Thursday, November 27, 2014

Camera Flash Tutorial (Part 1: Terminology)

Photo by Arkadiusz Sikorski
I wanted to help beginner photographers with getting into flash photography. There's actually a lot of technical and artistic components to using flash successfully so I wanted to share what I've learned to hopefully make their experiences better.

What is Flash Photography?
Flash (aka strobist) photography is all about using artificial light to supplement or even replace the existing lighting. The problem is that when most people use flash, their images look flat, washed out, or like a police mug shot. The result that we want, is for images to look like they are naturally lit, even though there might not be enough light in the right spot to take the photo. Knowing and controlling where we need to add light is ultimately what we're shooting for.

Flash Features
It can be hard to get started without knowing a couple of terms and what they mean. I'll start off with some of the most common terms you are likely to hear.

Flash Power
Flash Power is usually expressed as a fraction of its full power like 1/1, 1/2, ... 1/128. How much exactly is 1/2 then? It doesn't really matter because even when using flash manually, you take a test shot and dial it up or down depending on what you need.

My Flash can Zoom?
Yes, I recommend you only buy ones that can zoom, because zoom provides many benefits. First, its much more efficient if you can direct all its power on just the area you want. Second, a long zoom can help constrain the flash light to a localized area and out of the areas you don't want it.

What is TTL?
TTL stands for Through The Lens and it basically means that the flash power will be measured (aka "metered") through the lens and into the camera to determine the correct flash power to use to correctly expose the image. It works by firing a small flash and letting the camera meter the exposure with and without the flash right before the picture is taken. The camera then adjusts the flash power to correct the exposure and fires the flash again (this time at the right power level) to take the picture.
If your camera or the flash didn't support TTL, then you would have to manually adjust the flash power to get the desired exposure, which might mean taking a few test shots until you can dial it in.

Manual or TTL?
This question gets debated sometimes, but suffice to say, I recommend that you get TTL since you will always have to option to fallback to manual if you need to. Manual adjustment doesn't work so well if you're running around getting impromptu shots from different locations. In studio or product photography, TTL can sometimes change the exposure can even though the lighting parameters have stayed the same and in those cases you may want to switch to manual. However, I would personally start with TTL even in those cases and simply switch to manual if and only if there was a continuous problem.

High Speed Sync (HSS)?
Flash duration is actually really small, like 1/10,000 sec so you would think that you should be able to set your camera to a shutter speed like 1/500 and still use flash, but unfortunately, its not that simple. The problem lies in how the camera shutter works. There are actually two "curtains" in a camera that make up the shutter. One curtain drops down to allow light in, while a second one starts dropping a few moments later to stop the light. The delay between when the first and second curtain drops is your shutter speed. It would be quite an engineering challenge if these curtains had to completely open and close within the maximum shutter speed (up to 1/4000 sec) and be able to do so repeated tens of thousands of times. So instead, in high speed shots, the curtains move at a much slower speed and the second curtain is allowed to start dropping before the first curtain has fully dropped. What this does is allow only portions of the frame to be exposed at a time, yet each portion still only gets exposed for the high speed shutter time. This also means that there is no point at which the entire frame is exposed to light. So if a flash were to fire, only the portion of the frame that was exposed by the curtains at that moment would be exposed. High Speed Sync solves this by simulating a long pulse of light for the entire duration that parts of the frame are exposed. Because this requires more power, the maximum flash output is reduced in HSS mode. HSS is most useful when you want to use a large aperture to get a shallow depth of field and you have a lot of ambient light but still want to use flash. You can use neutral density filters to slow down the shutter speed in cases where you don't have HSS. If the price difference is minimal I would get it for the convenience, but it's not necessary.

Rear Curtain Sync
Rear Curtain Sync - Photo by
Now that we know what curtains are, rear-curtain sync simply means that the flash doesn't fire until the moment the second curtain starts closing. Its important to recognize that when the flash fires, everything illuminated by the flash will appear frozen at that instant and at that position. Take for instance, the photo on the right. The shutter was open for an extended amount of time which allowed the reflections on the car to cast the long streaks of motion blur. Here the flash fires just before the car exits the frame, illuminating and freezing the car at the end of the motion blur streaks. This gives us the impression that the car was moving from left to right. If the photographer were to use the default front curtain sync, then we would see the car at the beginning of the streaks instead of the end and the car would then appear to be going backwards.

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