Basic help: How does Flash exposure compensation works

[eninja]

Its been very long, I have been playing with flash. But never get serious and until now, I just guess and set FEC to between 0..+1 .

But how does really FEC work with regards to camera exposure.

This is how I thought it work, flash output power release will depend such that it attain the exposure set on FEC.
For example: FEC is set to +1, in camera manual mode, i set camera exposure to 0. thus the amount of flash output will release power enough to make exposure to to +1. (in ETTL mode of course)

Inline with this concept, it follows that when i set FEC to -1,
since camera exposure is 0, the flash should not even fire at all..
but the real result from testing is not like this. flash still outputs and make the scene brighter than 0 exposure..
Thus making my initial concept wrong.

Comment please.. Any expert care to elaborate the concept.. I want to grasp just the concept.

Thanks.

**please dont give comment like suggest reading a book or website. If you know something, please share it.

 

[Loswr]

The idea is that E-TTL flash metering tries to 'correctly' expose the subject (determined by the pre-flash), while your camera's meter (in evaluative metering) is trying to correctly expose for the ambient/background.

 

[privatebydesign]

What the flash will try to do will depend on many things, so a simple answer just won't be correct.

The exposure mode is very important, as is the scenes EV and if there is a subject closer to the camera than the background or not.

Take this example, camera in M mode, flash in E-TTL, a person is standing 5 ft away and the background is dark and 20ft away. Set the EV value to the metered reading (this will be an overall reading to correctly expose the background), set flash to FEC+3. When you push the shutter button you get a pre flash that will send out a set amount of light, it will hit the subject and the camera will recognise that the subject reflects more light than the background, it will set the flash output to correctly expose the subject only, it will also honour your FEC +3 so will put three stops more light out of the flash to do this. The exposure will be subject overexposed by three stops and the background correctly exposed.

Do the same thing without a subject and the preflash will assume the background is the subject so it will over expose the entire scene by three stops (assuming it has enough power).

The brighter the scene the less power the flash will put out, it is a sliding decrease as the ambient goes up, this is proprietary so nobody knows the exact figures, but all it means is that as the scene gets brighter in E-TTL the flash will behave not as the subject illumination but as a subtler fill flash.

FEC just means +/- 3 stops over and under correct subject exposure, even at FEC -3 you will still get some flash output and the preflash and actual flash are so close together it is normally impossible to tell them apart anyway (unless you use second curtain sync).

 

[eninja]

Do you guys mean that,

What ever the FEC setting is, when flash is ON, it will always makes the subject "well expose" at least?

Background:

Even if FEC is set to -3, flash will still give light such that 'subject' (refers to background if nothing in between) will be "well expose" (means standard exposure = 0)

 

[Logan]

as i understand it, flash and exposure adjustment work in the same way. the camera calculates the "correct" exposure, and then adds (or subtracts) the set compensation. if the camera would normally use flash power X, FEC+1 gives you flash power X+1, results vary by subject.

practically it functions the same as dialing +1 stop power into the flash in manual mode. it should make the same amount of difference.

the ETTL procedure they are describing above is how the camera determines the "correct" power, or the X in my example. just like how in aperture priority, the camera calculates the "correct" shutter speed, and setting EV+1 will give you 1 stop longer shutter speed, regardless of if it is 1/1000 or 1/10. FEC adds or subtracts regardless of whether the calculated power is 1/1 or 1/32.

in your example, the camera would use
(the shutter/aperture settings that it calculates based on the metering)+0,
(flash exposure calculated off the preflash)+1.

 

[privatebydesign]

Do you guys mean that,

What ever the FEC setting is, when flash is ON, it will always makes the subject "well expose" at least?

Background:

Even if FEC is set to -3, flash will still give light such that 'subject' (refers to background if nothing in between) will be "well expose" (means standard exposure = 0)

No.

With flash on, the camera will try to make two exposure readings, one for the ambient and one for the subject. The ambient will be taken care of in Av and Tv automatically, you take care of it in M, the flash will try to determine the subject and with FEC 0 will assume the subject is 18% grey and send enough power to do that. FEC +3 will make the subject three stops over exposed, FEC -3 will make the subject three stops under exposed, but you have to determine how far off 18% grey your subject is, a bride with a white dress is going to need FEC +1 to +1.5 for instance.

If the camera can't separate a subject from the background it will try to expose the background as you have the FEC set, again FEC -3 will be three stops under exposed etc assuming the flash has enough power.

 

[eninja]

Do you guys mean that,

What ever the FEC setting is, when flash is ON, it will always makes the subject "well expose" at least?

Background:

Even if FEC is set to -3, flash will still give light such that 'subject' (refers to background if nothing in between) will be "well expose" (means standard exposure = 0)

No.

With flash on, the camera will try to make two exposure readings, one for the ambient and one for the subject. The ambient will be taken care of in Av and Tv automatically, you take care of it in M, the flash will try to determine the subject and with FEC 0 will assume the subject is 18% grey and send enough power to do that. FEC +3 will make the subject three stops over exposed, FEC -3 will make the subject three stops under exposed, but you have to determine how far off 18% grey your subject is, a bride with a white dress is going to need FEC +1 to +1.5 for instance.

If the camera can't separate a subject from the background it will try to expose the background as you have the FEC set, again FEC -3 will be three stops under exposed etc assuming the flash has enough power.

ok, lets constraints our topic to, camera on M mode, and flash on ETTL. and No subject, means we shoot a white wall for instance or a book shelves.

I agree to what you say BUT why for instance.
I set Shutter Speed, Aperture, ISO such that camera meters to -1.
Then I set FEC to -2.

Why is the resulting image is well expose. (as i remembered last night while playing with the flash).

I should make this test again tonight to confirm it.

 

[privatebydesign]

OK, in that scenario what is supposed to happen is the flash would play no part in the exposure, you should, in theory, end up with a one stop under exposed wall.

However in my experience given that scenario, that is not what you do end up with, but in fairness that isn't what E-TTL is set up to do either, the assumption is that you have the flash on to light something so it tries its hardest to do just that! In my experience you end up with a slightly under exposed wall, but not one stop under, and the flash is clearly visible.

There are many situations where E-TTL will struggle, and not having a subject in the frame is one of them. Try your scenario with a more realistic set up, put something in the frame closer to give the flash metering something to work with.

 

[eninja]

OK, in that scenario what is supposed to happen is the flash would play no part in the exposure, you should, in theory, end up with a one stop under exposed wall.

However in my experience given that scenario, that is not what you do end up with, but in fairness that isn't what E-TTL is set up to do either, the assumption is that you have the flash on to light something so it tries its hardest to do just that! In my experience you end up with a slightly under exposed wall, but not one stop under, and the flash is clearly visible.

There are many situations where E-TTL will struggle, and not having a subject in the frame is one of them. Try your scenario with a more realistic set up, put something in the frame closer to give the flash metering something to work with.

Thank you very much for you input.

I will play a bit more with my speedlite tonight.
After that being said, I will note what you said, and see if I can understand my flash better. and get the most of it.

The main reason behind my question is: I wanted to full used my 90ex. even for just a bit of fill.
On Six burst shot:
On ETTL, Flash fire alternately.
On M mode (flash), 1/32, flash fire on every shot.

So last night, I was trying to get the balance right, use ETTL
and at the same time, flash must fire on every shot.
But even i set camera exposure to +1 and FEC to -3.
Flash fires strongly resulting to not fire on the next shot..

 

[privatebydesign]

The problem with the 90EX (I have one) is that in E-TTL it has to send out a pre flash before the actual exposure flash, this cuts into the recycle time as in a six shot burst you are actually trying to fire the flash twelve times. In flash M mode it doesn't fire the pre flashes so it only has to fire six times in your six shot burst. Either way you are asking a lot from two AAA batteries!

 

[eninja]

i got 90ex.. so i say, why not might as well use it on my second body. So I encountered this problem/limitation.
Oh yeah ur right, it sends a preflash.. but still no matter what setting I use, on ETTL, I observe that flash is stronger still, where as I want it to just put out a bit.

 

[AcutancePhotography]

**please dont give comment like suggest reading a book or website. If you know something, please share it.

I cringe when I read stuff like this.

You would rather get opinions from anonymous posters on an Internet Forum, then read articles/books from identified established photography educators?

To each his own, but I just don't get it.

Good luck with it. I hope you find what you are looking for.

But there are some really good educational books/articles out there.

 

[eninja]

**please dont give comment like suggest reading a book or website. If you know something, please share it.

I cringe when I read stuff like this.

You would rather get opinions from anonymous posters on an Internet Forum, then read articles/books from identified established photography educators?

To each his own, but I just don't get it.

Good luck with it. I hope you find what you are looking for.

But there are some really good educational books/articles out there.

I understand your point, but you didn't get mine. I have read books, but tbh, most of the books says very long thing and so many example, introductions, and as a whole only trying to point to the same one concept. You need to read between lines and between words to grasp that one concept.

Also those guys who suggest to read books, might already read that book, why not just tell me what they know. It isn't hard is it? and it will take me stressful to parse and look the answer to my question on a book that says so many things - in the end never says a concept.

And my question is simple, I just need some input before go make testing myself. Afterall you need build your theory first before doing the testing.

Also, everyone with a speedlight must encounter this question in their time using the speedlight.

 

[eninja]

This is what I've got. and a good place to start.
Before flash is fired:
. camera meters the scene
. pre-flash is release
. with these two exposure information
. real flash is release in accordance to the set FEC

Question next.
. what does camera meters the scene really means - what important information is really take from here
. same goes with pre flash..

just brain storming. (i still got no time to do the testing) maybe next week.

 

[eninja]

Damn i think i got it.

Imagine we got a sensor, divided into square, maybe 25x25 squares if you will (just for illustration).

Metering refers to the light (in terms of brightness captured by the sensor) AFTER going thru shutter speed, aperture, and ISO.

After the scene is being metered, now you got an image that consist of 25x25 squares, with different brightness on each square, (pitch dark to overexpose white)

When pre-flash is fired, we get another image.

Subtracting this two image, the camera can now define which is the subject and which is the background (Ambient)

flash power is fired such that exposure of subject is as many stops (depend on FEC) as compare to Ambient Exposure.

This raises few more questions. Anyway, I nice place to start.

 

[Viggo]

What I would do is set camera to M and flash to M and see how the aperture and shutter affects the flash at a given setting.

The flash in M is just a light with a dimmer switch, closer to 1/1 is more power, closer to 1/64 or 1/128 is less light. Then set your camera to , just as an example, 1/200s (shortest time you can use and still get max light out, also known as sync speed). Then start to use longer shutter and see how the background gets brighter. Maybe your subject is too dark, bring the flash closer to 1/1. If your subject is to bright, dial it down.

The thing to understand is that you can light an object close to flash completely different than the ambient or background. Smaller aperture will render ambient darker, and then your flash needs to light more, but can still light a subject correctly even if the background is dark.

Quick example here:

One shot is no flash and just what the camera meters the scene to. The other I used a small aperture of f11 or something and had a flash light up my subject, that way I control both the background and my subject and it is VERY easy.

 

[pwp]

This tutorial explains it pretty well:
http://neilvn.com/tangents/flash-photography-techniques/flash-exposure-compensation/

Have a dig around in Neil van Niekerk's blog, there is a LOT of great flash related info, some paid, a lot free.

-pw

 

[Marsu42]

In general ettl is ingenious to quickly dial in an ambient/object ratio. Not that the "correct" exposure at fec 0 often looks over-flashed, at least to me - you might want to start at -1 to get less broken shots. Raising shadows with fill flash in postprocessing is doable, but trying a over-flashed shot is a pita.

But even i set camera exposure to +1 and FEC to -3. Flash fires strongly resulting to not fire on the next shot..

The downside of ettl is that it relies on your camera's metering, so it's a good idea do be able to do both - ettl and m flash if you shoot the same or similar scenes again.

Note that there are two situations where ettl hurts:
[list type=decimal]
[*]To get *maximum* flash output, set the flash to m and full power. This way you don't waste power on the pre-flash which will only result in the camera telling the flash "dial to 11" anyway. The same applies if you want to do as many consecutive flashes as possible on lower output
[*]Automatic flash power might not be able to get the *minimal* flash output. If your subject is too near, and zooming the flash out, fanning and whatnot still over-flashes try to use m flash at min. power setting.
[/list]

Last not least, flash is indeed one of the subjects where reading a book or good website is really a good idea - unlike a lot of other photography stuff you can simply figure out reading/posting some forum threads and trial & error.

 

[privatebydesign]

Automatic flash power might not be able to get the *minimal* flash output. If your subject is too near, and zooming the flash out, fanning and whatnot still over-flashes try to use m flash at min. power setting.

Just as an aside, ETTL can go much lower than min Manual flash output. Manual is limited to 1/64 in the case of the 90EX, or 1/128 flash power with other models, but ETTL can output less than that.

 

[Marsu42]

Just as an aside, ETTL can go much lower than min Manual flash output. Manual is limited to 1/64 in the case of the 90EX, or 1/128 flash power with other models, but ETTL can output less than that.

I know ... and that's why it took me so long to find out that at least with the 6d's ettl metering the flash in some situations fails to reach the lower limit and it can be a good idea to set 1/128 manually. ymmv with real cameras and good metering though.

The problem is that there's now way (that I know of) to find out what power level(s) ettl chose, there's no tag in the image file or log on the flashes... so quickly switching m <-> ettl is always tricky. Or are there any tricks available?