Understanding my Flash

ktb53 · November 25, 2010

Question for all you pros out there. I'm pretty stumped about my flash and I'm hoping you can help. In low light situations (typically on location for an event shoot) if I use TTL for flash it seems I get much less power out of the flash then it could produce in manual mode and I have to turn the ISO much higher. I know there's the flash exposure compensation that will help me power the flash higher more then it thinks it needs but I'm trying to understand why the camera doesn't understand it has the power to get a shot if it uses more flash?

For example, in this particular lighting situation I could shoot 100 iso f/3.5 at 1/160 sec. With the flash at 1/4 power for a perfect exposure. However if I try to make any setting "auto" such as aperture value at f/3.5 the camera is guessing the shutter speed at like 1/8 a sec way off the 1/160 that is giving me a proper exposure. If I keep the camera on full manual and set the flash to TTL the shot won't have half the flash power that the manual flash was bringing and will come out dark.

It may be that the camera just isn't good at metering in these odd situations but if that's the case I need to come up with a way that i can get some automation if possible in this setting because I'm shooting some celebrity events and I am worried full manual including flash will be problematic for potential over/under exposure.

Thanks for reading, any advice is appreciated.

cpagility · November 25, 2010

Looking forward to reading the answers as I'm having difficulty figuring out my flash. I went to a party and had a really difficult time. I'd love to learn more.

Karen

ktb53 · December 11, 2010

guess no one knows the answer here.

Dan7312 · December 11, 2010

In eTTL mode when you set aperature priority the camera determines the shutter speed according to proprietary formula that determines how much ambient and how much flash light to use to illuminate the scene. The result in a not too bright scene is that the camera chooses a shutter speed that isn't a whole lot different from what it would use if there was no flash.

Try an experiment and use aperture priority with the flash turned on and off. You see that the shutter setting is different, but not by a whole lot.

The result is that the scene is illuminated with a mixture of ambient and flash light, like this image below shot with Aperture priority f10.

Another way to use the flash it so M mode where you set the aperture and shutter your self. This allows you to chose how much ambient and flash light to use. For example set your aperture to f10 and your shutter to 1/1000th. You image will still be exposed about right, but the color may be different, like this image.

That's because the second image was almost totally illuminated with the flash. In the first image above the ambient light was filtered by some shades which gave it a much different color than from the flash. The camera used AWB, but the camera saw a different color temperature in each image because in the first image the camera saw a lot of filtered light from outside and in the second it saw only the light from the flash.

So there is not absolute setting for a flash, it depends on the effect you want to get. eTTL does a really good job when you can't take the time to pick the flash power manually, for example when you are taking pictures of things where the scene is changing all the time.

P.S. There are people on dgrin that know the answer to the question you are asking way better than I do, but often are very busy and just can't get to it right away, but eventually they will. Dgrin is one of the best places there is to get technical and artistic information about photography.

rwells · December 11, 2010

ktb53 wrote: »

...snip...
For example, in this particular lighting situation I could shoot 100 iso f/3.5 at 1/160 sec. With the flash at 1/4 power for a perfect exposure. However if I try to make any setting "auto" such as aperture value at f/3.5 the camera is guessing the shutter speed at like 1/8 a sec way off the 1/160 that is giving me a proper exposure. If I keep the camera on full manual and set the flash to TTL the shot won't have half the flash power that the manual flash was bringing and will come out dark....snip...

At the moment, I really don't have time to go into a lot of depth on the subject, and there is a lot of depth to this subject! But I will say, you've mixed a lot of things together here, if your explanation is correct.

In Canon's world, in AV mode, Canon decided that you only want "Fill Flash" in ETTL, so that's why your seeing differences from your "manual" flash setting.

The most consistent method to use ETTL, (IMHO), is to have your camera in manual mode and set your ambient as you wish, keeping your max X-sync speed in mind, or you might wind-up with over-exposure due to the flash forcing the camera body back to your X-sync speed (You can go into HSS "High Speed Sync" ~ but I'll not go into that here). Now set your flash to ETTL, then adjust the flash output with FEC (Flash Exposure Compensation) as needed.

I really hate to say this, as it sounds put-off-ish, but there is a LOT of flash ETTL information threads here on dgrin. So a search might be in order

Neil van Niekerk's site has a wealth of super information about ETTL use. Look at the articles on the right side of page. This guy is a master at ETTL, and he explains it in very easy to understand articles.

Chuck Gardner's site also has very specific Canon ETTL "Why and How To" information.

Hopefully that helps a little bit...

pathfinder · December 11, 2010

At the top of this forum I have included a number of Sticky thread one of which is entitled "Some Links for EOS system flash Photography" - http://www.dgrin.com/showthread.php?t=70330

I strongly encourage anyone who wants to fully utilize the EOS system flash capability to read some of those links starting here - http://photonotes.org/articles/eos-flash/

The Canon system is quite good, but complex, and not built as many of use envision.

One must REALLY understand the underlying philosophy Canon used in setting up their flash system. It is tempting to think you can just pop the flash on your camera in AUTO and fire away, but you quickly learn that this does not always work out as expected, as the original poster has noted..

In short, when using ETTL flash with the camera in Av mode, the assumption is that you want the near ground in focus subject lit appropriately by the flash, AND that you want the background appropriately exposed as well by lengthening the shutter speed as much as needed. Canon ASSUMES that when you choose AV mode, you are knowledgeable enough about cameras to anticipate that you may need a tripod in Av mode. In P mode, you will find you shutter speed never drops below 1/60th I believe, because it is assumed you will be shooting handheld.

I found it counterintuitive that I wanted to shoot with the camera in Manual Mode, and the EOS Flash in ETTL, like Randy suggested. But that allows one to light the subject with the flash correctly, and capture the background with the exposure one desires, by altering aperture, ISO or shutter speed. One can light the subject nicely, and, at the same time, under expose the background, creating a nice theatrical lighting for the final image. How cool is that!!!

Read a few of the links I listed, and you will have a much better understanding of the EOS flash system and its many uses and potentialities. There are many.

Qarik · December 13, 2010

can you post some shots with exif data? Also are you spot metering? That may be an issue.

Understanding my Flash

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