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.

Camera settings and their actual effect.

A friend just bought his first dSLR and immediately started taking pictures to get to know it, which is the best thing to do once you get one. Don't ogle over it; just go out and start taking pictures, while noticing what changes in your photos when you change a setting at a time.

During his first tries though the results didn't go along with the mental images he had. And I'm guessing here that the manual may be at fault a bit. Most often the manual says that you should try and expose the image correctly with the exposure meter. Yes, you should, even the cheapest of the dSLRs now have great sensors and can forgive a step up or down. (A bit underexposed or over exposed. If the circumstances are such that you can't get a perfect exposure, you can fix it later in lightroom or photoshop. Though that should be the last resort...)

Aperture for emphasis.
Manuals most of the time say that the aperture setting is for changing the lens's iris, allow more or less light in and that will make the depth of field shallow or deep. Now i know you'll yell "semantics!" and you'll be right i guess, but it should be the other way around, that the aperture setting allows you isolate a subject or bring everything in focus. I'm putting it like that to make it easier to realize just by reading it from the first time, that if you want to make a single thing stand out, that's what you'll use, instead that "it changes the amount of light through the lens and in doing so makes stuff blurry or not." People tend to think of non-sharp photos when you say blurry, instead of thinking isolated subjects with bokeh.

Shutter speed as a time function.
Same thing with the shutter. It says to keep a fast shutter to avoid shakiness in your photos. Well yeah, if the results came out shaky and you don't have Image Stabilization, tripod or steady hands, using a faster shutter will help with that. But don't think of the shutter as the thing to make your images ... not shaken. Grip the camera firmly, have the shutter button semi pressed and ready to just press it just a tad more to release the shutter and you can do well with a slow shutter speed. You should think the shutter speed as a way to express time instead, add movement and life in a still photograph. There is this thing that people say; that you should keep your shutter speed at, or above the lens's focal length, meaning that you should have 1/50 for a 50mm lens. Let me tell you that you can go handheld a lot lower than that with just a tiny bit of practice and no IS. If you want to capture a car moving fast, you can use lets say 1/200 and get a steady car, with the wheels blurry from spinning, and the trees blurry from you tracking the car. But do that with a human walking and it's just a frozen thing. May as well be standing still. Go for a slower shutter speed, one that will get your subject's limbs just starting to go blurry from movement and then use the aperture to cut down light. Want to isolate the subject as well? Use a lower ISO setting. Don't have a lower ISO setting? Well at that point you'll need to get an ND filter to cut down light I'm afraid. But the point is that there's still a way.

So don't think the settings as the way to get your exposure meter at the center. Instead think of what you want to show in the picture and use the tools for that, there lots of times you'd want to have motion blur.