Using an external flash with your iPhone

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

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 work-around

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:

Universal underwater housing patent... Could it work?

Today, an interesting article popped up in my RSS reader, from Photorumors. The article discusses an universal camera housing for use underwater photography. As an avid diver (I'm a PADI Divemaster), my ears perked up: Universal? For SLR cameras? What a fantastic idea.

Whilst it looks like a great idea initially, anybody who has done much diving will be able to tell you that it wouldn't work. The patent application outlines a type of rubber 'gloves' that would enable you to manipulate your camera underwater. However, there many problems with this. The biggest one is the challenge of pressure. Imagine, for a moment, that you fill balloon with air, and take it under water with you. When you dive down to only 10 meters (only a fourth of the depth that recreational divers can go), the pressure will have reduced the balloon to half its previous size. Take it down by another 10 meters, and it'll be a third of its original size. at 30 meters, it'll be a fourth, and at 40 meters (the max depth for recreational divers), it'll be a fifth of its original size.

The opposite is also true. If you fill a balloon with air at 40 meters (It is possible: You look like a fool taking your regulator out to blow up a balloon, but it can be done), and you ascend, the balloon will grow to five times its original size. Or, with a balloon being a balloon, it'll pop before you get to the surface.

How does a normal underwater housing work?

Most underwater housings are rigid. The idea is simple: By letting the pressure of the water simply press on the outside of the housing, the pressure makes it water-proof. All it needs to do is to withstand a lot of external pressure.

At that point, the challenge is simply an engineering challenge: Create a plastic or metal housing that can withstand 5 atmospheres of pressure.

The challenge with this design

Now, the problem with this patent is obvious: you'd need a rigid part (so your lens can take photos), but also flexible parts (so you can operate your camera's controls). The problem happens when you have both rigid and flexible parts - the flexible rubber-like parts would come under a lot of stress from the water surrounding the camera housing, and would likely collapse under the pressure from the outside. In effect, the 'glove fingers' would fill with water, extending their size hugely, making the camera hard to use.

The patent application further explains that the manufacturers plan to cancel that effect out by having a "A pressure equalization system". The easiest way to equalise pressure would be to fill the enclosure with a liquid (since most liquids don't compress at the pressures we are talking about), but since we're talking about camera equipment here, that's probably not going to fly. Of course, there are liquids that could be used without short-circuiting the electronics in a camera, but non-conductive liquids tend to be expensive, and either way, they would affect the optical qualities of the lenses and the camera.

So, the other way to equalise pressure is to simply add more gas (air) to the enclosure when you dive down, and let the air back out when you go up. Sounds easy, but anyone who has ever tried using a Lift Bag underwater, will know that maintaining the correct amount of air to equalise, compensating for the constantly changing pressure based on the amount of water you have above you, is bloody hard.

It seems as if they're trying to solve the problem in this patent, by equalising the pressure inside the casing with CO2 cartridges:

But I can't help but thinking that, whilst an universal underwater housing is a great idea, and I want SalamanderSkinz's invention to work, but based on the patent application alone, it makes me wonder whether anybody would ever risk their pride-and-joy camera to the device, if and when it ever gets built. I don't want to sound like a stick-in-the-mud, but for now, I think the other nearly-universal solution that already exists is a better (and much cheaper) option...

Canon vs Red: The battle for amateur filmmakers is about to commence

In the Blue corner... Canon's C300

I don't want to say I told you so, but... I told you so. Specifically, I've been saying that shooting video on SLR cameras simply doesn't make sense - in some cases. Don't get me wrong; if you're a stills shooter who occasionally shoots video, knock yourself out. It's just the current wave of filmmakers shooting on SLRs that baffles me.

In that article (originally written some time last year), I'm arguing that it's only a matter of time before Canon launch a video-specific camera with an EF lens mount, so you get the best of both worlds: Access to affordable, high-quality glass, and all the features you expect from a video camera (but that tend to be lacking from an SLR camera; including things like decent video codecs, fine-adjustable frame rates, and audio recording that doesn't make you want to stab yourself in the eyeball with a 3.5mm jack plug)

It's fantastic news, then, that the EOS C300 comes along. If you look at it, it looks like the bastard lovechild of an EOS camera and a Sony Camcorder - but it's got a rather fantastic spec, which is what makes it all the more interesting.

I see that my Pixiq stablemate Jose has already beaten me to the details of the new camera, so I'd invite you over to his article to learn more, but I'd just like to talk for a minute about the implications of this...

What does it mean?

The new camera comes in two versions; The C300 comes with the EF lens mount, so you can use the incredible varied array of pre-existing EOS-series lenses with a device that's built from the bottom up to create high-quality video. In addition, there's a C300 PL version of the camera, which means you can marry up the new Canon body with any of the Arri Positive Lock lenses - that's the high-end lenses used on many a 16mm and 35mm film cameras.

The price tag of this new wünderkind is around €12,000, according to Amateur Photographer, so that probably means around £10,000 / $14,000 or thereabouts.

It's worth pointing out the obvious here: That's not pocket change. The whole reason for the video-on-SLR revolution is that it affords amateur and semi-professional filmmakers high quality video at entry-level pricing. If you're going to lay down that kind of money, there are other alternatives out there that are more tempting. Red's entry-level video snapper Scarlet, for example, comes with a £6,000 price tag, a much better set of accessories for filmmaking, and Red's cameras have had the option to use Canon EF lenses for a while, if that's your thing. It's almost obscene how thoroughly Canon has been beaten to the punch on this launch.

Not everything is lost though. Even though I have to admit that I'm disappointed by the price point, I think there's definitely a space in the market for both the C300 and the Red Scarlet. If Canon marketed their new video-cannon more in the $2,000 range, we'd have a true alternative to the 7D or 5D mk 2 for film makers. The tech included in the video camera isn't all that different from the net sum of bits and pieces that goes into one of Canon's high-end SLR cameras, so there's no particular reason for why they wouldn't be able to create the cameras at a loss for a while, until the economies of scale make the cameras profitable.

Canon certainly has its work cut out, but this is one battle where there can only be one winner: Amateur filmmakers. It's taken bloody long enough, but things are finally getting interesting in this market, and I, for one, am thoroughly looking forward to this battle!

Camera Exotica: the Pellicle mirror

Sony's new Alpha SLT 77 uses ancient tech in a new way. Nifty stuff.

A pellicle mirror is a semi-translucent mirror used in a few very rare and far-between cameras - until recently, when Sony re-introduced the technology in a few of their compact system cameras. So why are Sony reaching into photography history to make new cameras?

Let's take a look at the technology...

The idea of a pellicle mirror is that it takes the place of a moveable SLR mirror. In traditional SLR cameras, the tech looks something like this:

The lens focuses the image and flips it upside-down in the process. It then reflects off the mirror, into the pentaprism. The pentaprism flips the image rightside-up again, so you're looking at the world as you're used to seeing it.

When you are taking a photo, the mirror flips out of the way, so the light reaches the sensor, enabling you to take your photograph:

A pellicle mirror does things differently; it is, in fact, a semi-translucent mirror, which lets some light through to the imaging sensor, and some light through to whatever else needs to see the light: In Pellicle mirror SLR cameras, it sends part of the light into the pentaprism so you can view it through the viewfinder:

Why use a pellicle mirror?

The advantages of using a pellicle mirror are many: The viewfinder never goes dark, so you can see what happens all the time. There's no mirror slap - this is good for macro photography, where even the slightest shake of the camera can cause a blurred image - and it makes the camera significantly quieter as well.

Finally, back when pellicle mirrors were first introduced back in 1965, it was the only way to get high-speed photography done, enabling pictures to be taken at a faster continuous rate. Why? well, because the mirror does not have to go up and down for every image.

The EOS 1N RS (RS stands for Rapid Shooting), for example, can take 10 pictures every second

There are a few disadvantages to using a pellicle mirror - traditionally, pellicle mirrors caused about a 1/3 stop of light loss. (Some light has to go to the viewfinder). In addition,   The mirror has to be kept perfectly clean, or else the light sensor and other electronics (as well as the image quality, obviously) will suffer, but cleaning a pellicle mirror is a bit of a specialised job.

The reason why a pellicle mirror has to be kept so much cleaner than a 'normal' mirror, is that a normal mirror isn't part of the optical path to the film or sensor: If you have a dirty mirror in your SLR, that's annoying when you're using your viewfinder, but it flips out of the way before a photo is taken, which means that it doesn't really matter. In addition, when you change lenses, the mirror offers a little bit of protection for the shutters whilst the innards are exposed. On a pellicle-mirror camera, what you see is what you get: It's the front-most element of the camera, and if it gets dirty, your images will degrade in quality.

Pellicle mirrors have been used in the Canon Pellix QL (1965), the Canon F-1 High Speed (a limited edition camera introduced for the 1972 Olympics), the Canon EOS RT (1989), and the Canon EOS 1N RS (1994). On the Nikon side, pellicle mirrors were used in the Nikon F2 HS and the Nikon F3 HS; the latter was introduced for the 1998 Nagano Olympics.

Pellicle mirrors in the digital age

Sony have re-embraced the technology in a couple of new launches, including the top-of-the-line Sony SLT Alpha 77, and the Sony SLT Alpha 65:

The advantages of using a pellicle mirror in this case are many; Using the image sensor for the EVF causes it to slowly heat up, which degrades image quality through the addition of extra noise. By using a dedicated (presumably lower-power-consumption) secondary sensor for the image preview, you can get better battery life, and higher quality photographs to boot.