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Old Jan 18, 2005, 2:33 PM   #1
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What I'm asking, is say I was using a 50mm lens. What differences would I see with an f-stop of 1.7 compared to 2 or 3.5? I see people referring to larger aperatures as "faster" lenses. What does that mean? Thanks!
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Old Jan 18, 2005, 2:50 PM   #2
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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.

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.

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.

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:

http://home.earthlink.net/~terryleedawson/dcnotes/tables.htm

The term "faster" lens is because you can get faster shutter speeds with it.

For existing light use without a flash or tripod, the larger the aperture the better.

Of course, nothing is without tradeoffs. A brighter lens is larger, heavier and more expensive.

Here is a handy online exposure calculator that lets you see how this works in 1/3, 1/2 or 1 stop increments (you can change this via check boxes at the bottom).

http://www.robert-barrett.com/photo/exposure_calculator.html


A brighter (a.k.a., faster) lens also helps a camera to "see" better for Autofocus Purposes. Many cameras won't be able to focus with a slow lens in some lighting conditions. You often seen complaints of the Autofocus hunting when users try to buy inexpensive long zoom lenses (with maximum available apertures of f/5.6 or f/6.3).

For indoor or night sports use, you really need a lens with a constant aperture of f/2.8 or better to have a chance at decent shots, even at high ISO speeds. Otherwise, you'll have motion blur from camera shakeand subject movement.

With zoom lenses, you usually see two numbers listed. One is the maximum available aperture at wide angle, and the other is the maximum available aperture at full zoom. For example, f/3.5-5.6. For higher quality zoom lenses, you may only see one number listed (for example, f/2.8 ). These lenses are able to maintain a constant aperture throught their focal range.

For some uses (i.e. indoor concerts, etc.), many users go to faster (f/1.8, f/2.0) primes, since they can getter even faster shutter speeds this way. Some users invest in f/1.4, f/1.2 or even f/1.0 primes for some low light conditions (but an f/1.0 prime is very pricey -- for example, Canon made one for a while, and it's usually $2k plus on Ebay in used condition.

Aperture also impacts depth of field. Lenses with larger available apertures are desired for portraits (or similiar subjects), where you want a shallow depth of field to help your subject stand out from distracting backgrounds.

You can see how Aperture, Focus Distance and Focal Length work together for Depth of Field purposes with this handy online calculator:

http://www.dofmaster.com/dofjs.html

Many users make the mistake of buying inexpensive, slow, low quality lenses that are not suitable for many conditions. So, you'll need to pick your lenses carefully for the conditions you plan to shoot in, and the largest available aperture is one of the most important considerations.


BTW, a lens with a larger available aperture still lets you use smaller apertures when needed. So, if you want faster shutter speeds and/or a shallower depth of field, you can set it to a larger aperture value(smaller f/stop number); or set it to a smaller aperture (larger f/stop number) when greater depth of field is needed (which will result in slower shutter speeds).


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Old Jan 18, 2005, 7:28 PM   #3
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JimC...if the aperture is a ratio of the focal length to the iris dimension, then is that directly related to these parameters 'f/n'? Eg .. when we have the scale f/11, then is f supposed to be the focal length value, and the number n=11 is some iris diameter? Or am I on the wrong track here?

This has been one of the things that's put me off for a long time (ie in camera manuals etc), since the terms aperture, aperture value, f/stop etc never seem to be clearly defined for the camera users. And I always wondered why they just don't be more straight forward, and simply say 'smaller iris' or 'larger iris'.

But one thing I do kind of understand is that the scale is convenient.. the f/stop scale. So if you start on one of the f/stop numbers in a scale, and then move to the number to the left of it...then it means that the iris gets bigger and lets in twice the amount of light, as compared with the amount of light that was let in initially.
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Old Jan 18, 2005, 7:49 PM   #4
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That answered my question 110%! Thanks!
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Old Jan 18, 2005, 8:12 PM   #5
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Kenny_Leong wrote:
Quote:
JimC...if the aperture is a ratio of the focal length to the iris dimension, then is that directly related to these parameters 'f/n'? Eg .. when we have the scale f/11, then is f supposed to be the focal length value, and the number n=11 is some iris diameter? Or am I on the wrong track here?
Basically, the f is for focal length, and the idea being (after doing all of the math), is that the area of the hole formed by the aperture iris either doubles or halves as you go up or down 1 stop. So, if you go from f/2.0 to f/2.8 (one stop), half as much light gets through, because the area of the aperture iris is being reduced by one half.

Let's say you have a 50mm lens shooting at an aperture setting off/2.8. 50/2.8 ~= 17.86, where 17.86mm is the diameter of the iris opening.

Or, you can take the focal length, and divide it by the iris opening diameter and get your f/stop (50mm / 17.86mm ~= 2.8 )

That's one of the reasons you see lenses that have different aperture ratings as focal lengths get longer, even though the physical size of the iris may not be changing.

For example, if you had a 50-100mm zoom lens, with an iris that has a maximumdiameter of 17.86mm; that would work out to f/2.8 (as in the example above). Yet, if zoomed in to a 100mm focal length, and the size of the aperture iris did not change, you'd have f/5.6 (100mm/17.86mm ~= 5.6)

Now, it gets more complicated. The area (not the diameter) of the aperture iris is what is changing by half or doublewith each one stop change.

Here is an article on it that explains it in detail, complete with formulas:

http://www.uscoles.com/fstop.htm


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Old Jan 18, 2005, 8:33 PM   #6
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Thanks a lot JimC. You explained it super clearly, and now I finally understand what the f/number means physically. Very much appreciated over here. This helped me a lot. Thanks again for the time to explain that. I'll pass the info on if I see anybody asking what f/stop means.
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