Help understanding 'f' numbers please

[Paulq]

Hi

Last year I bought my first DSLR in a Canon EOS500d with the standard 18-55mm kit lens. I am happy enough with the camera (though do occasionally wonder whether the Sony I was considering would have been easier for me), but would like to invest in a telephoto lens for better distance shooting. Can anyone help me understand the following please:

1. One of the lenses I am considering is a 55-250 mm. I assume that the range of this lens starts at the point at which the kit lens stops? Is this likely to be a good lens or is a 70-300mm a better option? What exactly do the 'mm' dimensions mean?

2. The lenses I am looking at, in addition to the above descriptor, have an 'f' dimension too and I don't understand what this is or how I decide which is best. The 2 particular ones, both around £150, are:

CANON 55-250MM EF-S F4-5.6 IS

And

CANON 75-300MM EF F4-5.6 III USM LENS

All help much appreciated (in beginners language if poss).

Thanks

Paul

[ALwin]

The 'mm' dimensions are talking about the focal length of the lens.
http://en.wikipedia.org/wiki/Focal_leng ... hotography
Many photographers have a rough rule for lenses: Wide angles distort and telephotos compress.
http://en.wikipedia.org/wiki/Perspectiv ... ography%29

The f-numbers such as f3.5-4.5 or 5.6 are talking about the widest aperture available to the lens within the shortest and longest focal range.

Some lenses say 17-55mm f2.8 which means through the entire zoom range of the lens the widest aperture (smallest f-number) stays constant at f2.8. However note that when it comes to Macro/Micro lenses, this is a little misleading as even though they say f2.8, the widest aperture available differs upon focus distance. With Macro lenses, you only get the widest aperture of f2.8 at focus to infinity.

With other lenses that indicate something like f3.5-5.6, such as 70-300mm f3.5-5.6, the widest aperture value does not stay constant throughout the entire zoom range. At 70mm widest aperture is f3.5 and as you zoom in closer the aperture gets smaller till the widest you can get at 300mm is 5.6.

The width of the aperture increases inversely to the value of the f-number. Meaning f2.8 has a wider aperture than f22.

Visit wikipedia to understand what f-stop means: http://en.wikipedia.org/wiki/F-number
Also look at the article on aperture: http://en.wikipedia.org/wiki/Aperture
Roughly speaking, between each whole f-stop value, the amount of light reaching the sensor doubles or decreases by half. Always a factor of 2. So F2 to f2.8 which is a difference of 1 whole unit of f-stop, you are losing half the light as f2.8 has a smaller aperture. From f2 to f1.4 which is another 1 whole unit of f-stop, you are doubling the light as f1.4 has a wider aperture.

So whole units of f-stop values or f-numbers are the following:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 44, 64... (notice that every other number is rougly multiplied by 2).

[Paulq]

Thanks that's helpful - I'll read the articles you cite.