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With an ordinary telephoto, the glass elements inside are arranged in a straight line and the light travels straight through the lens from the front to the back. With a mirror lens, the light enters through the front element, is bounced off a concave mirror in the back of the lens, is then reflected back from another mirror behind the front element and finally exits through a small hole in the centre of that first mirror at the back. So why would you choose one over the other? The answer here is a question of price over usability. As described above, the mirror lens is shorter than it's equivalent ordinary lens! A mirror lens does tend to be cheaper for the same focal length. The down side is to do with the operating window; a mirror lens will typically have a narrower depth of field than its comparable ordinary lens. In other words around 2 stops less than a comparable lens; simply it will need more ambient or artificial light than a normal lens.
A prime lens has a fixed focal length. A 35mm prime lens shows you the kind of view and perspective that you are used to seeing with your eyes on a crop sensor camera. A 50mm prime lens shows you the kind of view and perspective that you are used to seeing with your eyes on a full frame camera. Wide angle lenses (can be had as telephoto or prime lenses) open up the field of view, bringing more of the top, bottom and sides of your subject into the picture. Telephoto lenses are rather like telescopes. They increase the magnification and bring far subjects closer. The longer the lens, the higher the number in its focal length and the greater the magnification. The wider the lens, the lower the number in the focal length and the more the scene is opened out for you. As a rule of thumb prime lenses are usually considered to be sharper, telephoto lenses mean that you do not need to keep changing your lens.
For different types of picture. If you are shooting fast action and want to freeze it, you use a fast shutter speed, coupled with a wide aperture (f8 or less). On the other hand, in some pictures depth of field is important. This is something that can be controlled with the aperture. So for landscapes you would use, say, a small aperture (f11 or greater) for a great depth of field and couple that with a slower shutter speed. In the end, as you so rightly say, the same amount of light reaches the sensor, but it has been controlled in different ways for different subjects and different effects.
Assuming that you have a correctly matched lens to camera and that the mechanics and electronics are functioning correctly, there are two further things to consider. Are you shooting at the correct shutter speed and Depth of Field (DOF). The rule of thumb for the shutter speed is to have a shutter speed equal to one over the focal length of the lens; eg 500mm lens = minimum shutter speed 1/500s. The depth of field question is more complex. Basically f2.8 is a very shallow depth of field. F22 is a large depth of field. So the rule of thumb here is the smaller the f number, the more of the image either side of the point of focus will be out of focus.
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