I was recently reading the article The Truth about Digital ISO in Digital PhotoPro magazine, and decided to investigate the camera sensor noise profile of the Canon Powershot G12 I use for underwater photography.
Noise plays a big part in underwater photography. Available light levels are lower underwater,with less available the deeper you go. Knowing the noise performance of your camera allows you to push the ISO and get maximum performance.
I started looking at noise in the Canon Powershot series of cameras in my previous post Noise Comparison – Canon Powershot G11 vs G10.
There has been some recent talk on the web about “native ISO”, where the lower ISO of a camera is not necessarily the best for noise performance due to in-camera amplification methods. I have always thought the underwater photos taken with my Canon G12 at ISO160 were better than ISO80 or ISO100, and thought this might have been the effect of native ISO.
After reading the article The Truth about Digital ISO in Digital PhotoPro magazine, I decided to test my theory.
I setup the camera in manual mode, with a shutter speed of 1/60 sec, aperture of f2.8, and fixed white balance. A fixed white balance is required so the camera does not try and change the colour tints of the high ISO noise, giving you an incorrect result.
I placed a filter barrel and cap over the lens to block out all light, and shot a sequence of RAW images at different ISO settings from 80 to 3200.
The first interesting thing I noticed was the size of the RAW files.
The size of the file increased as the ISO increased with more sensor noise. The ISO3200 RAW file is nearly twice the size of the ISO80 file.
Plotting the files sizes on a graph we see that they start to increase significantly after ISO 320. (Note:- the bottom scale of this graph is non-linear, and if plotted linearly, the graph approximates a straight line, however the ISO320 turning point is still visible.)
The other interesting aspects of this data are:-
- There is no dip in the file size to indicate any intermediate ISO is better than any adjacent setting.
- Files shot with a shutter speed of 1 sec are larger in size than those shot at the same ISO and 1/60 sec. This is expected as the longer the sensor is sampling, the more noise will be produced.
Opening the files in photoshop and visually comparing them with no change to the exposure, noise is only evident from ISO640 and above.
Exposure = 0EV. Click the image to expand.
To make the noise more visible, the files were reprocessed with +4 exposure in Adobe Camera RAW. All other setting where left unchanged.
Exposure = +4EV. Click the image to expand.
The noise can now be easily seen. Interestingly the noise performance is quite good up until ISO320, which approximates the file size graph above. After ISO320 the noise deterioration becomes more significant.
From both the file size data, and visual inspection of the sensor noise on a Canon Powershot G12, the best noise performance is found between ISO80 and IS320. To allow some margin, I would be comfortable using this camera up to ISO250 in my underwater photography.
With the huge improvement in noise correction in the latest versions of Adobe Lightroom and Photoshop, this is not as big an issue as in the past, and the camera ISO could be pushed higher if needed.
There is no evidence of “native ISO’ effects due to in-camera amplification methods.