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Lens Testing

Friday, January 11, 2008

After reviewing a few articles about lens sharpness, I thought I'd conduct my own little test.
But as I was setting up the test subject (card board box and paint swatch), I started to wonder: So is there a basic rule we need to follow on sharpness testing?

I have seen pictures of brick wall, news paper article, fertilizer box, etc. Is there a distance limit? Light condition? Should a standard scene be utilized? Just curious.

The test was performed with my favorite manual portrait lens, a Ricoh 55mm f1.4. I started with f1.4, and went up to f11. The smallest aperture is f16, I didn't go to that one because amount of light I have at time of testing. Camera is my Pentax DL.

The camera was mounted on a tripod, subject was 6 feet away from camera. No flash, just available light.

This is full size picture. The letter K (SINK) is near center.
At f1.4

At f2.0

At f2.8

At f4

At f5.6

At f8

At f11

I manually cropped each shot from Picasa, so the crop size varies from each sample.
I think the results are very interesting. Letter K being my center point, it is actually pretty sharp through out the aperture range. The corner (barcode and #31.5) of the lens are very soft from f1.4 to f2.8, then at f4, it started to get better.
I was expecting the image to be very sharp at f5.6. But f5.6 is still a little soft, but I am suspecting camera movement, since letter K shows a slight fuzziness.
Conclusion, well, nothing much than what I already suspected. For my portrait shots, most important point is the center of the shot, so corner softness is not so bad, the softness also add to the bokeh most people are after. I am more curious at what test methodology should utilize. Maybe there's a standard testing method that us poor home users can use.

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