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The G15 has a 1/1.7" sensor i.e.. 7.6 x 5.7 mm
The Canon APS-C sensor is 22.2 x 14.8mm
The G1X Sensor (for comparison) is 18.7 x 14mm
In terms of sensitivity the G15 offers 12 megapixel (same as the 550D DSLR) and supports ISO between 80 and 12800 with a max aperture of 1.8-2.8 through the zoom range. In practical terms this means the G15 will focus closer and faster with potential for a shallower depth of field than the G1X - the behaviour of a DSLR will vary with the lenses that are available.
Quote: Just a small point the Canon 550D is 18MP, the 1100D 12MP.
Absolutely right - my mistake.
Quote: Please excuse my relatively naive question regarding sensor size. What I would like to know is how does say the Canon G15 CSC camera sensor compare in sensitivity/size against a Canon DSLR with an Apsc sensor. Hope all that makes sense 
The G15 has a sensor with about 1/8th of the area of the APS-C DSLR. The sensitivity of the two sensors will be similar (per unit area) but, because the larger sensor has a greater area the APS-C sensor will appear to be more sensitive - i.e. it'll be able to take decent pictures at higher ISO ratings. In theory the APS-C sensor at ISO1600 should match the 1/1/7" sensor at ISO200, but in practice the smaller sensor cameras usually have more in-camera noise reduction to mitigate this. The number of pixels on each sensor doesn't really affect the sensitivity much, except at very high ISOs.
Here is a page that shows the sensor sizes by specification with some camera examples
http://www.ephotozine.com/article/sensor-size-explained-with-sample-photos-17813
If you go to a site like dpreview they will tell you the size and you can compare them on this site.
Steppenwolf's comments regarding sensitivity are spot on.
The 10-20 on APS-C has a field of view equivalent to 16-32 on 'full frame'. The G15 is equivalent to 28-140mm on full frame. The two are almost mutually exclusive so you need to work out why you want either of them.
In what way do you see the G15 as being an upgrade of the 30D? The 30D is a very good camera and I am not sure it will be an upgrade, other than it is more portable.
Have a play on this page, looking at ISO and jpeg/RAW - they only have the 50D but if you compare the 50D at 800ISO against the G15 at 400ISO you will get some idea of the comparison.
http://www.dpreview.com/reviews/canon-powershot-g15/12
Quote: The 10-20 on APS-C has a field of view equivalent to 16-32 on 'full frame'. The G15 is equivalent to 28-140mm on full frame. The two are almost mutually exclusive so you need to work out why you want either of them.
In what way do you see the G15 as being an upgrade of the 30D? The 30D is a very good camera and I am not sure it will be an upgrade, other than it is more portable.
Have a play on this page, looking at ISO and jpeg/RAW - they only have the 50D but if you compare the 50D at 800ISO against the G15 at 400ISO you will get some idea of the comparison.
http://www.dpreview.com/reviews/canon-powershot-g15/12
Thanks mikehit, I really dug a hole for myself there didn't I ha ha . Next time I will do the arithmetic instead of wasting the time of people kind enough to comment. Back to the drawing board.
Quote: The G15 has a 1/1.7" sensor i.e.. 7.6 x 5.7 mm
The Canon APS-C sensor is 22.2 x 14.8mm
The G1X Sensor (for comparison) is 18.7 x 14mm
In terms of sensitivity the G15 offers 12 megapixel (same as the 550D DSLR) and supports ISO between 80 and 12800 with a max aperture of 1.8-2.8 through the zoom range. In practical terms this means the G15 will focus closer and faster with potential for a shallower depth of field than the G1X - the behaviour of a DSLR will vary with the lenses that are available.
I'm confused about your claim that the G15 might offer a shallower DoF than the G1X. For example at 28mm equivalent if you open the G15's aperture up to f1.8 you would need an aperture of approximately 4.5 on the G1X to achieve the same limit on DoF. At that AoV, the G1X's maximum aperture is 2.8 - which would give a significantly shallower DoF.
If you go to the maximum reach of the G1X [112mm equivalent], the G15's advantage in lens speed does take it closer, but not quite a match for the G1X, which would still only need an aperture of around f7 to match the DoF control of the G15 at the same AoV at f2.8 [I know it would possibly be a marginally bigger aperture, but not by much].
Focus speed does seem to be an issue with the G1X - but it's not something that particularly afflicts modern interchangeable lens cameras with the very glaring exception of Canon's own M-series, so it hardly seems a relevant comparison, particularly as the OP didn't even mention the G1X.
The larger sensor on an APSc camera would give an even greater advantage in shallow DoF with the most modest of kit lenses giving far more control than the G15 is capable of.
Quote: The larger sensor on an APSc camera would give an even greater advantage in shallow DoF with the most modest of kit lenses giving far more control than the G15 is capable of
It all swings and roundabouts Chris, smaller sensors have there advantages as well, its better to have to much dof in low light than having not enough to play with
Quote: The sensitivity of the two sensors will be similar (per unit area) but, because the larger sensor has a greater area the APS-C sensor will appear to be more sensitive
To be precise in photography we don't use sensor as some average brightness/grey value meter but to take image so question is about signal quality of pixels... largely defined by how much light individual pixels receive when all technical factors are identical.
So it's larger pixel receiving more light which gives that increase in quality when sensor size increases.
Quote: It all swings and roundabouts Chris, smaller sensors have there advantages as well, its better to have to much dof in low light than having not enough to play with
Indeed...
Stopping bigger format down for same DOF (related to physical aperture size and not focal ratio) as smaller format lowers brightness of image (related to focal ratio) projected on sensor equally much as bigger sensor area can gather more light.
So bigger format might not give any light gathering advantage depending on type of photography.
Quote: The larger sensor on an APSc camera would give an even greater advantage in shallow DoF with the most modest of kit lenses giving far more control than the G15 is capable of
It all swings and roundabouts Chris, smaller sensors have there advantages as well, its better to have to much dof in low light than having not enough to play with
I wouldn't argue with that at all - it's all dependent on the circumstances and the type of photograph you want. In a lot of commercial and artistic photography the subject isolation of a shallow depth of field is very desirable - which is why compacts aren't particularly suited to that type of photography.
On the other hand there are circumstances where you'd want to maximise depth of field and might struggle to do that with a larger sensor format [although you can usually push the sensitivity further all things being equal].
It's another good reason why we don't all own the same cameras and some of us own more than one type [aside from having more money than sense, that is...
]
Quote: To be precise in photography we don't use sensor as some average brightness/grey value meter but to take image so question is about signal quality of pixels... largely defined by how much light individual pixels receive when all technical factors are identical.
So it's larger pixel receiving more light which gives that increase in quality when sensor size increases.
No. It's a common myth that larger pixels give better "quality" but the truth is that they only give better quality in low light - for the obvious reason that in low light the greater area you have the better. Larger sensor cameras work better at high ISOs simply because they have a greater area of sensor. The pixel area is not that significant - if it were manufacturers wouldn't be stuffing more and more pixels onto their sensors.
I use an A77 (24Mp, APS-C) and a Nikon V1 (10Mp, CX) and the pixel pitch of these two cameras is pretty similar. The A77 is way better at high ISO because it has larger sensor and collects more light.
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