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03-14-2015, 02:09 AM   #1
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Sigma 18-250 Sharpness

I bought the Sigma 18-250 Macro as my first lens for the K-3. Recently I've wondered whether a different lens might be better?

The results shown at dpreview show the following (albeit on a Canon EOS 7D), which I'll put on a 'boilerplate' to get the best out of this lens (instead of staying at f/8 all the time)

Zoom - Aperture - MTF centre - Sharp over
18 f/3.5 1800 2/3

28 f/5.6 1700 1/2
28 f/8 1500 2/3

35 f/5.6 1650 1/3
35 f/8 1500 2/3

50 f/5.6 1500 1/3
50 f/8 1500 2/3

80 f/8 1500 1/3

135 f/6.3 1400 1/3

250 f/8 1400 1/6


(f/11 is shown to be noticably worse)

It's interesing to compare with other results on dpreview.com/lensreviews, e.g.
Sigma 17-70 f2.8-4 = approx 1800 MTF (best 2000)
Tamron SP 24-70mm f/2.8 = approx 1750 MTF

So at around 1500 MTF, the 18-250 isn't too bad, and can be very good at 18 to 28mm.

Distortion and CA can be taken care of with Lightroom, so not important?

Wouldn't it be cool if the K-3 could automatically adjust the aperture based on a configurable table of aperture vs zoom?

03-14-2015, 03:32 AM - 1 Like   #2
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Location: Perth Western Australia
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I also have Sigma 18-250 non OS (filter 62mm) HSM Macro, great lense, as good as my favourite DA18-250.
You can zoom in into my pictures and look for pixels:
Sigma : https://www.flickr.com/photos/itrax/sets/72157632857154297/
Pentax DA 18-250 : https://www.flickr.com/photos/itrax/sets/72157629902479422/
03-15-2015, 07:27 AM   #3
PT1
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Location: Somerset, UK
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Original Poster
Yes, it has a lot of good reviews. Taking approx 1400 for 'Sharp over', the list should probably be

Zoom - Aperture - MTF centre - Sharp over - MTF corner
18 f/3.5 1800 2/3 1200

28 f/5.6 1700 1/2 900
28 f/8 1500 5/6 1300

35 f/5.6 1650 1/3 to 1/2 1000
35 f/8 1500 2/3 1300

50 f/5.6 1500 1/3 (or 1300 over 1/2) 1000
50 f/8 1500 2/3 1300

80 f/8 1500 1/3 (or 1300 over 1/2) 1150

135 f/6.3 1400 1/6 (or 1300 over 1/3) 950

250 f/8 1400 1/6 (or 1300 over 1/3) 1000
03-15-2015, 12:10 PM   #4
PT1
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If Sigma made the 24-105 without image stabilisation for Pentax... up to 2500 MTF!

They could make it weather sealed as well?

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