E-TTL II Custom Function on XT

[GovtLawyer]

It has taken me awhile to realize that the Rebel XT usually exposes for ambient exposure and flash separately. If I am not mistaken, in P, Av or Tv mode the flash is always exposed as a fill flash. So, I've been trying to figure out how to set the camera with my 580EX and occassionally the 420 EX as a slave, so that the flash is used to expose the whole photo. For example, a dimly lit indoor photo, as opposed to an outdoors night photo. In the situation of a dimly lit indoor shot I would rather have the camera use the flash to light the whole scene, so I can shoot at a faster shutter speed.

I just took a look at the custom functions and I see where there is one which allows you to choose an evaluative mode or the E-TTL II mode. The evaluative mode appears to use the flash for exposure of the entire photo. This may be what I wish to use. However, I am confused as to what benefits of E-TTL II I am losing by choosing the other mode. I thought the E-TTL II mode was an advanced communication mode between flash and camera in which more information; such as, distance, is taken into account in deciding flash output. If I am disabling E-TTL II by choosing the evaluative function, what am I left with?

[scottbergerphoto]

It seems a basic understanding of flash exposure is in order. It helps to conceptualize flash pictures as the combination of two exposures, the flash exposure and the ambient light exposure. The camera is always metering to get the correct exposure for ambient light via aperture and shutter speed. and correct flash exposure via ETTLII. Evaluative Mode(default) flash exposure has nothing to do with the evaluative metering used to measure ambient exposure. For an explanation of most aspects of flash photography please read through the EOS Flash Sticky.

[GovtLawyer]

It seems a basic understanding of flash exposure is in order. It helps to conceptualize flash pictures as the combination of two exposures, the flash exposure and the ambient light exposure. The camera is always metering to get the correct exposure for ambient light via aperture and shutter speed. and correct flash exposure via ETTLII.[/

Yes, I understand that. However, the flash can be used to either light up the entire scene, or just to fill in on the subject, without regard to the background or ambient light. I'm just trying to figure out which settings give me the flash fill and which settings light up the whole picture, as in a poorly lit indoors photo.


Evaluative Mode(default) flash exposure has nothing to do with the evaluative metering used to measure ambient exposure. For an explanation of most aspects of flash photography please read through the EOS Flash Sticky.
[/QUOTE]

So, what does the evaluative mode, vs the E-TTL II mode do? Yes, I will look at the sticky, again. I appreciate your responses, and have found them valuable. Unfortunately, much has been written on this subject and much of what has been written is confusing. A case in point is the very good article on (Photonet, I believe, which covers extensively the EOS flash system. There is even a section in that article called EOS Flash Confusion)

[PacAce]

It has taken me awhile to realize that the Rebel XT usually exposes for ambient exposure and flash separately. If I am not mistaken, in P, Av or Tv mode the flash is always exposed as a fill flash. So, I've been trying to figure out how to set the camera with my 580EX and occassionally the 420 EX as a slave, so that the flash is used to expose the whole photo. For example, a dimly lit indoor photo, as opposed to an outdoors night photo. In the situation of a dimly lit indoor shot I would rather have the camera use the flash to light the whole scene, so I can shoot at a faster shutter speed.

I just took a look at the custom functions and I see where there is one which allows you to choose an evaluative mode or the E-TTL II mode. The evaluative mode appears to use the flash for exposure of the entire photo. This may be what I wish to use. However, I am confused as to what benefits of E-TTL II I am losing by choosing the other mode. I thought the E-TTL II mode was an advanced communication mode between flash and camera in which more information; such as, distance, is taken into account in deciding flash output. If I am disabling E-TTL II by choosing the evaluative function, what am I left with?
You seem to be a little confused about the the E-TTL II custom function (C.Fn-8 ) so let me see if I can set you straight about this. :)

There are two options for setting E-TTL II using C.Fn-8. The default is for ETTL II to work in evaluative mode. In this mode, the camera will analyze the results of the image from a pre-flash and try to determine if the flash shot should be taken as is or if some form of flash exposure compensation of its own (in addition to what you yourself have set using FEC) is in order (much like the way Evaluative metering work with ambient lighting). The camera will attempt to determine where the main subject of the shot is and expose the shot accordingly. In this mode, distance information, if available from the lens, is also incorporated into the algorithm used to derive the appropriate flash exposure setting.

The other option is the average flash exposure metering mode. In this mode, the camera averages the flash exposure over the entire frame. No attempt is made by the camera to determine where the main subject is and set exposure bias. This would be a good setting to use if you were shooting indoors where everything in the frame has a good chance of being evenly (or close to being evenly) lit by the flash.

[GovtLawyer]

You seem to be a little confused about the the E-TTL II custom function (C.Fn-8 ) so let me see if I can set you straight about this. :)

There are two options for setting E-TTL II using C.Fn-8. The default is for ETTL II to work in evaluative mode. In this mode, the camera will analyze the results of the image from a pre-flash and try to determine if the flash shot should be taken as is or if some form of flash exposure compensation of its own (in addition to what you yourself have set using FEC) is in order (much like the way Evaluative metering work with ambient lighting). The camera will attempt to determine where the main subject of the shot is and expose the shot accordingly. In this mode, distance information, if available from the lens, is also incorporated into the algorithm used to derive the appropriate flash exposure setting.

The other option is the average flash exposure metering mode. In this mode, the camera averages the flash exposure over the entire frame. No attempt is made by the camera to determine where the main subject is and set exposure bias. This would be a good setting to use if you were shooting indoors where everything in the frame has a good chance of being evenly (or close to being evenly) lit by the flash.

That's how it appeared to me. So, if you were taking an indoors shot in someone's dimly lit den, and wanted the entire scene to be evenly lit, you could use the cn8 setting and use average. The camera would expose the flash based on the average of the entire scene, and not give any undue weight to the person sitting in front of the pool table. If you used the evaluative mode, the camera would assume teh person in the front was the main focus and set a flash exposure accordingly.

So the difference between the two have nothing to do with whether or not the flash is being used to fill or mix with ambient light; that's a function of the mode you're shooting in. It is the difference of how the camera measures the exposure for the flash.

[scottbergerphoto]

The Term Fill flash implies that you want to use the flash to "fill in" some shadows thatthe ambient light doesn't. You can use fill flash in Evaluative or Averaging modes of ETTLII. Simply dial down FEC 1-2 stops. There is a section of this in the Sticky:
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The most consistent way to use fill flash is to take the camera out of the equation as far as determining how to adjust the relationship between ambient and flash. I suggest you:
1. Disable the Automatic reduction of fill flash in Custom functions.*
*If your camera doesn't have this Custom Function, put the camera in Manual Mode, and leave the flash in ETTL/II.
2. Set the camera to Manual mode and set your ambient exposure using the camera meter.
3. Set FEC on the camera body(not available on the body of the 420EX) to from minus 1 to minus 1 and 2/3.
4. Adjust the FEC up and down to see what you like.
The historical expert on this technique is the late Galen Rowell. You can learn alot about flash techniques from "The Nikon Flash Guide" by Thom Hogan.

[PacAce]

As Scott said, the use of the term "fill flash" implies that your main source of lighting is ambient light and you will only be using the flash to fill in the shadow areas not directly lit by the ambient lighting.

What I think you are trying to say is that you would like to use the flash as your main source of lighting because the ambient lighting is not strong enough to allow for handheld exposures. If that's the case, then any Creative mode other than Av mode will do that for you although the modes that'll give you the most flexibility is the M mode, again as mentioned above by Scott, as you have full control of the shutter speed and the aperture to be used.

Personally, I do not think that setting the C.Fn-8 to average mode instead of evaluative mode mode will make that much of a difference unless you are really having problems with ETTL-II not getting it right for a particular type of "odd-ball" scenario you are shooting.

[robertwgross]

4. Adjust the FEC up and down to see what you like.
The historical expert on this technique is the late Galen Rowell.

Did he write that in a particular book? I have a couple of his books here, and I guess I need to haul them out and re-read them.

By the way, I met him at a slide show back in 1978, before he was famous.

---Bob Gross---

[scottbergerphoto]

From Galen Rowell:
Smart Flash is Addictive

by Galen Rowell

Outdoor Photographer, October 1996
"It's been eight years since I got hooked on smart flash. I especially remember an afternoon party where every slide came out with a perfect balance of fill and natural light. My Nikon 8008 was set on program mode with an SB-24 flash set at the -1.7 compensation ratio that I use to get natural- looking fill in landscape photographs.

When I later tried such casual shooting with either flash-equipped point-and-shoot cameras or more sophisticated SLRs with pop-up flashes, I could get acceptable one-hour prints for family use, but not well-exposed slides for publication. What caught my fancy, however, was that an occasional frame would come out just right.

In the last year, I've experimented with point-and-shoots and light SLRs for my lightweight adventures. What started me in this direction was Nikon's introduction of the N70 with the first programmable built-in smart flash. I tried "top-down" tests with the "smart" N70 and then began "bottom-up" tests with an old favorite Olympus XA and a super-light Canon Rebel-X SLR.

If you want to set the camera on autopilot and not worry about computing for the flash, there's no substitute for smart flash. If you prefer to have finer control over the image, however, the odds of balanced fill-flash with an unsmart camera can be greatly increased.

Without a smart flash, the desired -1.7 compensation ratio can be attained in a number of ways. A dedicated flash unit that ties the amount of light to the film speed setting on the camera can be fooled into doing the right thing for the wrong reason. First, set shutter speed and aperture for a manual exposure of the naturally lit background. Second, set the exposure compensation for -1.7 stops (or 1.5 if no 1.7 setting). For cameras without this adjustment, set the film speed 1.7 stops higher than normal; for example, ISO 320 instead of 100. Now you'll have a correct background exposure combined with flash that won't blow out natural shadows with overly bright fill. Remember to set your film speed back to the original ISO or you might end up with whole rolls of seriously underexposed pictures.

Things get more complicated when built-in flash units are relatively weak or not dedicated. Beginning from square one, you need to determine at what distance and apertures your unit will give you perfect looking flash fill. Don't trust the listed guide number for your flash unit. Logically, it should be too low; you need 1.7 stops less light for fill than for full-power flash in totally dark situations. Practically, guide numbers tend to be exaggerated, unless, that is, you're shooting in a narrow white hallway in a Hobbit house with five foot ceilings, where lots of light is reflected. In the open outdoors, effective guide numbers are cut in about half.

The concept of a guide number is far more simple than its relationship to light falling off in inverse proportion to the square of the distance makes it sound. Because the f-stops on your lens are also based on squares relating to the size of the lens opening, a flash guide number for a particular film speed remains constant as simply aperture times distance. A guide number of 64 means that at f8 a subject can be properly lit up to eight feet; at f16, four feet.

When I tested a Nikon N70 for outdoor fill with ISO 100 film, I began by setting the lens at f8 using the pop-up flash at distances varying from 2 feet to 8 feet. Without flash exposure compensation, only one of these distances would give a correct exposure, but the N70 with its built-in smart flash gave perfect exposures up to five feet, where they began to go dark. I concluded that 4.5 feet was the maximum distance, and eight times 4.5, or 36, was my guide number. To confirm this, I later tested fill up to 18 feet at f2 and down to about 2 feet at f16. I get a majority of properly exposed fill-flash images with the N70 simply by checking to make sure that I'm always within the limits of my experimental guide number.

With the Canon Rebel-X, exposures aren't so simple. The extremely light camera is very appealing for recreational use, but it lacks a flash exposure compensation setting and has a fill guide number that tests out at 30. Images made closer than the distance computed with the guide number are consistently overexposed for natural-looking fill. I go the extra step and use the guide number to select my distance and f-stop for accurate fill.

If successful fill-flash in the outdoors was merely a matter of choosing guide numbers, lots of people would be doing it with simple cameras. There are other complications. The two photographs printed here were made with and without fill-flash using a Canon Rebel-X on a climb of El Capitan in Yosemite. Without flash, the image is too dark. With flash at five feet at f5.6, the fill is excellent except for a strange round shadow on the rock at the left. The 20-35mm zoom lens interferes with the built-in flash's coverage area at close range with wide-angle settings.

There are three easy corrections for this problem. One is never to shoot wider than 28mm. The camera's manual recommends this limit as the coverage of the flash, but in practice the fill looks okay on this 20mm shot, even with a bit of fall-off at top and bottom. The second solution to the lens-shadow problem is to turn the camera 180 degrees so that the shadowed flash is against the natural light of the sky. The alternative of using the 28mm setting and backing farther away from my subject was not an option on a tiny ledge a thousand feet up a sheer rock face.

A major problem with overexposed backgrounds happens when a camera with a low flash synch speed and a weak flash is used in an auto-exposure mode. An aperture of f8 that works perfectly for fill at four feet will give a hideous two-stop or more overexposed background in bright daylight if the camera automatically limits shutter speeds to 1/90 sec. when the flash is popped up. The Nikon N70 is better at 1/125 sec. More sophisticated top-of-the-line Nikons and Canon SLRs have a far more versatile 1/250th synch speed, but lack a built-in flash.

In summary, any camera with a flash can give you perfect daylight fill if you know how to choose the situation. Depending on whether your flash unit is basic, dedicated, or smart, you will need to follow some or all of these procedures:

1. Compute your own experimental guide number and stay within its limit of shutter speed times aperture.

2. Don't let your camera choose a slow flash synch speed that will overexpose the natural light. (Many cameras have warning signals for this.)

3. Test wide-angle or large-diameter lenses for light fall-off and shadowing on close subjects.

4. When shooting verticals, turn the camera so that the flash is on the side of the lens that maximizes light on the subject and minimizes shadowing of wide or large lenses.

When all is said and done, it takes a focused mind as well as camera to make consistently good fill-flash exposures, unless you're willing to carry around that pro SLR with a 1/250 sec. synch speed and separate flash that allows you to shoot away at f8 or f11 in bright daylight. I'm still waiting for someone to come out with a perfect camera for my outdoor adventures. It should weigh less than a pound, zoom to 20mm with full flash coverage and no lens shadow, synch to 1/500sec. and have programmable flash exposure compensation. Until then, I've learned how to make do.

"
You can find more of his writing at:
http://www.mountainlight.com/index.html