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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 21 Jun 2004 (Monday) 15:46
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Auto Reduction of Fill Flash

 
DAMphyne
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Jun 21, 2004 15:46 |  #1

This question came to mind while doing some team photos for Little League.
I like to use the AV mode so I can control the depth of field, if I leave the flash in Auto, sometimes I get the "Ghosting" effect. So I set the flash to 1/200 sec, which gives a somewhat "flat" look to the photos.
My question is, if I turn on the "Auto Reduction of Fill Flash" will this reduce the duration of the flash, allowing me to better control the shadows, and still control my DOF?
What exactly does Auto Reduction of Fill Flash Do?
Thanks,
Dave


David
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slin100
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Jun 21, 2004 16:38 |  #2

Find the explanation for Auto Reduction of Fill Flash here (external link).

Turning on any kind of auto mode reduces your control. The same applies to Auto Reduction, which is enabled by default, btw. Whether or not it's enabled has no effect on your ability to control DOF in Av mode. DOF is determined by the aperture setting.

Auto Reduction affects the flash output at ambient light levels above EV 10. The rationale is that when the scene is bright, the flash output will be automatically dialed down a bit so as to soften the shadows a bit and not eliminate them entirely. The reason why it reduces your ability to control shadows is because the amount of dial-down depends on the ambient light level. At EV 11, the dial-down is 1/2 stop, EV 12 is 1 stop, and EV 13 or above 1 1/2 stops. So if the light level is varying between EV 11 and EV 13, the flash output can be difficult to predict.

You didn't say what the ambient light levels were. I suspect it was dark based on your "Ghosting" comment. The camera was probably selecting a low shutter speed, resulting in motion blur with a sharply defined image captured by the flash. When you set the shutter speed to 1/200 (you can't set the flash to 1/200), you got a flat look because the light level was below EV 10 that Auto Reduction did not come into play and the flash provided the primary illumination.

To handle this situation, you need to decide how you want the background to be exposed. Av mode will always attempt to expose the background properly. If you stick with Av, then you can to avoid ghosting in low light by using a tripod. Without a tripod, the only way to avoid ghosting is to increase the shutter speed. This will cause the background to darken and make your images look flat. One solution is to move the flash off-camera to provide some depth of illumination, although with group photos this can be challenging since you usually want even illumination across your subjects. You'll find yourself needing a multi-flash setup if you go down that path.

As far as Auto Reduction is concerned, you can turn it off and set your own flash exposure compensation. This will give you the most predictable control. I'm hoping you don't have a 300D and 420EX because you won't be able to do either.


Steven
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RichardtheSane
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Jun 21, 2004 16:43 |  #3

OK, I admit that I just have this enabled and it seems to do a great job for me... but I din't trylu understand it. The quoted text below is from the EOS flash document http://photonotes.org/​articles/eos-flash/ (external link)

Auto fill reduction.

Also called “automatic reduction of flash output†in some Canon documentation. Canon EOS cameras automatically use regular flash exposure with no compensation when ambient light levels are low - 10 EV or lower. However, when ambient light levels are brighter - 13 EV or higher - the camera will switch to fill flash mode and reduce the flash unit’s output level. It does so in TTL mode by dropping flash output by 1.5 stops. Between 10 and 13 EV the camera will smoothly lower the flash unit’s output by half a stop for each EV.

E-TTL flash works in a similar fashion, though apparently flash output will be lowered by as many as 2 stops when ambient lighting is bright. Canon have not, however, divulged their secret E-TTL fill reduction algorithm, so it’s total guesswork exactly how it works. Apparently, though, the algorithm compares the brightness level of each zone both before and after the preflash, in part to compensate for highly reflective areas.

Some mid to high end EOS cameras allow you to disable this auto fill reduction by means of a custom function. See the section on flash exposure compensation for details. Note that any flash compensation you may apply manually is in addition to this auto fill flash reduction, unless of course you’ve disabled it via a custom function.

Edited to say, slin100 beat me to it with a better answer... but to save you searching eos flash the quoted text above is all there is in there :)


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DAMphyne
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Jun 21, 2004 17:02 |  #4

Thanks for the help, I did mean set the shutter speed to 1/200 with the custom function. I guess I'll have to experiment with the compensation.
I turned off the pre-flash also, could this be part of the problem I'm having?


David
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slin100
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Jun 21, 2004 17:39 |  #5

C.Fn-3. This is strictly MHO, but I see little reason to use that. You might as well switch the camera to M mode.

Are you sure you're disabling pre-flash? You can't disable the pre-flash when using the built-in flash. On a 420EX or 550EX, disabling pre-flash means disabling ETTL, which means you're using the flash in manual mode.


Steven
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DaveG
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Jun 21, 2004 20:25 |  #6

slin100 wrote:
Find the explanation for Auto Reduction of Fill Flash here (external link).



Auto Reduction affects the flash output at ambient light levels above EV 10. The rationale is that when the scene is bright, the flash output will be automatically dialed down a bit so as to soften the shadows a bit and not eliminate them entirely. The reason why it reduces your ability to control shadows is because the amount of dial-down depends on the ambient light level. At EV 11, the dial-down is 1/2 stop, EV 12 is 1 stop, and EV 13 or above 1 1/2 stops. So if the light level is varying between EV 11 and EV 13, the flash output can be difficult to predict.

At ISO 100, what would the shutterspeed/aperture combination be to achieve EV 10. And how do you get to EV 11, open up a stop?


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DAMphyne
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Jun 21, 2004 20:34 |  #7

In answer to setting the shutter to 1/200 sec, I can then determine the aperture without getting the Ghost effect. I've found this usefull when shooting action pictures. Perhaps it is the same as using M mode, I never thought of it that way. When I have the camera set that way, I've never noticed a pre-flash, so I thought I had dis-abled it. What I did dis-able was the pre-flash for focus. I need to re-read the manual again. Sometimes when you think you get it, that's when you find out what you really don't know.
This flash system for the Canon EOS system is very difficult to understand sometimes.
I'd like to be sure I understand the answer you've given me.
If I use C.Fn-3 fixed shutter speed@1/200sec, in AV mode, the Auto Reduction of Fill Flash has no effect. Is that correct?


David
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slin100
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Jun 21, 2004 23:17 |  #8

DaveG wrote:
At ISO 100, what would the shutterspeed/aperture combination be to achieve EV 10. And how do you get to EV 11, open up a stop?

You can find an EV table here (external link). For example, 1/60 @ f/4 is EV 10. Close down one stop to get EV 11, so 1/60 @ f/5.6 or, equivalently, 1/125 @ f/4.

DAMphyne wrote:
When I have the camera set that way, I've never noticed a pre-flash, so I thought I had dis-abled it. What I did dis-able was the pre-flash for focus.

I'm not sure what you mean by disabling preflash for focus. Most people can't see the preflash. The only times when the preflash is easily seen is when doing Flash Exposure Lock (FEL) or when 2nd curtain sync is enabled and a slow shutter speed is used.

DAMphyne wrote:
If I use C.Fn-3 fixed shutter speed@1/200sec, in AV mode, the Auto Reduction of Fill Flash has no effect. Is that correct?

No. Auto Reduction can still operate.

One thing I'm not sure about is if Auto Reduction is determined by EV which dictate only shutterspeed and aperture, or if it's determined by the actual ambient light level which is dictated by shutterspeed, aperture and ISO. There's a big difference.


Steven
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DAMphyne
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Jun 22, 2004 04:50 |  #9

Steven,
Thanks for taking the time to help me work this out.

I'm not sure what you mean by disabling preflash for focus

C.Fn5/setting #1
I believe you are right about the Auto settings, it becomes a sort of guessing game. I'll try the flash exposure compensation and see how that works for me.
Someday I'll figure it out, in the meantime, I'm having a great time with this Digital Wonder.


David
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slin100
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Jul 02, 2004 23:30 |  #10

C.Fn5 controls the operation of the AF-assist beam, which is used to illuminate the subject in low-light so that the lens can focus. It has nothing to do with the preflash, which is used to determine the flash output.


Steven
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Auto Reduction of Fill Flash
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