Hello shooters. I know a student photographer who is basically starting out from the absolute beginning and doesn't quite understand why her new Canon 28-135 mm 3.5-5.6 IS zoom doesn't stay at a constant aperature. On the wide end, we know that the larger aperature of 3.5 is the constant, but when extending to 135mm, the largest f stop is 5.6. in other words, she is having trouble understanding why she can't shoot at 3.5-4.0 at 135mm. She is ordering the new Sigma 70-200 f2.8 which of course is a constant aperature. She may order a 1.4 extender as well
How can I explain why lenses with variable aperatures perform the way they do and you have no option to choose the f stop scale on the entire range of the lens focal length. One thing is, variable aperature lenses are much less expensive than f2.8 constant aperature lenses. She is starting to understand that on a higher quality f2.8 zoom, any shutter speed can be selected. Canon makes many variable aperature lenses and I am certainly getting sharp images from my own 28-135mm IS
Can someone give an explanation on how to better explain how and why variable aperature lenses work.
Thanks, for any help, as i am not so technically an expert, I just try to visualize the shot I try for and get it with whatever lens i have with in reason