Tuesday, December 8, 2009

Learning the 50D - Turning Off the ALO

I've just made the jump from the Canon 30D to a Canon 50D. Faster burst rates, twice the megapixels, bigger, brighter LCD, I couldn't wait to get out and do some shooting.

My first sets of test shots showed, me two things:

First, some of my lenses need to go in to the manufacturer for service. That's another post.

Second, and the subject of this post, was noisy shadows. Noisy, nasty shadows, even at ISO 100. The speckles of red, green and brown, were especially noticeable in areas that were supposed to go pure black.

"What the ___?" was my first thought. "Oh, crap, do I have a bad sensor?" was my second. I worried that I was going to have to send my brand-new camera in for warranty service inside the first week.

But before I completely panicked, I did some Googling, and found other folks discussing the same problem. The root cause was not a defective sensor, but rather Canon's Auto Lighting Optimizer (ALO).

This "feature" is turned on by default, even when you're shooting raw. It tries to bring up the shadow detail in photos, which introduces sensor noise there.

So, I dug through the custom functions menu and turned ALO off, ran back outside, set the flash back up, and reshot. Low and behold - the sensor noise the shadows was gone!

I'm sure there situation where ALO is a help. But if you're planning on shooting anything dramatic, with black backgrounds or deep shadows, you'll be a lot happier turning it off. Pictures like, say, this one:

Rainy Day Rose - v2

11 comments:

  1. Paul,

    I realize this is a rather old post, but it is still very relevant today (mid-2013) as the ALO feature has been included in every Canon DSLR since its introduction in the XSi in 2008, and the feature is turned on by default.

    The one part of this post which I believe is incorrect is that ALO is active in RAW mode. It is not active (per se) in any RAW or RAW+JPEG mode, or when in any manual exposure mode. See page 177 of the 50D manual (or the ALO page of any Canon manual). When shooting RAW the ALO feature embeds the appropriate metadata for Canon Digital Photo Professional (exclusively) to apply, but at the user's discretion. This means that the post processing option to use ALO data is available, but as an option.

    This post-processing of RAW images is unavailable for cameras which preceded the 50D, where ALO data was not recorded when in any RAW mode. Canon changed several aspects of ALO during 2009, when they also added multiple strength settings to ALO, where previously just "on and off" was offered.

    Thank you for letting me clarify this for other readers who will find this page while Googling for "Canon ALO", just as I had been.

    Mike Lee
    former Canon representative - Michigan

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