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[OM] Re: Question for Dr. Flash

Subject: [OM] Re: Question for Dr. Flash
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Tue, 15 Jan 2008 08:19:51 -0500
Yes, I would agree with priit that what you described sounds like 
dragging the shutter and possibly even with the shutter on "B". 
Whenever you take a flash picture there are always 2 exposures.  One is 
the flash exposure which is controlled by aperture but not by shutter 
speed because the flash duration is very much faster than the shutter. 
The second is the ambient exposure controlled by aperture and shutter 
speed.  But in typical flash pictures in a dark area the shutter isn't 
open long enough to capture any significant amount of ambient light. 
Therefore, all we're used to seeing is the flash exposure.  "Dragging 
the shutter" is simply the technique of leaving the shutter open long 
enough to be sure and capture the much weaker ambient light.  The longer 
the shutter is open the greater the ambient contribution.  But if it's 
open too long then the subject that you hoped would be frozen by the 
flash will also start to show up via the ambient light.

Here's a shot with 1/4 second exposure and a lot of flash power.
<http://www.chucknorcutt.com/party.php>
If you check the left hand of the woman in the red dress at left you 
will see that there is some motion blur of her fast moving hand at 1/4 
second but it is not terribly noticeable across the full scene.  If the 
shot had been taken at a more typical flash sync speed like 1/60 second 
the incandescent lights lining the inside of the tent would have been 
almost invisible.  The result would have been a much less satisfying 
shot since it wouldn't have conveyed what could be seen by eye.  Of 
course, if the camera is being waved about as described in the article 
that you read the background incandescent lights would all show as streaks.

Whether the flash exposure occurs at the beginning or end of the shutter 
cycle depends on whether the camera is built or set for first or second 
curtain sync.  With first curtain sync the flash fires just after the 
first curtain is completely open.  With second curtain sync the flash 
fires just before the second curtain closes.  Usually it makes no 
difference how the sync is set but the impact of this is realized when 
there is significant ambient exposure as well as flash exposure and also 
significant subject motion during the exposure.

Consider dragging the shutter while taking a flash shot of a cyclist in 
a race with the cyclist heading toward you.  With first curtain sync the 
flash fires and freezes an image of the cyclist.  But the exposure 
continues and produces a blurred and much weaker image of the cyclist 
still moving toward you.  The visual effect on the eye when viewing the 
photo is that the cyclist was moving backwards away from you.  With 
second curtain sync the effect is reversed.  The blurred, ghost image 
occurs first and leaves a trail behind the flash image that occurs when 
the cyclist is closest to you.  Now when viewing the photo the 
impression is that the cyclist is moving toward you and the blurred, 
trailing image gives the impression of speed.  OM bodies do not provide 
a first/second curtain sync option.  They operate as first curtain only.

Dr. Flash



priit@xxxxxxxxx wrote:
> 
> 
> On Mon, 14 Jan 2008, Ali Shah wrote:
> 
>> something about pressing the shutter while focusing on
>> the subject, in this case a bride and groom. While
>> keeping the shutter pressed to turn the camera causing
>> the lights to blur up and down while the subject stays
>> in place and perfectly focused??!? I am not exactly
>> sure if I read that right but I wanted to know how
>> that is done.
> 
> I'm no Dr. Flash, but that sounds like the good old 'dragging the shutter' 
> technique.
> 
>> The lights looked like they were taken at a very slow shutter speed 
>> while the camera or lights are moving but yet the photo was taken with a 
>> flash!?!
> 
> What may be confusing here is the assumption that the shutter speed is 
> very slow. Light sources are very bright compared to the rest of the 
> scene, as a rule, and they can easily be recorded on the film/sensor even 
> with a smaller aperture and not-so-slow shutter speed. You probably need 
> to experiment, but 1/4 to 1/10 range should be good enough to get the 
> lights motion blurred (just a guess).
> 
> The flash exposure, which is a tiny fraction of a second will light and 
> freeze your subject. If it's a bounce flash they won't even have that 
> dreaded 'deer in the headlights' look.
> 
> The clever bit about focusing on the subject that you mentioned in the 
> beginning will take care of how the subject is focused and framed even if 
> you jerk the camera around, because the flash exposure happens in the very 
> beginning (maybe technically not 100% accurate, but should be close 
> enough).
> 
> priit.
> 
> 
> 
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