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Old Sep 28, 2005, 3:49 AM   #1
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I was just wondering if anybody can throw any light on the subject of lenses and the numbers that they quote. For example if you buy a lens that quotes 50 - 200, what does that actually relate to? Somebody tried to explain to me but they started talking about focal length and multipliers and it all got a little baffling! Does anybody know a god website with a newbie explanation?

Thanks a lot,

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Old Sep 28, 2005, 5:58 AM   #2
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Dom, To understand the lens numbers, you will need to understand focal lengths, and their meanings. Rather than go into an explanation here I'd suggest you do a Google search for focal lengths.

You will find many sites there. Two I noticed are from Dpreview.com, which explain it all very clearly, and the meaning of multipliers.

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Old Sep 28, 2005, 9:11 AM   #3
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It also depends on whether you are talking actual focal lengths or 35mm equivalents.

On a 35mm camera a 50mm lens gives an angle of view approximately equal to the human eye. A longer lens gives a telephoto effect so 200mm would give a 4x magnification. Shorter lenses are wide angle because they give a wide angle view.

If you fit a lens to a DLSR then depending on the size of the sensor the angle of view changes so something around 30 - 35 mm gives a field of view similar to the human eye. On compact cameras they generally have very small sensors so the lenses tend to be 9 or 11 mm or less, in these cameras the 35mm equivalence is often quoted as it relates the lens to something a lot of people already understand.
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Old Sep 28, 2005, 9:46 AM   #4
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Thanks for the responses. It is starting to sink in..!
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Old Oct 2, 2005, 2:38 PM   #5
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If it won't confuse you more, you can think of focal lengths as 'normal' 'wide' and 'telephoto'.
The way to determine how a normal focal length will look is to compare it to the format you are using.
On 35mm, a 50mm lens is normal because the focal length is very similar to the diagonal measurement of the film. On an aps-c DSLR, 35mm is close to normal (and appears similar to a 50mm on a film camera) becasue the focal lenth is close to the diagonal measurement of the sensor.
Compare that to other focal lengths of lenses and you can determine what would appear to be wide or telephoto.
Hope that wasn't more confusing.
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Old Oct 2, 2005, 7:55 PM   #6
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There are many variables on this, as noted above, but by and large you can say 35mm is "normal" - less than that is 'wide angle'. the lower the number, the wider the shot - more than that is the 'zoom' level, the bigger the number, the more it zooms in.
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Old Oct 2, 2005, 8:06 PM   #7
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You may find this handy (select a focal length at the bottom and your view will change):


Your Fuji S602 Zoom has a lens with an actual focal length of 7.8-46.8mm.

But, you'll want to use 35-210mm when looking at the Canon link above (since it's designed for 35mm cameras).

Because of it's small sensor size, your Fuji can use a much shorter focal length lens to get the same angle of view compared to a 35mm model.

In the case of your Fuji, it's 7.8-46.8mm lens would give you the same angle of view as a 35-210mm lens on a 35mm camera.

Since most camera owners have used 35mm cameras, manufacturers of non-DSLR models will often give the 35mm equivalent focal length, too.

That way, buyers have a better idea of what to expect from an angle of view perspective.

The other numbers you see are aperture. The lens on your Fuji has a largest available aperture of f/2.8 at the wide end of the lens (shortest focal length setting), and a largest available aperture of f/3.1 on the long end of the lens (longest focal length setting). Smaller f/stop numbers are larger apertures.

Aperture is a ratio of the focal length to the iris diameter.

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.

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.

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:


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:


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

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). Also, note how the graphics show the aperture iris diameter changing with your aperture settings (to let more or less light through).


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 sports, night sports in a stadium, or similar conditions, you really need a lens with a constant aperture of f/2.8 or larger a better chance at decent shots of non-stationary subjects, even at high ISO speeds using a DSLR model. 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 effective aperture throught their focal range.

For some uses, 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:


Many users make the mistake of buying inexpensive, slow, 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.

A larger available aperture is often preferred for getting a shallower depth of field, too (so that you can make your subjects stand out more from distracting backgrounds by blurring them).

Although, since Depth of Field is based on the actual focal length (and your subject will occupy a much greater percentage of the frame at any given actual focal length compared to a 35mm camera), this is difficult to do with a camera like your Fuji. A DSLR model is preferred for this purpose.

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).

Your Fuji's lens is bright (f/2.8-3.1) for a zoom lens on a fixed lens digicam with it's focal range.

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Old Oct 2, 2005, 8:55 PM   #8
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After a while, you get a feel for it.

I have a 20D, and when I'm shooting portraiture I reach for my 28-75 lens.I can shoot people 20 feet to about 50 feet away.

If I'm shooting far away stuff, then I reach for my 75-200. I can grab most shots of people reasonably up to 100 yards away.

If I'm doing scenery and want the wide-expanse thing happening, then I reach for my 15-30mm.

So, you can look at all this quite academically, but after a while you get the feel for it.


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