Understading of Aperture control.

Hi All,
Can someone please clarify me with the below question.
I'm trying to understand fixed aperture with variable focal length lenses. Imagine I have a f2.8 70 - 200mm lense my understanding is aperture will remain at f2.8 when you change your focal length from 70 to 200mm but you can control the aperture size with your camera body by using it in aperture priority mode, so setting it to f8 in your camera the amount of light passing will be less in this case even if your lense aperture is f2.8.
Please correct me if my understanding is wrong.

Thanks.
Sam.

essentially, the fstop that you have set on the camera is the fstop that the picture will be taken. During you composition, it keeps it open, and when you take the shot, stops down to the set value for the exposure.

A variable fstop just means that they did not have enough light to maintain the more light fstop thru the whole lens zoom range. The tradeoff is lens area and cost.

So, the pro level lens often have the most open setting available thru the whole zoom lens, while the less expensive
designs have variable fstops. Lot more explanation out there on the net.

Thanks for your reply stmv, so my understanding is correct, my next question was variable fstop lense but you made it clear.
Thanks.
Sam.