M-lens metering with K10D redux ...

Started Jan 26, 2007 | Discussions thread
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Godfrey Forum Pro • Posts: 29,493
M-lens metering with K10D redux ...

Practical test of K10D with pre-A lenses, metering results

This text at:
http://homepage.mac.com/godders/mCAL/calibration-K10.txt

Several people took issue with the metering test I did the other day comparing the K10D and the DS bodies using pre-A metering modes (Manual exposure mode with the Green button or AE-Lock button metering actuation). That was done in fairly low light on a standard reference target and showed that the two bodies operated produced metering that was essentially the same in those circumstances. Text of that set of tests is available from http://homepage.mac.com/godders/mCAL/calibration-K10-DS.txt .

As I said in one of the many threads that this was talked about, there were several possibilities for my results being at odds with others', not least of which is that the setup of my test might not be valid to push any discrepancies or aberrations in the metering of the two bodies. Someone sent me an image file demonstrating the results he acquired with a Zenitar 16mm lens.

Since I'm interested in the subject, I decided to do a practical test of just the K10D body with both the Zeni 16 and the Pentax FA28/2.8, used off the A setting on the aperture ring, to determine what my K10D body did in this regard. I also realized that I'd checked the K10D and DS bodies' operation only in Center Weighted averaging mode and someone had mentioned that their problem was most acute in Spot metering mode.

So I set up a new test. I used the Zeni 16. I set the camera to ISO 400 so as to accommodate making an exposure at all apertures in the daylight available, and did two series of exposures at whole stops from f/2.8 to f/22 ... first in Spot metering mode and then in CW Average mode. I made another similar pair of series with the Zeni 16 indoors with ISO set to 800, in CW Average and Spot modes.

Then I switched to the FA28, I went back outdoors, set ISO 400, and did the same two series using the aperture ring and Green button metering. For a reference control of the camera's behavior, I then switched the lens to the A setting and made another series in Manual exposure mode, setting the aperture and shutter speeds from the body.

Results are available at
http://homepage.mac.com/godders/mCAL/calibration-K10.jpg

Data:

Looking at the results, it is evident that the "M-lens" exposure behavior does vary, the variation seems to depend on both ambient light level as well as metering mode.

  • Spot mode in daylight produces the greatest amount of variation from f/2.8 to f/22: results from f/2.8 to f/22 show a steady increase in over exposure by about 0.5 EV per stop.

  • Metering the same way with CW Averaging mode in daylight, the progression is similar but the variation is down to about 0.3 EV per stop.

  • Moving indoors, the evidence is not so conclusive. At the illumination level I tested at, the exposures in both Spot and CW Averaging modes showed little variation from f/2.8 to f/8 ... f/11 to f/22 were beyond this stop-down metering mode's range and exposure fell off as expected stop by stop.

  • The manual metering using the body control with the FA22 are all within a 0.3 EV range, aside from the f/22 setting which dropped off a little from the reference line ... that might be due to an iris actuation issue.

Summary:

The K10D's metering system is not 100% regular in providing perfect metering capabilities with pre-A series lenses. A characteristic behavior is to evaluate for more overexposure as smaller lens opening are used. This behavior differs to some degree between CW Averaging and Spot metering modes, and depends as well on the overall illumination that the camera is presented with.

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Since the overexposure behavior in this mode is pretty regular, one can work around it to a good degree by modifying the meter setting after performing a metering operation and bracketing. I'm not trying to trivialize the issue with that, it would be good if Pentax can do some work to improve this.

Personally, I don't feel it makes these legacy lenses unusuable in any way... after all, I was able to use the K-mount Zenitar, meter and make photographs with it without any problems once I knew the behavior of the camera's metering system. It does make metering with it a little more fiddly.

I feel that the best lenses to use with the K10D are the latest series of AF Pentax lenses if you want the best that the camera can offer. That's about as far as I'm going to take this thought.

enjoy,
Godfrey

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