I am often asked “Can I not just bring one 18-300 lens instead of bringing all these multiple lenses?”

Well, of course you can. There is some benefit from carrying just one lens. Convenience is important.

So why do we often forgo that convenience and use multiple lenses instead? What is the drawback to a “one does it all” type lens?

There are several.

1. Aperture. Your standard kit lens says “1:3.5-5.6” on the front. Or even “1:4.0-6.3”. That means when zoomed out to a wide angle, the lens can open its aperture to f/3.5 or f/4; and when zoomed all the way in, it can only go down to f/5.6 or f/6.3.
2. Sharpness. General purpose lenses tend to be less sharp than specialized lenses.
3. Distortion. General purpose lenses distort more at then far ends, and show more vignetting. They are neither a great wide angle lens nor a great telephoto lens.
4. The lens hood has to be the right shape for the widest angle, so at a zoomed-in setting, it will be less effective.
5. Size. They are big and heavy when they do not necessarily need to be – you always carry around your longest lens.
6. Build quality. These lenses tend to be less sturdy than “professional” lenses.

So the convenience comes at a cost. and the secret is to see when you need the benefits above, and when you do not. When you are on a vacation outdoors, a general lens may well do. But to take product photos, sports photos, night shots, or pictures for a magazine, I would go with a better lens.

Remember:the picture is not made by the camera, but by the lens (and by you!).