[文章] Canon X-sync vs high-speed sync

[KKH]

Tech Tips from The Digital Journalist -> http://digitaljournalist.org/issue0809/tech-tips.html

不要因為自己不懂用 flash 就說 flash 不準, 虛心些學習吧.

A friend of mine took a "bee on flower" macro with a Canon ring flash using a Canon 100mm macro at f/22 on a 40D set to ISO 200 in M mode with 1/200 second dialed-in. For all the world it looks like there's motion blur in some of the bee's legs. I can only surmise his flash was in High-Speed Sync mode, which caused the light to be on the whole of the 1/200 second shutter open time. I'm not near him, so I can't look over his flash settings nor do I own a Canon Ring Flash. Anyone familiar with how one determines/enables/disables High-Speed Sync mode on the Canon Ring Flash?

The controls for high-speed sync on the Macro Ring Lite MR-14EX are on the back of the power supply's control panel. However, high-speed sync is not the best solution for this problem. High-speed sync works by firing the flash at an extremely rapid rate for the entire duration of the exposure. It effectively turns the flash into a constant light source when used at shutter speeds higher than maximum X-sync speed, which is 1/250 for the EOS 40D and the 1D series digital SLRs. It has to be that way in order to provide even illumination across the frame as the moving slit of the focal plane shutter executes the exposure. That's also why the exposure level drops as the shutter speed increases, because you are in effect reducing the amount of time that the light is reaching the sensor. You can see a diagram that shows the difference between standard flash exposure and high-speed sync in our Flash Work Web site here:

http://web.canon.jp/imaging/flashwor...igh/index.html

Bottom line, the 'high speed' in high-speed sync refers to shutter speed, not flash duration. One additional item of information may be helpful: Even in normal flash mode (i.e., not high-speed sync), flash duration at full power with an MR-14EX can get as long as 1/750 sec. That might not be fast enough to freeze the motion of a bee's wing. One way to shorten the flash duration is to raise the camera's ISO speed setting. Another method for doing that is to open the aperture. Making the same shot at f/16 and ISO 400 instead of f/22 and ISO 200 would have cut the flash duration significantly with very little difference in noise levels or depth of field. Every little bit helps!

Thank you, Chuck. It always helps to know how things work to get the most out of using them. By the way, my friend made changes per your suggestion and retried his bee shots. The results were much better; the bees were all sharp and the 40D didn't deliver any distracting noise to speak of. I'm curious: Is flash duration information for various Canon Flashes available anywhere?

Canon does not publish detailed specifications on flash duration for its Speedlites, but the information I quoted for full-power flash duration on the MR-14EX is also applicable, generally speaking, for the MT-24EX and the 540EZ, 550EX, 580EX and 580EX II flash units. There are several reasons why full-power flash duration is designed to be approximately 1/750 second. Here are the top two:

1. Maximum X-sync shutter speed on high-end EOS SLRs like the 1D/1Ds series and the 40D/30D/20D cameras is either 1/250 or 1/300 second depending on the camera model, but the actual amount of time that the shutter blades are completely retracted during that time is only about 1/750 second. Makes sense if you think about it; it takes a certain amount of time for the first shutter curtain to move out of the way so that the entire image sensor is fully exposed, and it takes an equal amount of time for the second shutter curtain to cover the sensor and end the exposure. Keep in mind that except during high-speed sync mode, the flash can only fire when the shutter is completely open.

2. For any given Speedlite, the longer the flash duration, the greater the light output. Considering the relatively low-capacity flashtubes and capacitors used in these Speedlites, a 1/750 second flash duration provides the greatest possible output at all shutter speeds up to maximum X-sync. That's usually a good thing, except in the case of a very rapidly moving subject like the wings of a hummingbird or a bee.

The MR-14EX and MT-24EX each have two flashtubes, whereas the 540/550/580 series has only one. If both flashtubes are fired at full power simultaneously, their individual durations will be half that of the single tube Speedlites. However, it is also possible to fire only one flashtube at a time with the Macro Speedlites, which will bring the full power duration back to approximately 1/750 second. I could go on, but hopefully this information provides some additional useful perspective on the topic.