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

Started Jan 7, 2013 | Discussions thread
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 It is 2.8, sorry! In reply to joger, Jan 13, 2013

joger wrote:

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

Your calculations may look impressive, especially for people who do not own the lens. But your theory is easily proven false by an easy practical test which I have just done. I set the camera to manual and RAW and made 3 shots of the same object with 120-300 at120mm and at 300mm set to 2.8 and - to compare - with a current Canon L lens set to 2.8, using the same shutter speed for all 3 test shots. All 3 shots look exactly the same in terms of exposure (brightness, density - call it what you will). So the Sigma is indeed 2.8, not 3 or 4 or whatever else you think it is. I am writing this not in order to say something bad about you, but for other people who may be reading this thread and, not having the lens to try, may mistakenly believe you. 2.8 is 2.8 and 4 is 4, it makes a huge difference in terms of image quality, bokeh, etc. It is worth the extra money and the muscle needed to hold it. By the way, s[eaking of IQ - I have used many different good L lenses and I can confirm that Sigma 120-300 is a very good lens in terms of color, contrast, sharpness and handholdability, I'd say surprisingly good:

http://lordofthelens.co.nz/

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