I think you may be missing what I am saying here.
I know that there are physical hardware limits on the aperture opening size (it can't physically open or close any further) and that there are software set limits.
For the SX10, at wide angle there are 3 limit cases:
1. Non-CHDK gui: F2.8-F8.0
2. CHDK (current): F2.8-F11.0
I found this second limit was still being software limited by the FW and made a workaround and test compile:
3. Modified CHDK: F2.8-F16.0
From my testing it would appear that the extreme hardware limits are equivalent to about F2.8-F16.0 as pushing further does not yield different results (doing a direct comparison of image exposure level).
This corresponds to av96 values of 288-768.
The FW converts these values into a second value, which I assume is used for setting the aperture mechanism (I don't know what the actual significance of the value is, but they are *mostly* linear).
Now as we zoom out, the bottom aperture limits increases. As you have confirmed, this is basically due to how AV is calculated, it is a factor of both the focal length and the physical opening size of the aperture. Because of this, and because there is a bottom aperture *size* limit (av96 288 @ wide), this would make sense. At the largest aperture opening size at wide, the AV is F2.8, at full zoom for the same opening size the AV is F5.6.
What happens in the firmware is that at full zoom the av96 value for the current AV gets stepped down by 194 (eg F8.0 ---> av96 = 576 - 194 = 382) and this value is used to set the aperture size (ie F8.0 (576) at full zoom has the same aperture opening as F3.97 (384) at full wide). This offset value is constant for all AVs for a given zoom factor (zoom=0 (wide) factor = 0, zoom=128 (full) factor = 194, zoom=97 factor = 125, etc.).
I wanted to know the relationship between zoom factor and the AV offset factor, however this proved unnecessary, as the value was available in the FW code where I needed it. (For curiosity, I would still like to know the exact relationship. I know it isn't linear based on the values I have got: 0=0, 41=43, 97=125, 128=194. Eventually I may try to collect all 129 values and check the exact relationship.)
Now this offset has further implications when we look at zoom and it's aperture/AV relationship. At full zoom, the AV gets limited to F5.6-F8.0. This is equivalent to the aperture size for F2.8-F3.97 at wide or 288-382. However, we know the aperture is physically limited to 288-768, so we should be able to push the aperture up to 768 at full zoom. This would be equal to F32.2 (768+194=962 -> F32.2), but only at full zoom. At other zoom levels, the top and bottom limits would calculate out differently based on the offset value:
z= 0 (0): F2.83-F16.00
z= 41 (43): F3.30-F18.69
z= 97 (125): F4.44-F25.12
z=128 (194): F5.70-F32.23
I am not saying F32 is available at wide angle! That would be impossible! (For the given hardware...)
Hopefully my rationalisation is understandable here.
But outside all these theoretical numbers, the question would be is it true? And the answer is yes. I made some test modfications to the firmware. The values I needed were all available:
[SX10 1.03a] Upon return from the conversion function (FFA568A8) at FF93B4E8, register R0 holds the converted value (aperture setting), R5 holds the offset av96 value, R6 holds the offset value.
Using these values, I convert the av96 into the corresponding aperture setting and save it to R0 where necessary.
Testing the resulting exposure levels using this build was as expected:
At full wide: Exposures varied from F2.8 up to F16.0
At full zoom: Exposures varied from F5.6 up to F32.2
The last issue here, is whether or not it is usable. I had previously tested up to F16.0 at full wide and found a little diffraction at F11 and even more so at F16 (however these were at close-up shots ~12in).
I will still be trying some tests at full wide and zoom to see what the diffraction level is.
For every av value it was off by the same 194 units
Which is fractionally more than two stops.
The SX10 fully open aperture varies from 2.8 to 5.7 as you zoom, fractionally more than two stops !
I know this which was my point. Why is F2.8 @wide = F5.6 @zoom? I'm sure there is some calculatable function for this based on aperture size and focal distance.
As I noted above, it's not necessary to know the relationship as the offset value needed is readily available in the FW.
it may be able to set it as high as F32.2
With all due respect, you cannot be serious ?
CHDK hackers try to push the limits Canon have imposed without understanding why those limits exist.
(the same goes for absurd claimed shutter-speed overrides).
Yes I am serious, as outlined and tested above. Be clear on what I am referring to here though: the F32 AV is at full zoom only, not at full wide.
I am aware of the difference in being able to tell the camera to do something and it actually doing it. There are hardware (and software) limits on everything. However what I have stated here does not extend beyond what the know hardware limitations are, it just removes some of the software limits.
I have noted people stating ridiculous shutter speeds just because they can set it, without actually verifying it. Even with the SX10, on one of the Flickr groups, people were initially noting some pics taken at very high shutter speeds, whereas, I had checked and found that this was likely not actually happening as there were distinct software limits built into the FW that CHDK wasn't bypassing. (
http://chdk.setepontos.com/index.php/topic,4392.0.html). Fixing this, I was able to test and verify shutter speeds up to ~1/16000-1/32000 which is surely not adsurd if it can be proven.
Due to diffraction, image resolution would be terrible.
This is true and the bottom line on the usefeluness of all this. However, in the end, the usefulness is in the eye of the photographer. Someone may see or have a use for the expended AV settings, regardless of the diffraction level.
