AEB, M-mode vs AV-mode: Does your camera do this?

[C2S]

Hey all,

After seeing some strange results in auto-bracketed exposures on my camera, I figured I would make a simple comparison between M-mode AEB and Av-mode AEB.

In the following image, the auto-bracketing done in the aperture priority mode is on the left, and the bracketing done in manual mode is on the right. Aperture, shutter speed and ISO values are written on the left of each exposure. As you can see, each normal exposure, underexposure and overexposure is shot with identical settings, and the only difference is the usage of the two different modes.

Now, take a look at the underexposed frame in M-mode. There must be something wrong with the shutter/exposure timing here - is this typical for the 400D?

[yankees3791]

in manual mode, you make the exposure...there is no EC in manual mode, thats the whole point of it!

[av8tr]

Could this be due to the bright white building and black tires and the metering system trying to make them both grey?

[evandavies]

I just tried on my 40D
After doing the A-bracket I set the same values in M for each shot and for me the exposure came out the same.

Which exposure mode were you using?
I use CenterWeightedAveraged

[C2S]

Notice that the camera does fine on the standard exposure and on the overexposure, and only the underexposed frames look different, even though the EXIF-info claims that identical f/stop, shutter speed and ISO-value were used on both frames. The underexposed frame (or what it's supposed to be) in the M-mode looks pretty close to the overexposed frame (except for a little dark band at the top), which reduces the effectivity of the bracketing done.

I used the evaluative metering, but in my opinion it doesn't matter here, since the other two frames are fine, and the EXIF-data shows identical settings for each pair, yet different results for one pair. And no, the shooting conditions didn't change either, there was a thick layer of clouds and the lighting was absolutely constant. ;)

Well, it simply looks like my camera does AEB badly on the M-mode, so I should stick to Av-mode when I want to auto-bracket exposures. It's fine as long as I don't run out of the EC-range. The downside in full-manual bracketing are the possible alignment issues when combining the exposures, since it's user who has to change the shutter speed between the shots.

[E-K]

Did you manually bracket in the sense that you set each of the three exposures independently? Or did you set the exposure corresponding to the middle exposure (EC 0 in AV) and use AEB?

On my Rebel XT (350D), AEB in manual produces results equivalent to Av mode assuming I set the correct initial exposure to match the EC 0 one from Av.

e-k

[AdamLewis]

in manual mode, you make the exposure...there is no EC in manual mode, thats the whole point of it!

I pretty much only shoot in Manual mode and when I used AEB it works just fine.

[angryhampster]

LOoks to me like the light changed slightly. Look at the shadows of M mode underexposed vs M mode normal exposure.

[C2S]

E-K: Both were shot using AEB, and both AEB's had the same initial exposure, and same range. That is, I had decided that I'll use f/13 for this test. When the needle was centered in AV-mode, the camera chose 1/15 sec. I then switched to M-mode to apply these same settings, after which I AEB'd with both M and AV modes. Once again, the shooting settings are on the left of each exposure pair.

I repeated the test, but this time I also captured three additional, non-AEB'd shots with the similar settings to the AEB'd captures, in M-mode. AEB done in M-mode was still flawed, AEB done in Av-mode worked fine, and the single shots captured in M-mode matched with those AEB'd in Av-mode. (hopefully I didn't confuse anyone with the use of these abbreviations)

Steve: Nope, in the M-mode, the underexposure is quite similar to the overexposure (the two bottom images of the M-mode), except for a wide dark band at the top in the underexposure. I've looked back to my past auto-bracketed exposures I know I've done in M-mode, and in all of them the "underexposure" is similar to the overexposure, with a dark band at the top.

Looks like nobody else is experiencing this, so I guess my camera is just a bit flawed. Oh well, at least I've figured this out, so I can improve my results slightly from now on.

[C2S]

Ok, just one more quick update since I know you all are so genuinely interested in this. (yeah, right) ;)

It's a shutter timing issue. When the exposure times get slower, AEB works fine even in M-mode - I'm talking about exposure times of half a second and slower. Of course this doesn't help typical daytime landscape photography, for example, since there's usually plenty of light and the shutter speeds tend to be faster than that. (unless using any darkening filters)

Here's a new test compilation of an interior scene. All shots are captured using AEB, f/8, in M-mode. ISO-values used are on the left of each row. Shutter speeds used are overlaid on each image.

Again, standard exposures and overexposures are fine, but the underexposures get more flawed as faster shutter speeds are used. Compare the different underexposures, and see how they differ from each other even though the standard exposures and overexposures shot with different settings are fine.

And don't worry: looks like it's my camera only, AEB on your camera is probably working just fine in whatever mode you use it. I'll just use Av-mode when I need AEB.

[René Damkot]

What I don't get is how an image at 1/30s can be lighter then an image at 1/8s.
Any chance the aperture isn't always closing?

[C2S]

The shutter speeds written on the images are based on the EXIF information, and the EXIF information for the underexposures is incorrect when the shutter speeds get faster. In fact, I continued my testing and concluded that for the AEB'd underexposures in M-mode, my camera won't use a faster shutter speed than 1/4, approximately.

Proof: I AEB-shot a fluorescent light in my kitchen: the standard exposure was 1/1000 sec, the overexposure was 1/250 and the underexposure was supposed to be 1/4000. But the underexposure was the most blown out, and you could listen how the time between the mirror slams noticeably longer than with the other two exposures. I then shot an additional single shot using 1/4 sec, and this shot was identical to the AEB'd "underexposure", except for the small dark band at the top.

And in case you're interested, I've pinpointed that all this only happens when all of the following is true:

1. AEB is used, and
2. M-mode is used, and
3. Self-timer is used, and
4. Mirror lockup is used, and
5. Remote control is used

If I press the shutter button with my finger instead of using the remote control, all works fine. Strange, isn't it.

[René Damkot]

Very strange.

[Jon]

If you press the shutter with your finger, are you also looking through the viewfinder when you press it? Same question when you're using the remote control.

[C2S]

No to both questions. The camera was mounted on a tripod.

[runninmann]

If you press the shutter with your finger, are you also looking through the viewfinder when you press it? Same question when you're using the remote control.Ah ha! Good possibility!

[Jon]

No to both questions. The camera was mounted on a tripod.
It may or may not be a factor, but that's bad practice since light entering the pentaprism area through the eyepiece during metering can throw off the exposure. That's why Canon provides the eyepiece cover.

And does this occur with more than one lens? What drive mode are you using?

[C2S]

- It's not a factor in this case. I completely blocked the viewfinder with my thumb, and nothing changes.
- This happens with all of my three lenses.
- Since I'm shooting with a self-timer (and a remote control), I'm using the "Shooting with self-timer" drive mode.

[apersson850]

And in case you're interested, I've pinpointed that all this only happens when all of the following is true:

1. AEB is used, and
2. M-mode is used, and
3. Self-timer is used, and
4. Mirror lockup is used, and
5. Remote control is used

If I press the shutter button with my finger instead of using the remote control, all works fine. Strange, isn't it.Qute. That explains why I didn't see anything strange when I did the test. Now I'll do the same setup as your's and see if I can re-create the problem.

The viewfinder blocking thing reasonably shouldn't affect AEB when using manual mode. Mine simply varies the shutter speed (only) from the original setting.

[C2S]

Well, if you do that, let us also know which camera model you are using, as you have no gear information in your signature.

[apersson850]

Now I tried my 40D (firmware 1.0.8 ) with an EF-S 17-55 mm f/2.8 IS USM, and set it up like this:

AEB +/- 2 steps.
Manual mode.
Base exposure set to 1/125 @ f/2.8, ISO 800.
Self-timer first at 2 seconds, then at 10 seconds (doesn't make any difference).
Mirror lockup.
TC-80N3 remote controller.

The three exposures are taken at 1/125, 1/500 and 1/30.
Looking at the pictures as well as the histograms, they bare witness of a 0, -2 and +2 exposure.

So I can't reproduce your fault.
Oops! Now I posted this and went back to your original post to read more carefully. Wait a moment and I'll try it with the same kind of camera as you have.

All right, so once again, now with an EOS 400D (firmware 1.1.1) with an EF-S 17-85 mm f/4-5.6 IS USM, set up as close to the 40D setting above as possible:

AEB +/- 2 steps.
Manual mode.
Base exposure set to 1/160 @ f/4.0, ISO 800 (aiming at a brighter spot than above).
Self-timer at 10 seconds (but it becomes two seconds when mirror lockup is used).
Mirror lockup.
60-T3 remote controller (converted to match the 400D remote control socket).

No matter what I do now, the result is identical to what I got from the 40D. Normal, under- and overexposure. Shutter speeds 1/160, 1/640 and 1/40 s.

Let me know if I've misunderstood something, or you want me to test something in another way. I don't have any wireless remote, unfortunately.
I did try it once again, now with the EF-S 18-55 mm f/3.5-5.6 II, as that's the only one we have in common. No difference. The pictures came out as expected.

Maybe we ought to meet and compare our cameras, as we are at least not half the way around the globe from each other!

[C2S]

Good for you, apparently your 40D is working correctly. By the way, the remote controller I'm using is the wireless Canon RC-1. The firmware I'm using on my 400D currently is 1.1.1, but it doesn't fix this problem.

[pixelharmony]

What I don't get is how an image at 1/30s can be lighter then an image at 1/8s.
Any chance the aperture isn't always closing?

His ISO was 400 vs 1600.

[C2S]

Actually, there are some images with identical shutter speeds, and René is talking about the lowest image row, which is shot at ISO 1600.