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Old Apr 28, 2009, 8:11 AM   #1
Join Date: Feb 2009
Posts: 68

What does the specific aperture of a lens mean? I know what aperture is but I just bought a new lens that has an aperture of F4.5-5.8. When I put the lens on the camera & go to say aperture mode I can still change the aperture to a wide variety of different ones. So what does it mean if the lens only has an aperture of F4.5-5.8?

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Old Apr 28, 2009, 8:44 AM   #2
Join Date: Jun 2003
Location: Savannah, GA (USA)
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That's a relatively odd aperture rating for a zoom. A quick google search seems to indicate it's probably a Pentax 75-300mm f/4.5-5.8, which is a a typical consumer grade zoom lens, best used outdoors in bright light.

To answer your question, you can still use smaller apertures (higher f/stop numbers) with a lens like that. That's only the widest apertures available. f/4.5 is the widest available aperture at the widest zoom position, and f/5.8 is the narrowest (smallest) aperture available when zoomed in all the way (most lenses get dimmer as you zoom in more).

For example, you may find that using f/8 gets you a bit better image quality versus using the lens at it's widest aperture settings (provided you have enough light to maintain adequate shutter speeds). Most zoom lenses can use apertures as small as f/22 (and some can stop down to as small as f/45).

Aperture works similar to the pupils in your eyes, where you can open up the aperture iris wider to let in more light, or close it down to let in less light. If you let in more light with a wider aperture, you can expose the film or sensor faster. If you let in less light with a smaller opening, it takes longer to expose the film or sensor. Note that aperture is normally expressed as f/stop, which is a ratio between the focal length of the lens and the diameter of the aperture iris. So, smaller values represent a larger iris diameter.

With a prime (non zoom) lens, you will see one aperture listed (the widest available). But, you can still use smaller apertures (represented by higher f/stop numbers).

With a zoom lens, you usually see two apertures listed (the largest available aperture at wide angle zoom setting, and the largest available aperture at the full telephoto zoom position). When in between the widest and longest focal length of the lens, the largest available aperture will fall somewhere in between the apertures shown. Note that larger (i.e., brighter) apertures are represented by smaller f/stop numbers.

Many high quality zoom lenses can maintain a constant aperture throughout their zoom range (with f/2.8 being the most common). So, you'll only see one aperture listed for this type of higher quality zoom lens (the widest available, since you have that aperture available at all focal lengths supported if desired). But, you can still set it to smaller apertures (higher f/stop numbers). For lower light, primes (fixed focal length versus zooms) can be found that are brighter (smaller f/stop numbers).

These are best for lower light conditons, as a consumer grade zoom (like one with only f/5.6 available if you zoom in much) may be too dim to get fast enough shutter speeds to prevent motion blur from subject movement in lower light. A dimmer lens will also impact the camera's ability to "see" well enough to Autofocus in less than optimum lighting.

When you vary the aperture, you're controlling the iris in the lens (which like a pupil in your eye, can be opened up to let in more light or closed down to let less light in). So, this impacts the shutter speeds you'll need for proper exposure (since more or less light is getting through to the sensor). For example, a lens with f/2.8 available will be able to get shutter speeds 4 times as fast as one with only f/5.6 available for a given lighting condition and ISO Speed. Aperture also impacts Depth of Field (wider apertures will allow for a shallower depth of field when desired to help subjects stand out from distracting backgrounds).

The aperture scale in one stop increments (with larger than f/1 apertures possible but very rare in lenses) 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... With each one stop move to a smaller aperture (represented by higher f/stop numbers), you will need shutter speeds twice as long for proper exposure for the same lighting and ISO speed (only half the light gets through compared to a one stop larger aperture).

ISO speed. This is how sensitive the film or sensor is to light and is the same thing as the older ASA rating for film. The higher the ISO speed, the faster you can expose it (each time you double the ISO speed, you can use shutter speeds twice as fast for the same lighting and aperture.

Shutter Speed is how long the camera's shutter stays open to expose the film or sensor).

IOW, it all boils down to how sensitive the film or sensor is to light (which you control via the ISO or ASA speed of the film you use with film, or the ISO speed settings you use with digital), and how much light you need to let it see to "expose" the iimage (which you control via the aperture opening size and shutter speed).

So, you've got lots of fancy features on newer cameras to automate what settings it uses, and let you vary it's behavior to expose an image darker or brighter than the camera's metering would normally expose it. But, it really boils down to the camera changing the same things you had to worry about with a strictly manual camera without a fancy metering system, Automatic Exposure modes, etc. These exposure calculators and simulators may help you understand it better.



Note that aperture also influences depth of field. See this handy calculator for more information about it:


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