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Ok, I'm a little confused. How can f2.8 = f11? I do understand f-stop, basically the number= the number of times that size opening could fit into the overall lens opening. i.e. f11 means the opening at that setting is 1/11th of the total possible, it would fit into the maximum opening 11 times. (larger f-stop number= smaller opening= greater DOF)
But that raises it's own question for me....Isn't the f-stop then a particular percentage of ONLY that particular lens or similar lenses? The answer is obviously yes, so then you can not directly compare f-stop between lenses unless they are VERY similar. That's something of a revelation for me....and I've been taking photo's for decades.....
Guess I'm just not too into the technicalities of photography...(that's odd for me, I'm pretty anal)
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The f number is the ratio of the focal length to the diameter of the opening that the light enters through. For example, a 50mm lense at f2 has an opening 25mm across.
To be perfectly clear, there is no difference in f-stop number between different formats (sensor sizes) when it comes to how much light the medium is exposed to. F2 is f2 when it comes to exposure. The 35mm equivalent that dimitrz is referring to is the equivalent depth of field. The depth of field is not a function of just the aperture but also of focal length. The dependence on focal length, typically much smaller in a digicam, to have such a large depth of field.