I am just starting to learn about photography. I have been trying to understand aperture and I have a question. I think I understand what aperture is and how it affects depth of field.
Ive been watching tutorials and stuff on YouTube about variable and fixed aperture lenses where they talked about a 70-300 f/4-5.6 At 70mm the aperture is 4 and at 300 its at 5.6. The videos also talked about zoom lenses that have a fixed aperture like a 70-200 f/2.8.
Why do I set the aperture on my camera if my lens is already at a certain aperture?
Thanks in advance for your time.

Unless your using a digital camera the apature is always associated with the lens. If you can examine a lens from a 35mm SLR and look through it you can change the apature and see the result. Also depending on the number and position of the lenses in a zoom lens it what can make the difference as to where the focal point is and in effect change the appature rating based on value of zoom or power factor.

A digital camera uses a sensor so it can by artificial means create a given apature through exposure. So keep that in mind. Straight film can not do that. Once exposed that's all you can have from it.

Depth of field and exposure also go hand in hand with the speeds your working with. The greater the F-stop the slower the speeds your working with. Given the value of the film. One example you can test it with is a fountain. Try the different settings and keep a log book. You will see the effects are dramatic in showing the flow of the water. The faster speeds might catch the action where the slower speeds will catch the motion.

If you have never taken a basic photography class then its best if you can to check with your local stores and see if the offer a 1 day class. Start with the basics and keep a log. Above all have fun.