F number is the Aperturehttp://en.wikipedia.org/wiki/Aperturemm is the range of the lens 70-300 means the lens goes from 70mm to 300mm.
If a lens is 70-300mm F4-5.6 that means that at 70mm the aperture is F4 at 300mm the aperture is F5.6A lens thats 70-200mm F2.8 that means the lens is F2.8 at 70mm and 200mmLenses that are 50mm F1.4 are fixed zoom lenses there is no zoominghttp://en.wikipedia.org/wiki/Zoom_lens.
Seems you are very new to the photo worldMight want to check out this site to learn the basicshttp://www.shortcourses.com/use/Sparky_caGo take photos ..
Pick up Bryan Peterson's book "Understanding Exposure" if you can, it's all in there .
Or go to Wikipedia.com and search "focal lenth" "lenses" "aperture" "photography" etc.You will learn lots .
I shot my wife last night... I mean 'took photos of' ..
Well 500mm or 100mm or 300mm is the focal point of the lens, that is at 50mm the light passing through the lens becomes a single point of light, remember the old trick with a magnifying glass and a piece of paper, let the sun shine through, move it up and down a bit until it became a single point at which stage it burned the paper, the distance of the glass from the paper at this stage was the focal length..
So thats Focal length (generally speaking the smaller the focal length the wider field of view, so a 10mm lens might be able to "see" 180 degrees but a 300mm might only see 5 1/2 degrees.
The f1.4 f 2.8 f3.5.f5.6 etc are the aperture values whcih are a ratio of size of hole to the focal length but thats all too complicated so a lens with f1.4 is what is known as a "fast" lens has a very large hole, as the numbers increase f2.8 f3.5 f5.6 these are decreasing the the "area" of the hole by 1/2, therefore each "f stop" halves the amount of light it lets in..
Lets say we have an exposure of f8 @ 1/500th second..
If I move up to the next f Stop which halves the amount of light then it's logical to double the time the light hits the sensor, soan exposure of f11 at 1/250th second is absolutely identical.
You may ask well whats the point of having exposures with different numbers that are all identical, well we now start to introduce the third variable related to exposure and thats depth of field DOFf1.4 @ 1/1500th secondf2 @ 1/750th secondf2.8 @ 1/375th secondf4 @ 1/187th secondf5.6 @ 1/94th second.
F22 @ 1/6th second are all identical but the f22 has a long depth of field, that is that a lot of the Photo is in focus both in front and behind the subject but the f1.4 @ 1/1500th second, only the face of the subject (if a portrait) would be in focus and items in front and behind would be beautifully blurred called "Bokeh".
Now you know about 50mm and f1.4 let me tell you it's get complicated from here because we havent talked about ISO another doubling or halving of sensitivety of the film or sensor.Go look athttp://www.pbase.com/ericsorensen/image/52955921/largePrint it out and keep it as a reference..
You asked "OK so you buy a lens....50mm f14 or 70-300mm f4-5.6 and on and on....what does all that mean?"..
Well, I hope you meant 50mm f/14 and not f14 ! Anyway, that's a standard sort of lens to put in the box with the 35 mm film camera. Only that's a posh (dearer) one as they are usuall f/2 or there abouts. You get them f/18 but that's like pricing something at 4-99 and not 5 imo..
As for what it means, well 50 mm is the focal length and is - in theory and talking about very simple lenses - the distance away that parallel rays are focused. So hold the thing 50 mm from a piece of paper and the sun's rays will focus on the paper and burn it. The usual symbol for the focal length (of 50mm) is f and f/2 means just that: the focal length divided by 2 or 25 mm which is the size of the aperture the light comes through. As the area of lens determines the light coming through but the focal length also comes into it we can say for all lenses that f/2 is the same in terms of light getting though. In other words a standard ratio and not a measurement..
Look at the standard apertures like 1, 14, 2, 28, 4, 56 and 8 and you might just see that, when squared (as we are talking about area) the sequence doubles. Square those numbers and we get (roughly - again) 12, 2, 4, 8, 16, 32 and 64. So they double each time..
In between - and to confuse the issue - we get what were called half stops or third stops. They are useful to give more precise control of the light coming through..
A bit of history: the thirds ran between f/28 and f//8 like this: 28 32 35 4 45 5 56 63 71 and 8. And until the 50's the lens would show a different set of thirds like this f/35 then f/45 then f/63 and so on. Look at old lenses on ebay and you'll see them clearly marked on the lenses..
Lastly, on prime lenses the dial would go "click" for each main or half aperture and so people would talk about opening it up a half click more and so on. All good harmless fun..
First of all, you buy a lens for a a certain purpose, only if you need or want to take certain type of pictures, like:A macro for.
A portrait for.
A tele for.
A wideangle for:.
A fisheye for:.
Second - you buy depending on the quality. A cheapie zoom for convenience or an expensive prime for better sharpness, clarity, colour, contrast, less CA, less distortion, etc..
See more at.