Understanding automatic settings

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Fulhair

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Name
Andy
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I'm asking this purely out of curiosity to understand what my camera is 'thinking'.

I've a Canon 550d and replaced the kit lens with a Sigma f2.8 17-50 lens. Not having the camera that long and my inexperience I sometimes fell back to using the full automatic mode (i.e. green square).

Thing is when I put it in this mode the camera always seems to open the aperture to full - f2.8. When I put my 430ex flash on it limits it to f4. This often results in blurred photos given the shallow DOF and I can't alter any of the settings.

I'm now using the manual setting more and more but I've been wondering, what is the camera trying to do? :thinking:
 
If the camera is using 2.8 I am guessing the light is fairly low, what shutter speed and ISO is the camera using when it uses 2.8?
I am guessing it is not using 2.8 (with say ISO 100 and shutter of 1/4000) if you are outside on a bright day?

Cameras use some pretty basic calculations but they don't all use exactly the same. You also have the ability to influence the calculation on some cameras, for example don't use lower than 1/200.

As an example, I tend to use Aperture priority quite a lot with Auto ISO up to 1600. The camera will do everything it can to stay above 1/60. Only when it runs out of ISO will it start to go under 1/60.
 
The flash can only be fired at shutter speeds of 1/200s and longer, It will probably be closing the aperture down to f4 to let in less light and slow down the shutter speed.
 
Who knows what the little Japanese person inside your camera thinks.:thinking:

Instead of using auto try using Aperture priority instead, That is something your little in camera Japanese man can work out better.:thumbs:

Realspeed
 
The thing is, there's a really complicated answer to this question, but considering that you're using green box mode because you don't want to learn the fairly basic rules you need to use the more creative modes, I'm not sure it's worth going into. There should be a complex matrix available. Early program mode cameras had them in the manual.

But simply as above, you'll be in fairly low light if your camera is choosing 2.8 by default.

The priority for the camera (when you don't know what you're doing) is to give a high enough shutter speed for a sharp picture, the next priority is to keep the ISO as low as possible for image quality.

In short though, you'll never take a photograph in green box mode, because the picture is taken by the camera. Take control and make yourself happy.

In P mode, you can select what you want the camera to focus on, and if you want, you can shift the aperture or shutter speed. You can use this to get an idea of how the controls effect your image.;) it's a safety net but you have some input.

Reading through that it might sound a bit harsh, but you've bought a camera presumably because you want to take better photos, so go out and do it.:thumbs:
 
Agree with Phil. The green square mode could be doing almost anything, according to its complex algorithms, the conditions, and its rather small brain. Don't use it. Don't use manual either, unless you either know what you're doing or are a masochist and want to learn the hard way, from your mistakes.

Use P, and evaluative/matrix metering. It will always get you a decent result, then check the settings against the images and understand what's going on. Then start to use program-shift that actually gives you a lot of control without anything going seriously wrong, and also exposure compensation. After that, have a go at aperture-priority.

I'm off to a wedding today, as a guest. I'll use aperture-priority all day I expect, with exposure compensation. This is the preferred default mode for most folks I think, with AF on centre-point using focus-recompose technique. Might use manual, perhaps with flash in the evening, but I doubt it.
 
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