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Old Jan 29, 2009, 12:29 PM   #1
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Adorama has just posted an analysis of accuracies/inaccuracies of camera manufacturer's ISO ratings of 50 DSLRs. Knowing the inaccuracies of your camera can allow you to correct by compensating with an exposure correction if your camera consistently over or underexposed images:

http://www.adorama.com/catalog.tpl?a...op=academy_new

This analysis is based on data from DXO Labs, which have an online feature that enables a detailed comparison of cameras image quality based on their lab tests of those 50 cameras:

http://www.dxomark.com/index.php/eng...ality-Database

It is interesting to see how real world experience might differ from lab tests. Although the differences between current Pentax models are relatively slight if you run the cursor up the bar on the right side of the charts, they are perceptible. If one were to base a purchase decision solely on these test figures alone, one would not upgrade from a K10 to a K20, as the K10 rates higher in most tested qualities, with the K200 close behind or equal to the K10. These tests are on RAW images only, as that is what DXO Labs are concerned with, and not jpegs - I would like to see similar tests run on jpegs. I have not shot RAW images on each of these cameras for my own comparisons, but from comments on this forum, I doubt that the tests reflect user opinion.
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Old Jan 29, 2009, 5:21 PM   #2
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I won't even pretend to understand what the data means, however it is interesting to compare the K20D with the Samsung GX20:

K20D GX20

IMAGE QUALITY 65.4 68.7

COLOR DEPTH 22.9 23.2

DYNAMIC RANGE 11.1 11.2

LOW LIGHT ISO 740 771

??????
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Old Jan 29, 2009, 5:48 PM   #3
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I think I remember reading (from DXO) that a DXO mark that are within 5 are considered to not be reinvent as in you can't really see the difference. Next AFAIK DXO has not published any information on how much error (sample variants, testing error etcetera) but it looks like that could be 3-5 more. As how many Mb the sensor has is a variable over and above things like noise all sensor data is normalized to a sensor with 8 Mb. What that would mean is that if 2 sensors are both 10Mb and within 5 DXO marks that they are basically the same. If these 2 are DXO mark differents of 15 the one that is 15 above the other is significantly better. If 1 sensor is 8 Mb and the other 15 Mb (if you need more Mbs) but both have a DXO mark within 5 the sensor with the more Mbs is better because everything else is about equal.

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Old Jan 29, 2009, 10:21 PM   #4
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Numbers don't always tell the whole story. Though I do think the K10 has a bigger dynamic range than the K20. The K20 makes up for it in other ways, and it has less noise at higher ISO levels. It's interesting to note that there are a number of very expensive, highly touted cameras made by other manufacturers that aren't as accurate on their ISO specifications. Not that I think it matters much - as long as the camera is capable of producing correctly exposed shots.

One of the interesting things I've found is that the K20's exposure varies with the lens. It tends to underexpose 1/3 stop for my film lenses (including the FA 77). I haven't used the kit lens enough on the camera to make a judgement, but it doesn't underexpose as much with the DA 55-300. I have to watch that because I tend to leave the camera set to a +.3 EV and that will overexpose a bit when using the 55-300. It's been interesting to watch how the camera chooses different apertures or different shutter speeds when doing comparison shots between the A*300 and the DA.
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