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Old Apr 2, 2006, 9:58 PM   #1
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I just bought an inexpensive zoom lens for my Canon digital Rebel XT. The lens is 70-300m but then it is described as f4-5.6. Does that mean that this lens only works well with f/stops in between those two numbers?
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Old Apr 4, 2006, 9:32 AM   #2
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This means that at "wide angle" (70mm) the widest opening for the aperture is f4 and when at full "telephoto"(300mm) the aperture is f5.6.

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Old Apr 4, 2006, 3:52 PM   #3
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The answer to your queston is no.

As Paul described, most Zoom Lenses are just rated by their largest available apertures (smallest f/stop numbers) on their widest and longest end.

In between the widest and longest focal length, the largest aperture will fall somewhere in between f/4 and f/5.6 for the lens you describe.

Most lenses will go to at least f/22, and some can use apertures as small as f/32 or even f/45 (some Macro lenses allow you to use very small apertures compared to most lenses).

Maximum sharpness for most lenses usually occurs about 3 or 4 stops down from the largest aperture available.

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Old Apr 4, 2006, 9:49 PM   #4
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Jim,

So if I'm at full zoom (300mm) -- and want the clearest possible pics -- I should have my aperture down two stops from 5.6 and adjust my shutter speed for the best exposure from there?
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Old Apr 5, 2006, 11:02 AM   #5
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It depends on what you're shooting, and in what conditions.

For one thing, you've got depth of field to consider.

A larger aperture (lower f/stop number) will have a shallower depth of field (useful for helping subjects stand out from distracting backgrounds).

A smaller aperture (higher f/stop number) will give you greater depth of field.

You've also got shutter speed to consider.

Smaller apertures require slower shutter speeds for proper exposure. So, it wouldn't do you any good to use a smaller aperture if your shutter speeds would be too slow to prevent motion blur from subject movement or camera shake.


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Old Apr 8, 2006, 10:06 PM   #6
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I would mostly be shooting wakeboarding and snowboarding with that lens.

oh and kiteboarding....
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Old Apr 8, 2006, 11:05 PM   #7
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The rule of thumb for reducing blur from camera shake is to make sure your shuttter speeds are 1/focal length or faster if you're not using a tripod.

For example, if shooting at 100mm (35mm equivalent versus actual), you'd want shutter speeds of 1/100 second or faster (but this may not be fast enough to stop motion blur from subject movement), or if shooting at 300mm (35mm equivalent), you'd want shutter speeds of 1/300 second or faster.

If shooting at maximum focal length (zoomed in all the way) with your new lens, you'd be at a 35mm equivalent focal length of approximately 480mm (same angle of view that you'd have using a 480mm lens on a 35mm camera).

So, it would be a good idea to keep shutter speeds at 1/500 second or faster at max zoom to keep camera shake to a minimum if you are not using a tripod.

That would also be a good speed to aim for to help prevent blur from subject movement. If you go too much slower than that, you may end up with blur from subject movement with rapidly moving subjects (especially if they are going across the frame versus towards or away from you).

In brighter outdoor conditions, you could probably get away with going a couple of stops down (to around f/11) on the long end if you wanted to shooting around ISO 400 and still keep shutter speeds relatively fast. Just keep an eye on shutter speeds and either use a larger aperture (smaller f/stop number) or increase ISO speeds as needed for lighting conditions.

But, you may want to try and isolate your subject more by using a larger aperture at times for a shallower depth of field. There is no right or wrong way. Any number of aperture/shutter speed combinations could be used. Experiment and see what works best for you, for the way you want the photos to look (more or less depth of field, etc.).

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