The zoom lens is by design variable. It is called just a "Zoom" lens. Variable would refer more to those zooms that have a variable aperture at different focal lengths. A fixed zoom retains the same aperture throughout its range. Zooms can vary between wide angle to barely telephoto and then full telephoto. ie 15-55 is generally a wide angle to "normal" 100-300 is telephoto zoom range.
Many zooms also have a macro feature. Some are better than others. But will do if a prime macro is unavailable.
A telephoto lens usually denotes a prime lens in the 100mm and up category.
A wide angle is generally under 35mm
A "normal" lens is generally 50-55mm but some photogs consider a 70mm normal for portraiture.
A prime lens is any fixed focal length lens and are generally considered better quality than a zoom because they are better quality.
DOF refers to the range (distance) in front of and behind the actual focal point that is still in focus. Lenses in general will have a shorter DOF in front of the focal point and more behind it. This of course is shortened with a larger opening.
FOV. Field of view. If a camera is a "cropped" sensor the field of view of the sensor is less than a full 35mm film frame. Lenses made for FF or 35mm (on certain systems) will work on a cropped sensor. A lens designed for the cropped sensor will usually cause vignetting on a FF camera. A cropped sensor gives the illusion of a longer focal length lens but in reality you're just capturing lens then what the lens is capable of.
Tripods. Buy a good tripod. A cheap unsteady tripod is worse than no tripod at all.
In digital it is noise. Grain is for film.
There are 3 diseases aquarium photographers are prone to. MTS, LBA and last but not least CBA. It is recommended to avoid these at all costs. Interventions rarely work.
Thanks for the info, I'm not going to get too indepth on lenses and sensor details but I will correct to "noise" and I will correct Variable Zoom lenses to just "zoom lenses" But I am curious as to what MTS LBA and CBA are?