Understanding crop factor vs. sensor size 1/1.7"

RovingEyePhoto

Probably pretty lame, but relating sensor size on P&S's and compacts to those of SLR's always has confused me. I understand the concept of crop factor as relates to pixel count and individual pixel size, but crop factor isn't included in specs on P&S's and compacts, thus the confusion. For example, Canon G-11 has 1/1.7" CCD, but no crop factor specified. So, in trying to relate, I divide spec'd 35mm equiv 28mm widest FL to 6.1mm indicated widest FL, and get 4.6 crop factor. Same result when I divide spec'd 35mm equiv 140mm longest FL to 30.5mm indicated indicated logest FL, 4.6 crop factor. Does this in fact mean that the G11 has a crop factor of 4.6, or am I totally misunderstanding? Reason this has come up is that a friend is considering either the G11 or Panasonic/Oly micro 4/3, and body sizes aren't that much different (both neck/shoulder carried, neither P&S pocketable), but crop factor, at least the way I've described it, is hugely different. And at 10mp for all, individual pixel size also hugely different, along with related implications as to dynamic range, high ISO noise, etc. Considering only price and convenience in 28-140 carryaround range, it's easy to make the case for the G11, but considering all the other factors having to do with sensor/pixel size, micro 4/3 carries a heavy hammer.

Anyway, any thoughts relative to my understanding here would be greatly appreciated. Or if I'm just totally off base in my thinking, telling me that would be appreciated also. Many thanks in advance.

craig_d

You can calculate the crop factor of a P&S based on the 35mm equivalents of its lens, but the calculation is only approximately accurate because both the actual and 35mm equivalent focal lengths are probably rounded off. What you really want to know is the physical size of the sensor.

This article at Wikipedia:

http://en.wikipedia.org/wiki/Image_sensor_format

provides sizes and crop factors for a variety of digital camera sensors. The 1/1.7 sensor used in the G11 is said to be 7.6x5.7mm, with a crop factor of 4.55.

For a smaller secondary camera, I am tempted by the Canon S90, which is very similar to the G11 (same sensor in a smaller body with a very nice control layout), but I will probably end up going with a somewhat larger Micro Four Thirds camera with a pancake lens.

RovingEyePhoto

craig_d wrote:

You can calculate the crop factor of a P&S based on the 35mm equivalents of its lens, but the calculation is only approximately accurate because both the actual and 35mm equivalent focal lengths are probably rounded off. What you really want to know is the physical size of the sensor.

This article at Wikipedia:

http://en.wikipedia.org/wiki/Image_sensor_format

provides sizes and crop factors for a variety of digital camera sensors. The 1/1.7 sensor used in the G11 is said to be 7.6x5.7mm, with a crop factor of 4.55.

For a smaller secondary camera, I am tempted by the Canon S90, which is very similar to the G11 (same sensor in a smaller body with a very nice control layout), but I will probably end up going with a somewhat larger Micro Four Thirds camera with a pancake lens.

Many thanks, this confirms my understanding. One fact the Wikipedia page reminds me of is that we're talking here of diagonal measures in calculating crop factor. In terms of total sensor area, halve the crop factor (i.e., 4.6 for G11 vs 2.0 for micro 4/3) and you get 4 times the sensor area (L x W)! Isn't this the key measure, total sensor area, i.e. how many of the same size individual pixels you can pack into a given space?

I'll pass on your comment regarding the Powershot S90, although I recall reading a review knocking the S90's free-moving back ring (no detents) and complexities in using the front ring. Seemed to suggest a lot of things that could go wrong unless looking pretty closely at settings (especially ISO and WB) every shot, and that's not something I would do, nor expect my friend would do, for most vacation and other secondary shooting.

I personally would lean toward micro 4/3, considering most of what I shoot has my feet do the zooming (generally 24-50mm). For others enamored with telephoto for vacation and other secondary shooting, however, I certainly see the come-on of cameras like the G11 and S90, notwithstanding their much smaller sensor area.

craig_d

Yes, one normally uses the diagonal length in calculating the crop factor because not all sensors have the same native aspect ratio. DSLRs are usually 3:2, but P&S's (and Four Thirds) are usually 4:3. If you were only looking at one class of sensors and they all had the same aspect ratio, then you could use the diagonal, the horizontal, or the vertical, and it would all come out the same as long as you were consistent.

You're right that total area is actually more interesting than the crop factor; for one thing, area relates more directly to light-gathering ability. But you still need to know the crop factor to figure out how focal lengths compare to full-frame.

You will note that camera manufacturers tend to use whatever number is most beneficial to them at any given time. They like to talk about megapixels because that increases faster than linear resolution, but they like to talk about crop factor because that decreases slower than area. A 40% increase in MP seems more impressive than a 20% increase in linear resolution, and a 1.5x crop factor somehow doesn't seem as bad as saying that the sensor is half-size.

What it all basically comes down to in the end is that, relative to 35mm full-frame, APS-C is a bit less than half-size, Four Thirds is quarter-size, and P&S sensors are so small you could easily lose one if you drop it on thick carpet.

ivar

See also: http://en.wikipedia.org/wiki/Crop_factor

The Dutch one also has a nice list of examples at the bottom: http://nl.wikipedia.org/wiki/Crop-factor