exposure - Why doesn't the picture become darker the more you zoom in?

As your lens's focal length gets longer, fewer photons pass through the lens to hit the mirror/sensor.

Why don't you see darkening when you look into the viewfinder and zoom in with a zoom lens, and brightening vice versa?

Why don't telephoto lenses need longer shutter times than wide-angle lenses?

Answer

The answer to this question revolves around explaining how zoom lenses function because you are correct in your observation: As you zoom to higher and higher magnifications the image dims unless somehow compensation is applied. Suppose you zoom from 25mm to 50mm, should the working diameter of the aperture remain unchanged, image brightness would suffer a 4x loss as to its intensity. Stated differently, each doubling of the focal length will dim, it will be just 25% as bright as it was before the zoom. If true, how is this light loss prevented?

The amount of light energy that can enter the lens is directly related to the working diameter of the iris diaphragm (aperture). The larger the working diameter the more surface area, the more light the lens can gather.

The modern zoom lens has a trick up its sleeve that keeps the image brightness the same thorough most of the zoom. Some high end zooms keep the image brightness throughout the zoom. How this works: The diameter of the aperture as seen when looking into the lens from the front appears larger than it actually is. This is because the front group of lens elements of the zoom lens magnifies thus the diameter of this entrance circle appears larger than reality.

Further, as you zoom, the distance from the front lens group and the iris diaphragm also change. This induces an apparent diameter change. The fact that it is apparent and not a real change is unimportant. From the outside looking in, this change appears real and this action allows more and more light energy to enter as you zoom.

As I said earlier, some high end zooms are good to go through the entire zoom. These are called constant aperture zooms. Lower priced zooms keep a constant aperture until the last 80% or so of the zoom, these fail and suffer the light loss you are asking about.