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11-19-2009, 11:33 AM   #1
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Questions on fast 50s

Hello,

I have the A50mm f1.7 and it is becoming my default lens. I love it! I am fixing an A50 f2 and I just purchased an M50mm f1.4 which is on its way.
I will test it for myself eventually but I have a question:
We all know lenses are softer wide open and start to sharpen up as they close up.
Having these 3 lenses: Would the f1.4 be sharper at f2 than the f1.7 at f2?
Would the f1.4 be sharper at say f3.5 than the other 2 at the same aperture?
Just curious. I will eventually do the test myself when I get my hands at all three.
Why so many 50's? Have you heard of LBA?

Thanks,

11-19-2009, 11:39 AM   #2
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QuoteOriginally posted by ismaelg Quote
Hello,

I have the A50mm f1.7 and it is becoming my default lens. I love it! I am fixing an A50 f2 and I just purchased an M50mm f1.4 which is on its way.
I will test it for myself eventually but I have a question:
We all know lenses are softer wide open and start to sharpen up as they close up.
Having these 3 lenses: Would the f1.4 be sharper at f2 than the f1.7 at f2?
Would the f1.4 be sharper at say f3.5 than the other 2 at the same aperture?
Just curious. I will eventually do the test myself when I get my hands at all three.
Why so many 50's? Have you heard of LBA?

Thanks,
The 50mm F/1.7's are considered better optically because they are sharper at F/1.7 than the 50mm F/1.4's at F/1.7. I think the 50mm 1.7's stay sharper until something like f/5.6. Dont quote me on that though
11-19-2009, 12:39 PM   #3
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What Yeatzee said is the going wisdom. But to answer the question more broadly, you are mixing two separate factors that impact lens sharpness.

1- all lenses tend to lose some resolution when they are wide open. That is because the entire lens diameter is being used (especially that outside edge), and every imperfection of the lens impacts the image.

2- In general it is easier to make a slower lens than a faster lens, which often results in slower lenses being sharper than fast lenses, when wide open. This is not always the case, however, because there are so many other trade offs that can be made; a faster lens can be designed to excel at center focus at the expense focus at the edge, or at the expense of field flatness.

So when does a faster lens overcome a slower lens' sharpness by stopping down? Answer: it all depends on the two lenses in question and the kind of sharpness you are looking for.

Take a browse through the lens review database. Many of the reviewers are very specific about the various qualities of the lenses you have. When you test your lenses side by side, look not only at the focus at the center of the image, but at the edge. Look for flare, purple fringing, field curvature and other factors that will influence the images you capture.

-David
11-19-2009, 12:51 PM   #4
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The 1.4 and 1.7 are both really sharp lenses. To be honest, I can't tell a lick of difference. I have a feeling a lot of the f/1.4 "softness" might have more to do with focusing errors with such narrow dof. That, and that I only use f/1.4 when it is really dark and I have slow shutter speeds, which introduce blur.

There are a lot of 50/2 haters out there, but it wipes the floor with many much more expensive lenses.

11-19-2009, 01:14 PM   #5
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QuoteOriginally posted by ismaelg Quote
Hello,

Why so many 50's? Have you heard of LBA?

Thanks,
No such thing as too many 50s!

I say that as one who has the M50 f1.7, and the K 55mm f1.8, but also intends to buy the A 50 f1.7 and wouldn't mind a K 50 f1.4...

Last edited by Dangermouse; 11-19-2009 at 01:15 PM. Reason: Clarification
11-19-2009, 01:43 PM   #6
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Well, my pentax-F f/1.7 *is* sharper than my pentax-m f/1.4 and the half stop of light difference doesn't make *too* much diference in the real world.

Don't take that to mean you made a mistake in purchacing the faster lens, as both of the two lenses you now own render scenes in very different ways. It's really hard to explain but the 1.7 is more clinical than the 1.4.

The pentax-m 50mm 1.4 has a lot of soul!
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