High Speed Sync Part II
This is a follow up to my post from a couple days ago, in it I will expand on the concept of High Speed Sync, why it’s used, and some ways to get by without it.
Most SLR camera shutters consist of two parts known as “curtains” These curtains work together to begin and end the exposure. As a reminder, this is the sequence of shutter operation at speeds of 1/200 or less:
- The mirror flips up to reveal the shutter
- The first curtain of the shutter moves out of the way expose the sensor
- There is a very brief pause during which the sensor is completely exposed
- While the shutter is still completely open, the flash fires it’s 1/10,0000th of a second burst
- The second curtain comes down to end the exposure
- The mirror slaps back down making a very gratifying “Kerthump” noise.
When you exceed Max Sync Speed (Canon 1/200 - Nikon 1/250) the mechanism just isn’t physically fast enough to make the exposure the same way. The second curtain no longer has the luxury of being able to wait around for it’s twin to finish falling before it too has to start. The only hope it has of making it down in time is to begin falling before the first curtain has made it all the way down.
And for flash photography, that’s the problem.
If that second curtain starts falling before the first one is all the way down, the sensor is never completely exposed. There will always be at least a bit of it obscured by either the first or second curtain. So, if you’re using a flash, part of the sensor will never see it because one of the curtains of the shutter is always in the way. This will manifest as “Black Bars” or “Black Bands” on your image.
High Speed Sync to the rescue
There are certain limitations to shooting at 1/200 or slower. One of these is that it makes it harder to underexpose the ambient light, which makes it harder to balance your flash with the sun.
To overcome these issues, camera makers needed to find a way to lengthen the flash duration to whatever the shutter speed was. The solution they arrived at was to use a series of light pulses that last the entire time the shutter is open, instead of using one quick burst of light. So, as that opening between the two curtains moves along the sensor to make your exposure, the flash will be continually pulsing, exposing your scene right along with it. That’s why they call it “High Speed Sync”, because they’ve synchronized the flash and shutter so that your flash exposes the entire scene regardless of speed. The “High Speed” they’re referring to is the shutter speed, i.e, faster than 1/200.
Typical use of High Speed Sync
Consider this photograph:
The sky looks rather dark in the image, but in real life it was much brighter. In order to darken the sky, I cranked the shutter way over to 1/1600 and then pumped a ton of light on my subject to bring her out of shadow. This technique is known as “Killing the Ambient”, and it’s a great way to add drama to your scene. I’ve touched on this concept before in previous articles. Without High Speed Sync, I would have been limited to a shutter speed of 1/200. That’s three full stops of ambient light! Kind of a big deal, because each stop lets in half as much light in as the previous stop, my sky would have been much brighter and it would have ruined the mood I was trying to create.
The good news is that being stuck at 1/200 isn’t the end of the world, there are other ways to achieve the objective of killing the ambient light.
The Old Fashioned Way
Consider this image:
Like the image before, I’ve underexposed the sky to serve as my backdrop and and fired a ton of speedlites on my subject to brighten her up. But instead of darkening my sky by cranking my shutter over to the right, I stopped my aperture down to f/16, achieving a similar effect.
In my next post I will touch on additional methods of killing the ambient. I will also offer my analysis of the pro’s and con’s of each of the five methods available. Yes, there are five, extra credit if you can guess them:
- High Speed Sync
- The Old Fashioned Way
The Napa Photographer