what does a fast lens actually mean

[ollyk]

Am I right in thinking that the f number is derived from distance from lens halfway point to film plane divided by equivelent diameter of the Iris? It certainly explains why the f# is reversed!
Regarding Aperture and Depth of Field, I would like to be able to visulise exactly what is going on to increase the DoF when you allow more light in. I have seen a couple of diagrams and can't quite see why allowing more light in allows for a narrow focus range and less light allows for a larger focus range...? Can anyone explain this to me please?

[Kev Lewis]

Everything you want to know about aperture and shutter speeds
Everything you want to know about Depth of Field

The exam will take place at 7pm and will consist of 128 multi-answer questions followed by the practical test

Originally Posted by ollyk

Am I right in thinking that the f number is derived from distance from lens halfway point to film plane divided by equivelent diameter of the Iris? It certainly explains why the f# is reversed!
Regarding Aperture and Depth of Field, I would like to be able to visulise exactly what is going on to increase the DoF when you allow more light in. I have seen a couple of diagrams and can't quite see why allowing more light in allows for a narrow focus range and less light allows for a larger focus range...? Can anyone explain this to me please?

[nightshade]

Expensive prime lenses,with large maximum apertures usually give good to excellent
results without having to stop down isolating the subject and allowing a high shutter
speed just what the doctor ordered but at a high monetary premium

[eeyore]

another advantage is that they allow you to use TCs without losing your autofocus (most AF won't focus effectively if the maximum effective apperture is greater than f5.6) thus a 500mm f5.6 lens with a 2x TC gives you an effective 1000mm f11 which hasnt a prayer of autofocusing, but a 500mm f2.8 with the same set up gives you 1000mm f5.6 where af will work no problem

[ollyk]

Originally Posted by Kev Lewis

Everything you want to know about aperture and shutter speeds
Everything you want to know about Depth of Field

The exam will take place at 7pm and will consist of 128 multi-answer questions followed by the practical test

Looks like I have some reading to do - Thanks Kev! I can't make the 7pm exam though, is that an automatic fail?

[mart_d50]

thankyou all again, I'm learning so much, it's so important to me to be able to imagine how it all works in my head, what effect this has on that etc.

brilliant