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02-20-2007, 03:47 PM   #1
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Focal Length Disagreements?

I recently read in the DCResource forums about how large power zoom lenses like the Sigma 18-200 was coming up short on its stated focal length. Then I check out my Sigma 24-135mm and noticed it was coming up short too! Since I had an array of options at the 135mm focal length, I decided to check it out. Am I missing something and this is just a mere relation to the lens diameter or does my Sigma really come up short?

All tests done with a tripoded GX-1L. ISO 200, F8, 2.5sec, -0.3EV, Cloudy WB preset, Self timer 2 sec w/ mirror lock up. No PP, just resize (Web Large - 640x480)

SIGMA 24-135mm f2.8-4.5 Asph. IF

RICOH XR Rikenon 135mm f2.8

SMC PENTAX-M 135mm f3.5

MC Pro Rokinon Auto Zoom 80-200mm f4.5

And here are the lenses themselves.

Going Left to Right: Rokinon 80-200, Sigma 24-135, Ricoh 135, Pentax 135

Shoot I just noticed I forget to zoom the Sigma out to 135mm for size comparison. Oh well.

So what are your thoughts and ideas on this issue?

02-20-2007, 06:39 PM   #2
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In various tests I have read over time zooms typically come up short in either end. The lens factories allow themselves some slack, I guess, and always to the side that exaggerates performance! But your example appears extreme I'd say.
02-20-2007, 10:51 PM   #3
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Not just the FL issue, but the color casts are interesting, too.

As with hard disk capacity, I'm guessing lens FL has some fudge factor in them. I think my Sigma 18-50 is really 19-47 (so I'm told), and probably NOT f2.8 as well.
02-20-2007, 11:09 PM   #4
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Haha on yea. (oh if you haven't figured out already, its CptOfGondor from DCR :P)

Aw man, I thought I had hope when Cameta Camera started selling Sigma 18-50/2.8 EX on ebay for like $350 CAN. I was thinking to myself..if I got a job, that would be it!

Then I read some wonderful praise here about those supposedly cheapo russian lenses on eBay and found out that most are zeiss derivatives and its totally reset my lens outlook. Its LBA from Russia with Love haha.

Got a Vivitar 28-90/2.8-3.5 coming in so now I'm like flat out broke.

Yea about those colour casts on the test images. I didn't change anything. 2.5 seconds and set all the lenses on f8. Its weird cuz in the photo I took with my Ricoh Pentax MZ-M seemingly lost all colour. But check the film package and the colour looks ok.

02-21-2007, 11:27 AM   #5
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QuoteOriginally posted by FotoPete Quote
Am I missing something and this is just a mere relation to the lens diameter or does my Sigma really come up short?
Your observation is correct, BUT the focal length changes with focus distance! So if you take a shot at infinity, you'd probably find that it is indeed very close to the focal length specified (how the focal length specification is justified; they just don't tell you the "but"). The shorter the distance, the more it deviates from the specified focal length.

This, I believe, is also one of the reasons why some users have experienced less than optimal SR results with the Sigma lens (e.g. 18-200mm). Because at shorter distance, the SR focal length settings would not be correct.

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