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Old Sep 9, 2004, 11:42 PM   #1
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its just something on my mind thats been keeping me up at night...... :?
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Old Sep 10, 2004, 3:00 AM   #2
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Not sure exactly what you are asking. If it is about depth of field, two settings determine the DOF. Aperture & also the lens focal length.

Apart from the aperture, the lens focal length greatly affects the DOF. i.e when the lens is zoomed to a greater focal length, the DOF is reduced, compared with a shorter focal length wideangle zoom.
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Old Sep 10, 2004, 7:33 AM   #3
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trueno92 wrote:
how does zooming in affect aparture?
F-stop/Aperture is the ratio between the diameter of a lens opening and its focal lenght.
When one zooms in (or out) the actual focal lenght changes, but the lens barrel stays fix -> this ratio (ie f-stop) changes as one zoom!

The more expensive zooms will keep this ratio constant by using larger optical elements with a combination of bigger lens barrel (hence heavier). This becomes only critical when one is in the studio or if the camera is on manual since zooming can affect the exposure by over 1-2 stop! :evil:
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Old Sep 10, 2004, 7:38 AM   #4
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Aperture is a ratio, and is determined by dividing the focal length of the lens by the size of the iris opening.

So, if you had a 50mm lens with an 18mm iris opening, then it's aperture would be f/2.8 (50/18~= 2.8 ). Yet, if the physical size of the iris opening (18mm in this example) did not change, and the lens was zoomed out to 100mm, then your aperture would become approximately f/5.6 (100/18=5.6).

As a general rule, metered aperture (amount of light reaching the film or sensor) and physical aperture ratio are roughly the same. That is, less light reaches the sensor through the lens when longer focal lengths are used (for a given physical size of the iris opening).

The aperture scale (in one stop increments) goes F/1.4, F/2.0, F/2.8, F/4.0, F/5.6, F/8.0, F/11, F/16, F/22... With each one stop move to a smaller aperture (represented by larger f/stop numbers), you will need shutter speeds twice as long for proper exposure.

Here is achart you can use to get anidea of the shutter speeds required for any EV and Aperture (but make sure to use your camera's metering, as lighting can vary -- this is only to give you an idea of how it works). It's based on ISO 100. So, each time double the ISO speed, you can use shutter speeds twice as fast:


Aperture also impacts Depth of Field. Thelarger the aperture (represented by smallerf/stop numbers), and the closer you are to your subject (focus distance), and the longer your focal length (amount of zoom used), the less depth of field you will have (less of the scene in focus, as you get further away from your focus point).

Here is an online depth of field calculator. Plug in your camera model, then change focal length, aperture and focus distance to see what impact aperture has. Note that you must use the actual (versus 35mm equivalent) focal length of the lens for computing depth of field (you'll see a link to a table with information for popular cameras).

This focal length is much shorter than the 35mm Equivalent Focal Lengthon most Digital Camera models (since thesensor is much smaller than 35mm film, the focal length can be shorter):


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