Since we launched the Triggertrap Mobile, we've added lots of awesome extra functionality to the iPhone camera - and given our customers the world's most powerful application for triggering their SLR cameras. One question that keeps coming up again time and time again, however, is if there's a way of using the iPhone's built-in camera with an external strobe.
To answer this question, we need to understand a few things about how cameras work. The iPhone - and especially the iPhone 4S - is an extremely capable camera. But it is not perfect: The biggest problem with it is that it uses a rolling shutter.
The iPhone's camera and how it works
What this means is that instead of reading the whole photo at the same time (as you would do with a film-based SLR camera, for example), the camera effectively 'scans' the photo from top to bottom, one row at the time. This process is very fast, but it's not instantaneous.
The other problem with the camera on the iPhone, is that there's no way of influencing its settings; you can focus and take a light metering from a particular point, but that's it: You cannot chose a shutter speed or ISO, and the aperture is fixed at f/2.8. It can't be changed even if Apple gave you a software slider to adjust it: This is a physical limitation of the iPhone's camera.
There are other phones that get around this by implementing much better photo cameras, complete with shutters, apertures, focus mechanisms, and even built-in xenon flashes - but not the iPhone.
How a flash works
A flash, however, is a very different piece of kit; to ensure the correct amount of power output, the flash charges its capacitors, and then dumps a high voltage through a flash tube. The amount of light coming out of the flash tube is directly proportional to the amount of power it dumps through the flash tube - a lot of power means a lot of light.
Advanced flashes even have light-meters built in, where they are able to 'turn off' the flash after microseconds worth of light, for precise light level control. This is a small part of how E-TTL flash metering works with the flash power output to get perfect exposure.
All flashes have the 'one flash only' approach, except so-called 'high sync speed' flashes. Instead of using a single flash, these flashes can synchronise with shutters at speeds of up to 1/16,000th of a second (that's not a typo- but it is absolutely incredible, from a technology point of view). I've written a separate article about high-speed flash sync, if you're curious.
So, can you use an external strobe to take a photo with the iPhone?
In a word, no.
In many words: There are several problems with getting the flash to sync with your iPhone:
The first problem we would have to overcome is to find a way of syncing the flash with the iPhone. The challenge there is that we don't actually have any way of doing that. Remember that a 'normal' shutter speed for the iPhone will be in the region of 1/15th of a second to 1/30,000th of a second (or, at least, that is what the EXIF data of your files reports when you take a photo directly into the sun) or so.
The only indications the iPhone gives that it is taking a photo is the flash (which goes on for about 300 milliseconds - that's about 160 times longer than the duration of a flash). The other indication we can access is the sound the recorded "cah-chunk" sound the iPhone plays when you are taking a photo. Whilst slightly shorter than the LED flash built into the iPhone, it is still way too long to be able to derive an exact shutter duration from - especially if you are planning to use an electronic flash. It goes without saying that trying to synchronise a 1/30,000 second shutter duration (on an iPhone) to a 1/50,000 flash pulse (on a high-end, high-speed flash gun) based exclusively on a light flash that lasts 1/3 seconds is an exercise in utter futility.
However, if we somehow found a way of getting the iPhone to report exactly when it is taking a photo, we have a secondary problem: Since the iPhone uses a rolling shutter, you cannot use the single flash approach. Even if you were able to somehow fire the flash at exactly the right time, it would only affect a very small 'sliver' of the exposure. The problem is obvious, and would look a little bit like this:
So, the first hurdle is finding out when the iPhone camera is triggering. The second hurdle is the rolling shutter. But if you shomehow managed to overcome both of those; we're facing a third problem: Since the iPhone doesn't support any manual settings, and since flashes are too fast for the iPhone once the exposure has started, your photos will almost certainly come out over-exposed.
From the iPhone's point of view, a flash of light (or, more likely, a rapid series of flashes of light, to counteract the rolling shutter issue), is far, far more light than it expects. In the studio, you would 'expose for the backgrounds, and stop down for the flash' - which, in practice, means that if you shoot with a fast shutter speed, you kill off the ambient light (as described in my Darkening a room by adding light article). You would regulate how much of the strobe light you want to have an effect on your photos by choosing a smaller aperture.
Unfortunately, with the iPhone, you only get one aperture (f/2.8), and you have no influence over the shutter speed or ISO. So: When you take an image, your photos would be exposed for the pre-flash lighting situation. Once the flash goes off, the image would be grossly over-exposed and utterly useless for anything.
In summary; there are three reasons for why you cannot use an external flash with an iPhone: There's no way of knowing exactly when your iPhone is exposing the photo, and even if you did, you would need to ensure the photo is exposed not with a flash, but for the whole duration of the exposure. The iPhone will not report how long an exposure is (it can vary between 1/15th of a second to 1/30,000th of a second...). Even if you managed to overcome these two problems, your iPhone wouldn't be expecting the light, and you'd dramatically over-expose your images.
The obvious work-around for improving your lighting quality for iPhone photos, is to not rely on flash synchronisation at all, but to instead use continuous light. This photo, for example, was taken with a desk lamp and the light from an iPad's backlight (!) - for near studio quality lighting: