Understanding "speed" of a lens... please help.

Started Aug 4, 2011 | Discussions thread
Eric MORAND
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Re: Understanding "speed" of a lens... please help.
In reply to trekkeruss, Aug 14, 2011

trekkeruss wrote:

No estimation is necessary. f/1.8 is two stops faster than f/3.5, so the shutter speed for f/1.8 would need to be two stops faster than 1/30 of a second, which is 1/125 of a second.

Could you explain why it is "two" stops faster ? On my lens, I have access to f1.8, f2, f2.2, f2.5, f2.8, f3.2 and then f3.5. As a beginner, it looks like there are 6 stops between f1.8 and f3.5. I know this is not true and I know you are right, but I don't get why...

Also, I was sure that the amount of light that was hitting the sensor was influenced by the focal length divided by the f value. I always thought that a 20mm/1.7 was slower than a 25mm/1.7 for example, because the second lens would let more light hit the sensor. It seems like I'm wrong once again, but could you explain why ?

Thanks by advance,

Eric.

 Eric MORAND's gear list:Eric MORAND's gear list
Panasonic Lumix DMC-GX1 Panasonic Lumix G 20mm F1.7 ASPH
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