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Canon Exposure and Flash Compensation
Old 09-18-2007, 04:49 PM   #1 (permalink)
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Hi,

I am using a Canon 20D and 580EX flash.

I experimented this weekend with Exposure Compensation (EC) set at -1 É stop and Flash Compensation (FC) set at + 2/3rds É stop. I used the Tv setting. I like the effect--darker background and lighter model--though in some of the photos the model was a bit dark. With some minor post production work of simply increasing the overall exposure, the pictures look better.

My question is, does the FC setting take into consideration the EC setting?

For example, if EC is set to -2 É stops and FC is set to + 1 É stops, is the flash on the subject still somewhat underexposed by 1 É stop? Or should the subject be brightly lit with a slight overexposure of 1 É stop? In my quick experimenting, it seems that FC ignores the EC setting.

In my situation with EC set to -1 É and FC set to +2/3rds É, the background was slightly underexposed (good) but so was the model, though to a lesser degree. I will have to experiment more to determine a good range of settings. But I'd like to know how the EC and FC work together.

I also know that on-camera flash is not the best idea. I plan to address this issue next year. I am a fair weather outdoor photographer, and I note that tomorrow's forecast mentions flurries.
Cloudy. 40 percent chance of rain showers or flurries early in the morning.
My photography for 2007 is rapidly coming to a close.

Any guidance on how EC and FC are supposed to work together is much appreciated.

Regards,
Kevin
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Re: Canon Exposure and Flash Compensation
Old 09-18-2007, 05:26 PM   #2 (permalink)
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No, the camera doesn't correlate FC with EC or vice-versa. Exposure compensation and TTL flash compensation work independently of each other and are based off of the same basic exposure calculated by the camera's meter for the scene which it is evaluating. The camera exposure is calculated *immediately prior* to the shutter opening (or when you press exposure lock). Exposure compensation is applied to that exposure value. TTL works *during* exposure - the camera quenches the flash when the camera decides that the image sensor has received enough light. Flash compensation is applied against the original exposure calculation and varies the amount of light the camera expects to "see" from the flash.

The reason you're seeing variation from shot to shot is that you don't really know how the camera is evaluating the scene from shot to shot. Each shot is composed just a little bit differently and the camera makes different exposure decisions depending on how much (and where) light falls on each of the meter segments. The camera is also making TTL decisions differently from shot to shot for the same reasons, so in the situation you describe you are allowing the camera to make two exposure decisions for you.

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Re: Canon Exposure and Flash Compensation
Old 09-18-2007, 05:46 PM   #3 (permalink)
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Quote:
Originally Posted by ChipBulgin View Post
No, the camera doesn't correlate FC with EC or vice-versa. Exposure compensation and TTL flash compensation work independently of each other and are based off of the same basic exposure calculated by the camera's meter for the scene which it is evaluating. The camera exposure is calculated *immediately prior* to the shutter opening (or when you press exposure lock). Exposure compensation is applied to that exposure value. TTL works *during* exposure - the camera quenches the flash when the camera decides that the image sensor has received enough light. Flash compensation is applied against the original exposure calculation and varies the amount of light the camera expects to "see" from the flash.

The reason you're seeing variation from shot to shot is that you don't really know how the camera is evaluating the scene from shot to shot. Each shot is composed just a little bit differently and the camera makes different exposure decisions depending on how much (and where) light falls on each of the meter segments. The camera is also making TTL decisions differently from shot to shot for the same reasons, so in the situation you describe you are allowing the camera to make two exposure decisions for you.
Hi Chip,

Thank you for your provided detailed answer that addressed my question.

When experimenting, I can more confidently interpret what I am seeing.

Best regards,
Kevin
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Re: Canon Exposure and Flash Compensation
Old 09-18-2007, 09:16 PM   #4 (permalink)
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In addition to the remarks already made, I would note that when using these kinds of flashes, which basically try to guess what might work in the photo based on a set of exposures values that the photographer may not be in full control of, that one often gets a surprise when viewing the end result. That's why its often better, especially with digital, to just change to manual mode across the board. Then you can control the flash and the exposure and values yourself. Making good use of the histogram in this process will aid in this kind of manual shooting.
Cheers,
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Re: Canon Exposure and Flash Compensation
Old 09-18-2007, 09:51 PM   #5 (permalink)
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Quote:
Originally Posted by R_Fredrick_Smith View Post
In addition to the remarks already made, I would note that when using these kinds of flashes, which basically try to guess what might work in the photo based on a set of exposures values that the photographer may not be in full control of, that one often gets a surprise when viewing the end result. That's why its often better, especially with digital, to just change to manual mode across the board. Then you can control the flash and the exposure and values yourself. Making good use of the histogram in this process will aid in this kind of manual shooting.
Cheers,
Roger,

Agreed. I am experimenting to find a quick method where I can dim the ambient and accentuate the model slightly. Using either a target, camera meter, or light meter, I can get determine the ambient exposure. Then reduce the ambient and just use trial and error with the flash.

Rolando wrote an article where he suggested reducing the ambient by at least one É stop.

http://www.glamour1.com/forums/view....sunrise_beauty

Quote:
Another trick, especially if youíre using on-camera flash, with the camera set at fully-manual, take your flash power down to a fraction of itís power, or if you use TTL then set your flash to underexpose the image by at least one-stop. I recommend you make tests ahead of time and find the right starting combination, you certainly donít want to go out in the ocean and take a ďwhimĒ shot at it like I did unless you know what youíre doing and can work really fast.
Others have recommended that I just meter the ambient to determine my manual settings and then use FEL for the flash. That is, focus on the subject, use FEL to determine flash exposure, recompose and shoot.

Speaking of FEL, I have a couple of questions for you.

According to the Canon's 20D manual on page 95 -

For FEL:

1) Check that the flash icon is lit.

2) Focus on the subject. Press shutter halfway. Keep pressing the shutter button halfway until step 4.

3) Press the FEL button. The speedlite will fire a preflash and the required flash output is retained in memory for 16 seconds.

In the viewfinder, "FEL" is displayed and the FEL symbol will light.

Each time you press the FEL button, a preflash is fired and the required flash output is retained in memory.

4) Take the Picture

Compose the shot and press the shutter fully.

The flash is fired to take the picture.

My question is, do you have to keep the shutter pressed halfway while hitting the FEL button as the manual seems to indicate? And can you "adjust" the FEL by increasing or decreasing exposure?

Best regards,
Kevin
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