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Old Jan 6, 2005, 5:40 PM   #1
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Please help me to understand lenses what means those numbers for example 8 mm.f/4 or 14 / 2.8 I know what is aperture but I cant understand how to calculate lenses.

Thanks a lot if somebody helps.

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Old Jan 6, 2005, 7:06 PM   #2
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The focal length of a lens determines the angle of view (amount of apparent magnification you have). The angle of view will depend on the type of camera a lens is being used on. For example, a lens with an actual focal length of 50mm on a small digital camera would appear to have a narrower angle of view (much more apparent magnification), compared to the same focal length lens on a 35mm camera.

That's one of the reasons you often seen "35mm equivalent focal lengths" quoted in specifications (because 35mm cameras are popular, so many users know how the angle of view looks for a given focal length lens). As a general rule, a 50mm lens on a 35mm camera is about 1x magnification (about what the human eye sees).

Aperture is a ratio, and is determined by dividing the focal length of the lens by the size of the iris opening. Most Zoom Lenses have two numbers for aperture: the largest available aperture at the wide angle setting, and the largest available aperture at full zoom.You may see this expressed as f/2.8-4.9 (f/2.8 at wide angle, dropping down to f/4.9 at full zoom). Some higher quality zoom lenses only have one number (they can maintain a constant aperture throughout their zoom range).

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, andthis lens was a zoom lens andzoomedto 100mm, then it's 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/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, etc. 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). Of course, lenses with larger available apertures can also be set to use smaller apertures.

A lens with larger available apertures is desirable for a couple of reasons - one is that they allow faster shutter speeds for any given lighting condition. Another is that they can make it easier to blur distracting backgrounds so that a subject stands out from them by using larger aperture settings. But, due to the much shorter actual focal lengths of the lenses on non-DSLR models, the ability to blur backgrounds is very limited (a DSLR is really needed for most subjects for this purpose).

Here is an online depth of field calculator. Plug ina 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).


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Old Jan 21, 2005, 6:47 PM   #3
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thank you
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