Black Line/Complete blackness when Using a Flash

[AshleyBaldwinPhoto]

Hi,

When I use my flash or flash units I get a black line or a black photo, depending on my settings. I am pretty sure it has something to do with aperature and/or exposure but I can't figure out how to control this. Can anyone help?

 

[bigtwinky]

Its your shutter speed that isn't in sync with the flash.

I believe your shutter is going too fast. It opens and the flash hasn't had the time to illuminate the whole scene before it closes. Slow that shutter down, check your manual for the right sync speed

 

[NateS]

Details about how you are using the flash....is this off camera using pocket wizards or ebay triggers? If so, you are probably going higher than the max flash sync speed on your ss which is causing what is said above. If it's mounted on the camera or on-board flash then that's odd as I thought camera's wouldn't let you go past your max flash sync speed (unless using FP sync).

 

[Big Mike]

That's correct.

The shutter is made up of two curtains. The first open opens, exposing the sensor/film, then the second one follows it to close the shutter. The time in between is the 'shutter speed'.

For very fast shutter speeds, the 2nd curtain might start closing before the first curtain is all the way open. This is a problem when using flash because the burst of light is very short, and of course, light travels at 'the speed of light'....so if the flash fires when the shutter isn't fully open, you end up with parts of the image that are severely underexposed (black line).

The camera will have a 'max sync speed', which is the fasted shutter speed at which the shutter will be fully open. It's probably in the neighborhood of 1/60 to 1/250.

So your solution is to always keep the shutter speed at or under your max sync speed when using flash.

 

[musicaleCA]

Or use the high-speed sync mode on the flash, if it has that.

 

[Dao]

Check the camera user manual and look for max flash sync speed or something similar. Of course, you can google your camera model with max flash sync speed and see if you can find the result online.

Once you know your camera max flash sync speed, just make sure you do not have your shutter speed faster than that.


For example, my Canon camera max flash sync speed is 1/250. So if I am using a external flash, I will dial the shutter speed to 1/250 sec or slower.

 

[Actor]

Let's turn the clock back to the days of flash bulbs. Flash bulbs had a nest of flammable metal in a pure oxygen atmosphere which would burn with intense brightness when ignited. The burn would take 1/30 second with maximum brightness being reached at 1/60 second.

Strobe flash lasts for only 1/2000 second or less but is much brighter than a flash bulb. To maintain compatibility with cameras designed for flash bulbs, strobe flash designers designed their units to flash 1/60 second after the shutter opened. I believe this practice continues to the present. If you set your shutter speed to less than 1/60 second the shutter will close before the flash occurs.

The rules for setting shutter speed and aperture are different for flash. Set the shutter speed to 1/45 second if your camera has that setting, 1/30 second otherwise. Then find the guide number (in feet or meters) of your flash. Divide the guide number by the subject to camera distance to get the correct aperture. Also verify that the guide number is for ASA 100 (it usually is). If it's for another film speed, or you're using film with a different speed, you'll have to adjust for that too.

Example: Your subject is at 5 meters and your guide number is 40. 40 divided by 5 is 8, so set your aperture to 8. But the guide number is for ASA 100 and you're using ASA 400. Stop down two stops to an aperture of 16 and take your shot.

To get the subject to camera distance, focus on the subject and read the distance off the lens. It's close enough.

 

[musicaleCA]

Looking at that...thank goodness for ETTL. Makes life so much easier...most of the time. (when running and gunning, it's great, but things like a model shoot, it gets really wonky)

The fastest shutter speed you can use with a modern flash unit (without black lines from the shutter, and at full power) is the max sync speed of the camera, which may be as low as 1/250 of a second. Faster than that, the flash can usually be used in what's called "high-speed-sync", where the flash is pulsed really quickly to simulate continuous light. It works pretty well, but the guide number of the flash falls off dramatically as the shutter speed gets faster.