FF Commentary by Thom

Started Nov 4, 2013 | Discussions thread
Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
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.

Yea the above isnt true. The whole point of the F stop equation is to normalize the numbers so every lens is given a rating we understand. Diameter changes based on FL for a given F stop, yet SS would not. This means the amount of light doesnt change either.

For anybody who doesnt know, F2 on a 50mm lens means the choke diameter is 25mm (50/2). F2 on a 20mm lens would be 10mm. Both, at F2, allow the same light to hit the sensor, even though diameter is different.  So no, aperture diameter is useless unless we multiply it by FL, which is exactly what F stop does. Incidently this is why fast tele is so dang spendy.

 Ontario Gone's gear list:Ontario Gone's gear list
Nikon D7000 Canon EOS 70D Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF-S Nikkor 50mm f/1.8G Canon EF 40mm f/2.8 STM
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