Why did I do this?
Well, sometimes a guy wants to open the aperture wider in order to get a blurrier background. The lens that I usually carry only opens to f4. I have a nifty fifty (50mm) lens that opens to f1.8 which allows the background to blur out more, which in turn focuses on the subject better.
You can see the effect in this photo of asters.
|The one aster is quite well focused but the blurring soon begins|
and the far background has almost no detail at all.
Here's another example of how the background begins to blur giving a pleasing effect — to my eyes anyway.
The next day, I took my 40mm lens. It only opens to f2.8 but it might be slight better for closer work. First, however, I took this photo. Actually, it is 8 photos stitched together in Lightroom: four across and two rows up with the camera in portrait orientation.
The background blur was pretty good at f2.8.
In this one, it was the red veins in the maple leaves which caught my eye. I have never noticed this so much before.
Just in case you are perplexed with apertures and all of these f numbers, here is a little graphic that might helps. As the number gets bigger (32 as opposed to 1.4) the opening (aperture) gets bigger and the background gets fuzzier. There is great depth of field when the aperture is small. In the graphic below, with the camera always focused on the person, the background mountain are fairly clear at the left end but very fuzzy at the right end.
Here's one more image that helps to illustrate the relationship between aperture and depth of field: the smaller the lens opening (aperture) the greater the range of things in focus from front to back (depth of field) and vice versa.
Anyway, to wrap up. I like the convenience of zoom lenses which is why I almost always use them. I have three for my main camera. My long lens and wide angle lens both open to f2.8 but they are more useful for special conditions and not so much for general photo ops. My lens that is most suitable for everyday use only opens to f4, so that is why I switched to my fixed lenses on the walks in question when I knew that I would be concentrating mostly on flowers and leaves.