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Focal Length & Magnification Factors

Tuesday, February 15th, 2005 by | Filed Under: Digital Photography
 
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When people talk about lenses, they talk about focal length. This is basically a measurement that will help you determine how far your zoom can “reach” or how much of that great scenery your wide-angle lens will encompass.

For those who wonder how it’s figured out, it’s the distance from “optical center” of the lens to the point where it is focused when set to infinity. (No we’re not talking luxury cars here—setting a lens’s focus to “infinity” simply means to focus a lens at its furthest distance). Don’t worry, you don’t need to know or understand any of that to figure out how this works :-)

Here’s the practical application: the smaller the focal length of a lens, the more it can “see” and the more “stuff” you can get in the photo. Small focal length lenses are commonly referred to as “wide angle” lenses.

On the other hand, the higher the number, the less the lens is able to “see” and the closer your subject looks. These are usually referred to as “long” or “telephoto” lenses.

Back when 35mm camera ruled the roost and digicams were only cute “toys”, focal length made it easy to compare one lens to another. If it was a 28mm lens, then it was a wide angle lens. If it was a 300mm, it was a telephoto lens. It was simple because the size of a 35mm negative was the same from camera to camera. A 50mm lens on one camera “picked up” the same scene as a 50mm lens on a different camera

Now, that digicams are taking over, the game has changed. This is because of the size difference with the various image sensors out there.

You see, a smaller sensor doesn’t “see” as much of the scene as a larger sensor does. In essence, it’s cropping in—call it a magnification factor. Since sensor sizes range all over the board, the image you get with a 28mm setting on one camera will probably look completely different on another.

Note in the photos above that although the lens sees the exact same thing, the camera with the red sensor captures more of the scene than the camera with the blue sensor. Both were taken with a 28mm lens, but the results are very different.

Fortunately, a lot of manufactures are starting to give the 35mm equivalent ranges when they discuss their equipment. So, instead of saying a camera has a 7mm~28m lens, they give you the 35mm equivalent: 28mm~112mm.

So, if you see a camera that has 35mm equivalent focal lengths listed, here’s a chart to help you decide if you have enough lens:

35mm Lens Chart:

12-20 – Ultra Wide Angle

24-35 – Wide Angle

40-80 – Normal lens

90-135 – Short Telephoto

150-250 – Medium Telephotos

300+ Longer Telephotos

OK, if you’re shooting a regular digicam, that’s about all you need to know. However, if you have a digital SLR (you can take the lenses on an off), you’ll want to read on.

Most digital SLRs do not have a full frame sensor. This means that there is a magnification factor with any given lens. Normally this is around 1.5, so we’ll use that for our example. Check your owner’s manual for the exact number.

Anyhow, here’s what happens with this magnification factor. Let’s say you have a 200mm lens. If your camera has a mag factor of 1.5x that means you take the focal length of the lens—200—and multiply it by 1.5. In this case, your 200mm is now equivalent to a 300mm. Cool!

Now the bad news—If you like taking wide-angle shots, this works against you. For example, that 24mm lens you liked so much on your 35mm camera now looks like a 36mm lens (gasp)! In order to get back to 24, you’d need to fork over lots of dead presidents for a 16mm lens! That pain you just felt was from your wallet.

So, if you’re into long lenses, you’ll probably like the magnification factor, if not, you may need to start stitching images together. Short of that, most manufacturers now have special “digital only” lenses that don’t cost as much as the standard 35mm equipment (won’t work on it properly either). They aren’t necessarily cheap, but it may be worth looking into.

Oh, there are cameras that have a full frame CMOS sensor so there is no magnification factor. Unfortunately, they sell for $7000+ so it’s probably cheaper to buy some new lenses :-)

Geeze I talk too much! Happy shooting!

~Steve

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