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.
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
--------------
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.
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
---------------
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.
No comments:
Post a Comment