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06-15-2009, 06:36 AM   #1
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a dud maybe? sigma 28-70 f2.8 ex DF

Hi guys

i had a chance to take the sigma out for the first time since i got it.. and i had quite a frustrating time!

prior to taking the camera out i checked out the lens with the focus charts and came up with a -7 focus adjustment.

within the first few shots i realised something was wrong as most shots were out of focus or soft.. very soft. i struggled to get any distance shots that were in focus, or not soft.. even at small apertures

sooo.. i got home and set the camera up on the kitchen island looking across at a pineapple! (it will be the new focus test )

i messed with the front and back focus adjustment until i was happy at both 28mm and 70mm (turned out it was zero to +3 so i left it at 0) and then messed with aperture and fine sharpness as i was shooting tethered in JPEG they are far better than the outdoor ones... but still noticeably soft at 2.8 and 4

the following link is a gallery of those test shots...

could you tell me if this is acceptable or if im pixel peeping too much? i'll follow up with some lousy shots from the weekend after too if we think its required!

is there anything that can go wrong that would render the lens useless for distance and ok for close up?

linky here Picasa Web Albums - Rachael & Steve - focus test

if you go to download the full size file should be available!

Thanks

Steve

06-15-2009, 07:54 AM   #2
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QuoteOriginally posted by 5teve Quote

could you tell me if this is acceptable
Sigma AF 28-70/2.8 EX DF (not DG, right ?) is the version before Sigma AF 28-70/2.8 EX DG was released... I had a small discussion w/ another member @ DPReview about Sigma AF 28-70/2.8 EX DG

Is the Sigma 24-60mm f/2.8 discontinued? [Page 1]: Pentax SLR Talk Forum: Digital Photography Review

I stand that Sigma 28-70/2.8 (all incarnations, not to be mixed w/ 24-70s or 24-60s) is an equivalent of Pentax 28-70/4 (not 2.8) "iq"-wise, that means consumer level lens where you should not expect great resolution wide open... but Eric stands that it is a better lens, which I can only attribute to him using it on a FF/12mp sensor (Canon 5D)... I did not see any great pictures from K20D using those lenses @ 2.8 yet...
06-16-2009, 05:35 AM   #3
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I have been using this lens on my K100D and K20D for about a year.
Consider it the best lens I have particularly for low light outdoor and indoor shots.
At f2.8 the sharp focus range is VERY small. Look at the attached image where the fingertips are very sharp but the lips and eyes have just missed - but not by much!
Image shot with K20D f 2.8, 1/90 and ISO 800 late afternoon natural light only.

06-16-2009, 05:42 AM   #4
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I have the DF lens ... and yes it can be a little soft wide open at the 70mm end ... but nothing where it is that noticably soft ... it isn't sharp/sharp though.

It is a handy lens though ... i got it mainly because it was a decent price 2nd hand ... and i couldn't find a good Tamron 28-75 in my area either.







As I said ... not sharp wide open


06-16-2009, 06:32 AM   #5
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Original Poster
Thanks guys

i guess its down to sigma's usual quality control

mines nice close up.. its just at distance it seems to struggle..

are any of the pics posted Processed at all? i can get the sharpness up it just takes a bit of effort in lightroom.

im also wondering if being under 20w halogen bulbs has any effect on the sharpness of the picture due to focus being affected (is halogen the same as tungsten?)

i may make some enquiries as to having the lens service to see if they can improve it..

Thanks

Steve
06-16-2009, 06:41 AM   #6
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Halogen lighting! Similar to Tungsten in temperature which give trouble with Pentax AF according to reports. Have you tried manual focus and/or onboard flash for your tests.

Sigma EX series lenses are very well regarded in the other camera brand worlds.
06-16-2009, 08:43 AM   #7
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The shots of my niece above were taken with a Sigma EF 530 flash gun bounced off the ceiling as well ... I thought I should put that in ... the other 2 are natural light shots.
06-16-2009, 02:33 PM   #8
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I thought the biggest problem with the pineapple test was the countertop. In other words, just below the pineapple is a horizontal line with reasonable contrast running the width of the frame. You can't tell what the camera decided to focus on. The actual focus sensor is bigger than the red dot in the viewfinder and may be slightly shifted from its indicated position, so simply putting the red dot on a subject does not guarantee the sensors won't get distracted by something they like better.

If you suspect distant focus is the problem, you might first try to manually focus, just to see if there is an optical issue. Then test AF, but with a more isolated subject - even putting the pineapple on the edge of the counter would be better. The more information you can give a service center, the more likely it is that they will fix it.

06-16-2009, 03:28 PM   #9
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Original Poster
thanks for the info guys and thanks Dave for the suggestion, i cannot do the pineapple thing as its now been eaten!

I'm fairly confident that it focussed correctly, however i will repeat the test once we have another pineapple (around friday, market day!) with manual focus as a sharpness test.

I wasnt suspecting mis-focus i was just asking the question re halogen lighting... so i'll try to eliminate everything first (natural daylight would be nice but being winter and busy at work i dont see much!)

Thanks again

Steve
06-16-2009, 07:30 PM   #10
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QuoteOriginally posted by 5teve Quote
are any of the pics posted Processed at all?
all of them are seriously resized, you need to look at unresized image to make a call.
06-21-2009, 08:56 AM   #11
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ok guys i took your advice and went ahead and looked at manual focus (improved results) and also outdoor shots (unimaginative and needed lots of AF adjustment!) for the indoor ones manual focussed i used a cereal box and shot via remote assistant using 4*jpeg with default everything shot in AV mode in some cases i took several shots until i had the best result. Not the best test subject but it seemed to get me some results.

album is here, i would link the photo's direct but picasa doesnt transfer exif but does support full res downloads.

Picasa Web Albums - Rachael & Steve - manual focus ...

following that i then took the camera outside and took some very unimaginitive pictures of a friends work van (distance shots) and our beat up runabout (closer focus) adjusting the focus until i get the best shots repeatedly (i ended up with +8).

Picasa Web Albums - Rachael & Steve - outdoor af test

again full size are downloadable.. or exif can be viewed by using the more info link to the right of the picture on picasa. the shots dont seem too bad but this is where im losing it now..

i mentioned that there is something about the lens shots i dont like, im not convinced but i think its the Bokeh (maybe not strictly Bokeh but the out of focus areas front or rear of the focal plane) especially toward wide open i feel like i have double vision and it feels like if i look for too long i'll get a headache! im hoping its just be as im not used to looking at bokeh (p&s DOF was huge!) but im really wondering if something is decentred.

the following album hopefully highlights it nicely! 3 shots 2 of a hat .. .look at the text that says rinnai, the f4 doesnt look too bad but the f2.8 is rough... but the adidas embroidery is sharp where its in focus at f2.8

the canon logo is a bit soft (probably slight misfocus) but look at the screen printed white text for the controls.. hurts my eyes!

album is here! Picasa Web Albums - Rachael & Steve - double vision

would appreciate your honest opinions please guys..

Thanks

Steve
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