Focal Length :
1. The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e.g., 28 mm, 50 mm, or 100 mm).
2. In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example, 18–55 mm.
3. for landscape shots—you should buy a lens that offers focal lengths shorter than 18 mm, for example, a 10–24 mm lens. If on the other hand, you want to photograph at greater distances, you should choose a lens that offers focal lengths longer than 55 mm, for example, 55–200 mm.
Aperture :
1 .The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter.
2. A lens typically has a set of marked "f-stops" that the f-number can be set to.
3. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor.
4 Aperture set at f/2.8 = less depth of field means far object will be blurry
aperture set at f/11 = more depth of field means far object and near object both will be sharper
How Aperture Affects Shutter Speed :
The f/stop also affects shutter speed. Using a low f/stop means more light is entering the lens and therefore the shutter doesn't need to stay open as long to make a correct exposure which translates into a faster shutter speed. Again, the reverse is true: using a high f/stop means that less light is entering the lens and therefore the shutter will need to stay open a little longer which translates into a slower shutter speed.
