How the iPhone copes with only having a f/2.8 aperture

If you ask any photographer whether they would be willing to take photos with a fixed focal length lens, many would say 'yes'. Prime lenses are as old as photography itself, and there are many excellent reasons to embrace them. Ask the same photographer if they'd be happen to work with a fixed-aperture lens, however, and you wouldn't get many good responses.

And yet, this is the reality of taking photos with an iPhone 4: It doesn't matter how bright or dark your scene is, you're stuck with a f/2.8 aperture lens. This is a problem if you want to use the iPhone with an external flash (not that you could anyway - here is why) - but how does the iPhone cope with extremely bright situations?

The lower limits

As you (probably) know, an exposure is controlled by 3 factors: ISO, Aperture and shutter speed. If aperture is fixed, you have to deal with any lighting situations with the other two. In low light, the iPhone will ramp up the ISO.

In fact, if I press my iPhone against a dark surface and take a photo, the camera reveals its limits:

This incredibly boring photo reveals the limits of the iPhone's camera: It won't use slower shutter speeds than 1/15th of a second, and it won't go beyond ISO 1,000:

The upper limits

Similarly, it's easy to test the iPhone's upper limits, by pointing the camera at a ridiculously bright light source. The sun will do. This photo:

... Reveals the other set of extremes:

... Which is that whilst the iPhone is still stuck at f/2.8, the maximum shutter speed is an incredible 1/30,000th of a second.

Putting these two figures together (about 4 EV steps of ISO and another 11 EV steps of shutter speed) reveals the exposure range available to an iPhone  photographer: an impressive 15 stops of difference from the lowest light to the brightest lighting situations.

Of course, this is nothing compared to the extreme shutter speed, aperture, and ISO ranges of modern SLR cameras, but hey - it's not bad for a device you keep in your pocket at all times!

Equivalent exposures

Changing, say, the aperture on your camera changes things beyond the amount of light that gets into your camera. As such, you might decide that you want a smaller aperture, in order to get a greater depth of field. A smaller aperture means that less light comes in to the camera. How do we solve this?

Your camera has three different adjustments for exposure: Shutter speed, aperture, and ISO. If you adjust one so the exposure would be brighter, you can adjust another one to compensate for the additional light captured.

For example: Let’s start with an exposure taken at 1/100 second, f/4.0 and ISO 200. Now, you can change your camera settings to 1/200 second. That would let half the amount of light into your camera compared to 1/100 second, because the shutter is only open for half the duration. Your photo will now be darker. If you change your ISO to 400, the sensitivity of the sensor is doubled, and the photo will come out looking more or less the same, from a brightness point of view, as with your original exposure.

You can change any of the settings to compensate for any of the other settings: A smaller aperture can make up for a higher ISO, a faster shutter speed can make up for a larger aperture, and a lower ISO can make up for a slower shutter speed.

Other effects of exposure changes

Of course, ISO, Aperture and shutter speed don't just affect the brightness of the image... Here's a handy reminder for what else they affect:

Easy peasy!