Tuesday, June 28, 2011

Heart of the Pentax Q - Sony IMX078CQK Imager

For what I have searched and found myself, this is the Sony BSI CMOS imager which is used by Pentax in the Q, I believe:-


Below is a key comparison of the Sony imager against the specifications of the Q:-

Key Parameter
Sony IMX078CQK
Pentax Q
1/2.3" Sized Backlit CMOS
1/2.3" Sized Backlit CMOS
Pixel Pitch Size:
1.55 um x 1.55 um
Not Specified
Total Pixels:
Effective Pixels:
Movie Recording:
1920H x 1080V @ 60 fps
1920H x 1080V @ 30 fps
Aspect Ratio:
Maximum Recorded
Picture Resolution /
Colour Depth / Speed
4000 x 3000 x 12 bits @ 20 fps
4000 x 3000 x 10 bits @42 fps
2000 x 1125 (16:9) x 10 bits @ 60 fps
1332 x 74 x 9 bits @ 1000 fps

4000 x 3000 x 8 bits @ 5 fps (5 JPEGs)
4000 x 3000 x 12 bits @ Unspecified (RAW)

So, how does this imager perform actually? Let's look back at the above marketing datasheet. The following improvements/key features are stated:-

1. The colour response has been improved and it is more balanced and the individual response for R, G and B wavelengths of light are now more even;

2. The saturated signal level per pixel (aka "full well capacity/depth"), which is usually measured in mV, is now increased by 10% when compared to the previous generation of product of the same pixel size. Do note that now its unit is specified is in number of "digit", which is something that I don't know and understand..

I could not find any information on the signal-to-noise (S/N) ratio of the imager anyway. I did have tried to search for other datasheets of other Sony imagers and I did find others could contain that S/N data that we are mostly interested in, e.g.:-


.. which is the marketing datasheet for a 1/1.7 10MP CCD Sony imager and look at the S/N chart as per Fig. 1.

It can be seen that the S/N can reach near 40 dB (which equals to a Dynamic Range of 6.64 steps in EV term) at the best quality ISO of 100. The saturated full well capacity is also specified in mV as usual. However, since a new uncommon unit has been used for the new IMX078, I can compare nothing after all, no matter for that performance aspect nor for the typical S/N ratio.

Still, we can look at how the effect of decreasing the pixel pitch size on the imager performance. Let's look at one more also latest Sony 1/1.7 type CCD but which is of a higher pixel density near 14MP, with a pitch size of 1.75 um (versus the 2.03 um of the above ICX685):-


It can be seen that the saturation voltage drops drastically from 800 mV to 420 mV! But just bear in mind that the pixel pitch of the IMX078, i.e. the Pentax Q imager, is even smaller, in only 1.55 um. It is of a different design and is back illuminated, though. Whilst for a back illuminated design, the full well capacity and hence the S/N can be increased, the optical pixel cross talks between pixels are increased. The problem is particularly severe for different colours being leaked into adjacent pixels, resulting in more colour noise. As a result, the S/N ratio is decreased again, on the other hand.

In short, as for the IMX078, my impression is that it is an all-round product that is optimised for providing all the latest features for both still picture taking as well as video recording. However, it also seems that it is optimised for speed rather than Picture Quality, otherwise Sony should have emphasized those PQ strong points in the marketing sheet already (but there is none mentioned, except for that improved and more balanced colour response).

Read Also:-

The Secrets of the K10D (Part 1 of 3) - The Heart: CCD Imager

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