October 28, 2009

Lumolabs: 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:

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 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 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.


  1. It looks that the K-x has some colour inconsistency against lightness so that the greys reproduced are different at different lightness levels.

    As such, the greys reproduced by the K-x are not the same and they are not accurate enough for different key tones, measurements based on incorrect colour balance and reproduction may not give you the most accurate results on the 18% grey tests, but anyway those were what the camera actually produced.

  2. Rice, K-7 and K-x had the same lighting and I imported the RAW files using a constant WB for each camera. Nethertheless, I cannot entirely exclude a possibility that the hue did vary slightly by image coordinate because my studio doesn't have black walls and because I used multiple light sources (although equal types).

    I have a number of small but systematic sources of measurements errors in my setup and I don't believe this one is more severe than some of the others ;)

  3. Dear Falk, my work is more modest.

    JPEG from LR. DA*16-50 with tripod at 50 mm.

    Light Room Beta 3.
    NR levels set to ZERO.
    WB 3800°K.
    Shaprness 40 with radius 0.8, details 50.

    Photos from ISO1600 are with colour NR (set to 25) and without NR at all.

    All photos are FULL-SIZED jpeg saved with quality 100 from LR.

    To say honest, K-7 is a winner in terms of resolution and sharpness.
    If to add colour NR in Light Room, ISO1600 looks really cool from K-7.
    K-x has lower noise, but worse in per-pixel sharpness and resolution.

  4. I'd say that this isn't surprising since I think the base ISO of the Kx is ISO 200 only and the K7 starts at ISO 100.

  5. Very Good test and interesting results.
    Could you check shutter speed for a given iso and aperture : I found in imaging-ressource and dpreview samples that the K7 takes +2/3 stop exposure time for the same shot than others dslr such as D90 or K20. Which is disputable way to increase the snr at a given iso.

  6. Are the Dynamic Range improvements from the K-x coming at the highlights end or in deep shadows?

  7. @pan900: I hope you read from my article that I agree with your statement. At ISO 1600, the difference in SNR is just 3dB or about one stop. SNRs in excess of 30dB are hardly an issue when applying good NR (or printing). On the other hand, the resolving power of the K-7 is better, as stated in my article.

    @Jehu: I compared at identical exposure settings. The K-x creates about +0.27 EV brighter patches which means that SNR values are recorded at 1.2x the luminosity, compared to K-7. This can be compensated by the reader by shifting the K-x SNR curves from ISO 100 to ISO 120 etc. This shift is small, though. I have no absolute ISO sensitivity calibration target.

    @Steve: Deep shadows only. Improvements at the highlights would mean lower base ISO values, like ISO 50 or ISO 25.

  8. Looks line NR for KX is turned on and for K7 to off, noise seems to be blored. Can you please provide some real test images?

  9. I think K-x trades sharpness for noise. Would be nice to see the test with lowered sharpness on K-7

  10. No K-x has new Sony EXMOR sensor with 1 stop advantage

  11. The noise level beats cameras like D90,D300s, K7,EOS 50D, 5D1 and also shines with details, there is hardly any filtering on raw data up to iso 1600 and really good detail even at iso 6400.

  12. I must admit that I'm not close so skill as most of the writers are.
    I have quite many Pentax lenses which I want to take in use with a good digital body.
    If I do not need more than ISO800-1600, could I be happy with K-X?
    I also have many Minolta lenses, and for them I bought an A700, can K-X or K7 compete with the Sony A700, or even beat it?

  13. @Anonymous, it may be best to consult some full camera tests or to learn more about K-7 and K-x. IMHO, the K-7 is at least a camera as good as the A700. The A700 sensor scores almost exactly in the middle between the K-7 (worse) and K-x (better) when one ignores the fact that the K-7 sensor has a better resolution and better video. Using ISO 1600 is no problem. The Sony may have a slightly faster AF module while the Pentax is environmentally sealed and has a better body (100% VF, quieter, more compact, metal shell etc.).


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