Modern lenses for SLR cameras have an Automatic feature built in that keeps the aperture open to the widest aperture ( the maximum speed of the lens) to let in as much light for viewing and focusing - until the shutter is pressed.

 

However, when the shutter is released, it activates a mechanism that stops the lens down to the set aperture or working aperture for the exposure - and then returns it to the open position for ease of viewing the next scene to be shot. Today when we take images with a modern camera, many photographers are not conscious that this happens - that the aperture stops down and lets less light into the lens than when they are looking through the viewfinder.

 

However they may discover this when the viewfinder goes dark - perhaps they add an accessory like an extension ring or bellows unit that does not carry over the auto feature from the lens to the camera - this means they have to use the lens and perhaps the exposure system similar to a preset lens.