3rd December 2003, 11:19 AM
I was just wondering why on my A80 when on Wide Angle end the largest Aperture size i can go is f2.8 but when on telephoto end the largest aperature size is f4.9 can anyone explain to me why? Thanks
3rd December 2003, 11:27 AM
That depends on the build quality of the lens. Most lenses have different f/stop for different focal lengths. Like what you mentioned. For constant f2.8 throughout the focal lengths, eg the Nikkor f2.8 70-200, those lenses tend to be very much more expensive. Some thing to do with how the lens is made etc.
3rd December 2003, 07:16 PM
3rd December 2003, 07:48 PM
3rd December 2003, 08:28 PM
as mentioned, it depends on how the lens was designed. constant aperture lenses are more expensive and difficult to manufacture, that's why most cameras with fixed lens have variable aperture lenses.
3rd December 2003, 08:34 PM
Just A rough one....
Here i have a qiuck sketch of the possible reason...
Normally Lenses that changes in its physical length when zoomed
tend to have a smaller aperture at its longest zoom position. Those
with constant apertures normally have wider front lens element ......
and internal zoom/focus.....
However, there are lens like the Tamron 200-400mm but its at f5.6
fixed aperture....obvious as its like taking the worst case with respect to
the front lens element.....
Even a 200mm F2.8 + 2x Teleconverter physically will be as long as the 100-400mm and same aperture ...i.e f5.6
Experts....out there...correct me if i'm wrong....
At the end of the day....do expect lowered light gathering ability if the physical lens extends it self like pinocchio's nose...
Remember if you take the MRT ans the train is coming out of the tunnel.....the nearer it gets to the opening...the brighter the outside light.....
3rd December 2003, 09:01 PM
Ah, I just read about this in John Shaw's Nature Photography Field Guide. Thing it has something to do with aperture being related to focal length... thus without expensive correcting optics, the normal behaviour of a zoom is to reduce the aperture as you increase the focal length. Hence constant aperture zoom lens are 'pro-grade' lenses and are highly expensive...
I can look for the exact quote if anybody needs clarification (can't remember the reason to clearly myself...)!
3rd December 2003, 11:03 PM
4th December 2003, 06:14 PM
oh ok i get the rough idea now