Quiz Nights. You can’t beat them so we joined ‘em. Thanks to Myk Garton for putting the evening together, I know from first hand experience just how much effort goes into making it look effortless. And of course all participants showed themselves masters of their hobby. This week on the blog we will wrap up two of the three remaining questions from the Week 2 Q by looking at: What is front curtain, rear curtain and slow flash? And what is Back button focus? This week has also been the big European photographic trade show, Photokina, held in Germany, one year I will have sufficient time and money to go …..
Still, daydreaming aside, what is front curtain, rear curtain and slow synch flash? Flash, aka strobes, aka those lighty things, is an area that is both technical and often ignored by the amateur. Yet it is one of the most effective accessories we can purchase. After all most of us have one built into our cameras, even if it does get overlooked most of the time. There is more to this than we will look at here, indeed we will visit this in a future blog, but for now we will explore the question that was put concerning these three often found menu options.
For this specific question we do not have to differentiate between off camera flash (or strobes if you prefer) and on camera (that is pop-up or otherwise built in). The reason behind this is that this is a specific question of the firing order of the flash in relation to the curtain or shutter. Mostly this is done off the position of what is known as the front curtain. The majority of cameras have a two curtain (shutter) set up. This enables faster shutter speeds, the flash synch speed of your camera is actually around about the fastest speed that the whole of the sensor is exposed. Otherwise it is exposed through a slit formed by the front and the rear shutter curtains moving across the sensor plane at a fixed distance apart. The faster the shutter speed the closer the gap.
If you expose using a strobe/flash unit above the flash synchronisation speed you get a dark band of varying width according to the shutter speed as the duration of a flash is extremely short. Because the lens projects things upside down onto the sensor and the shutters move from top to bottom this band will appear from the bottom upwards. Not so much of a problem if you only need light at the top of the frame, but not actually something that is easy to fine control both because of the position of the shutter at any given time and how much darker the rest of the frame is.
So why have a front curtain flash and a rear curtain flash? It’s to do with the motion blur in the frame and where the pulse of light freezes that in time. The least subtle explanation, nonetheless one that actually holds true, is that you use front shutter curtain to freeze the action. You might use your maximum shutter synch speed (often automatically set but that depends upon make and model of your camera body) to give yourself the maximum chance of freezing the action. You use the rear shutter curtain and, usually, a lower shutter speed to freeze the foreground and retain some motion blur in the scene. And slow synch flash? Well that is an automatic camera mode that forces a slower shutter speed and synchronises the flash. You can get some very different looking results from the same scene using these variations. As ever, try it out for yourself.
Back button focusing, once you’ve tried it you will never go back. At least that is what its fans say and there is no denying that it is a very useful tool. We looked at this on the blog last August viz “Back Button Focusing (refer to your manual for the native translation in your Camera’s Brand-Speak) does exactly what it says on the incredibly expensive magnesium alloy tin, or plastic camera body as befits your pockets/needs/delusions of grandeur. It is a button on the back of your camera body that activates the camera’s focusing system in isolation from the shutter release. When you operate via the shutter release a half pressure triggers the autofocusing system (assuming you are not mounting a manual lens) and a full depress activates the shutter release. Usually the shutter will not fire until the camera processor detects all the algorithms are in place to produce a point of focus and an acceptable circle of confusion (i.e. something is in focus …). The button itself is usually marked AF or a version thereof and is normally accessible with the right thumb (I’ve never seen one on the left but then I haven’t conducted a survey in any depth). And it’s on the back of the camera.”
Yeah, that is what it is but what use is it? Well first off there is the fact that, whilst depressed, the AF button means that you hold whatever it is you have focused on. In the automatic modes focus shifts when you shift what you are looking at, which can be time consuming. In order to keep the focusing, for instance if you want to shift the main point of focus to the edge of the frame and blur the background. You might want to add to that you can use the continuous mode (AF-C or AF-Servo depending on your camera body manufacturer) of focusing on your camera when following action and use the AF button to freeze the most advantageous point (takes some practice but worth the effort with a high degree of movement in the frame). Or basically no more having to focus every time you let go of the shutter which takes time and can mean that you loose your shot. Annoying when it was already in focus the last time you half depressed the shutter.
So now you know. Next meeting is Table Top photography, a practical so bring your cameras and tripods. Maybe your flashes too!