The aperture controls both the amount of light and the level of lens blur.
Shooting portraits can be a difficult task. Not only do you need to worry about the subject looking good, but you also need to make sure the technical qualities of the photo are good. The biggest challenge of indoor portraits is typically the low amount of light. Shooting a photo with a small aperture results in an underexposed image that will not have the acceptable level of detail for a portrait.
Natural or Household Lighting
A photo that only has natural light, such as a window, or household lighting has a relatively low amount of light compared to other situations. For this setup, the widest aperture you can get will be better. Most lenses are capable of f/3.5, but as much as an aperture of f/1.8 is preferable. That extra amount of light will allow you to shoot at a lower ISO and have an image with less noise.
Studio Lights
If you have studio lights, your aperture needs will be much different. Strobe lights produce a relatively high amount of light, especially at their peak levels. Typically, an aperture of f/3.5 or higher is necessary, depending on your artistic style. For a photo with a shallow depth of field, you'll need to lower the light levels to shoot at f/2.8 or lower.
Shallow Depth of Field
Depth of field is the amount of foreground and background blurring in a photo. Some portraits look amazing with shallow depth of field, which means only a small area of the subject is in focus. A good example of this is when a subject's eyes are in focus, while his or her ears have a small amount of blur. This depth of field can only be achieved by an aperture of f/2.8 or lower. When using an open aperture, be sure that your focus point is actually in focus!
An Entire Subject
When shooting someone's entire body, your aperture should be more open to compensate for the potential lens blur. An aperture between f/3.5 and f/5.0 should be acceptable. Generally, the more subjects and subject matter that is in your photo, the smaller the aperture should be to prevent blur. If possible, an aperture of f/8.0 generally results in the sharpest photo for most lenses.
Tags: amount light, aperture should, depth field, lens blur, photo with, shoot lower