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05-06-2012, 07:17 PM   #1
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Comparing 50mm focal lengths... Again.

Thought I'd see how the 50mm ends of my zooms compare with my primes, and there are some interesting results... Especially regarding the FA 50mm f/1.4, which seems to be a stop faster than the others at all the same exposure settings.

Take a peek here

And please forgive any typos, I've not written like this before...

05-07-2012, 03:44 AM   #2
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Interesting about the FA 1.4 overexposing.

As an aside, it really bugs me when focal lengths that should be the same aren't. Why can't lens makes either engineer things to give exactly 50mm focal length, or just change the writing on the lens so it matches the actual length. For example, there's no way you could say the long end of the 16-50 is the same as the wide end of the 50-135, based on your test. But according to Pentax they're both 50mm. Bonkers.
05-07-2012, 11:57 AM   #3
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Many lenses, especially zoons, are the stated focal length only when focus at infinity. So when testing at closer distances, it's normal there would be more variation.
05-07-2012, 12:03 PM   #4
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Yeah I figured that, I know the 50-135 changes in focal length as the focus distance changes. I'll have a poke around at infinity focus and see what turns up. I'm still surprised by the FA 50mm though. I can't for the life of me conceive how it's over a stop faster at all the same settings.

Maybe all my other lenses are a stop slower? D:

Nah.

QuoteOriginally posted by Marc Sabatella Quote
Many lenses, especially zoons
I want a zoon.

05-07-2012, 12:08 PM   #5
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Could be that it's aperture blades are a little sticky and closing all the way. Or it could be that the simpler design allows more light to pass through the glass compared to other lenses thaat have more glass and absorb more light (look up the term "T-stop").
05-07-2012, 12:41 PM   #6
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I thought that, but I've checked the aperture blades, and you can see them close in at f/2.0 and f/2.8 but not at f/1.4, as it should be. So it's not that it's not stopping down...

And yeah, I've read about T-stops, I just never imagined it could be so dramatic as an entire stop of light's difference. Maybe more. I've got a new found appreciation for the FA 50.
05-07-2012, 05:26 PM   #7
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Well, clearly the blades are closing down to some. But it woild be impossible to tell just by looking if they are closing *enough*, or *fast enough*.
05-08-2012, 05:21 AM   #8
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Yeah I guess you can't measure the actual aperture size because it'll be distorted by the glass in the lens or something won't it? It does seem to get to a similar size as the A 50mm f/1.7 at all apertures above f/2 though.

I hadn't thought about whether the blades were closing fast enough. I just tried taking a shot whilst holding the lens on to the camera set at f/2.8, so the aperture wouldn't open up before or after the shot. You have to twist it slightly otherwise the aperture lever starts having a slight effect. Even like this the lens over-exposed as before, so it doesn't appear to be an issue with the aperture, unless the blades aren't calibrated properly or something.

I'm gonna have to try a DoF test or something to see if the blades stop down enough. Curiouser and curiouser...

05-08-2012, 05:57 PM   #9
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QuoteOriginally posted by Mareket Quote
I'm gonna have to try a DoF test or something to see if the blades stop down enough. Curiouser and curiouser...
Very interesting. Why don't you shoot the same scene @ 1.4 and 1.7 and see if the difference in exposure is what it is supposed to be? Or compare to another 1.4 lens if you have one available.

T-stop to F-stop difference usually* hovers around 0.1 to 0.2 stops in primes (there are exceptions, like the sigma 30 1.4, which in t-stops is really a 30 1.8*), in zooms the variability is higher: anything from -0.1 to +0.4 is common*. Anyways, there shouldn't be one full stop difference when comparing two lenses.

*source: dxomark

EDIT: sorry, see you already compared 1.4 to 2.8
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