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09-11-2015, 01:31 PM   #1
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Sigma 70-210 AF-K random shots...

I just got this lens and tested it on sea-side while also testing a Vivitar 70-210 3.5 series 1...

This sigma lens is realy strange, it has a very fast but inacurate AF so I'd better use it in MF, but this is not the only fact :
When I use it in Av mode, I have totally random results, most photos are over exposed with low contrast, while about 1 photo from 10 is almost perfect, ultra sharp, great colors... etc. One of these is even sharper (landscape) than the sharper I could get from the vivitar !

Is there a trick to use wisely this lens ? My K200D sees it as a smc PENTAX-F 70-210 4-5.6 and that's the sigma specs. Is there a compatibility issue between this lens and my K200D ? Is there something to tweak on my K200 to use this lens properly ?

Thanks for advices if any !

---------- Post added 09-11-15 at 01:38 PM ----------

I guess there is something wrong with aperture commands given to the lens, could it be a bad "A" plot contact ? I've only used it with ring on A position, I'll test it on fixed positions, not yet sure this is the "bug" but feel it is^^

This is one of the good shots : https://flic.kr/p/xxvBpE


Last edited by bygp; 09-11-2015 at 01:52 PM.
09-11-2015, 01:45 PM   #2
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Remember that no aperture commands are sent to the lens in the Pentax system. The A position locks it in the smallest aperture position (but still wide open) until the armature is moved. The camera then decides based on the way the A position lenses are supposed to behave how far to depress the lever to simulate that aperture position. The question I would have is how quick are the blades reacting. If you take the lens off and flick the aperture actuator do the blades slam closed and open rapidly or slowly?
09-11-2015, 10:27 PM   #3
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QuoteOriginally posted by UncleVanya Quote
Remember that no aperture commands are sent to the lens in the Pentax system. The A position locks it in the smallest aperture position (but still wide open) until the armature is moved. The camera then decides based on the way the A position lenses are supposed to behave how far to depress the lever to simulate that aperture position. The question I would have is how quick are the blades reacting. If you take the lens off and flick the aperture actuator do the blades slam closed and open rapidly or slowly?
Well, it seems as fine as on my kit lens 18-55 DAL, only the lever spring seems a bit stronger so you need a bit more force to lock aperture wide open but when you release the lever, it closes as fast as any of my lenses, the blades also seem in mint condition (visual inspection). Anyway I'll digg that if all other options don't fix it.

One important thing I remind is that all good shots where taken with sun in my back, could a low-cost bad quality CPL filter cause such effect in high light conditions ?
Missed shots look like if you add a 50% opacity pale-yellow layer on a picture in photoshop...

For the AF issues, I've just seen my camera was set on AF.C mode which is not good at all for what I wanted to shot, mainly with 1 point center AF, my bad, I'll test that again today.

Missed shot example (photo untouched, only resized) :



And a good exposed shot (facing north, so sun behind me, untouched photo) :


Last edited by bygp; 09-11-2015 at 10:48 PM.
09-11-2015, 11:49 PM - 1 Like   #4
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Tested this morning with better settings, I'll use it only in MF and I can get a very sharp focusing with this lens in MF, the ring is well calibrated but sadly I have a model with a tiny ring just around the filter thread so I'll mod it. Using it without any filter seems to give much better results, guess I've wasted money with cheap chinese filters pack sold on e-bay. Using full frame AE seems also the best choice with this lens. Anyway it gives decent pics for a low-cost zoom, this one is quite slow but compact and has smooth rings/pump. I'm still intersted by links to advices on how to get the best of modern zooms as I've learned photography on old full-manual slr (in the 80's) and love old lenses, but old HQ zooms are often very heavy and big.

09-12-2015, 04:42 AM   #5
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Glad you solved the problem.
09-14-2015, 08:56 AM   #6
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The problem is still there and I still don't understand what causes this lack of contrast, using a hood does not seem to make any change, the shot I'm posting here was taken from a place under shadow with no incoming light.

If all shots were bad, I could say it is a lens weakness, low-cost, etc... but I had a few very nice contrasted and saturated shots although 95% have low contrast & low saturation, this is weird, this lens seems in mint condition, a tiny fungus but on the front lens and on the border of the lens, almost no dust, diaphragm looks fast...

Could a too slow speed setting produce such flat photos ? If yes, this would mean that my K200D does a wrong speed setting when I use this lens in Av mode. In Av mode, the camera don't release the aperture level to calculate the right speed, it is done by AE mesure, aperture choice on camera and lens model informations, am I right ?

Here in Av mode (it is a sharp lens, this is a 100% crop testing infinity focusing at ~80mm) :



---------- Post added 09-14-15 at 09:11 AM ----------

Just done some more tests with lower light condition (clouds) and manual mode, but no changes, I presume it is just because that's a low-cost lens and internal optics may have been degraded by U.V., it has maybe always be used without any UV filter... It's sad because the lens build is nice and photos are still sharp but you need to post-prod all of them to get the right curves... I'll sell it the same price I bought it or keep it for black&white shooting, as it has almost no CA or distortion...

---------- Post added 09-14-15 at 09:36 AM ----------

Still valuable lens if I choose to use it mainly for B&W photography with LR post-production (curves tweaking), very sharp for a 25$ zoom :


Last edited by bygp; 09-14-2015 at 09:14 AM.
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