NX200 test shots - lenses, focus, and ISO 400

Started Nov 9, 2011 | Discussions thread
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Andy Westlake
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NX200 test shots - lenses, focus, and ISO 400
Nov 9, 2011

OK, with the latest round of test shots up, it's time to explain what's been going on.

The first round of shots were taken with the Pentax 50mm F1.4 on the Samsung NX-K mount adapter. The lens was set to F8, but what has become clear is that it was actually stopped down to about F11. This had three consequences:

  1. Longer shutter speeds

  2. Extended depth of field

  3. Very slight softening (mainly visible as loss of contrast) due to diffraction

These gave rise to various erroneous interpretations on these forums:

From 1) it was assumed that the NX200 mis-reported its ISOs, whereas in reality it is, just like pretty well every camera out there, within 1/3 stop of the ISO 12232:2006 SOS spec that's relevant to our testing methods. (Never, of course, to be confused with DxOMark's saturation-based test, which gives different answers.)

From 2) it was assumed the lens was front focused, causing the images to be soft. This was incorrect - the lens was correctly focused, but DOF extended further forward. Most of what people considered to be incorrect focus was simply the result of high ISO noise reduction (noone was ever able to demonstrate an obvious problem with the ISO 100 shot). The addition of RAWs should now make that more clear.

With the new round of shots taken with the NX 60mm macro, we're confident the aperture setting is now correct (F8), and shutter speeds consequently match other cameras to within 1/3 stop at any given ISO. However the longer focal length and larger aperture inevitably combine to reduce DOF compared to the previous set. Most obviously, the Queen of Hearts and corner checkerboards are now just beyond the depth of field, and look slightly soft as a result. There's little we can do about this given the 3D-nature of our test scene.

The ISO 400 shots have been withdrawn because we've identified slight motion blur at the shutter speed used (1/80 sec). The pattern is highly consistent between all of the duplicate shots we took, despite the fact that the camera is (as always) on a heavy studio tripod, with OIS turned off and using the self-timer for release. Note that the blur is so slight you'd never notice it in real-world shots - you can only see it when you place the ISO 200, 400 and 800 shots side-by-side and pixel-peep intensely. It's important to understand that it's unlikely to have any real impact in practical use. However it was clear from comments that it was immediately being massively over-analysed, so we withdrew the shot to avoid confusion.

As for what's causing this blur, our current working theory is that it may be shutter-induced vibration. This would explain why it affects only a narrow range of shutter speeds, and appears pretty consistent in both amplitude and direction from shot to shot. The relatively light bodies and double shutter action (close then open) makes this type of camera theoretically more prone to this effect than either SLRs or compacts (which use in-lens rather than focal plane shutters).

It's not just theory, either; we've seen essentially the same effect previously with the Olympus E-PL3 and E-PM1. However on these cameras it can be mitigated using Olympus's 'Anti-shock' setting, which closes the shutter, waits a specified time, then opens the shutter again to make the exposure. The NX200 has no analogous setting.

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Andy Westlake
dpreview.com

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