image quality - Lack of sharpness / focus in low light

I recently was playing around with my camera (Canon 60D) in a low light situation trying to focus on some subjects across the room. I noticed once getting home and getting the images onto the computer that the image quality was absolutely horrible. I was so very frustrated with such poor quality.

This is atypical, as my lens / camera combination have taken some very sharp pictures in the past (and do most of the time). I could understand if I had an issue with camera shake / long exposures, etc, but that is not the case here.

Below are the specs for the shot:

• Camera: Canon 60D


• Lens: Canon 50mm f/1.4 USM with a UV filter


• Aperture shot at: f/1.4


• Exposure: 1/160 sec


• ISO: 640

I shot over 100 images and they almost all look the same, extremely soft, almost like there isn't a focus at all to the image. You'll also notice TONS of chromatic aberrations (In the window, its entirely outlined in purple).

Did my lens not focus? Does low light simply not take as sharp of photos even with decent exposure time/ISO/aperture? Is this caused by a dirty lens? If so, why am I not seeing this in brighter situations?

Any help would be appreciated!

(Large version: http://i.stack.imgur.com/ydWqf.jpg)

Update

FYI - I used 1 point of focus versus all focal points on the camera (excuse my lack of detail here, I'm not positive what those points are called exactly).

Answer

It seems to me that the image is quite adequately sharp given the lighting conditions. Some things affecting the sharpness of a photo in low light:

• 
  

  Almost any lens is going to be somewhat soft at its maximum aperture. As far as I know, the Canon 50mm/1.4 at f/1.4 is a bit on the soft side relative to other fast primes (even compared to the cheapo 50mm/1.8!)

  
  



• 
  

  The larger the aperture, the tighter the depth of field - most of the things in your photo are simply not in focus due to the razor-thin DoF at f/1.4.

  
  


• 
  

  The less light, the less contrast and the less accurate the autofocus mechanism is. And the larger the aperture, the easier it is for even a tiny focusing error to throw the focal plane slightly off.

  
  


• 
  

  Related to the previous two bulletpoints, moving the camera even slightly (eg. when doing "focus-and-recompose") might well be enough to cause noticeable misfocus. Using the non-center autofocus points might help, but they are often less accurate than the center one.

  
  


• 
  
  

  The higher the ISO, the softer the image due to noise reduction. This is something that can be tweaked in post-processing if you shoot RAW, but it's always going to be a balancing act between noise and softness.