if you look at you kit zoom lense it will have on it some thing like Fx.x-Fx.x this means that as you adjust the zoom the apature will change also, in a prime lense this will not happen.
This is why i suggested spending on a good 50mm prime and opting to get macro extention tubes (hollow tubes to extend the focal lenght of the lense) to go between it and the camera body thus mimicking a Macro lense at say 60/90/100mm depending which tube you use
as you put it with your kit lense at 50mm the field of view will be the same as the 50mm prime but the sharpness may not be.
it would be zoomed in not out (takes a little getting used to) small number = wide shot, big number = tight shot E.G. 24mm lense will give a wide angle shot and a 100mm lense will give a tight shot. to see this stand infront of you tank and look through the viewfinder and go from 15mm to 55mm and you will see what i mean.
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