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09-29-2012, 04:33 PM   #1
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K-30 sharpness & focus test

I have compared a Pentax-M SMC 1:1.4 50mm and a Sigma 17-70mm F2.8-4 DC Macro OS zoom lens.
Camera settings: all default, except ISO 200, WB daylight, format RAW.
Lens settings: f=50mm, F 4.0
3 shots were taken, 1: Pentax, 2: Sigma with A/F, 3: Sigma with M/F.
In all cases I focused at the center where the pin was inserted.
It's no surprise that the prime lens is sharper, but that much? The Sigma lens got some good reviews after all.
I would have liked to test the Pentax 18-135mm zoom as well but I haven't got one.
With A/F there is some visible front-focus.
Also the prime lens produces whiter whites than the Sigma lens.
Pentax:
Pentax-M
PentaxM | Flickr - Photo Sharing!
Sigma A/F:
SigmaAuto | Flickr - Photo Sharing!
Sigma M/F:
SigmaMan | Flickr - Photo Sharing!

09-29-2012, 11:46 PM   #2
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QuoteOriginally posted by kruser1947 Quote
It's no surprise that the prime lens is sharper, but that much?
The problem doesn't appear to be lack of sharpness but focus.

Your Sigma images are not focused on the needle but in front of it in the a/f image and behind it in the m/f image. In the a/f case 1) if the K-30 is focusing on the needle, then your K-30's a/f system needs microadjustment to calibrate it properly to the Sigma lens. BUT I suspect it's not focusing on the needle. 2) For the a/f tests it would be much better to stick the needle in a blank sheet of paper, because your K-30's a/f system will focus where it finds the most contrast, i.e. on the black/white borders of the squares. Having a blank sheet would guarantee that the a/f system would focus on the needle and not anywhere in front or behind.
In the m/f test case: it's operator focusing error.

Once you fix your focusing you'll be able to properly compare the sharpness of each lens.

In all three tests, the white squares have a blue cast on my screen, more so with the Sigma, as point out. But the whites appear a long way from being "whiter whites" IMO. Depending on the lighting, you would need to adjust the K-30's white balance. Here is the original Pentax-M image (on the left) side by side with a colour-cast-corrected copy.



Last edited by Gray; 09-29-2012 at 11:59 PM.
09-30-2012, 01:56 AM   #3
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QuoteOriginally posted by Gray Quote
The problem doesn't appear to be lack of sharpness but focus.

Your Sigma images are not focused on the needle but in front of it in the a/f image and behind it in the m/f image. In the a/f case 1) if the K-30 is focusing on the needle, then your K-30's a/f system needs microadjustment to calibrate it properly to the Sigma lens. BUT I suspect it's not focusing on the needle. 2) For the a/f tests it would be much better to stick the needle in a blank sheet of paper, because your K-30's a/f system will focus where it finds the most contrast, i.e. on the black/white borders of the squares. Having a blank sheet would guarantee that the a/f system would focus on the needle and not anywhere in front or behind.
In the m/f test case: it's operator focusing error.

Once you fix your focusing you'll be able to properly compare the sharpness of each lens.

In all three tests, the white squares have a blue cast on my screen, more so with the Sigma, as point out. But the whites appear a long way from being "whiter whites" IMO. Depending on the lighting, you would need to adjust the K-30's white balance. Here is the original Pentax-M image (on the left) side by side with a colour-cast-corrected copy.

Agreed. Severe lack of red in all the images rather than too much blue. What lighting was used I wonder?
09-30-2012, 02:23 AM   #4
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I was using a white LED lamp. It appears to be pretty much daylight but I might be wrong. The color temperature could be over 5000 ēK.
The Sigma lens is very hard to focus manually, the adjustment is very sensitive. The Pentax-M lens is much easier to focus. I'm using the O-ME53 magnifying eyecup and I'm wearing prescription glasses. I don't know if these could affect manual focusing.

09-30-2012, 05:10 AM   #5
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You may find using the K-30's LCD and focus peeking (rather than the viewfinder and matt optical screen) much easier for manual focusing.

To adjust the white balance under LED lighting, choosing "cool white fluorescent" may help offset the blue cast. You'll need to experiment a little with your camera's WB settings to see what works best.

Last edited by Gray; 09-30-2012 at 05:19 AM.
09-30-2012, 05:38 AM   #6
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I tried focus peaking but couldn't get better results even with magnified view. Otherwise the LCD screen is pretty much useless outdoors in bright daylight. I will probably invest in a KatzEye focusing screen.
I don't worry about white balance at the camera, I can adjust it when developing the RAW image. I just pointed out that the two lenses had different tint.
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