Pentax K-x vs. K-7, sensor and video
Pentax has recently released a new SLR, the Pentax K-x. And while it is positioned at the entry-level market and very competitively priced, it is yet rumoured to have a very good noise performance.
After having done all my lab tests, I may say that the rumors are not true.
The truth is that the K-x may be the 35mm SLR camera which has the best high ISO noise and dynamic range performance to date. This isn't "very good". This is outstanding!
This is my concluding summary which is based on the following individual articles:
- Lumolabs: Pentax K-7 HD video quality
- Lumolabs: Pentax K-x HD video quality
- Lumolabs: Pentax K-7 sensor quality
- Lumolabs: Pentax K-x sensor quality
Noise and Dynamic Range comparison:
At the top of the article is a compilation of various gray sample patches for both cameras, for direct comparison. The patches are from linear raw files (cf. "further reading" at the end of the article).
The resulting 18% gray level chart is like this:
Signal to noise ratio of 18% gray luminosity and color temperature 2900°K (halogen tungsten). Using manufacturer ISO stops 100, 200, 400, 800, 1600, 3200, 6400, 12800. The interpretation of results is the same as for the corresponding graphs on dxomark.com. E.g., print-normalization is for 8 megapixels. The dotted lines indicate the slope for pure photon shot noise.
Obviously, the K-x has no true ISO 100 and both cameras reduce noise even in RAW at ISO 3200 and higher. The ISO sensitivity of the K-x seems to be slightly higher, like at ISO 100, true ISO were more like 87 for the K-7 (DxO's value for the K20D) and 105 for the K-x. However, the above curves have not been left/right-shifted to take this into account.
Gray level noise at ISO 1600/3200 for the K-x is about the same as at ISO 1000 for the K-7. While gray level noise at ISO 1600 and 3200 are about the same for the K-x, there is a minimal loss of resolution when looking at the pattern part in the full test charts. Nothing serious, though.
Signal to noise ratio (SNR) for the Pentax K-7 and K-x. Full measurement for various luminosities and ISO settings at color temperature 2900°K. Measurement similiar to dxomark.com. The dotted lines indicate the slope for pure photon shot noise. The flattening at 30% gray and brighter is believed to be due to systematic measurement errors like imperfect gray patches. The dashed curves for the K-x have been properly normalized to match the higher sensor resolution of the K-7.
The curve for K-7 only and test images are at:
Full SNR Pentax K-7
The curve for K-x only and test images are at:
Full SNR Pentax K-x
The K-x/ISO 200 and K-7/ISO 100 curves are almost identical down to 1% luminosity.
The full SNR graph clearly exhibits the true strength of the K-x sensor: it maintains the good SNR at low luminosity levels. This is an indication that the read-out noise from this sensor is very low.
Therefore, the K-x sensor outperforms the K-7 in low light situations whereas the K-7 outperforms the K-x in good light. This is further confirmed by the better resolution and more artifact-free images from the K-7.
The quality from HD video is very good. The resolution is as high as the 720p mode allows. It somewhat more moiré and color moiré artifacts than the K-7 and therefore, may not exactly match its analog and film-like quality in 720p mode. But it is very good nevertheless.
The Pentax K-x has a sensor which rivals the best. That Pentax makes it available in the entry-level class is a good thing. On the other hand, the Pentax K-7 excells in many respects and even its noise performance isn't bad. It is in line with the rest of the competition which is beaten by the K-x in this respect now.
Pentax K-x: "Editor's choice" (entry level, available light)
Pentax K-7: "Editor's choice" (semi professional)
Further reading: Lumolabs testing methodology.
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