a bigger diameter, why?

[stylish225]

Hi there

one question has come up while i was looking for a lens which has excellent optical performance.

as mine is nikon d3s and because i needed a standard zoom range lens for the friend's wedding, I looked to choose between new tamron's 24-70 f2.8 and nikon's 24-70 f2.8.

of course they both are already famous for their optical performance but i am wondering why they are designed differently with their filter diagram. Tamron is 82mm and Nikon is 77mm but so far as i know as long as their focal length and aperture are the same each other, their effective apertures are also the same according to the simple fomula (F = focal length / effective aperture)

then why does Tamron design their stuff to have bigger filter diameter than Nikon even though their effective apertures are not different? bigger filter diameter has a lot of disadvantage in weight and size. yes, i know some people who like bigger lens because they think its cool but for me it is just a burden for my neg and shoulder.

can anybody explain this? i think it should be associated with optical performance but it is beyond me to catch it with my little knowledge.

 

[480sparky38 37]

There's other factors involved.

Such as the VC mechanism. And AF motor.

 

[enzodm]

for 70/2.8 the theoretical diameter could be 25mm (much less than both 77mm and 82mm: to both you could ask why), however a zoom may have a complex optical scheme to control aberrations, and no one will be the same. By the way, Tamron has stabilization, Nikon is longer, etc. Different lenses .

 

[Gavjenks]

Here is an example of a lens design with light path diagram:
http://www.freepatentsonline.com/6937401-0-large.jpg
Hopefully this should address your question about why things are not as blissfully simple as "F = focal length / aperture)... it's stupidly bloody complicated.

Slightly longer answer: The "Aperture" in your equation is NOT the diameter of the front element. So although their wide open apertures are (nominally) the same, that has no reason to suggest anything about front elements. They're simply different companies with different patents that there are super complex rules about infringing on, and they may have different glass suppliers that make different element sizes more or less profitable, and blah blah. MOST importantly, the Tamron is several hundred dollars less than the Nikon. That alone completely justifies the difference: You can save a lot of space and bulk by using things like exotic glass aspherical lens elements or fresnel lens elements or whatever, but that stuff costs MONEY. So the bigger element may be annoying, but that's probably part of reason why it's more affordable.

for 70/2.8 the theoretical diameter could be 25mm

The theoretical diameter COULD be 15mm for all we know... There is virtually no obvious hard optical constraint on something like simply the front element. At least not that you could possibly know simply by being told the focal length.
Imagine something like a ridicioulsously rocket-shaped curves protruding element that sticks out 3 inches, has multiple nested lenses inside of it made out of space age materials and obscenely bends large swaths of light in toward the sensor. Who knows.

 

[Braineack]

then why does Tamron design their stuff to have bigger filter diameter than Nikon even though their effective apertures are not different? bigger filter diameter has a lot of disadvantage in weight and size.

Except the Tamron is smaller AND lighter...

4.3in long and 29oz w/VC vs. 5.2in and 32oz w/o VR.

what's more annoying is that the 70-200 is a 77mm filter thread so I need a step up to fit my 82mm CPL and can't use the hood with it...

 

[Gavjenks]

what's more annoying is that the 70-200 is a 77mm filter thread so I need a step up to fit my 82mm CPL and can't use the hood with it...

You could just MAKE a hood...

Posterboard with black denim or whatever you have lying around (ideally cloth, if not, matte paint or even sharpie) works about as well as any commercial hood. The fancier, the more permanent.