Why would you say the 50mm is faster? And why sharper?
I'm assuming it's sharper because it's focused on one distant than having more than one.
And f/ 1.8 makes a difference? How so? My lowest is 3.5. When I played around with it, my shots are darker when higher? I've read that I should have a low f/ to blur backgrounds and have my subject that's closer sharper. Is that the only purpose?
Thanks
I'll try to make this easy, but you really need to get a basic photography book from the library and read it.
Forget f1,8 for a second and let's call it f2 and call your 55mm far zoom 50mm. With a variable maximum aperture such as your 18-55, setting it at 50mm would be slightly faster than the f5.6 number for the full 55mm. We knocked off 1/3 of a full stop on the f1.8 rounding to f2 so all computations will be extremely close and probably the 50 f1.8 would come out slightly better in the math gymnastics we are about to engage in because I doubt backing off 5mm on the zoom will get you 1/3 of a stop.
First off, you don't have an f3.5 at 50mm ... you have an f5.6 at 50MM.
Now, what this means is that the f2 lens will be capable of using the same shutter speed at the same ISO in half the light of an identical lens with an f2.8 maximum aperture which will be capable of using the same shutter speed at the same ISO in half the light of an identical lens with an f4 maximum aperture which will be capable of using the same shutter speed at the same ISO in half the light of an identical lens with an f5.6 maximum aperture which is what you have.
The f2 lens will also have half the depth of field of an identical lens with an f2.8 maximum aperture which will have half the depth of field of an identical lens with an f4 maximum aperture which will have half the depth of field of an identical lens with an f5.6 maximum aperture which is what you have.
Don't ask me exactly how the math works because it takes some explaining which is why I suggested a good book. Once you read a decent book and play with the camera the math becomes far less intimidating quickly. Please, trust me on this.
Now, to break this into laymen's terms the advantage of the 50mm lens would be:
A - You can shoot in 1/8 as much light at the same shutter speed and ISO setting as the 50 f5.6.
A1 - This allows you to extend the range of your flash.
A2 - This allows you to shoot more often without flash.
A2 - This allows you to shoot handheld when your f5.6 would require a tripod or monopod. (Assuming you have the non VR version.)
B - You can shoot at 8 times as fast a shutter speed at the same ISO under the same light as the f5.6.
B1 - This obviously allows much better freezes of action.
C - You can shoot and capture an image with 1/8 the depth of field of the f5.6.
C1 - This is what gives certain photos that *POP* where the subject is in tack sharp focus while the background is blurred out.
D - It is a shorter lens.
E - It is a lighter weight lens.
F - It is a less expensive lens if you ever had to replace it.
G - It is sharper and less prone to color fringing because it is a better/simpler optical design.
I - It qill pincusgion far less as well as have far less barrel distortion
J - You will also have f2.8 and f4 available as aperture selections, which you don't have now.
K - You will still have access to f5.6 and smaller apertures when needed, just as you do now.
Ad vantages of the 18-55 you have now would be:
A - It zooms with the lens instead of your feet. I could just as well make a case that this is a detriment, because moving as a photographer starts your brain to changing the way you visualize things.
B - It has 5mm more reach on the long end. This edge is so slight it is hardly worth mentioning.
LWW
