Defective flash?

santiago · Post by **santiago** » Sat Dec 01, 2018 9:10 am

After reading this excellent post, I made a "flash vs shutter speed" motion blur test by taking pictures of a spinning disc.
To my surprise, I saw a strange "ghosting" effect on the images taken using the Canon Speedlite 580EX flash:

The images taken using the Kuangren Macro Twin Lite KX-800 (only 1 lamp was used) don't have this issue:

Some background information:

The images were taken in a pitch-black room. The subject is a circle with some doodles inside which is glued to a cardboard disc mounted on a tripod. The disc is rotating counterclockwise at an acceptably constant speed (DC motor from a broken toy powered with an AC adapter). The camera is mounted on a tripod. The exposure time on the camera is set to 1 second, live view & silent mode are on. To make the exposures, I pressed the shutter button remotely and manually fired the flash (also remotely) shortly after to fit the burst within the exposure time. The flash is in M (manual) mode. Because the disc was rotating counterclockwise, the “ghost” markings were produced before the main burst, as if the lamp had weakly flashed twice before the peak of intensity.

All the intensities of the 580EX:

All the intensities of the Kuangren:

Is this the sign of a defective flash? Has anybody experienced this before?

Thanks in advance!

If someone is interested, here are the images of the same rotating disc, this time using continuous LED light:

(The purpose of this test was to have some kind of answer to the question: "If I took a picture of a moving/vibrating subject using flash at i.e. 1/1 power (ca. 4ms duration) and would be satisfied with the results (image acceptably frozen), which shutter speed(s) would produce "similarly satisfactory" results, when using continuous light?)

Lou Jost · Post by **Lou Jost** » Sat Dec 01, 2018 9:16 am

Many flashes have settings that include a pre-flash to measure reflectance of the subject, so the flash can adjust its output appropriately.

santiago · Post by **santiago** » Sat Dec 01, 2018 9:22 am

Lou Jost wrote:Many flashes have settings that include a pre-flash to measure reflectance of the subject, so the flash can adjust its output appropriately.

Thanks for the reply. I just read that the preflash does not work on manual mode, but maybe it does?

I never use this flash, only the Kuangren, I'll have to read the manual and check the settings...!

santiago · Post by **santiago** » Sat Dec 01, 2018 9:42 am

Lou Jost wrote:Many flashes have settings that include a pre-flash to measure reflectance of the subject, so the flash can adjust its output appropriately.

So it turns out that even in M mode there is some preflashing happening! Fortunately if you set the switch on the base of the unit to "OFF" the preflash gets disabled. I just tested this and the ghosting is gone!

Thanks Lou

Lou Jost · Post by **Lou Jost** » Sat Dec 01, 2018 9:52 am

Good, I am glad you figured out how to turn the pre-flash off!

santiago · Post by **santiago** » Sat Dec 01, 2018 10:00 am

Lou Jost wrote:Good, I am glad you figured out how to turn the pre-flash off!

Figuring that out took 1 minute of googling, I just didn't know that a thing such as "pre-flash" existed!

Thanks again for the quick reply

Lou Jost · Post by **Lou Jost** » Sat Dec 01, 2018 10:56 am

Glad I could help.

rjlittlefield · Post by **rjlittlefield** » Sat Dec 01, 2018 12:36 pm

OK, I'm confused.

If the 580EX II is set in manual mode, it should not be preflashing to do any sort of exposure metering.

I'm wondering if what's being talked about is attempted communication between flash units, with this one in Master mode.

What "switch on the base of the unit" turned off the preflashing?

--Rik

santiago · Post by **santiago** » Sat Dec 01, 2018 1:21 pm

rjlittlefield wrote:OK, I'm confused.

If the 580EX II is set in manual mode, it should not be preflashing to do any sort of exposure metering.

I'm wondering if what's being talked about is attempted communication between flash units, with this one in Master mode.

What "switch on the base of the unit" turned off the preflashing?

--Rik

The flash was indeed in Master mode. The switch I meant is the "OFF-MASTER-SLAVE" switch:

The only way to disable the preflash is to set it to OFF (from this thread, third post):

https://www.dpreview.com/forums/thread/1942053

And it does indeed work. I took a quick picture and the ghosting was gone! I didn't know that this preflashing was used for communication, but it's clear that under certain circumstances this can basically ruin your picture!

At least there's a way to disable it...

santiago · Post by **santiago** » Sat Dec 01, 2018 1:30 pm

Here's the "issue fixed" picture (1/128 power):

rjlittlefield · Post by **rjlittlefield** » Sat Dec 01, 2018 1:40 pm

Got it, thanks. My 580EX II does not have a physical switch for that, hence the confusion.

See comparison HERE, the third image in the article.

--Rik

santiago · Post by **santiago** » Sat Dec 01, 2018 1:52 pm

rjlittlefield wrote:Got it, thanks. My 580EX II does not have a physical switch for that, hence the confusion.

See comparison HERE, the third image in the article.

--Rik

I see. In the 580EXII this is set via the menu.