One thing that has really caused me to scratch my head since last June, is the difference between how the Ambient Light Sensors (ALS) on iPhone and Mac respond. After doing a lot of testing, I’ve found that the difference can be explained in software.

But, that still doesn’t explain exactly why Apple has chose to do radically different interpretations of ALS data for the two platforms. Theoretically, since Mac and iPhone both are running Mac OS X, they should act the same.

You can start to see the difference yourself with just a little testing. Crank the brightness on iPhone and Mac up to the maximum. Now, turn all the lights off in your room. Notice, the Mac will dim its screen, but iPhone will not.

Now (with the lights still off), turn the brightness down to near zero on both Mac and iPhone. Now, turn the lights back on. iPhone will brighten its display… but Mac will not.

What’s going on here!? If you use both Mac and iPhone, this can cause a lot of frustration, since your Mac and iPhone probably have similar brightness settings. What is happening, is that Apple decided to break from the similarities between Mac and iPhone, to make iPhone’s brightness setting be more like a traditional cell phone.

On Mac, the brightness setting is a maximum. You’re telling the Mac “don’t go brighter than this level”. On iPhone, the brightness setting is a minimum. You’re telling iPhone “don’t go less bright than this level”.

I can see why Apple did this. On other smart devices, the brightness setting is a minimum, and it would be confusing for someone in the Windows (Mobile) world to switch to this setting. Of course, Apple didn’t seem to grasp that it would be confusing to their most loyal customers… who have been using ALS since the PowerBook 15-inch added it.

Thus, if you want to maximize your brightness control, you need to keep this in mind. This means keeping your Mac on a brighter-setting (so that ALS can dim the screen when needed), and on iPhone set the display to a lower setting (so that it can increase when light demands).

But, there’s one more thing. iPhone has ALS standard deviations. What this means is that if you set your brightness level to the darkest… it won’t crank up to full brightness in normal light. It will only do so in direct sunlight. So, you have to set it at a more balanced setting, and compensate for the increase. This, as you might imagine, is frustrating for a whole new reason… you can’t have a dim display in dark environments, and then have iPhone crank its brightness up to a high level when in normal light.

I wish Apple would allow for some control of the ALS restrictions on iPhone. I want my iPhone dim in dark settings, and bright everywhere else. SDK project anyone?