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Old January 18th, 2008, 02:58 PM   #1
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C/CS lens & DOF

I'm contemplating an Elphel and wonder about DOF using C mount lenses.
Would you get DOF out of those smaller lens in the same manner as 35mm with a wide aperture or are even wide apertures on C mounts to small to achieve this with?


This is really making me start to wonder about the opposite too--using medium and large format lenses--.
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Old January 18th, 2008, 11:23 PM   #2
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Samuel.

This is a question which is asked quite often and is one of those dead ends for adaptor builders.

The depth-of-field remains essentially the same for a specific focal length lens. The area of the format determines the field-of-view available from a specific focal length lens. Additionally, lenses vary in the actual field-of-view they project onto an imaging surface for a specific focal length.

C or CS mount lenses likely will offer slightly more DOF compared to medium format lenses imaging directly to a small format chip as the available apertures are wider. However for an identical aperture setting, all lenses of a specific focal length will offer a near identical depth-of-field. The medium format and 35mm camera lens bokeh may be superior as the larger physical structure of the iris mechanism enables better fit of the iris blades, more iris blades and a more regular circular shape to the aperture.

CS mount lenses are incapable of covering a digital SLR sized imager. I am assuming in ignorance the Elphel sensor is a DSLR sized sensor. So ignore my comments completely if I am wrong.

Some C mount lenses are capable.

Fujinon's website has a selection of C Mount lenses for webcam, CCTV and machine vision. The published spec tables have columns for formats which some but not all of these lenses can cover as 1/3", 1/2", 2/3" and 1" if my memory is correct.

Last edited by Bob Hart; January 18th, 2008 at 11:27 PM. Reason: error
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Old January 19th, 2008, 01:38 PM   #3
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I think I'm being misunderstood. Forget about sensors and Elphels.
This is just an optics question, and I can't get my head around the math behind it, because I don't know enough on theory. I'm just looking for a check on my own logic.

The smaller the aperture, the wider the DOF--more in focus at once--.

So I want to know some info on f-stops.
The way I understood is that f-stops are a ratio between the lens and the aperture. So a theoretical f-stop of 1 means the aperture is as wide as the lens and you would have such a shallow depth of field, that you couldn't really focus so well. <---Is that true, do I have that right?

So now I want to know how that corresponds to the physical size of a lens.
I know on a pinhole camera, you don't need a lens because there is such a small aperture and no matter how large the lens is, 35mm, 70mm, 1000mm in diameter, they will all focus everything they see if they have an aperture that is shut almost all they way

So what about apertures that are all the way open. I'm just going to use the theoretical f-stop of one, basically, no aperture at all in the lens.
(my real question) Will a 16mm lens have a difference in it's DOF than a 35mm lens or 70mm lens?
Is the physical size of the lens akin to a certain aperture--larger lens is like a larger aperture, smaller lens is like a smaller aperture when aperture is at same f-stop?
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Old January 19th, 2008, 09:36 PM   #4
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Samuel.


I probably have to defer to the real DPs among us as like yourself I am no expert, not an industry player even.

As I understand things from a practical standpoint.

16mm, 20mm, 35mm or any mm number quoted should be taken as the focal length of the lens, no matter what its physical size or diameter.

The f number quoted should be taken as the ability of the lens to pass a measure of light to land evenly across the area the lens was designed to project its image onto.

The adjustable iris values should be taken as the ability of the lens, regardless of its physical size to pass controlled measures of light to land evenly across the surface the image lands upon.


A lens for an airbourne survey camera is absolutely huge. It is designed to pass an evenly lit image onto a very large film area.

A CS-mount lens for a 1/4" CCD security camera is very small. It is designed to pass an evenly lit image onto a very small area.

If you have a camera lens designed to cover the 5 inches or so width of a view camera plate, aperture set to f4, is 70mm focal length and is focussed on an object 10 feet away, you could study an area 1/4" diameter in centre of this image, copy it and put it alongside an image made by a CS mount lens designed to cover a 1/4" sized sensor, aperture set to f4, is of 70mm focal length and focussed on an identical object 10 feet away. The 1/4' wide image from the CS lens and the 1/4" image cut from the centre of the plate camera image should be near identical.

A 16mm focal length lens set to f2.8 will have a naturally deeper depth-of-field compared to a 70mm focal length lens set to f2.8. Set the iris to a higher aperture number on both lenses and the depth-of-field will become deeper. The depth-of-field differences between both lenses will be less apparent at f16 than f2.8.

Last edited by Bob Hart; January 19th, 2008 at 09:42 PM. Reason: error
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