Does the SLR lens focal length and the aperture have a relationship?
The reason why the definition of aperture parameters, because this parameter can be a comprehensive representative of the amount of light. That is no matter how the focal length, as long as the same aperture, then the amount of light into the same. So whether it is 18 or 55 focal length, in the same aperture, the exposure is the same.
From another point of view, if the aperture does not change, the larger the focal length, then the greater the effective aperture. So some of the telephoto cannon caliber is very large.
So some of the low-end lenses that do not do much on the aperture (for example, you say 18-55), the maximum aperture at the 18 end is larger than the 55th end (the smaller the aperture is, the smaller the F value).
There are relationships.
Aperture F value = effective aperture / focal length of the lens
So it is clear that if the effective aperture of the lens does not change, then the larger the focal length, the smaller the F value.
