Exposure locks when pressing shutter release halfway in evaluative metering mode

Hello,

I have a problem with my Canon EOS 400D/Rebel XTi camera. I use Av mode most of the time and realised that in evaluative metering mode the camera locks the exposure value (exposure time) when I press the shutter release button halfway to fix the focus. This is problem, because 99% I recompose the picture after locking the focus and the lighting changes with turning the camera. I suppose the * button should only fix the exposure. With other metering modes (center weighted and partial) the exposure is not locked and the exposure time changes by recomposing the image.

I made a little survey amongst my friends having Canon cameras, and it turned out that some cameras lock the exposure time in evaluative mode while others not.

In Custome Functions the setting for the shutter release button is set to 0 (default) all the time in all the examined cameras.

So it seems that this behaviour depends on the camera type/firmware.
Is this a bug or what?

I believe that's how it's supposed to be, my 450D works like your camera. I think it has to do with that the evaluative metering accounts for what you focus on and tries to get that exposed properly, but as you say you can use the -button to get around this, compose, lock exposure, recompose and lock focus, recompose back again and shoot.

When using Evaluative metering, exposure needs to be locked whenever autofocus is locked. That's because Evaluative metering considers the image at the locked AF point(s) to be the subject, and recomposing between AF and metering would give the wrong impression of what is the subject.

If you are using a non-locking AF mode (AI Servo, for example) the exposure will not lock. If you're using AI Focus AF, exposure will lock initially but will unlock if the camera switches to servo AF.

This behavior is alluded to on page 168 of the XTi/400D manual:

"AE Lock: Auto: Applied in One-Shot AF mode with evaluative metering when focus is achieved…"

The 400D XT/XTI etc... they use like these gentlemen said a focus point for their metering.

the XSi no longer does this.

However, for my personal shooting, i use evaluative or spot metering exclusively.

I use custom function 10 (1) on the XSi which moves Focus to the rear * key. This helps me in that I can focus whenever, and lock metering whenever.

So in the end, I would just lock exposure, then recompose and refocus. I think you will find this solves your issue hopefully.

let us know.

Gary Atkinson said:

The 400D XT/XTI etc... they use like these gentlemen said a
focus point for their metering.

the XSi no longer does this.

However, for my personal shooting, i use evaluative or spot
metering exclusively.

I use custom function 10 (1) on the XSi which moves Focus to the
rear * key. This helps me in that I can focus whenever, and lock
metering whenever.

So in the end, I would just lock exposure, then recompose and
refocus. I think you will find this solves your issue hopefully.

let us know.

Click to expand...

Well my 450D works exactly as the original poster described.

Hello,

Thanks for the answers so far.

I checked this thing again.

In Av mode the settings are:
1. Custom Function is set to 0;
2. Evaluative Metering;
3. One shot auto focus;

And with these settings, all the cameras (what we have at hand: 400D, 20D, 40D) lock the exposure by pressing the shutter release button halfway.

This behaviour seems a bit pointless for me.

A basic situation: First I focus on somebody's face, then I recompose the image according to the rule of thirds. At this point the camera should meter the light again (continuously, as I turn the camera), to get the correct exposure for the final composition. It should only lock the exposition if I pressed the * button beforehand.

I think, center weighted metering and partial metering is not the best to get the image as a whole well-exposed (as their names say: mainly the center/only part of the image is taken into consideration with these other 2 modes).

A friend of mine owning a Nikon D300 checked this stuff, and it turned out that in the Nikon you can set what you want: the exposure locked or not with these very same circumstances (one shot focus, matrix metering).

The only solution:

1. Point the camera to the final composition, lock the exposure by pressing * button;

2. Turn back the camera to the main subject, focus by pressing the shutter release button halfway;
3. Finally, recompose the image again and then shoot.

It would be easier if the camera did not lock the exposure at focusing, or I have to use the other two metering modes (but that's also not the best solution).

what you should do is...

use the custom function (1) to send the focus to the back * key.
this will allow you to focus and lock AE independently.

You can also choose the option (4) that shoudl do no AE Lock. this means at no time is AE Locked until shutter pressed.

Gary Atkinson said:

what you should do is...

use the custom function (1) to send the focus to the back * key.
this will allow you to focus and lock AE independently.

You can also choose the option (4) that shoudl do no AE Lock. this
means at no time is AE Locked until shutter pressed.

Click to expand...

--

OK, I will try these other possibilities.

Anyway, I'm very used to my shooting mode and still cannot get the point of this behaviour (exposure locking in evaluative mode).
For me, it would make more sense not to lock it. Maybe my fault

Well, going back to your previous example of focusing on somebody's face and recomposing, I would argue that I want to have that face in good focus and also have proper exposure as well, which presumably would be my main subject, so I would kindly disagree that locking the focus together with exposure is such a bad idea after all. As a matter of fact in such scenario I would actually use spot metering to make sure I get proper exposure of my main subjects face and wouldn't really care that much about the background exposure, unless I'm missing something? However, I'm little surprised how strong influence the focusing point has on overall exposure, after all I would expect strong correlation from center weighted and spot metering modes, but evaluative mode would suggest evaluation of the whole scene and average the exposure for whole picture (at least I thought that what it supposed to mean) and therefore focus point shouldn't affect the exposure by that much, unless you're introducing large area of the sky or similar, during recompose?

Boldi said:

Gary Atkinson said:

what you should do is...

use the custom function (1) to send the focus to the back * key.
this will allow you to focus and lock AE independently.

You can also choose the option (4) that shoudl do no AE Lock. this
means at no time is AE Locked until shutter pressed.

Click to expand...

--

OK, I will try these other possibilities.
Anyway, I'm very used to my shooting mode and still cannot get the
point of this behaviour (exposure locking in evaluative mode).
For me, it would make more sense not to lock it. Maybe my fault

Click to expand...

Agree with Pete4's explanation. Evaluative tries to determine the subject and its proper exposure via the active focus point and the other metering segments. Recomposing does not change what will be the proper exposure of that original subject.

Pete4 said:

... However, I'm little surprised how strong influence
the focusing point has on overall exposure, after all I would expect
strong correlation from center weighted and spot metering modes, but
evaluative mode would suggest evaluation of the whole scene and
average the exposure for whole picture (at least I thought that what
it supposed to mean) and therefore focus point shouldn't affect the
exposure by that much, unless you're introducing large area of the
sky or similar, during recompose?

Click to expand...

Evaluative is not a simple averaging routine like CW. In measuring and comparing the 35 segments and the focus point it tries to make a determination of whether the subject is back or front lit, dark or light toned, etc. and applies its own exposure judgement rather than just averaging to 18% gray. When you re-compose and re-meter, the active focus point is on a new subject and a new determination is made as to what it might be and how to expose it.

Boldi said:

still cannot get the point of this behaviour (exposure locking in evaluative mode).
For me, it would make more sense not to lock it. Maybe my fault

Click to expand...

Let's take an extreme example so that it's easier to follow.

You're taking a photograph of a person standing in front of a large picture window. The window is providing a rather strong backlighting.

You focus on the person and lock the focus, then recompose so that the AF point is over part of the window. You want the metering to be based on the person rather than on the window, right?

That's what the camera is doing. When you lock the focus, the camera locks the metering so that you're metering what's at the focus point at the time that you focused. If it didn't lock the metering when you locked the focus, the camera would be setting the exposure based on what's at the focus point after you've recomposed.

Pete4 said:

However, I'm little surprised how strong influence
the focusing point has on overall exposure

Click to expand...

Evaluative metering is based on dividing the image into subject and background, a division that is made based on the presumption that the AF point(s) that locked are over the subject. The AF point(s) are fundamental to the evaluative metering process.

I thought evaluative metering worked a bit differently. Something like averaging the light measurement in the 35 segment of the picture.

For connecting the exposure value to the active auto focus point, I would use partial metering or even more spot metering (my 400D does not have spot metering unfortunately). As I experienced, the partial metering is not sufficient at all. Too large area what it takes into consideration. Many times there is minimal difference in the exposure value after metering (comparing partial and evaluative) and it's not enough to get the right exposure for that subject.

I understand the person in front of a strongly illuminated window example, but for that I would also use spot metering, to get the right exposure for the person's face (and also exposure compensation if needed, fill flash etc).
My previous example (a portrait) was not the best.

What I really wanted to emphasize, that many times (almost all the time) first I point the camera's center focus point at a subject just to get the right focus, and then I recompose the picture to get the right composition. And most of the time I do not particularly care about the perfect exposition of the subject used to catch the right focus, but I do care about the final composition.

Simply, many times the auto focus cannot work because there is not enough contrast in the center of the final picture, so you have to find something with good contrast somewhere in the picture, focus, and then recompose.