Aperture Size


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Ooze

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Nov 6, 2003
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#1
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
 

coke21

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#2
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.
 

SuRfTeC

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#3
Most DC has a different aperture size when you use a different focal length...!

Don't know why too.. but i guess thats the limit of a DC... :confused:

If you want a fix aperture lense... then maybe can change to DSLR where there are lenses that has fixed aperture...! :D

cheers
 

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#4
hi SuRfTeC, it might be the way the lens focus the light in telephoto mode that it does not allow sharp image focusing. so cannot have constant aperture.
:)
 

mpenza

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#5
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.
 

sulhan

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#6
HIee....


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.....

rgds,
me
 

justarius

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#7
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...)!
 

sehsuan

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#8
aperture is a function of focal length (in mm) divided by the diameter of the aperture itself (in mm) - hence theoretically a 85 f/1.8 lens has a focal length of 85mm and a maximum aperture diameter of 47.2mm.

similarly, a 70-200mm f/2.8 lens has the following specs:
at 70mm, f/2.8:
focal length - 70mm
aperture diameter - 25mm

at 200mm, f/2.8:
focal length - 200mm
aperture diameter - 71.4mm

hence a telescopic, constant aperture lens like the canon 70-200 f/2.8L has an increasing aperture size relative to the focal length.

correct me if i'm wrong (quite possible)... of course there are other considerations, such as the dispersion qualities of the glass that may determine if the glass is more suited to a smaller aperture like f/4.5 or f/5.6 to prevent massive flaws like abberation etc. :)
 

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