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02-21-2011, 05:52 AM   #1
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flash synch speeds

While reading about cheap radio control versus cable or optical slave, I realized that I don't know much about flash synch, so I thought I'd come over here to the Beginner's corner.

Suppose I am using the camera (K100D ) in Manual or AP, and am using a non TTL flash. Does it really matter if the flash synchs at 1/125 rather than 1/180?

I'm guessing that if using a fully auto flash synch would be important to avoid the slight over exposure ?

thanks

02-21-2011, 06:24 AM   #2
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You have to choose an f stop and ISO combination that will not over expose the shot with a 125 shutter speed and your flash. Best advice is to do a practice set up and test the combination until you learn what works.
02-21-2011, 07:51 AM   #3
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The amount of light that a flash produces is determined by the time it emits light. This ranges from e.g. from 1/200s (full power) to 1/20000 (minimum power).

Those times are shorter than the shutter speed and therefore the shutter speed does not have an influence on the light coming from the flash.

If you use the flash in an environment with some ambient light, the shutter speed determines how much ambient light will be 'visible' in the photo.
02-21-2011, 07:56 AM   #4
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as long as you keep it under 1/180 it'll be fine

02-21-2011, 06:24 PM   #5
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QuoteOriginally posted by sterretje Quote
The amount of light that a flash produces is determined by the time it emits light. This ranges from e.g. from 1/200s (full power) to 1/20000 (minimum power).

Those times are shorter than the shutter speed and therefore the shutter speed does not have an influence on the light coming from the flash.

If you use the flash in an environment with some ambient light, the shutter speed determines how much ambient light will be 'visible' in the photo.
If full power flash emits light for 1/200th of a second and some budget radio triggers synch at 125th (or slower) , it sounds like the usefulness of the full power setting of the flash would be just slightly diminished?

(Thanks Everyone)
02-21-2011, 06:46 PM   #6
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QuoteOriginally posted by manybobs Quote
If full power flash emits light for 1/200th of a second and some budget radio triggers synch at 125th (or slower) , it sounds like the usefulness of the full power setting of the flash would be just slightly diminished?
(head scratching ) I must be slow today.....

Why?
02-21-2011, 07:53 PM   #7
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It's just that some folks on this forum seem to be very concerned about this and are even taking the radio units apart and adding resisters to fix this. I'm just trying to understand why they are doing this because the difference between 180 and 125 didn't seem big to me and I thought I must be missing something ? <grin>
02-21-2011, 07:59 PM   #8
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QuoteOriginally posted by manybobs Quote
While reading about cheap radio control versus cable or optical slave, I realized that I don't know much about flash synch, so I thought I'd come over here to the Beginner's corner.

Suppose I am using the camera (K100D ) in Manual or AP, and am using a non TTL flash. Does it really matter if the flash synchs at 1/125 rather than 1/180?

I'm guessing that if using a fully auto flash synch would be important to avoid the slight over exposure ?

thanks
In most cases, flash exposure is determined solely by the flash duration and the aperture. The camera's shutter usually doesn't enter into the equation at all.

This is because, with an electronic flash, the flash duration is extremely short. Typically, the full-power maximum duration is on the order of 1/1,000 second. At lower power, or if the subject is very close, so that the light is reflected back very quickly, the duration can be as short as 1/30,000. Some are even faster.

The reason for the synch speed limit (1/180 in the case of Pentax dslrs), is that a focal plane shutter consists of two metal curtains. Now days, most shutters travel vertically. At slow speeds (at or below 1/180), the first (leading) curtain opens, then the second (trailing shutter closes). At speeds above 1/180, the trailing curtain begins closing before the leading curtain is completely open. The result is a moving slit the travels across the sensor. Since the flash duration is so short, part of the image will be cut off.

At 1/180 or slower, the entire sensor is exposed for a brief period of time. During that time, the flash is triggered. Above 1/180, the entire sensor is never exposed at once. Unlike Canon and Nikon, which allow you to fire the flash at any speed, if you're willing to accept the cutoff, Pentax cameras will not fire the shutter if the camera is set for a speed faster than 1/180.

There are, however, occasions in which you might want to use a slow shutter speed. For example, say you are in a large room, with a foreground subject that you wish to have properly exposed. The flash will take care of that. But let's say that you'd also like the background to be at least recognizable. Let's further suppose that the aperture for the flash exposure is f/8. If there is enough ambient light in the room to form a decent exposure at, say, 1/60 at f/8, then both the foreground and the background will be properly exposed. However, if the foreground subject is moving, the flash will freeze that motion. A shutter speed of 1/60 isn't much good for freezing motion.

Have you ever seen a photograph in which a person, for example, is moving, and seems to have a "ghost" behind them, while they are perfectly frozen and exposed? This is done with a technique called "dragging the shutter". It also requires "trailing curtain sync".

"Trailing curtain sync" means that the flash fire an instant before the trailing curtain begins to close. Normally, the camera triggers the flash as soon as the leading curtain is completely open. You can demonstrate this easily. Set the shutter speed to one second and charge your flash. Trigger the shutter. The flash will go off immediately, while the shutter will stay open for the following second. Now, find the option in the menu for "trailing curtain sync" and enable it. Perform the experiment again. This time, you will see and hear the shutter open and, about 9/10 second later, the flash will fire.

To use this technique to create the ghost, simply set the shutter speed very low, say 1/10 second, with the camera set for trailing curtain sync. Now, as the subject moves into position, open the shutter. The slow speed will allow the subject to register on the sensor as a motion-blurred object. At the end, the flash will pop and give you a frozen subject that is perfectly exposed.

I think you misread STERRETJE's post. He referred to 1/20,000 second, not 1/200. I've never heard of a commercially available flash with a duration as slow as that.

02-21-2011, 08:38 PM   #9
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OK I just reread this 12 page thread https://www.pentaxforums.com/forums/pentax-camera-field-accessories/69648-rf-...-triggers.html

The reason why some are concerned with the 1/125th synch speed on some radio units is because of the addition of uncontrolled ambient light.

That makes a certain amount of sense. Now I get it. Thanks

edit: Sterretje already explained it, I just didn't realize it's importance.

"If you use the flash in an environment with some ambient light, the shutter speed determines how much ambient light will be 'visible' in the photo."

Last edited by manybobs; 02-21-2011 at 08:47 PM.
02-22-2011, 07:39 AM   #10
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No need to concern.

I have many flash units and use purely manual. I use radio trigger pt04 and found out quickly 1/180 does have a synch problem some time. 1/160 is ok 99%, but 1/125 is safe, and this is the speed I m using.

Depends on how much ambient you want. I prefer 1-2 stops darker and use flash colored gel to blend in my light sources. The flash pulse will practically freeze the object for you, as noblepa stated.

If ambient light is too bright, let's say you want to overflod the daylight, it's a different story.
In this case, I use ND8 (neutral filter), even stacking 2 ND, and bundle multiple flash units, and still keep the synch speed 1/125 (or 1/160).

I also use this technique to open up the lens to get bokeh and 3d effects in full daylight.

Hope this helps.

But do concern the cheap triggers will be discontinued one day or supply with a different radio frequency. I have several triggers and more receivers than my flashes. I consider cheap insurance for investment and as backup in case of weak batteries.
02-24-2011, 10:45 AM   #11
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Check out the Cactus 4 or 5 remotes too. Cheap triggers that seem to work. The 1/180th limitation is because of the Pentax body, not the flash or the trigger on any of the brands.
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