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03-06-2011, 09:54 AM   #1
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Sigma 17-70 f2.8-4.5 (FL and f-stops)

Hi to everyone, but question is for sigma 17-70 owners
I'm going to buy this lens but would like to know the steps of f-stops through all focal lenght.
Especially which FL is ending with 2.8 and 4 and what aperture does 50mm have.

Thanks for your answers.
Best regards.

03-06-2011, 11:07 AM   #2
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It doesn't vary in fixed increments, but is actually analog. The camera will usually round up in the viewfinder, so shortly after 17mm you'll see 3.5. At 50mm the true aperture will be around 4, but in the viewfinder you may see 4.5- it really depends on the lens. I haven't used this one so I can't tell you for sure.
03-06-2011, 12:20 PM   #3
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Thank you!

For exapmle, camera shows that f4 covers 30-60mm, so real f4 @ 30mm won't be the same real f4 @ 60mm (aperture continuously closing as focal length increasing and camera changes number of aperture only when passing certain FLs) ? Am I right ?

I hope you understand what I mean
03-06-2011, 12:24 PM   #4
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Right- but the difference is probably neglibible anyway. Lens manufacturers tend to approximate apertures and focal length, so there's definitely leeway in there.

03-06-2011, 12:49 PM   #5
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Thanks again, now it's clear for me
03-07-2011, 04:45 PM   #6
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These are some rough values I got from varying the focal length and seeing what max aperture I could get;

f/2.8 from 17 to ~21mm
f/3.5 from ~23 to ~34mm
f/4.0 from ~34mm to ~70mm
f/4.5 at ~70mm

So basically when I turn the zoom ring, at just a bit before 70mm, the figure on the display jumps up to f/4.5.

QuoteOriginally posted by fLyIr Quote
Especially which FL is ending with 2.8 and 4 and what aperture does 50mm have.
So the lens quite quickly loses it's f/2.8 max aperture, and at 50mm, the max aperture is f/4.0. The focal range makes the lens a quite useful everyday/walk-around lens, but if you NEED f/2.8 in that range, then you're probably better off getting one of the 16-50 or 17-50 or 18-50 constant f/2.8 lenses out there.

Hope that helps.
03-09-2011, 09:42 AM   #7
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QuoteOriginally posted by pop4 Quote
These are some rough values I got from varying the focal length and seeing what max aperture I could get;

f/2.8 from 17 to ~21mm
f/3.5 from ~23 to ~34mm
f/4.0 from ~34mm to ~70mm
f/4.5 at ~70mm

So basically when I turn the zoom ring, at just a bit before 70mm, the figure on the display jumps up to f/4.5.

So the lens quite quickly loses it's f/2.8 max aperture, and at 50mm, the max aperture is f/4.0. The focal range makes the lens a quite useful everyday/walk-around lens, but if you NEED f/2.8 in that range, then you're probably better off getting one of the 16-50 or 17-50 or 18-50 constant f/2.8 lenses out there.

Hope that helps.
Thanks for the info. I did not have the time to do this in mine. But it is nice to know other people observations.
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