Miton.. or the surrouning area's around me isnt the kinda place u wanna walk about with a big cam either lol (neds are us).
I was in glasgow city center the other day shooting a band and relaised i was walking around with over £1500 worth of gear.. im sure ppl would have no problem muggin me for that and probably sell it for a score bag or two lol.
Anyway back on topic...
A lens is the most important part of a camera, Its the lens that renders the image for the sensor to pick up... everything from how many glass elements.. what type of glass, the shape of the aperture blades and how many blades there is.. even the coating on the len's can change the image.
Theres also how the lens will focus (scew drive to connect to in camera motors.. or new lens have einbuilt motors.) Some lens have floating elements controled by gyroscopic sensors to stabilise the image (some cameras just move the sensor for the same effect).
You probably know these things but ill just pretend you don't for a second.
Lens elements with coatings (The better the glass and coatings used.. the more corrected (less distorsion.. flare.. ghosting) you will get on an image.. The better the lens elements and design will give you sharper images, better contrast, better colour rendering and dreamer bokeh (bokeh rendering is based on all elements of the lens design).
Aperture.. This works along with the lens elements. The larger the Fstop (smaller number. ie F1.4 is larger than f2.
the more light a lens can gather but the more glass you need to capture the light. The aperture blades come in all shapes and sizes, some have 5 and arnt very curved at all (giving you hexagon shapes in out of focus areas when the lens is stopped down.. ie an f1.4 lens at f3 as there is now a hexagon shape blocking the light). Some have 9 blades curved.. some old Russian ones even have like 18 blades. Also when you get to like f11 or so and your shooting at night a 5 blade lens will give you a 10sided starshape from highlights (odd number blades double the star effect) and a 8 sided blade aperture will give you
. The lower the FStop the bigger the starshape become. If the aperture ring was shaped like heart.. you would get heart shaped out of focus area's (the lens baby composer lets you do this).
Also the larger the fstop.. the less you can have in focus (f1.4 at close range can have someone's eye in focus and there nose out of focus)
Camera sensor.. The larger the sensor the less it can keep in focus at a time (shooting an fz8 with f2.8 and a 1.5x crop dslr.. is like me shooting the same image at f12 or so to have the same area infocus... but i would need a longer shutter speed as ive basicly killed the amount of light getting to the sensor). The larger sensors have much bigger photsites so gather more light with less noise and the larger area of the sensor (just my opinion) helps to make photos feel like they have more depth.
Camera sensors change alot in a short time but len's dont change so much... 40+ year old lens can still render amazing images and better sensors in cameras help capture the image better.. You can get to a point tho when the sensor becomes to good for a lens (as a lens has a resolution to) but generally you lens gets better with each new body you buy. Thats why lens are the best thing to invest money into
AS for the diffrence between the two lens i posted a picture off...
One is a sigma 24-60mm f2.8 the other is a pentax DA* 50-135mm f2.8
Both have a totally different focus range..although both can gather the same amount of light (the lens in the fz8 is a f2.8 to f3.5), the sigma uses screw drive which connects to the camera's motor to focus the lens and the pentax has both screwdrive for old camera support and a new ultra sonic motor which is VERY quite and slightly faster. The pentax is also fully weather sealed so i can go out in a proper storm if i wanted to and take photos
When shooting a gig, the sigma lets me to do groups to single person shoots, and the pentax lets me do person to head shots.