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02-21-2014, 08:16 AM   #16
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The f/3.2 will show up as an available stop on the short end of the zoom as a step above f/2.8 but I haven't been able to access it as the widest stop in the 20-28mm range.

02-21-2014, 08:22 AM   #17
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QuoteOriginally posted by MadMathMind Quote
This is not correct, btw. There's no 3.2 minimum anywhere.
I don't own this lens but have wanted to for a while now. The dpreview review of the 17-70 has a small chart on the very bottom of this page, which shows f/3.2 for between 24mm-27mm.

Sigma 17-70mm f/2.8-4 DC Macro OS HSM | C review: Digital Photography Review

Also, the PentaxForums review of the second version of the 17-70 indicates f/3.2 between 24mm and 33mm.

Pentax 17-70mm Lens Comparison - Sigma 17-70 v2: Construction - PentaxForums.com

So who's right??
02-21-2014, 08:48 AM   #18
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If it's there, I can't access it on my K5. The only way I can get the f/3.2 is when I have f/2.8 as the widest stop. Perhaps they're just copying a spec from an earlier model? It doesn't really matter to me as there is minimal difference between f/3.2 and f/3.5 and I won't have any real effect on the image as to DOF or bokeh in that focal range. There have been threads stating that sometimes an aperture shows up in EXIF data on some Sigma lenses that is faster than what the camera displayed. That is likely the result of the camera misreading the lens data.
02-21-2014, 12:59 PM   #19
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QuoteOriginally posted by reeftool Quote
If it's there, I can't access it on my K5. The only way I can get the f/3.2 is when I have f/2.8 as the widest stop. Perhaps they're just copying a spec from an earlier model?
Me either. No go on a K-5 IIs. I've tried multiple times. I set to the minimum f/2.8 at 17 and slowly increase the zoom and at 22mm, it jumps to 3.5. Dial it down all you want. It won't go to 3.2 unless 2.8 is also available.

02-21-2014, 01:27 PM   #20
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That's weird. The dpreview is of the Canon mount version. Could it be that the Canon version somehow reports to the camera f/3.2 whereas the Pentax cameras do not show this? But then the Pentax Forums review indicates that there is f/3.2 max aperture for version 2. I thought that the Contemporary version had the same aperture ranges as version 2. Anyway, I don't have any of these lenses but was under the impression, based on reviews and not by actual everyday users, that f/3.2 was available from at least 24mm-27mm, if not even up to 33mm...
02-21-2014, 01:39 PM   #21
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QuoteOriginally posted by seventysixersfan Quote
So who's right??
Neither, as the reported figures are completely meaningless
The aperture changes linearly from 2.8 to 4.0 throughout the zoom range, there are no "jumps". You just have to actually look at the aperture when you zoom to notice this. The "jumps" you see is just the rounding done by the camera because it cannot display 2.9, 3.0, 3.1..., so it just shows you the nearest (half or third, depending on your camera settings) f-stop.
02-22-2014, 06:46 AM   #22
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QuoteOriginally posted by sTi Quote
Neither, as the reported figures are completely meaningless
The aperture changes linearly from 2.8 to 4.0 throughout the zoom range, there are no "jumps". You just have to actually look at the aperture when you zoom to notice this. The "jumps" you see is just the rounding done by the camera because it cannot display 2.9, 3.0, 3.1..., so it just shows you the nearest (half or third, depending on your camera settings) f-stop.
The camera can and does display f/3.2. It doesn't offer it as a maximum aperture in the mid 20's mm range as some of the published reviews claim. DP's review states that f/3.2 is the maximum DISPLAYED aperture at 24mm on the Canon version they did their tests on. Our Pentax bodies only display f/3.2 as a stop between f/2.8 and f/3.5 at 17mm - 20 mm (appox. as there is no marking on the lens focal length scale between 17 and 24). At 24mm, mine displays f/3.5. I'm not going to loose sleep over this. I looked through Sigma's advertised tech specs and it only mentions the minimum aperture of f/22 and no mention whatsoever of the exact largest aperture at any particular length other than the assumed f/2.8 at the shortest and f/4 at the longest.
02-22-2014, 11:32 AM   #23
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QuoteOriginally posted by reeftool Quote
The camera can and does display f/3.2. It doesn't offer it as a maximum aperture in the mid 20's mm range as some of the published reviews claim. DP's review states that f/3.2 is the maximum DISPLAYED aperture at 24mm on the Canon version they did their tests on. Our Pentax bodies only display f/3.2 as a stop between f/2.8 and f/3.5 at 17mm - 20 mm (appox. as there is no marking on the lens focal length scale between 17 and 24). At 24mm, mine displays f/3.5. I'm not going to loose sleep over this. I looked through Sigma's advertised tech specs and it only mentions the minimum aperture of f/22 and no mention whatsoever of the exact largest aperture at any particular length other than the assumed f/2.8 at the shortest and f/4 at the longest.
With the F 50mm 1.7, I can see the following: 1.7, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 3.5, 4.0, etc.

Obviously, this is not a camera limitation but rather a lens one. It would seem strange that a lens released some 20 years before my camera was even an R&D project would be better at relaying f-stop information to the camera than one released last year, even if the lens is a third-party product. (They surely tested the lens on a K-5 before releasing!) I'd find it hard to believe that Sigma would be so lazy not to report fractions of f-stops that the lens is actually doing, although that could be true.

Also, I don't think it's necessarily true that f stop on variable aperture zooms necessarily increases linearly. It really depends on how the lens is designed. A constant aperture lens has magnification elements that move as you zoom the lens, and proper placement ensures the correct effective f-stop is attained. Certainly, a simple lens with only a single magnifying element would necessarily have linear and continuous f-stop increments, as the magnifying element would be moving from point A at the wide end to point B at the short end (closer to the lens), inherently reducing the magnification factor. But if the lens contains a few of these elements that move over shorter distances could achieve piecewise-constant f stop. I'm not an expert on lens design, but I'd like to think that sheer laziness of improper reporting of f-stop is not the explanation for what we are seeing here.

02-23-2014, 10:04 PM   #24
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I get f2.8, f3.5 and f4 on the K-5D, but I don't much care :-)
11-02-2014, 02:42 AM   #25
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So are people liking this lens?
I'm off to Paris for 3 days in Dec so I'm planning a zoom lens purchase. Where I live, both the 18-135 and the sigma are the same price, and I'm getting the impression that the sigma is the sharper out of the two from comments elsewhere here. I'm not sure I will need the extra length of the pentax in a city environment and feel the extra stops of the sigma might be useful late evening, but I feel I should be buying Pentax to support the brand.
I'm only taking whichever zoom I buy, no primes so for 3 days in a city, which way would you go....
11-02-2014, 07:18 AM   #26
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QuoteOriginally posted by edgedemon Quote
So are people liking this lens?
I'm off to Paris for 3 days in Dec so I'm planning a zoom lens purchase. Where I live, both the 18-135 and the sigma are the same price, and I'm getting the impression that the sigma is the sharper out of the two from comments elsewhere here. I'm not sure I will need the extra length of the pentax in a city environment and feel the extra stops of the sigma might be useful late evening, but I feel I should be buying Pentax to support the brand.
I'm only taking whichever zoom I buy, no primes so for 3 days in a city, which way would you go....
I certainly like mine! It has been a very good walk around lens for me. With the 18-135 being the same price makes the decision a little harder. The Sigma was $100 cheaper when I bought mine. It does distort some at 17mm. I mostly seem to be shooting in the 24mm area and it is very good. I have no regrets choosing this lens although the extra reach of the 18-135 might come in handy for my uses. Before you buy, consider your overall use for the lens, not just the vacation you are going on. I bought the 17-70 to replace a F35-70 which I had used for a few years. I wanted the extra reach on the short end.
11-02-2014, 09:49 AM   #27
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QuoteOriginally posted by reeftool Quote
I certainly like mine! It has been a very good walk around lens for me. With the 18-135 being the same price makes the decision a little harder. The Sigma was $100 cheaper when I bought mine. It does distort some at 17mm. I mostly seem to be shooting in the 24mm area and it is very good. I have no regrets choosing this lens although the extra reach of the 18-135 might come in handy for my uses. Before you buy, consider your overall use for the lens, not just the vacation you are going on. I bought the 17-70 to replace a F35-70 which I had used for a few years. I wanted the extra reach on the short end.


Apart from Macro I have never gone past 55mm so don't think the extra reach is needed right now. I might wait to see what the 16-85 reviews are like, reading a few posts I think Im down to


Sigma 17-50 F2.8 - cheapest
Sigma 17-70 F.2.8 - 4
Pentax 18-135


The Pentax is the same is the same cost as the 17-70 but I think the 17-70 looks slightly sharper than the 18-135 across the same range from the samples I have seen. The 17-50 has the extra stops and looks good as well, but as I have the budget I might as well spend the little bit extra. Im almost tempted by the 18-35 F1.8 if it wasn't for the size and that Id feel dirty 'cheating' on my intended mistress the FA31
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