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Old Jun 2, 2006, 1:24 PM   #11
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The 3.5 and 6.3 are both max's. 3.5 is the max fro 18mm end of your lens, and 6.3 is the max for the 200 end of your lens. The max for the others, say 100mm for example, falls somewhere in between.

Again, I suggest you go through the link I posted:

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Old Jun 2, 2006, 1:57 PM   #12
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Lenses with larger available apertures (represented by smaller f/stop numbers) can still be used at smaller apertures. Most lenses go to around f/22 (and some goto f/32 or even f/48 ).

Think of the aperture opening as the pupils in your eyes. If you open up the aperture wider, more light gets through, allowing you to expose the image faster for the same lighting and ISO speed (ISO speed represents how sensitive the sensor or film is to light).

Lenses are rated by their largest available apertures (smaller f/stop numbers), and for most (but not all) zoom lenses, you'll see two aperture ratings... the first one is for the widest aperture at the wide end of the lens (least apparent magnification), and the second is the widest aperture at the long end of the lens (most apparent magnification).

The largest available aperture (smallest f/stop number) will fall somewhere in between these two numbers at focal lengths in between the two extremes.

Your Tamron loses a lot of light as you zoom in more. So, keep that in mind in less than optimum lighting. If you can't use a flash indoors, you'll most likely need to stay on the wide end of the lens where it's brighter, and use higher ISO speeds to have a chance at getting good photos of non-stationary subjects.

Some zoom lenses can maintain a constant aperture throughout their focal range (with f/2.8 being the most common). Of course, a brighter zoom lens is larger, heavier and more expensive. For indoor use without a flash, a lens with a constant f/2.8 aperture is preferred in a zoom (but, a brighter prime is even better, allowing faster shutter speeds and/or lower ISO speeds for the same conditions).

Aperture as expressed by f/stop is a ratio, and is determined by dividing the focal length of the lens by the 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 byhigher f/stop numbers), you will need shutter speeds twice as long for proper exposure.

So, a lens with a larger available aperture is desired to get fast enough shutter speeds to reduce motion blur (either from camera shake or subject movement) in many conditions.

Of course, the downside to using larger apertures is a shallower depth of field. This can be a pro or a con, depending on what you're shooting. But, for portraiture, you typically want a shallower depth of field (to help your subject stand out from distracting backgrounds). Again, just because a lens has larger available apertures, doesn't mean you need to shoot that way.

For more information on how aperture impacts Depth of Field, see this Depth of Field Calculator:


The lenses can also use smaller apertures than the largest available (and a lens isn't going to be as sharp at wide open apertures anyway, so "stopping down" a bit can yield sharper photos, and you've got more room to stop down with a prime starting out with larger apertures versus a zoom that may start out at f/2.8 or smaller). But, you need to keep an eye on shutter speeds in less than optimum lighting, no matter what lens you use.

Here is a handy online exposure calculator that lets you see how larger apertures impact shutter speeds, in 1/3, 1/2 or 1 stop increments (you can change this via check boxes at the bottom). Film speed in this calculator is the same thing as ISO speed.


You may also want to get a book on basic photography and read it (or use some of the online sites like members posted links to). The basic concepts of photography are going to be the same for film or digital.

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