I hate that if the phone is put in a dark room while sleeping, when woken up it won't reduce brightness until it has been unlocked - it's too late then and the brightness raped my eyes while I was sliding to unlock. 
Got a tip for us?
Let us know
Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
How the hell does Auto-Brightness work?
- Thread starter WhySoSerious
- Start date
- Sort by reaction score
Brightness
I actually find the following:
Wake iPhone, screen is bright. Unlock the phone, screen dims quickly if I am in a dark room and auto-brghtness is set. This is only more exaggerated if I use my finger to cover the light sensor. As soon as I remove my finger, display brightens. However, if I then cover the light sensor again, it DOES NOT dim back down. A few other people said this. It seems to adjust one way (dark to light), but not the other way (light to dark)...
I actually find the following:
Wake iPhone, screen is bright. Unlock the phone, screen dims quickly if I am in a dark room and auto-brghtness is set. This is only more exaggerated if I use my finger to cover the light sensor. As soon as I remove my finger, display brightens. However, if I then cover the light sensor again, it DOES NOT dim back down. A few other people said this. It seems to adjust one way (dark to light), but not the other way (light to dark)...
iPhone 4s here and I originally thought I had a problem with the auto-brightness because the slider never moved from its original setting. I later realized that auto-brightness does work but the slider never moves. Set your slider to 75% then toggle auto-brightness on/off and you will see the slight difference in brightness. It is less dramatic than you think it would be.
My guess would be that the ambient light sensor is not always on. This might be why one generally only notices the change in brightness one time (when the phone is unlocked or shortly thereafter). Maybe the sensor takes a reading every n minutes consequently after the first. Not having the light sensor on all the time can save battery, I'm sure.
I also think that the slider setting is an Mean value, wherein the actual brightness may be above or below the user setting by a percentage. That percentage is probably calculated by the amount of light the sensor picks up (more light=lower screen brightness and vice versa).
This is simply a hypothesis, I don't have evidence to back up my statements other than years of iPhone use.
I also think that the slider setting is an Mean value, wherein the actual brightness may be above or below the user setting by a percentage. That percentage is probably calculated by the amount of light the sensor picks up (more light=lower screen brightness and vice versa).
This is simply a hypothesis, I don't have evidence to back up my statements other than years of iPhone use.
Some actual measurements
For a more scientific answer, I took some measurements of the screen brightness of my iPhone 4S with my light meter, when using the iLlumination app (setting the screen to be 100% white).
First to get reference brightness levels for 0% and 100%, I took measurements with Auto-Brightness off.
As you can see, the scale is not linear and you need to be past the middle point (as the screen shot shows) to get truly 50% of the range (323 lux).
And now, the measurements with Auto-Brightness on. I took measurements with the slider set at 0%, ~65% and 100% under a moderate low light (38 lux) and then took the iPhone under total darkness, a moderate low light (38 lux at iPhone position), and under a bright light (1820 lux at the iPhone position)
According to the measurements:
For a more scientific answer, I took some measurements of the screen brightness of my iPhone 4S with my light meter, when using the iLlumination app (setting the screen to be 100% white).
First to get reference brightness levels for 0% and 100%, I took measurements with Auto-Brightness off.
- Slider at 0%: 4 lux
- Slider at ~50%: 230 lux
- Slider at ~65%: 323 lux
- Slider at 100%: 650 lux
As you can see, the scale is not linear and you need to be past the middle point (as the screen shot shows) to get truly 50% of the range (323 lux).
And now, the measurements with Auto-Brightness on. I took measurements with the slider set at 0%, ~65% and 100% under a moderate low light (38 lux) and then took the iPhone under total darkness, a moderate low light (38 lux at iPhone position), and under a bright light (1820 lux at the iPhone position)
- Slider at 0%:
- Total darkness: 4 lux
- Moderate low light: 4 lux
- Bright light: 20 lux
- Slider at ~65%:
- Total darkness: 107 lux
- Moderate low light: 224 lux
- Bright light: 650 lux
- Slider at 100%:
- Total darkness: 615 lux
- Moderate low light: 636 lux
- Bright light: 650 lux
According to the measurements:
- When the slider is at 0%, turning Auto-Brightness on, only brightens the display when under the bright light (by 16 lux)
- When the slider is at 65%, turning Auto-Brightness on, dims the display immediately under the same moderate low light by 99 lux, and futher by 216 lux in total darkness. It also brightens the display by 327 lux under the bright light.
- When the slider is at 100%, turning Auto-Brightness on, dims the display under the moderate low light by 14 lux and in total darkness by 35 lux.
- There must be a specific level of ambient light above which the screen doesn't dim immediately when turning Auto-Brightness on
Attachments
ogs, thanks. I would have done the same if I had the correct equipment. Any info on how often the light sensor is reached for a reading? In other words, how often the brightness is actually changed?
As marksman wrote, great info, ogs123, but I'm still struggling with making a generalization in regard to the slider settings with Auto-Brightness on. It appears that as one slides from left to right, the slider shifts from limiting the maximum brightness to limiting the minimum brightness. Readings with the slider at 20 - 30% would be interesting.
