10d Exposure Anomaly w/Studio Flash

[bthomson]

Received my 10D yesterday and am so far very impressed with it. I have a strange problem that has really got me going though. When using the EOS 10D with studio flash, I take a flash meter reading and take a shot at the indicated exposure using the Manual setting (1/60 at f/5.6). When testing what should be similar exposure settings (1/30 at f/8, 1/15 at f/11, etc.), the camera appears takes shots that get progressively underexposed. Strangely enough, if I keep the aperture at f/5.6 and modify only the shutter speed (again 1/30, 1/15, etc.), the picture is exposed correctly! This peculiarity only seems to happen with studio flash, testing exposures without flash works as expected. Any ideas?

Thanks,

Bill Thomson

[teddynet]

That's exactly how studio flash is supposed to work, you set the shutter to any setting longer than the camera's sync speed and alter the exposure by changing the aperture.

Try thinking of the flash duration as being similar to a fixed shutter speed.


Rich.

[scottbergerphoto]

That's exactly right. The shutter speed is really only for exposing the background, the area beyond the range of the flash. The flash duration is so short, less then 1/1000th of a second that shutter speed has no effect on exposure of the foreground. You must keep below the maximum sync speed of the camera to avoid taking a picture of part of the shutter.

[daveh]

The above responses are correct and your flashmeter will tell you this. Dial a different shutter speed on the flashmeter and watch the recommended aperture not change. (Until you go so far that ambient light is a factor.)

If you want to use a smaller aperture with a studio flash, you increase the flash power setting and meter again.

[bthomson]

All,

Got it, since the flash speed is faster than the shutter speed, exposure is not impacted by the shutter. I can't believe that I have been operating with this misunderstanding for years! One of the main reasons I purchased a digital camera was because I knew that I would improve my photographic skills and knowledge much faster with the limited amount of time I had to apply to it. It's working!!!

So, another quick question. The manual indicates a flash sync speed of 1/60 with studio flash. My tests seem to indicate I can get up to 1/250 before I detect the curtain. Anybody else exceeding the 1/60 documented limit with their studio flashes?

Bill

[daveh]

bthomson wrote:
The manual indicates a flash sync speed of 1/60 with studio flash.

They're just being conservative because they don't know what you're going to hook it up to. Most studio flashes list their durations in the manual. The flashes I'm using have about 1/3000 sec duration at full power so they have no trouble syncing at 1/250th.

[justme_dc]

another reason that they indicate 1/60th is that some older and even newer chepo strobes have a much longer flash duration and the slow shutter speed is to allow for strobes that "bloom" meaning the color temperature and light instensity change over the duration of the flash. Some strobes are like a light switch full on, full off, others kinda ramp up to full intensity.