* The Pro 1 has 8-megapixel resolution, but just 7x optical zoom.
* The S1iS has just 3.2 megapixels, but 10x optical zoom.
The S1iS seems to have 40% better zoom than the Pro 1, but as I examined the specs more closely, I found the difference was 90% -- the S1 gets nearly twice as close as the Pro 1. Here's why.
Comparing Optical Zooms
You need to look at the 35mm-equivalent numbers, and compare them to the "norm" of 50mm -- which is like pH7, neither zoomed or wide angle. Here are the numbers:
Pro 1: 28 - 200mm (0.56x wideangle -> 4.0x telephoto)
S1iS : 38 - 380mm (0.76 wideangle -> 7.6x telephoto)
The Pro 1 gets more wide angle than the S1iS at the expense of significantly less zoom. I prefer more zoom over more wideangle. (I can always get more wideangle by stitching 2x2 photos together.)
Comparing Digital Zooms
Both cameras have the same digital zoom, a puzzle because the Pro 1 has so much better resolution than the S1iS. Let's do the same calculation after multiplying in the digital zoom factor of 3.2x:
Pro 1: 200mm x 3.2 = 640mm (12.8x telephoto)
S1iS : 380mm x 3.2 = 1216mm (24.3x telephoto)
The reason that Pro 1 has less zoom than the S1iS is because there is a trade-off between long zooms and high resolution. For me, I prefer long zooms.
Reading some more of Canon's specifications for the S1iS, it apparently has RAW mode -- something that's stated twice in the Tech Guide. But we know it doesn't have RAW mode, so I am wondering if it had been planned for the S1iS, then pulled so as not to compete with Canon's more expensive models.
drz at dpreview.com/forums notes that "this was discussed extensively when the camera first came out. It upset a few of us. If you look into the firmware updates that have come out, you can see RAW as an option in some menus -- but obviously those menu options are turned off somewhere. I keep hoping that either Canon releases a firmware update that turns it on or that someone figures out how to enable it -- as has been done for other options on other Canon cameras."
(I'm using this post to show off a remarkable image of a Pro 1 sliced lengthwise along its lens barrel. I was most impressed at how the lens goes almost all the way through camera body. Also, a partial see-through photo of the S1; the yellow cylinder at the lower-right is the ultrasonic motor that powers the zoom lens. Click both images to see the larger photos.)