Shallow depth of field is desirable, particularly in portrait photography, due to its subject and background separation, and its pleasing aesthetic. But while your gut reaction for depth might be to go for the fastest prime you can, you might be surprised at this side-by-side comparison.
There was a lesson I learned early on in photography, quite by accident. In fact, I still have the image that marks the lesson. I was fairly new to the craft, with only a year or two under my belt, and I wanted a 70-200mm f/2.8. The problem was, the Canon edition I wanted was far too expensive for me to be able to afford back then and although I looked at other lens manufacturers, I still struggled. Then I saw the f/4 variant, which while still an L-range lens, isn't anywhere near as expensive due to its narrower widest aperture. I got such a good price that I knew I could resell it if it was a complete disappointment.
When it arrived, I took it out at sunset on a freezing cold winter's afternoon and found some local horses to act as the subject. To my delight and complete surprise, at 200mm, f/4 created some pleasing bokeh and decent subject separation. I hadn't understood the relationship between focal length and depth in my images, or at least not to the degree I thought I did.
This video by Pye Jirsa of SLR Lounge is a more expensive and more recent example of the same revelation. Jirsa compares the Canon RF 85mm f/1.2 with the Canon RF 70-200mm f/2.8, and the results are surprising. At around the 7:50 mark of this video, Jirsa points out that most people would get the two example images the wrong way around with which lens was used, and I wholeheartedly agree. While the 85mm is brilliant, the 70-200mm wins in a lot of categories. What do you think?