FF Commentary by Thom

Started Nov 4, 2013 | Discussions thread
Bmoon Regular Member • Posts: 291
Re: FF Commentary by Thom

Mark Ransom wrote:

Roland Karlsson wrote:

It is not the F-number that determines how fast a lens is, it is the aperture diameter.

Therefore, an F8 lens might be faster than an F2 lens, if the F8 lens camera is more than 4x bigger.

Not sure I understand. Doesn't "faster" in this context mean you can use a faster shutter speed for the same light level and exposure? That's independent of sensor size.

Lets put it into context, looking at the new fast 1.8 zoom from sigma it shares the same aperture dia as its big brother 24-70 2.8 on a ff. Using that 1.8 on a dx camera would allow more total light to hit the sensor allowing you to use a faster shutter speed that a 2.8 lens on the same dx camera because you have increased the Ap dia. We know that 1.8 dx lens allows that same total light to fall on that dx sensor as the big brother 2.8 lens on FF so this would mean that the same shutter speed could be used on the dx at F1.8 as the FF at F2.8 and they would capture the same total light and would allow for the same shutter speed. The F stop is only regulates the light intensity striking the sensor and its the Ap dia that determines the total amount of light striking the sensor and is really that what determines the shutter speed. Many will argue that they can shoot DX at iso 100 F2.8 1/100sec and on ff iso100 2.8 1/100sec so that 2.8 lens n dx is as fast as the FF lens but they fail to see that the FF image captured 2.25 times more light so that image would contain lmore than half the noise so to equalize the total light striking the sensor one could shoot that FF at iso 225 and increase the shutter speed by 1 stop and still capture the same noise ( total light) as its DX brother but at a faster shutter speed. F stop is really meaningless when comparing between formats as it only gauges the light intensity striking the sensor with 2 vastly different sensors dimensions.

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