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PostPosted: Thu May 12, 2011 3:55 pm 
When using a 2.8 lens and a 3.5 lens both turned up to 5.
are they both going to have the same shutter speed?

I know that i could probably try this myself but if anyone know the answer it would be great if they could tell me the real benefits of a 2.8 lens if most of them will only perform good a notch or two (3.5-4) above their lowest setting when pixelpeeping.


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PostPosted: Thu May 12, 2011 4:03 pm 
At f5 they both let in the same amount of light, so the shutter speed will be the same.

The benefit of an f2.8 lens is that it can open a stop or two brighter than slower lenses. This is helpful if you are in a dark room, and ISO is already turned up, with an f5.6 kit lens, your shutter speed may be 1/20, which if you are shooting people, we will see motion blur as people move their hands and things, where as at f2.8 we will be able to shoot nearer 1/100 and freeze all that motion.

This is obviously useful for sports photographers as well.

Another point is that at f2.8 you can "blow the background out" a lot more than with a slower lens, for that traditional portrait look.

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PostPosted: Thu May 12, 2011 10:37 pm 
in the same light?

no, they won't stay the same.

if you achieved proper exposure at 2.8 then you stop it down, you'll have to compensate for removing almost two stops of light.

so i may be shooting an object at 1/5000sec at 2.8 but if I find that the background is too bokehlicious, I bring it to 5.6 I'll have to slow it down significantly.

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PostPosted: Fri May 13, 2011 7:30 am 
Well in response to jeremy's answer, if I shoot people (more then 1) indoors in low light using F2.8 they would have to sit next to eachother, have their faces in the exact same distance towards the camera for them both to be sharp. How often do people that sit around a table do that? ;) I couldnt shoot that with 2.8 if I wanted them all to look sharp. So I dont really see the benefit, being able to freeze moving objects but only a fraction of the image will look "OK,sharp" seeing how most 2.8 lenses require a few stops up to look real sharp. :)

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PostPosted: Fri May 13, 2011 9:40 am 
Hey dont shoot the messenger, Im just outlining the advantages of an f2.8 lens.

Fact of the matter is, if you need more depth of field you can always stop down and f2.8 lens to f4 or f5.6, but if you need more light you cant open up your kit lens to f2.8.

Chances are on an ASP-C sensor you'll have enough depth of field. My xmas day shots were shot at 35mm, f1.8, and there was enough depth of field for three people next to each other.

Also Palakaboy is wrong/has misunderstood the question. If two lenses are set to f5, then they will let in the same amount of light and give the same image regardless of maximum aperture.

 Post subject:
PostPosted: Fri May 13, 2011 4:59 pm 
oh... hahaah

yeah you're right. I totally missed thst part of the question.

 Post subject:
PostPosted: Sat May 14, 2011 1:37 am 

It is not true that you can assume same shutter-speed by comparing F-stops only. The have to have same focal length.

F2.8 on a 10.5mm fish-eye is a different story than F2.8 on a 70-200mm lens, for example.

Also, not every lens has the same transmissivity. For example - many lens review note varying degrees of corner darkening or "vignetting" - but the concept applies to the center of the lens as well.

This latter part has somewhat less practical implications, but the focal length does have a significant impact on the resulting shutter speed.

This may all be just a tangent on my part, since drax0r probably implicitly meant to compare two lenses at the same focal lengths.

Cheers :-)

 Post subject:
PostPosted: Mon May 16, 2011 6:51 pm 
Hehe yes, LahLahSr. I meant at the same focal length. I think I was a bit naiv thinking gettin a 2.8 lens would let me use faster shutter speeds indoor and yes even though that is true, I still end up using atleast f4 or more which is the same as the kit lens just to get more sharpness in the whole frame. Thanks everyone.

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