This is my understanding of how the K5 handles high ISO. If I am mistaken, please correct me!!!
Ok, so first of all,
your K-5’s highest “native” ISO is 1600. That’s why ISO 1600 is the highest selectable ISO in bulb mode. All higher sensitivities are done with smoke and mirrors, I’m afraid!
The interesting thing about this is your RAW files aren’t really RAW!
Here’s how it works (in slightly simplified language):
At ISO 1600 and lower, when you press the shutter button the Prime II processor sends a request to the sensor for a read-out of all the pixel locations. On previous generations of sensors, this information was sent to the processor in analogue form to be processed into digital. However, on this new sensor things are done a bit differently. The sensor gathers analogue data from the photosites. This information is immediately converted to digital data (1’s and 0’s)
on the sensor. Please note that no new information is added to the image from this point on. The digital data is then sent to the Prime II for processing into Jpegs or demosaic’d (I think?) and sent straight to the card in the case of RAW.
To explain what happens above ISO 1600 I will use the example of shooting at ISO 3200. Because the sensor is incapable of capturing images any faster, the camera does some tricks to get this “higher sensitivity”. In this case, when you press the shutter, the Prime II asks the sensor to take a readout
at -1EV. In other words,
the image is underexposed. This under-exposed analogue image is immediately converted to digital on the sensor and sent to the Prime II for processing. This is where it gets interesting when shooting in RAW.
Theoretically, RAW images are digital conversions of the analogue signal received by the sensor, with as little processing as possible, so that the user can do their own custom processing on their computer. However, when the K-5 is set to ISO3200 the digitized image from the sensor is actually underexposed by 1 stop! The Prime II then adds 1 stop (+1EV) to the image (,adds NR?) then sends the image to the card as a RAW file. This is a fair bit of processing for a “RAW” image! What we’re getting here is what you might call a “pseudo-RAW” file! So much for a RAW option which gives the user the power to process the image as they prefer!
The situation gets worse when you get to ISO 52100. By this stage, the sensor is supplying an image to the PRIME II that is 5 stops underexposed (-5EV)!!! This is then brightened by +5EV and (most likely) NR’d by the PRIME II before sending to the card as a RAW file. All this processing explains the wide variation of file sizes for RAW images. Generally, the higher the sensitivity, the larger the file size! Which basically means more noise information is being recorded.
In film terms,
what the K-5 is essentially doing at ISO 52100 is push-processing ISO 1600 "film" by 5 stops in-camera!
The point being,
you can get a more “true” RAW file at ISO 52100 by shooting at ISO 1600 at -5EV! And if you did, you wouldn’t have the reduced-buffer issue seen at higher in-camera ISO settings! The problem is, of course, reviewing the shots in-camera would be nearly impossible to do (especially accurately) because the images you’ve just captured are so under-exposed! I would expect the LCD would look almost black. Histograms would be useless of course.
In my view this calls for a feature request from Pentax in an upcoming firmware release: Can we please have “true” RAW image capture at high sensitivities? This would mean brightening the image with the PRIME II,
but only for in-camera review (maybe include a jpg “thumbnail” that has been processed to correct EV only?).
The RAW file actually stored on the card must be the under-exposed image, with as little processing as possible done to it. We will take it from there, thanks!
Now, if all the above is correct, we should be able to get results just as good from a RAW image shot at -5EV at ISO 1600 as we can get from a normally-exposed image shot at ISO 52100. Can someone with a K-5 please post one of each in RAW so that an NR whiz can do their thing? Thanks!
And thanks for reading. I'll be interested to see the responses I get.
Last edited by secateurs; 10-19-2010 at 07:23 AM.
Reason: small corrections and additions