PDA

View Full Version : Strange phenomena


Anita Saunders
March 23rd, 2007, 01:43 PM
I wonder if anyone has any ideas on what may have caused an unwanted effect during a shoot recently?

I was using a Canon 100mm macro on a Canon 5D, with two flashes (one through a softbox and another for fill).

The phenomena I found in the resulting photos was a dark gradient shadow the full length of one of the long edges of the photo. It was on the left side when shooting portrait format, yet on the bottom edge if shooting landscape format. Moving lights closer, further, left or right etc did not make any difference whatsoever to the effect.

The strange thing is, this effect did not occur when shooting earlier and later, after this particular shoot, with the same cam & lens but different exposure settings.

Examples are below, with the church pic taken a few hours earlier. Pics are straight conversions from RAW to jpeg without any edit other than resize for web.

Can anybody explain this?

http://photo-art-gallery.com/images/LHa-web.jpg

http://photo-art-gallery.com/images/LHb-web.jpg

http://photo-art-gallery.com/images/LHc-web.jpg

Anil Mungal
March 23rd, 2007, 02:11 PM
The black bar may be your shutter blade, which can be visible when setting the shutter speed faster than the sync speed.

What shutter speed did you use?

Anita Saunders
March 23rd, 2007, 03:48 PM
Hi Anil - that's an interesting possibility I hadn't thought of.

My shutter speed was 1/250th which I thought was the fastest for my on-cam flash and assumed my Portaflash Mini Studio Flash would be the same..... which has 3 settings - high, medium or low. I assumed this was power output, not synch speed, so I set it to medium.

Oh, I better check this out, you may be onto something - I just don't know...... :(

Anita Saunders
March 23rd, 2007, 04:19 PM
Anyone heard of RTFM???

Please will someone explain to me the relation between flash output, cam limits on synch speed and actual synch speed between two flashes?? Aaaargh.... I have used this set up many times but never had this problem.

My flash unit is only a cheapie and the 'manual' is just a 2-page leaflet that says "Set your camera's shutter speed to the correct flash synchronisation speed. This speed (typically 1/60th - 1/250th sec) is slow enough to ensure that the whole frame is exposed to the flash and is fast enough to eliminate camera shake".

Incidentally there was little or no natural light (which is another problem because I couldn't focus the lens in the dark).

Charles L Webster
March 23rd, 2007, 04:35 PM
I don't know which camera you use, but check the camera manual to find the highest flash sync speed. My 300D is 1/250 sec.

On camera strobes can sometimes use shorter sync speeds than can off camera (for reasons someone else will have to explain because I have no experience in that are).

I would assume the settings (Hi Mid Low) on the flash are power, not speed. But power settings on strobes often affect flash duration.

Try lowering your shutter speed to 1/125 sec and shoot a test. The bar should be gone. Then shoot one at 1/250 sec to see if it reappears. If it does, then you can't use the faster speed, regardless of the manual.

Anil Mungal
March 23rd, 2007, 04:40 PM
I don't have a 5D but I think it has a sync speed of 1/200... you used 1/250 which seems to be the reason behind your problem.

Will Thompson
March 23rd, 2007, 08:57 PM
Canon has stated in the past that large studio strobes should be used at a max of 125 sec. with all current Canon cameras.

Per page 102 of the 5D instruction manual:

"Sync Speed

The EOS 5D can synchronize with compact, non-Canon flash units at 1/200 sec. or slower shutter speeds. With large studio flash, the sync speed is 1/125 sec. or slower. Be shure to test the flash to see if it synchronizes properly with the camera."

Hope this helps.

Chuck Fry
March 24th, 2007, 12:29 AM
My 5D won't work with 3rd party flashes any faster than 1/200th. I get exactly what you're seeing at 1/250th and faster.

Anita Saunders
March 24th, 2007, 02:50 AM
Well thank you all for the answer. What a relief. At first I thought something was wrong with my camera. Previously I was using a D60 which syncs at 1/250th and is fine.

Alan T. Price
April 22nd, 2007, 11:13 PM
Your shutter could be playing up, but it should happen consistently at one edge of the sensor.

If you were using a hot-shoe mounted flash then it is probably not a synch speed issue, as that would result in a sharper light cut-off (since the flash burst is much much faster than the shutter blades are moving). If you were using external studio flash units then it could well be a synch speed issue.


Synch speed is a camera feature that is independent of the flash assuming that you are using a flash designed for hot-shoe operation. Synch speed is the fastest shutter speed that has the entire image sensor (or film) exposed to light long enough for a flash burst to register properly. i.e. the exposure and the flash are synchronised. Any faster speed either has an open slit between the front and rear curtains moving across the sensor, exposing each part for the required duration but not exposing all parts at the same instant, or else it exposes the entire sensor at once but not when the flash is firing. Both situations result in partial or no lighting by the flash.

Some flash units can produce longer-lasting flash bursts that you might think need a longer-duration shutter opening, but generally this is not the case. Flashes operate at around 1/10,000 seconds give or take and that is way faster than any synch shutter speed (1/500 or slower).

A more likely limitation of synch speed occurs when working with studio lights. Here the limitation is not so much the duration of the flash burst but the delay that occurs before the flash output reaches a useful level. Then the effective synch speed may be reduced to 1/125 or even 1/60 because a faster exposure has been and gone before the flash operates. This'll be true of any camera even if it has a rated synch much faster than that. The ratings are for hot-shoe mounted flash units.