E-TTL flash and metering mode

[kaustubh]

I know this has been discussed before. However, I could not find a

single thread with answers to all questions below.

 

I have a canon eos elan 2e and a 420ex flash. I would like to know

 

(a) if the metering modes (partial, evaluative, average) are respected

when I use the flash ?

(b) If so, are they respected in all modes (P, Av, Tv) ?

© If not, then, is evaluative mode used irrespective of the setting ?

(d) does elan 7e do things differently ? I am planning to buy one

(e) does flash 550ex give a different behaviour ?

 

I have read on some threads that eos-3 always meters in evaluative

mode when flash is used. am not sure what elan 2e does.

 

I have read bob atkins info and some other websites...but no one says

this varies from one camera body model to another

 

Thanks in advance

[Ken Katz]

For the Elan II, the ambient meter pattern is fully selectable when using any flash (at least in P, Av, Tv, M modes).

 

I don't owne a 7e so I can't comment on that.

 

Your comment on the EOS 3 is correct.

[whayne_padden]

For the EOS 3 I know the flash uses evaluative metering, but I can not find evidence one way or another for the ambient reading. If I'm in spot meter mode before I turn on the flash, surely I am still in this mode after I turn on the flash; why would it change? If not then it seems one should meter the ambient scene in manual mode before the flash is turned on and then turn the flash on and meter the subject.

 

Of course if you use evaluaitve anyway, it's no big deal, but I mainly use spot and I want to be sure that's what I'm getting when I turn the flash on and I havene't fully depressed the shutter.

[bill_goldman]

The following is quoted word-for-word from Canon's publication "Flash Work":

 

E-TTL Autoflash System Operation Flow

 

1. Shutter button pressed halfway. Autofocusing and evaluative metering (with the multi-zone sensor linked to the focusing point) are executed simultaneously. The existing light is thereby metered.

 

2. Shutter button pressed completely. A preflash is fired and the reflected light is metered by the multi-zone evaluative metering sensor.

 

3. The meter readings of the existing light and the preflash are compared and the main flash's output is calculated and stored in memory.

 

4. The reflex mirror goes up. The first shutter curtain starts to open, the main flash fires, the film is exposed, the second shutter curtain closes, and the reflex mirror goes back down.

 

5. The flash exposure confirmation lamp lights.

 

That's the story according to Canon. It should be noted that in step 4, to be technically correct, it should say "The first shutter curtain opens completely" instead of "The first shutter curtain starts to open".

[bill_goldman]

The E-TTL flash sequence from the Flash Work document makes sense to me since the stated objective of the E-TTL system is to balance the existing light with the flash illumination of the main subject, at least in P mode, where the camera has the most control.

 

If the system is going to compare the existing light and flash illumination, it makes sense to use the same metering system for both.

 

In Av and Tv modes, the camera meters exposure for the ambient light regardless of whether flash is used or not (with some caveats). In this case the flash serves only as fill for the main subject (using evaluative metering). Subsequently, one wonders about how the ambient exposure is actually being measured, although it doesn't affect the flash measurement.

[Ken Katz]

The EOS 3 (and 1v I assume), switches the ambient exposure pattern to Evaluative when a flash is turned on, in all modes, and irregardless of what meter pattern was set by the user. If I'm shooting a backlit subject, I want to spot meter exposure on that subject, not guess how much the evaluative meter weights exposure to the subject, or depend on fill flash to provide sufficent exposure.

 

I prefer the control the Elan II's set up provides in this regard.

[kitipong_morkchareonpong]

Kaustubh,

 

I think other people answered most your questions. Since you already own 420EX and are planing to buy Elan 7, I have to share.

 

As fas as my experience goes, 420EX + Elan 7 combo have slightly different flash exposure algolrithm from 550EX + EOS 1V. I guess Canon tailors them to suit casual/amature shooters and to compensate for lacking of LCD and some control buttons in 550EX. I alway use FEC +0.3 and -1 for shooting indoor and outdoor fill-flash, respectively, on 550EX + EOS 1V in Av mode and always get nice/natural/pleasing results. Please be noted that I disable "automatic reduction of fill flash" is the combo.

 

However, this fomula doesn't work on 420EX + Elan 7 combo. To get the similar results, I'll have to use 0 (yes, zero) FEC for both indoor and outdoor. That makes sense, doesn't it? (so users don't have to fiddle with FEC on camera, which can be somewhat inconvenient.

 

I can't remember if need (+0.5 and -1) FEC with 550EX + Elan 7 combo.

 

My Elan 7 works pretty much the same as EOS 1V, except "automatic reduction of fill flash output" can't be disabled.

 

Good Luck,

Kitipong.

[whayne_padden]

Ok I'm confused now. If I have a subject say a person sitting against some landscape and I turn my flash on and use Av mode to set desired aperture and I point it at the subject, it's using evaluative metering. But the exposure is weighted toward the AF point over the subject. If my bacground scene is complex and has light and dark regions how is the metering working for this, what will the exposure be based on?

 

It seems to me in this case it may be better to meter the background independently with the flash off and in Manual mode, set the exposure how you like (say -1 to -0.5 stops under), then turn on flash, and point at subject and set FEC appropriately and then shoot.

 

I haven't had any problems where the subject is filling a large part of the frame, but haven't really tried this on scenario above.

[bill_goldman]

Whayne; I think you're complicating things unnecessarily. Don't forget that Av (and Tv mode for that matter) always meter for the ambient light and the flash only provides fill for the main subject.

 

Evaluative metering handles complex scenes with light and dark objects very well so you will most likely get a proper exposure for the background with or without the flash. Av mode with flash often produces the most subtle flash effects and results in very natural looking pictures as long as the ambient light level is high enough to allow high enough shutter speeds to avoid camera shake.

 

Your proposed approach of metering the background and setting the camera to manual mode before turning on the flash may certainly work for you. My suggestion is to try both approaches and see if one works better for you. I doubt you will see much difference.

[whayne_padden]

You are probably right Bill, I'll certainly compare the two approaches. I guess it stems from the fact I rarely use evaluative mode so I'm not sure how it wil behave in this case. I've grown up on spot metering.

 

Thanks for the patience.

[eosdoc]

<a href=mailto:uce@ftc.gov></a>

See also the methdo used to check metering in this thread:<br>

<a href=

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=004brM

>http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=004brM</a>

[kaustubh]

perhaps this is too basic a question.....what does the camera set after the preflash is done ?

 

I don't think it alters either the shutter speed or the aperture. (or does it ?) My guess then is that it sets the duration of the flash. am not sure if 420ex varies the output in any other fashion.

 

let us assume I am using partial metering and am metering off a middle-toned object in Av mode. Distance of camera/flash from object is 3 feet. I think it will set the shutter-speed so as to expose the middle-toned object correctly (i.e. middletone). From the answers so far, it is clear than elan2e sticks to partial metering when the flash is on.

 

After preflash, if the camera is not altering the shutter-speed and aperture, how does it prevent overexposure of the object ? One wud think that even the slightest duration flash wold be sufficient to overexpose it.

 

Note that I am not unhappy with the results I get....however, I cannot seem to see what the camera is thinking in each different situation that I am testing.

 

Thanks a lot. This is very educational for me.

 

Kaustubh

[bill_goldman]

Here are the sequence details for an E-TTL flash exposure:

 

When the shutter release is partially pressed, the camera performs AF and AE simultaneously and in the case of Av mode, sets a proper shutter speed for the ambient light depending on the aperture selected. These values are displayed in the viewfinder and on the LCD panel and memorized by the camera.

 

When the shutter release is fully pressed, a low power preflash is fired. The camera uses evaluative metering based on the focusing point(s) achieving focus to measure the reflected light from the subject but since this is the combined ambient and flash light, it subtracts the previously memorized ambient light in order to compute the required flash intensity for a correct exposure at the set aperture value.

 

The camera then makes another ambient light measurement to verify that the ambient exposure has not changed or the camera has not moved (e.g., to reframe the picture). If the ambient exposure has changed, the camera cancels the focus point linkage and give primary emphasis to the brightest area in the scene, on the assumption that that is the main subject.

 

Finally, the mirror rises, the first shutter curtain opens, the exposure is made (including the main flash firing), the second shutter curtain closes and the mirror returns to the down position.

 

This all takes place very quickly and so the preflash and main flash appear to take place almost simultaneously.

 

The above sequence of events for an E-TTL flash is based on the description in Canon's "Flash Work" brochure as modified by information gleaned from an article published by the Canon European CPS site.

[kaustubh]

Bill,

 

thx for the answer. so you are saying the preflash and metering thereafter has no effect on the shutter-speed and aperture used for the actual shot. assuming camera did not move or the ambient light did not change.

 

suppose I am shooting a middle-toned subject on partial metering from 2 feet away. The camera meters the scene (or rather the 9.5% area around the focussing point) and sets f/8 and shutter speed 1/90. This means the subject will appear middle-toned even if flash were not to be used. (or doesn't it ?)

 

In this situation, won't even a small duration flash overexpose it ? note that i am shooting with a 420ex from 2 feet away.

 

Ideally, I would like the camera to increase the shutter speed after metering after preflash to prevent overexposure. I am still having a difficult time to believe that without altering the shutter-speed and aperture, the camera exposes the object correctly. I have tested this and the camera does seem to get the result right.

 

Kaustubh

[bill_goldman]

It's something of a mystery to me also as to how it gets it right and doesn't overexpose due to the flash. However, with the EOS-1v and EOS-3 (and some other cameras), the ambient exposure is reduced slightly depending on the light intensity and the ISO value. I assume it is done to prevent overexposure and to make the subject stand out from the background. The fellows who studied this call it the NEVEC effect. You can do a search and read about it.

[exrty_exrty]

Hello,

 

My opinion is : even you take a picture and the camera set aperture and speed according like it was no flash, you will not get a real surexposure because it is just fill-in flash and it is weak flash that just light shadows. I've read that fill-in flash power should be approximately -2IL above 13EV. Well i think it can be a reason why the pictures are not overexposed.

 

regards

 

exrty