05-04-2019 03:31 PM - last edited on 05-04-2019 03:48 PM by Danny
I Use my Canon 5D Mark III in conjunction with a 600ER-RT flash. The camera is in manual mode and the flash is in ETTL mode. That combination works fine until I put a diffuser cap on the flash head. Then the images are under exposed. I tested the same set up with a borrowed 600EX-RT on my camera and got the same results. My 600 EX-RT on another body, in ETTL mode, with and without a diffuser, gave good results.
I am using a 24-105 mm lens. As I go to longer focal lengths the under exposure gets worse.
I reset all of the camera and flash custom functions but the problem persisted.
05-04-2019 08:51 PM
We are sorry to hear that this happens. It's most likely due to the nature of the diffuser and the flash not taking it into account. Who makes the diffuser? If the issue doesn't happen without the diffuser attached, then there isn't much troubleshooting we can offer on this particular issue.
05-04-2019 09:59 PM
When you start using flash modifiers (like diffusers) on an E-TTL flash head like a Speedlite then you are getting into situations where the E-TTL system is far less likely to successfully automatically compensate like it can using its build in "wide screen" and/or other built in flash modifiers that signal the system what changes have been made. A diffuser is generally going to "eat" at least one full stop of light and many will cause an even greater decrease so it will be very noticeable and as you change the pattern of light dispersal then your choice of metering setup on the camera also becomes more critical. With a light modifier, the flash isn't producing the amount of usable illumination that the camera expects when it commands a specific flash power setting and the diffuser also largely defeats the functionality of the flash "zoom" compensation by broadening the narrowed light beam the flash tries to produce thus the bigger issue with the lens zoomed to more extreme focal length.
You could try dialing in exposure compensation but since you already have the camera in manual mode I would also put your Speedlite in manual mode so that you have full direct control over all of the exposure variables. This is a case where an external light meter can take a LOT of wasted time and frustration out of the process. You don't need an expensive complex meter and an under $200 meter from one of the established providers will serve you well for a very long time.
Also keep in mind that as you reduce the light output of the Speedlite via a diffuser, its power level has to be increased to compensate and this can get you into heating issues very quickly that are exacerbated by a diffuser that further reduces cooling of the Speedlite. Smaller hot shoe mounted flashes are very useful but forcing them to produce a continued series of full power flashes will stress the system rapidly. Unlike larger studio flashes which have forced air cooling to keep the xenon flash tube, transistor inverter supply, and capacitor bank cool a Speedlite is largely sealed to protect it from dirt and moisture but this impedes cooling. Using it at maximum power with a diffuser in place means you should keep in mind that the minimum recycle time it can provide with its external battery pack doesn't mean it is a good idea to fire it at a high duty cycle with very short periods in between flashes. This is another case where the light meter is nearly worth its weight in gold because it allows you to quickly dial in the correct flash power and camera "exposure triangle" settings instead of the test fire and review approach.
Since forum rules will strip most links, do a search for: Gossen exposure metering compendium PDF. Gossen is an old line German manufacturer of light meters among other products and their compendium will provide you with one of the best written and easiest to understand explanation of photographic exposure and lighting that you will find anywhere. It is a free download and one of those rare cases where some highly technical subject matter is presented in an extremely understandable manner.
05-05-2019 10:29 AM
The specific diffuser that I was using is the Gary Fong Lightsphere. My "understanding" of ETTL is that the system would compensate for the diffuser by increasing the light output.
05-05-2019 02:32 PM
How far away from your subject are you when using the diffuser? You will take a big hit compared to the bare flash when using this diffuser with the lens at maximum zoom. Depending upon your manually chosen F stop, shutter, and ISO settings you may well be running out of headroom with the flash.
I believe Fong recommends bumping ISO up to 800 or higher and using a lens aperture of F4 in his instructions because depending upon how you have the Light Sphere set up you will lose anywhere from about 1 stop to a little over a stop with the diffuser in place.
With your camera set to manual, the only compensation E-TTL 2 can do is to bump up the flash power and it may already be at that limit. I rarely use my Canon Speedlites and I can't remember if they report firing power data in the EXIF file but if so take a look at those and see if your flash is being commanded to fire at full power. If so then you are going to have to decrease shutter speed, open up the lens, and/or increase ISO if it is underexposing.
05-05-2019 07:42 PM - edited 05-05-2019 07:43 PM
Good luck with it! TTL makes using flash a lot easier but in many situations it is still less "automatic" than using existing or natural light, especially in order to get the image quality of which your DSLR body and lens is capable.
I have been doing various types of photography for decades but I first started seriously experimenting with a set of three 500 watt Hensel studio strobes a few weeks ago and it has been a humbling learning experience in becoming comfortable with the setup and use of various light modifiers. It has been an overall good experience and reminded me of why a separate light meter really is an extremely useful tool in getting the type of lighting setup you want with less time and aggravation. But I am also glad I haven't needed to use them for a critical situation yet because I am confident I have many mistakes yet to discover
05-13-2019 01:39 PM
Just got my camera back from Canon for routine maintenance. They checked the ETTL function and it was working OK. I tested the camera with the 600EX-RT, in manual at various ISO settings with and without the flash modifier. The ETTL is now working properly. I'm glad that is the case but would have liked to know why I previously had a problem.