REALLY dumb quesiton regarding apeture...

[shabba]

Ok this is SO stupid probably, but if you don't ask..........

I'm right in assuming that a 50mm 1.8 lens is just as fast as any other lens at a given apeture, it's just that the lower a lens can stop down the 'faster' you can go.

When I say faster I mean shutter speed. For example if I set up a few cameras in exactly the same lighting conditions, same bodies just different apeture lenses - but all set at F4 then they would all produce the same shutter speed (ISO etc all the same) right?

I.e so the only benefit of the Canon 50mm 1.4 over the 1.8 (other than the build quality etc) is the ability to shoot at 1.4 and therefore increase my shutter speed at smaller 1.4 apeture (and get a smaller DOF obviously, and possibly it's a slightly sharper lens).

I hope that makes sense, feel stupid asking but never known 100%.

 

[Canon Bob]

In short....yes

Bob

 

[shabba]

Ok thought so, sometimes I want to avoid shooting at high ISO so was debating getting a 1.4 in place of my 1.8, but then I don't like the idea of shooting at 1.4 really.

 

[StewartR]

... so the only benefit of the Canon 50mm 1.4 over the 1.8 (other than the build quality etc) is the ability to shoot at 1.4 and therefore increase my shutter speed at smaller 1.4 apeture (and get a smaller DOF obviously, and possibly it's a slightly sharper lens).

Yes. It is sharper and it is better built, as well as the other factors that are derived inherently from it being f/1.4.

Another benefit is that the 50mm f/1.4 has USM, which means the focussing is fast and silent, and you have full-time manual focus override available.

 

[shabba]

Thanks for the info, I've read a fair bit about it - would go for the Sigma I think - but my wait and see if the Canon 50mm 1.4 mkII comes out and how that rates, or affects the prices of the others.

 

[Cuchulainn]

Pretty much the case and you won't go far wrong assuming that f8 is f8 is f8 regardless of the lens. Strictly speaking though, different lens designs could have different overall transmittances regardless of being at the same aperture - to take an extreme example, a simple triplet lens with well-coated surfaces is clearly going to let more light through than some 14 element monstrosity with poor quality coatings and cheap glass with bubbles and other defects in it! That's one reason why you'll also sometime see people refer to "t-stops" - these are like f-stops, but also take into account the total amount of light transmitted through the lens with all losses considered. To be honest though, if you need to worry about t-stops, then you'll already know about them and know why you are worrying about them. For pretty much all dad-to-day use, you can be happy using f-stops!