What is written on every lens (i know its focal length and aperture)but plz explain me what it means

Question

Im a beginner on DSLR plz help me.

Answer 1

Focal length might be considered the "strength" of the lens.  Low numbers are wide angle and high numbers are telephoto.  Aperture is the size of the opening in the lens, measured as a ratio to the focal length.

My stock answers will help you:

Focal Length - Zoom Power

There is not a lens-to-distance equation.  If we are talking about a "35 mm equivalent," a 50 mm focal length is approximately "normal" view without any magnification.  For most DSLR's, the lens factor of 1.5-1.6 means a "normal" focal length would be about 31-33 mm.  If we go back to a 50 mm "normal" lens then, a 100 mm lens would be a "2 power" lens, 200 mm would be 4X, and so on.

Just divide the smaller number into the bigger number and you get the "X" power of the lens.  For instance, a 35-80 mm lens is 2.3X zoom.  Divide 80 by 35 and you'll get the result. 35-80 mm lens is 2.3X zoom.  Divide 80 by 35 and you'll get the result.

It is usually better to know what the focal length of a lens in "35 mm equivalent" is and judge by that, rather than relying on the "X" power of the lens.  For instance, most point and shoot cameras start at about 35 mm and have either a 3X or 4X zoom.  This would make it a 35-105 or a 35-140.  I've seen some that start at 28 mm, though.  A 3X starting at 28 mm is 28-84 and a 4X is 28-112.  Neither one is a particularly strong telephoto lens and the 4X is just about the same as the 3X that starts out at 35 mm.

It's also important to realize that tradition dictates that lens focal lengths are usually expressed in terms of "35 mm equivalent," where "35 mm" refers to a 35 mm film camera.  This is because of the relation between the sensor size and the actual focal length of the lens and the resultant angle of view of the lens.

I have one point & shoot that is actually a 5.8-24 mm zoom.  This is a 4X zoom.  The 35 mm equivalent is 28-116 mm.  The sensor is 7.2x5.3 mm. (1/1.8")  (And I wish I knew someone who could explain how the heck they came up with sensor size terminology!)

I have another point & shoot that is actually a 5.7-17.1 mm zoom.  This is a 3X zoom.  The 35 mm equivalent is 34-102 mm.  "How could a shorter focal length give a longer 35 mm equivalent?" you might ask.  It's because the sensor is only about 5x4 mm. (1/2.5")

I have a few Nikon DSLR's and - thankfully - they all have the same size sensor.  They all have a "lens factor" of 1.5.  This means that you just multiply the actual focal length of the lens to get the 35 mm equivalent and then you can make comparisons accurately from camera-to-camera.  Most Canon's, for instance, have a lens factor of 1.6.  On a Nikon DSLR, a 28 mm lens is the "35 mm equivalent" of a 42 mm lens. On most Canon DSLR's, the same 28 mm lens is the equivalent of a 45 mm lens.

These example are just to show you how freaking confusing it can all become if you try to make sense of the "X" power of a zoom lens.

Bottom line...

Check the 35 mm equivalent specifications for the lens.  This way, you will be leveling the field and comparing apples to apples.  More or less.

Here is a mini-tutorial I made myself to compare focal lengths. This is NOT a lens test or a camera test!  It is merely intended to show the difference between various focal lengths.  The lens was the Nikon 18-200 VR lens, which is (by definition) an 11X lens, but that 11X does not tell you what the final image will look like.  I added one more frame taken with a 300 mm lens.  The camera was a Nikon D200 so there is a 1.5X "crop factor," "lens factor," or "focal length multiplier."  There is further explanation on the image itself.  It would help if you click on "All Sizes" above the image.

http://www.flickr.com/photos/7189769@N04...

This was done with two different lenses: the Nikon 17-55 and 70-300:

http://www.flickr.com/photos/samfeinstei...

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Aperture

Aperture is the ratio of the lens opening size to the focal length of the lens.  Here's the formula:

focal length

-(divided by)-      =     aperture diameter

f-stop

Here's an easy example.  Say you have a 50 mm lens and adjust the aperture to f/2.  The actual size of the opening is 50 divided by 2 (50/2) or 25 mm.

Here's another.  Say you have an 85 mm lens set at f/4.  Do the math.  85/4 = 21.25.  The actual opening would be 21.25 mm.

To complicate matters, it's not always this simple.  Camera lenses play all kinds of tricks with physics in the shapes and arrangements of the glass in the construction of the lens and the f-stop we refer to is actually properly called an "effective aperture," but I can't explain that one for you, so I will just stick to the classic examples.