on a hasselblad, its 80mm
on a 4x5 view camera, its 150mm…
which is assuming a “pinhole” lens.. ie simple lens…
but a modern lens is multi-element,—
so a 85 mm movie camera lens does not cast a 85mm circle of light… they concentrate the light coming through the lens, down to a smaller circle , to improve the f-stop/T-stop,  which can screw with the whole depth of field thing also…

Posted by billS  on  08/18  at  07:07 AM

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Here is a link to a very detailed article on DOF.  The “Background blur” section is my favorite.

“Much of the confusion in DOF discussions arises because people base their judgment on out-of-focus parts of the image. A shallow DOF is not synonymous with a generously blurred background. A shallow DOF implies that there is a shallow region in object space that is rendered acceptably sharp, regardless of whether the background is just not sharp or completely blurred.”

http://www.vanwalree.com/optics/dof.html

Posted by .(JavaScript must be enabled to view this email address)  on  08/21  at  09:09 PM

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Sorry to repeat myself from the Red One article, but you are getting a lot of this wrong:

Depth-of-field actually has to do with magnification, subject-to-camera distance and aperture opening.

“The smaller size of the sensor means that lenses must be approximately two times shorter in focal length to capture the same image size as seen on a 35mm sensor.”

BUT - the smaller size of the sensor means that the image must be magnified more to present the same-size projected (or printed) image. This increases the size of the “circles of confusion”, thereby decreasing the depth of field.

In actual fact, the size of the sensor has little (directly) to do with depth-of-field, if equivalent (for the sensor size) focal lengths and apertures are used, and the image is projected to the same size, you’ll get similar depth-of-field.

In practice it’s a little different, because you’d need correspondingly larger apertures on the wider lenses to get the equivalent shallow depth-of field.

You’re confusing focal length and sensor size with magnification. The focal length and sensor size have no direct bearing on depth of field. The magnification, subject distance and aperture do.

Posted by .(JavaScript must be enabled to view this email address)  on  08/28  at  04:06 PM

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DOF vs. format size
http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size
To a first approximation, DOF is inversely proportional to format size. More precisely, if photographs with the same final-image size are taken in two different camera formats at the same subject distance with the same field of view and f-number, the DOF is, to a first approximation, inversely proportional to the format size. Strictly speaking, this is true only when the subject distance is large in comparison with the focal length and small in comparison with the hyperfocal distance, for both formats, but it nonetheless is generally useful for comparing results obtained from different formats.
To maintain the same field of view, the lens focal lengths must be in proportion to the format sizes. Assuming, for purposes of comparison, that the 4×5 format is four times the size of 35 mm format, if a 4×5 camera used a 300 mm lens, a 35 mm camera would need a 75 mm lens for the same field of view. For the same f-number, the image made with the 35 mm camera would have four times the DOF of the image made with the 4×5 camera.

Posted by billS  on  08/28  at  06:05 PM

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You use wikipedia as a source? Wow. This wouldn’t be the first time Wikipedia was wrong.

The Wikipedia article doesn’t take into account the magnification of the negative or sensor for viewing, either by projection, or printing.

You know what happens when you enlarge something? It gets softer. So, the smaller format needs more enlargement to reach the final viewing size.

In reality, images are magnified to the same viewing size, regardless of the source. So, if your target is an 8x10 print, the 35mm frame needs more enlargement than a medium or large-format frame.

This theory is only true if you are looking at the unmagnified negative or slide - which doesn’t happen in reality.

Another thing that people seem to be missing, is that there’s no such thing as “sharp”. There’s only varying sizes of “circles of confusion”. Enlargement affects the size of these circles. Even your “in-focus” parts of the picture won’t look sharp if you enlarge them enough.

Posted by .(JavaScript must be enabled to view this email address)  on  08/28  at  08:28 PM

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