Yatlapball said:
to be more precise... how does aperture size play an important role.
In short.. given the same conditions (i.e. amount of light) bigger apertures allow faster shutter speed combinations to be used.
Bigger apertures = shallower Depth of Field. Search the internet or even this forum for the numerous discussion/articles on Depth of Field
Lemme see if I have the energy to retype what I did a few years ago... hee.
You're right in what the aperture does, but it also signifies quality when you buy a f/2.8.
Do note that f/2.8 wide open means that image quality must maintain at a reasonable standard when shot at f/2.8, production and polishing of the glass elements must be very high and tight. Across the entire zoom range, quality must maintain at f/2.8 optimised even further mostly at f/5.6~8. If a consumer glass achieves optimal sharpness at f/8, expensive f/2.8s will largely and far-ly outperform the consumsers at f/8. Probably at f/4 it already outperforms most (I know AF-S 17-35 does).
f/2.8 will be the hardest to maintain quality at wide open across the zoom, thus you can imagine the price. Aperture stepped down, is concentrating the light rays towards the middle of the glass, which is always the sweetest location across the glass.
Again, for a f/2.8 wide open glass, image quality is constant and very acceptable at f/2.8, the accuracy of the glass through out the range, is kept at very high standards. Aperture is the hole opening. Look through the glass at f/2.8 and see how large the hole is, that's how much quality must be maintained when taking the shot. Phyiscs will come into place here.
f/2.8 allows a lot of light to be used, when stepping down, the light is concentrated towards the middle of the glass, thus the image quality is very further optimised as compared to f/2.8. So comparing a f/2.8 with any variable-zoom glass (ie f/3.5-4.5) is stupid because the aperture is already tattletale of it's glass element's sharpness, colors, distortion control and others.
Don't forget elements produced for f/2.8 must maintain strict quality from the middle of the glass to the outer portions. Variable-aperture zoom glasses must step down the aperture when zooming so that the imperfections of the glass is minimised. Yes, variable-aperture zooms has imperfect glass at the outer areas of the elements so stepping down is necessary to achieve optimal quality when zooming. Why do you think variable-aperture glasses are so cheap? Heh...
So now hopefully, you'll understand why a 70-200 f/2.8VR is so much more expensive than a 18-200 f/3.5-5.6VR. Aperture, glass quality, build is what that makes the difference.
Finale, f/2.8s will definitely be EX as compared to those f/x.x~x.x.