Flashing and curtains. (I'm talking about flash and the sensor curtains of your dSLR...)

That flash. I'm talking about that thing over there to the right. The external flash that you hook up to your dSLR.
Many people think that you just strap it on and then the camera will take care of the rest.
That's true up to a point. The whole deal with flash is to allow you to manipulate light, not just make dark things brighter. It can help you with motion, it can make bright walls even brighter to allow you for some silhouette shots, make things appear warmer and softer or colder and harsher.

It can also change the direction of things. What do i mean by that? Most of us have seen those images of things falling into water, or water droplets in fashion shots. You know, the ones where there is a model in front of a dark background and lots of water raining down. Many of us amateur photographers just point the flash and fire it. Don't do that. I've seen posts in photography forums of water shots with a slow shutter speed (to get that water movement) and when someone says "Your rain looks as if it's going upwards" they reply with "that's because of the flash."

NO. It is NOT because a flash was used and "that's how things work".
It's the settings used on the flash unit.

There is a certain trait of the way SLR cameras work and that is (are) the curtains. They are your shutter, they are what is being affected by your "shutter speed" setting, and they really are curtains. Or rather flaps nowadays that cover the sensor. (Click here for an awesome slow mo video on shutter movement from The Slow Mo Guys on YouTube™.)
What they do is that they change the duration that light actually hits the sensor and exposes the scene. How they work is by first dropping the first part which lets light come in and hit the sensor and after some time, the second one drops covering it back up. Then they rewind back up to the top and re-arm themselves for the next shot. The defaults on a flash is to fire up when the first curtain drops (which is fine if nothing is moving). By the time the second curtain drops to cover the sensor (or film, all SLRs work the same) the flash is all done. In fact it's done way before the curtain drops.

1st Curtain
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So what does that mean and what does it have to do with movement?
Everything. The sequence of events is as follows: 1) 1st Curtain drops, exposure begins. 2) Flash fires and instantly fills everything with bright light. 3) Sensor is still fully exposed taking in ambient light, flash stopped firing light. 4) 2nd Curtain drops to cover the sensor.
So, the the sensor took in the majority of light at the very beginning because of the flash and after the flash was done, it was still taking in ambient light.
Imagine a water droplet coming down. It reflected lots of light at the very beginning from the flash and when the flash was done it reflected back the less intensive ambient light. That is what makes the ghosting effect, and because the majority of light reflected back was at the very beginning, the brightest and sharpest imprint of the image is when the subject falling was at the very top position; when the flash fired and then the ambient light subsequently left a fading-out trail as the droplet keeps falling down. And our brain interprets that as moving upwards because of persistence of vision. The same thing that when you move something fast right to left, you see the object at the far left with a tail behind it.

If the flash unit used does not have any controls on it (as is the case with some really small units) then the camera can change those settings through a dedicated flash menu.

And yes, that is my oooold Canon 450D that is missing an eyepiece, the rubber is falling out, it has been drenched in sea water, rain, beer, coffee and tea.

The image shows the settings for the external flash, but they are still present for the pop out flash as well.


2nd Curtain
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All anyone has to do on the flash is to set it to fire at the 2nd Curtain. That's it. The camera will trigger the flash just moments before releasing the 2nd Curtain to cover up the sensor and in doing so ensures that the majority of light hits the subject at the end, and thus reverting the motion of the moving objects to what our brain is expecting.

The sequence of events now changes to: 1) 1st Curtain drops starting the exposure. 2) Ambient light comes in, no flash. Moving objects reflect only the softer ambient light. 3) Flash fires, moving objects get a burst of bright light for an instant and then immediately after ... 4) 2nd Curtain drops, ending exposure.

This is not something that is only available in pro equipment. They are normal functions of consumer grade SLR cameras and external flashes. (Note: With Canon cameras you can't set 2nd Curtain if you are in optical wireless mode. Cheap RF wireless triggers or studio strobes can be used, but that's a story for another time. If you can go wireless then you probably don't need lessons on what the 2nd Curtain is.)

The above (shitty) example shows the difference with 1st and 2nd curtain. In both instances the fan is spinning counter-clockwise. But notice the difference in the ghosting/blurring between the two images. Sure, most people won't care about the direction a fan is spinning as long as it's spinning, but this is just to show the difference. 1st Curtain of course can be used creatively. If for example someone wants to make someone flying upwards with motion blur, then they can have the person fall downwards and shoot with the flash set to 1st Curtain. So it'll be like those water droplets we discussed previously, where the flash would freeze the subject at the top of their motion and then ambient light would create motion blur going downwards.

Flashing your subjects

Sorry for the wall of text, there are 2 photo examples in there, i promise.
Also, someone mentioned that the images are not loading sometimes. Try refreshing the page, i've already contacted support for this. We'll see what they say.

Most people understand that it's for the best to have lots of light when taking a picture. After all - photography basically means to draw with light. What you actually need to do with light is to control it as much as possible, not just to have lots of it. Too much light may in fact ruin a picture if it is coming from the wrong angle. Ways of controlling the angle of light is your subject’s location, a reflector of sorts or to add a light source of your own, your camera’s flash. The flash is not just for adding light in low light situations, sometimes it may even ruin a night time photo while a daytime photo might be in need of a flash. Yeap, a daytime photo that most people wouldn’t think of using a flash (because you have the sun), may need some extra light coming from the right place. Camera sensors are not even close in behaviour when compared to our eyes. What you see, is not what the camera sees. Once someone realizes that, it will be easier to get the desired effect. And no, i won’t be going into serious flash photography where you use one or more flash units to light your subject and then another flash to light the background in a way you want. (It is though a really interesting subject, but that is not the point of this post.)

The first and most important use for a flash is of course fill flash, meaning to use the flash to add light to shadowy areas. That could very well be that friend of yours that you want to take a picture of, but is standing with the sun behind his back leaving you facing the sun. Sun blinds people but it blinds camera’s even more. That auto exposure and “limited” digital sensor kick in to lower the overall brightness of the scene to manageable levels and so the image ends up with a nice sky, but with everything else really really dark. What will you do if you can’t change your location? Use your flash of course. That’s why it’s there, it will try to fill in and brighten up the dark areas.

The fill flash can be done with any camera, compacts and dSLRs alike, it just needs a flash. The next two uses are easier with an external flash like the ones for dSLR cameras.

You can also use flash to simulate a softer, more natural light source. Swivel that head. Its got joints for a reason. As humans we are used for natural light to come from a big light source located far away, and the light hitting us from above, remember the cliche scenes from movies where they tell ghost stories with a flashlight shining from below? There's a reason some find it unsettling, the light is coming from an unnatural source. (Meaning we are used to seeing the shadows in a certain way.)
So turn that light source upwards and to the left or right and bounce the light on the ceiling and walls. The reason to do this is to give light a chance to diffuse and soften up a bit. Depending on the paint, the light may come down with a slightly different color and may even make the photo look warmer. (You can also use special gels to make the light coming from the flash match the ambient lighting more closely, but if you knew that, you wouldn’t be reading this anyway...) An added bonus for bouncing light on a wall is that your friends won’t want to hurt you for making their eyes bleed. A nice trick for nice results is to let light to come in from lets say a window from the left and then bounce light from a wall to your right to try and balance things out a bit. You may even play with the power of the flash to make things interesting with shadows. (Don’t forget, shadows are one of the things our brain uses to get info about a scene, especially if it is in 2D like a photo.) If you are using a compact camera and want to test bounce your flash, just use a piece of glossy photopaper in front of the flash, tilted 45 degrees towards the ceiling.

Just compare the two examples. On the first one the flash is aimed directly at the souvenirs, notice the harsh light and shadows. On the second one the flash head was aimed upwards. Light is softer and warmer because of the ceiling's paint. To make it seem as if the sun is at 45 degrees instead of noon, all one has to do is aim the flash head upwards and to the left or right. (Settings on both snaps are the same. Only the head's orientation was changed)

Another use for that rush of light is stopping action. It’s not as easy as putting the flash on 100% power and a fast shutter speed. Most probably your camera won’t be able to sync the flash with the shutter fast enough! Most consumer grade cameras have a flash sync of 200. Meaning it won’t be able to do its thing fast enough. The trick is in the flash. You see that bulb always flashes at the same exact power. What changes is the duration that the bulb fires. When you put the flash at 50% it is going to leave the bulb on longer than when putting it at 25% of output. That is what it really means. So if you put your flash at 100%, it is going to stay on longer, and so your moving subject will appear as if it has a trail. You could play with that setting and have a long or short trail from your subject, but if you want to freeze the action, then what you need to do is a bit opposite than what most people would think about. Up your ISO, select a slow shutter speed and for the flash’s power output, put it at around 1/32 of its power. Not half, not one eight. That 1/32 (or even 1/64 power) is going to be fast, making your effective shutter speed so fast that your camera can’t even think of reaching normally. Of course, with this trick, don’t expect to light up the whole scene. Just aim the flash head at what you want to “freeze”.

This snap was taken with a shutter as slow as 1.6secs. That's so slow you need a tripod. Yet, even with having the shutter open for more than a whole second, and just by holding the flash and manually triggering it when the cherries hit the water, the drops are clearly captured. Nothing blurred. (Yes i was throwing cherries in a glass of water. I know its not classy enough :P)

P.S. click the pic for a bigger view.
Oh and the f stop was f8, with the flash manually triggered at 1/32 of its power.