For many kinds of portraits, it is desirable to have a strong, creamy background bokeh because it simplifies the background and makes the subject stand out. However, there are certain kinds of portraiture where you want to preserve more details of the shoot location. Here's why f/4 makes a good guideline in such situations. (Reported by Teppei Kohno)

 

When the contextual details of the location are just as important as the portrait subject

Perhaps you are on a dream vacation with your loved ones and want to preserve their joyful faces against the sights and scenes of your destination. Or maybe you are trying to shoot an environmental portrait, a portraiture style that seeks to capture someone at a location that is significant to their work, lifestyle, or personality.

In these situations, making the portrait subject stand out is important, but it is just as important to preserve details that tell the viewer about where the picture was shot, as well as anything else in the background that enhances the story. If the background bokeh is too strong, such contextual details can get blurred beyond recognition. But if the background bokeh is too weak, the background could distract the viewer from the subject.

For harmony between the background and the subject, try using a moderately-wide aperture such as f/4.

 

What happens when you use an aperture that's too narrow?

 

Know this: The distance between the subject and the background matters too!

The f-number is just one of four factors that affect the intensity of background bokeh. Background elements that are further away from the subject will have stronger bokeh compared to those that are nearer, all other factors remaining constant. f/4 is just a rule-of-thumb to start with, so feel free to adjust your aperture setting accordingly.

Find out more about the factors that affect bokeh in: 
Lens Basics #3: Creating Bokeh

 

Bonus technique: Know which lens to use

Professional portrait photographers tend to use a medium telephoto length (full-frame equivalent focal length: 80 to 100mm ) as it ensures the least distortion and doesn't require you to be too close to the subject. We recommend a full-frame equivalent focal length of 80 to 90mm for the best results.

If you are using an APS-C format camera, the 1.6x APS-C format crop factor means that the focal length stated in your lens name will have to cover 50 to 55mm. You have several options, such as:

i) A standard zoom lens (such as the EF-S18-55mm f/3.5-5.6 IS STM, a popular DSLR kit lens)
ii) One of the "nifty fifty" EF50mm prime lenses (such as the EF50mm f/1.8 STM)
iii) A telephoto zoom lens (such as the EF-S55-250mm f/4-5.6 IS STM)

Not all the options may be equal, especially if they include variable aperture zoom lenses.  On such lenses, the maximum aperture that you can use is always the widest on the wide-angle end and the narrowest on the telephoto end.

For example,
- On the EF-S18-55mm f/3.5-5.6 IS STM, at 55mm, the maximum aperture is f/5.6
- On the EF-S55-250mm f/4-5.6 IS STM, at 55mm, the maximum aperture is f/4

As the images below show, f/5.6 is only one f-stop narrower than f/4 but the results can be quite different.

55mm (88mm full-frame equivalent) @ f/5.6

FL: 55mm (88mm equivalent)/ Aperture-priority AE (f/5.6, 1/60 sec)/ ISO 400/ WB: Auto
Shot at the telephoto end of the EF-S18-55mm f/3.5-5.6 IS STM.

55mm (88mm full-frame equivalent) @ f/4

FL: 55mm (88mm equivalent)/ Aperture-priority AE (f/4, 1/125 sec)/ ISO 400/ WB: Auto
Shot at the wide-angle end of the EF-S55-250mm f/4-5.6 IS STM.

Here's a closeup of the background. Notice how the bokeh in the f/4 example is stronger even though both images were shot at the same focal length.

Lesson: On an APS-C format camera, choose either the EF-S55-250mm f/4-5.6 IS STM or an EF50mm lens to have the option of using a wider aperture at 55mm.

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Not sure how to change your aperture settings? Click here for step-by-step instructions.

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Need more tips on shooting portraits that incorporate the shooting location? Check out these articles:
5 Portrait Photography Techniques to Take You from Day to Night
Shoot Beautiful Night Portraits with No Tripod, No Flash

Challenge yourself! Find out how you can level up your outdoor portraits with the help of an external flash in:
2 Instant Techniques to Liven Up Your Outdoor Portraits

 

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