Focusing and exposure in low light levels

 When you’re shooting in low light levels, you will often find the exposure process is not intuitive. Autofocusing can also be difficult and sometimes the lens fails to lock onto the subject and you end up with a blurred image – unless you have taken precautions to prevent this.

While fast lenses (with large maximum apertures) will allow more light into the camera than slower ones, they come with a few disadvantages. In order to capture more light, the glass elements used in fast lenses are usually large, which means fast lenses are always bigger and heavier than lenses that are just one f-stop slower. They are also significantly more expensive.