new Sigma 120-300 f/2.8 again? But now with weather sealing for sports?

Started Jan 7, 2013 | Discussions thread
joger
OP joger Veteran Member • Posts: 4,588
Re: new Sigma 120-300 f/2.8 - awesome!

Gearóid Ó Laoi, Garry Lee wrote:

Another chap on-line reckons it's a 290mm f2.9.

o.k. let's do the math (I am btw talking about T-Stop and not f-stop!!)

So we got some 98 mm front lens diameter - with 290 mm focal length - that makes f/3.0 (300/98=3,06) - if Sigma manages it to do a 105 mm filter thread with a bigger diameter significantly more then 98 mm I'd be very surprised - 3 mm to hold a lens is very tight!

The 300 f/2.8 II is advertised as f/2.8 and the front lens has a diameter of 115 mm which leads to a theoretical f-stop of f /2.6 (300/115 mm). DXOmark measures the light transmission to T/3.2 (I trust them - that result is easy to measure) - light in - light out

So we got with 16 lenses in the 300 f/2.8 II a 'misbehavior' of 23 %

Let's assume the 23 lenses of the Sigma 120-300 are extremely good optimized and we only add 5 % additional loss of light instead of the 23 vs 16 lenses of the 300 f/2.8 II which would lead to 43 % additional loss.

so we got following formula: 290 / 98 * 3.2 / 2.6 * 1.05 = 3.82 - which is in favor of the 120-300 - the real world result might be in the range of T/4.0 or even beyond . . .

If you're not into fooling yourself you should understand by now that Sigma is advertising something that's not existing - it is still a good lens but so are the the 70-200rds from other vendors

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