matplotlib-users Mailing List for matplotlib

Hi everyone, 
Using 'imshow' to plot a few arrays (arranged with gridspec) that I then export directly to a .pdf file using savefig, I noticed that when I print the .pdf file (i.e. on real paper) some of the subplots are blurry. 
To clarify: 
- in all my figures, there is always 1 subplot very sharp and crisp, and all the others are slightly blurry,
- the effect is not visible on the screen at any zoom level,
- the effect is very slight, and hardly noticeable ... until you plot a fine dotted grid - and then, it becomes evident (the grid points pop out a LOT more in the sharp plot),
- the entire 'sub-image' is blurred (axes, on-image text, grids, etc...) but the axis labels and tick labels are fine.
This effect is hard to describe, so I placed an example here: http://www.mso.anu.edu.au/~fvogt/mpl_tmp/
You can compare the original pdf (straight out of Python) and the 'scanned-printed' version. The scan is not of the highest quality, but you can definitely see that the grid on the left is fainter - and if you zoom in, the figure's edges are also less sharp. 
I tried (and failed) to find info on the web so far (I'm still searching). In the meantime, has anyone seen this effect before ? Is this a documented bug/feature ?
I suspect that it may be related to how subplots (containing distinct images) are exported to .pdf, but it is a long way beyond my current knowledge of the savefig function. Note that playing with the dpi setting (I went up to 1200) doesn't help (I can see the improvement on the images themselves but the print blurriness remains). And just in case you wonder, the print bluriness also remains whether I print the .pdf straight out of Python or include it inside a Latex document first. 
Any ideas on how to fix/go around this ? 
Cheers,
Fréd
P.S.: The effect is not present if I export my figures in png. But for quality purposes, I'd much rather try to avoid going through png if I can ...