Mastering aperture takes time and a lot of practice. This partly because the aperture you select affects your other camera settings, especially shutter speed if you want your photo to be properly exposed.
For your images for this Backgrounds Photo Composition Exercise, set your camera to AV (Aperture Value). This will allow you to select the aperture each time while your camera chooses the correct shutter speed for the light available. Keep everything else on automatic, including the ISO, until you feel confident.
Go outside – perhaps in your garden or a public park and choose a wide range of apertures. Your goal is to produce photos where the background is soft and dreamy. The background will lack detail. Other images you take will result in the background being pin sharp.
Start at the lowest F number (such as f 3.5 or 5.6) and keep changing the aperture until you reach the highest F number (f 16 f22, etc…)
You will take a photo at each F-Stop or number.
When you are satisfied, compare your photos at the different aperture settings.
Do note with longer focal lengths the depth of field (the amount in focus) will lessen.
When you have completed the Backgrounds Challenge, please share your images with me on Twitter. My Twitter home is @MarkPaulda