Concerning PRIMES (fixed focal length lenses) vs. ZOOM (variable focal length lenses):
Almost ALL zooms are limited to a maximum aperture of
f/2.8. To understand roughly what this means ... consider the f/stop scale:
Consider source light has a value of
1
Therefore, an aperture setting of:
f/1 = ½ source lighting
f/1.4 = ¼th “ “
f/2 = 1/8th “ “
f/2.8 = 1/16th “ “
f/4 = 1/32th “ “
f/5.6 = 1/64th “ “
f/8 = 1/128th “ “
f/11= 1/250th “ “
f/16 = 1/500th “ “
f/22 = 1/1000th “ “
f/32 = 1/2000th “ “
f/44= 1/4000th “ “
f/64= 1/8000th “ “
So, as you can see ... you are rapidly losing light as you "tighten"up" the aperture. As such, back to the zoom lens. At f/2.8, you are only getting 1/16th of the available light. In daylight or a well lit room, that is probably fine. But in the evening, when the lights grow dim ... f/2.8 becomes a challenge. If you only have an F/4 lens? Forget about it, without a flash, unless the subject is very still and you have a tripod.
So, let's pop on an
f/1.8 lens. Does not sound like a lot of change, but consider this ... you now are working with
1/6 instead of 1/16 of the available light. That is an amazing difference in the number of photons that are going to strike your camera's sensor.
So, why PRIME? Because, now, your camera can actually detect something, indoors, at a reasonable shutter speed, where before you would have had to have a tripod and prayed that NO ONE MOVED. That can be all the difference in your images ... and play a terrific part in photographing an orchestra (with all those moving arms and fingers) or a dance recital (as the subject gracefully moves, instead of a
motion blur of some kind.). Some venue where a flash simply cannot be used.
The neat part of one of these wider-aperture prime lenses is that they work very effectively on the intro-DSLRs ... allowing these cameras to operate at a much lower ISO, reducing the "noise" that you probably would encounter at ISO-800 or 1600. You can get a lot more mileage, per se, out of the intro-DSLR with a good lens on it. When you are ready to move up to a more advanced camera, that great lens goes right along with you, where it can be appreciated even more.
Camera bodies come and go ... but, great glass is ... well, always great glass.
