Re: Iris and depth of focus

On 5 Jan 2006 15:07:31 -0800, "ronviers@xxxxxxxxx" <ronviers@xxxxxxxxx> wrote:

>Dear Steve,
>Thank you for the explanation. Obviously you have a very solid
>understanding of this topic. Unfortunately I am not quite prepared for
>the answer.
>I have been trying to avoid the focal length bit of photography. I was
>hoping to gloss over it then pick it up as I went along. Now it seems
>my laziness has given way to mental block. I know it involves the
>lower case f. Sometimes I see f placed beside numbers as if it is to
>be multiplied by the number and other times I see it is placed over the
>numbers as if it is to be divided. The explanation on Wikipedia seems
>to be a bunch of rigamarow with little to do with cameras.

Don't get the focal length of lenses confused with the f number, which is a
ratio used to determine the amount of light and DOF the lens will have...

The focal LENGTH is the distance between the lens center and the plane of the
image it would project, when looking at an image of maximum distance away,
infinity being the best! (A far away scene, maybe 100 feet, will do nicely!)

The larger the focal length of the lens, the larger the picture it will project.
If you put film or a sensor on this image, the larger picture will cover less of
the sensor, giving a telephoto effect. A small lens gives a small picture, and
the entire picture could cover the sensor, giving wide angle effect.

As for the f number, it is the ratio of the length with the diameter of the
lens. Big diameter lenses pick up more light. But if they are long focal
lengths, they project a bigger picture with that light, and the small amount
falling on the sensor means less light for your picture! So the focal length and
the diameter work together, hence the ratio f number.

Each number is twice (or half) the light. They are referred to as f stops since
they 'stop' the light.

As the aperture is stopped down, less of the curvature of the lens is involved,
that's why a stopped down lens is NOT the same as a small lens with less
'stopping'. Lenses work best at a low size apeture, but not necessarily the
lowest.

If you stop a lens down too much, you get a pinhole effect, where the photons
and light waves crowd thru in such a way that the image is focused without the
lens! The presence of the lens just screws things up!

As for the DOF, actually there is only one plane of focus for any lens at any
setting, the fact you can see a wide focus range is because the distortion is
too small to see, A small f number gives less distortion over a longer area...
but it is still there, it is called the circle of confusion, which is the
smallest point you can discern with the lens under it's particular setting.


> I tried
>checking out a book on photography at my local library but they avoid
>all books on photography because of nudity.

Man that's just sick! If God wanted us to be naked, we would have been born that
way!


> What I would really like
>to find is not just an explanation of focal length but an explanation
>with lots of exercises or problems I could work though. Solving
>problems and working toward a solution is what works for me.
>For now I will commit myself to nailing down the focal length aspect of
>photography then return to the issue of iris and depth of focus.

.

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