There's a small button located below the lens-release on the front of the camera. That button is the "Depth of Field Preview" button.
If you push that button, the camera will stop down the aperture blades to the f-stop that the camera will use for the the shot
e.g. if you are in M or Av mode and you set the aperture to f/22, then press the DoF Preview button, the aperture blades will stop down to f/22.
If you are looking through the camera when you do this, you'll notice things get very dark when looking through the camera ... but also you'll notice things that may have appeared out-of-focus will appear less out-of-focus.
Creating a blurred background with a sharp subject is based on creating a narrow depth-of-field (the range at which things appearr to be in acceptable focus) and placing your subject IN that depth-of-field, while things you want to appear blurr are not only outside the depth-of-field... but preferably WELL-OUTSIDE the depth-of-field (e.g. close focused subject, but very distant background.)
Three factors control the depth of field:
#1 Focus distance: The closer the focus distance, the shallower the depth of field. As you focus out near infinity, the depth of field becomes much much larger. So if you want a shallow depth of field, put your subject fairly close to the camera.
#2 Lens Aperture (or Focal Ratio): Very low focal ratios produce shallow depth of field. Very large focal ratios create large depth of field. Note that a focal "ratio" is is the ratio of the lens focal length (measured in millimeters) divided by the clear aperture opening diameter (also measured in millimeters) E.g. if a 100mm lens has a 25mm aperture opening then that's "f/4" because 100 ÷ 25 = 4. But this also means the higher the number, the tinier the opening.
#3 Focal length: Longer focal length lenses naturally have shallower depth of field. Wide-angle (short focal-length lenses) naturally have very broad depth of field. I have a 14mm f/2.8 ... and it has a huge depth of field ... getting background blur with that lens is pretty much hopeless. Conversely, the 70-200mm f/2.8 lenses that Canon has are actually very popoular for portraits because at 200mm and using f/2.8 (or even f/4) you can get very pleasant backgrond blur.
If you combine all of this together, then a "long focal length" with a "low focal ratio" and a "close subject" with "distant background" and you'll get very strong background blur.
You'll sometimes encounter the term "bokeh" ... bokeh (derived from a Japanese word which means "hazy") refers to the quality of the blur ... not the quantity.
With your 24-105, you might get a little background blur if you use f/4 at 105mm, place your subject close ... and have a distant background. It would be better at f/2.8 but you don't have an f/2.8 lens.
Here's a link to an example I found using PixelPeeper.com to search. I told it to look for images taken with the 24-105 using focal length's in the 100-135mm range (that's the only way to make sure it showed 100mm or above), and f/4 ... shot with a full-frame camera. (PixelPeeper indexes images from Flickr.)
https://www.flickr.com/photos/hz536n/18981890283/sizes/c/
That shot works because the flowers are very close. When shooting human subjects, you have to back out a bit farther... so here's another example:
https://www.flickr.com/photos/hz536n/19421075494/sizes/c/
Notice that in this shot, the photographer is slightly farther away than in the shot with the flowers... and even though they are using the same lens, focal length, and aperture... the background blur is reduced (because of focus distance). The farther away the subject, the less blur you get.
For this reason, if you want strong background blur, consider the 70-200mm f/2.8 lens ... or something like the 85mm f/1.8 or even the f/1.4. Canon's 135mm f/2 also creates fairly extreme blur.
Tim Campbell
5D III, 5D IV, 60Da
