2.8?

P

Park Edwards

Veteran
might be weird to ask, but shouldn't a 50mm 1.4 lens at 2.8 be the same as 28 2.8? Not the FL, but the aperture blade? Why I'm asking is cause my 50mm 1.4 stopped down to 5.6 is what my 28mm is wide open...strange.
 
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theres no such thing as as a 50mm 1.4 2.8. Its a 50mm 1.4 1.4 is the F stop of the lens. 2.8 is a whole different lenses F stop.
 
odd, i assumed that if both lens were stopped down the exact aperture that both lens would have the same amount of light entering. but I guess that's not the case. sucks, cause 2.8 on my 28 is a whole lot slower than it is on my 50 1.4
 
Car3o said:
odd, i assumed that if both lens were stopped down the exact aperture that both lens would have the same amount of light entering. but I guess that's not the case. sucks, cause 2.8 on my 28 is a whole lot slower than it is on my 50 1.4
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because the more glass (elements) the more light loss...i mean a 28mm vs 50mm would seem like the 28mm would be wider (more glass?) maybe im totally off
 
dunno that's why i posted but sucks, cause all your lens need to be around the same speed or else your shots, if changing lens, are all going to be exposed differently. now shi* just got more expensive.
 
f-stop (or t-stop in some rare cases), controls how much light hits the mirror/sensor and not the convergence/divergence of the lens
 
Welcome to the wonderful world of T/Stops (http://en.wikipedia.org/wiki/T_stops#T-stops)

An f/stop is just a numerical equation: Focal Length/ Lens diameter

On (proper ;) cine lenses, the lens is put on a calibrator before it leaves the factory and the stops are punched into the barrel of the lens based on how much light is actually being transmitted through the rear element to the focal plane. This means that T/2 on any lens is the same as T/2 on any other.

In photography, T/Stops are not really necessary, as there is no continutity of images and you can adjust your shutter speed or aperature to get correct exposeure. In cine, if your punching in for a close up, it needs to match the other shots in the scene and the only thing you have control over is aperature (assuming that your iso and fps is fixed as it would be with film).

Hope that helps.
 
As the above poster stated, its just an equation. And my understanding is that it would be the same amount of light. It looks smaller just because it is a shorter focal length which means the blades have to be smaller to let in the same amount of light.

That's my understanding at least.

So an f/2.8 is the same amount of light no matter the focal length.
 
artforme said:
So an f/2.8 is the same amount of light no matter the focal length.
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No, it's not. You're neglecting the actual light transmission through the glass. F-stop is not constant between lenses. It even varies across the same focal length (i.e a 50mm prime at f2.8 doesn't let in the same amount of light as a zoom lens set at 50mm f2.8). The lens itself absorbs some of the lights that are hitting it depending on the number of lens elements, which is why we have T-stop to account for that.
 
Huy Vu said:
No, it's not. You're neglecting the actual light transmission through the glass. F-stop is not constant between lenses. It even varies across the same focal length (i.e a 50mm prime at f2.8 doesn't let in the same amount of light as a zoom lens set at 50mm f2.8). The lens itself absorbs some of the lights that are hitting it depending on the number of lens elements, which is why we have T-stop to account for that.
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You are correct, and my statement was incomplete.

With increased technology in lenses, and if your shooting digital, T-stops are not as important as during the days of shooting in film with little to no post exposure compensation.

But you are right.

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What I was trying to point out is that the theoretical amount of light if glass elements were not used would be the same. Which is why the iris looks smaller on his wide angle lens. That has nothing to do with t-stops. Which is what the OP was really asking about.
 
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some lenses have t-stops as well f-stops.T-stops are calibrated for the individual lens.When calculating the exposure or consulting the tables, f-stops and T-stops can be considered equivalent
 
Car3o said:
dunno that's why i posted but sucks, cause all your lens need to be around the same speed or else your shots, if changing lens, are all going to be exposed differently. now shi* just got more expensive.
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hehe **** always does.

I wonder how the 50mm f/2 compares at f/2.8 to the 28mm, you can still get them cheap at least.

Apparently the zeiss lens are all calibrated, great if you want to sell a kidney to pay for them!
 
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