coming from an astronomical telescope background have a big objective lens was one of the things i looked for to gather more light. the way camera lens seem (to me) to be measured in light speed is the ƒstop. thats ok if you like a thin field of view and chase bokeh, i'm finding i prefer a decent field of view to my images so i probably should be aiming for 2.8 and over in a lens, which brings me to my question. does having a huge objective glass in a camera lens make a difference to the field of view vs ƒ stop. ie: if i had a lens with an objective of 100mm and i stopped it down to ƒ8.0 be the same as having a 30mm stopped down to ƒ8.0 or would that extra glass width gather more light or more data? Do the lenses with the large objectives have them there to benefit them being wide open then the huge glass benefit is lost when stopped down? as you guys been around camera lenses alot longer than i have are there any large objective lenses that are standard or standard / zoom you can mention, somewhere in the 18 - 130 mm focal lengths be it zooms or primes also another thing, i can easily add tubes and converters to my sigma 100-400 but i cant with my sigma 18-35 would this be the fact the 100-400 was a full frame compatible lens where the 18-35 is a crop only so the rear lens is right at the back of it. therefore would it be best to stick to getting full frame lenses for my crop if i want to add tubes to them. or do any full frame lenses have less space at the rear because the way the lenses are focused internally?