So, let’s look at aperture.
In photography, aperture is the hole or opening through which the light is emitted. It is used to control the depth of field or the amount of the photo in focus. It is also used to control the exposure together with shutter speed and iso but we will just look at today just from the DOF (depth of Field) point of view.
The aperture is the F numbers you see on your camera. When you choose a small F number (like 2.8) you are choosing a wider aperture or letting in more light as the hole is bigger. A smaller F number gives a smaller depth of field or amount of the photo in focus while a larger f number like f22 gives lets in less light and gives a much greater dof.
To work out what would be in focus in a photo would depend on what F stop you have chosen. Here is a link to an online dof table http://www.dofmaster.com/doftable.html As you can see, your DOF depends on how far away from your subject (and also the lens you're using - this table is based on the 50mm on a crop format camera - 400D, 450D, D80 etc etc)
So, what does this mean? Basically if you are wanting to take a picture of your child with a nice blurry background you will want to chose a small F stop number (what is referred to as a wide aperture). Generally for portrait work with individual people this is often between f2.8 and F4. Of course when you have got your focussing down pat and if your lens allows, you can open up wider (to 1.2 - 1.8) but your focus plain will be even smaller. Also remember that generally when you focus the depth of field is approximately 1/3 in front of that point and 2/3 behind the focal point.
You will also want your subject a couple of metres or more from the background to have a nice blurry background. The quality of the background blur (or area out of focus) is referred to as Bokeh. Better quality lens will give you better quality Bokeh.
Also look for distracting elements that may be at the same focal plane as your subject as these items will also be in sharp focus and may be distracting, unless you are doing a more environmental portrait and then a strategically place branch (or something) in the same focal plane as the subject can add a 3 dimensional feel to the image.
Okay so enough rambling, here's my suggestion to help you take your first photos not using auto.
1. Find your camera manual and work out how to put your camera on aperture priority. Also look at how to change your aperture (which dial to turn) and where on your camera the F stops show.
2. Set you camera on Aperture Priority Mode and choose a F stop of 2.8 or the smallest number you can on your lens (kit lens is 3.5 from memory).
3. Find your subject and position them a few metres from the background (just make sure that there's not a tree coming out of the top of their head or something like that). Take the photo from the same level as them if you’re taking a picture of a child rather than from above them pointing down (unless that's the look you’re particularly going for).
4. Now take the same picture using f 5.6 f8, f11, f16 and f20 for comparison later. You will notice that the amount of in focus area increases with the larger F stop numbers (stopping down)
5. Now move your subject closer to the background and take the series of photos again
6. Now move them further away and again take the series of photos.
Hopefully you'll have a better understanding of aperture and its relationship with DOF when you upload and study the photos.
Things to note
- Because you are in aperture priority your camera will choose your shutter speed. Make sure you are taking your photos where there is lots of light otherwise the shutter speed may drop too low and your photos will end up blurry.
Happy to answer questions here or on my blog
Andrea
I am still getting the hang of my 450D so this kind of info is wonderful!
