In conjunction with Sigma and Hunt’s Photo, I recently lead a garden photography workshop at The Botanic Garden of Smith College in Northampton, MA. There and anywhere I lead a workshop dealing with macro photography, I always stress the importance of choosing your f-stops wisely.
The animated GIF above shows what a difference just one f-stop can make in flower photography. The frame with more detail in the background was shot at f/5.6. The second image with the softer background was shot at /4. Both were taken on a tripod with a cropped sensor (1.5x) Sigma SD1 Merrill using a Sigma 150mm f/2.8 lens.
While I might have anticipated that the increased depth-of-field of f/5.6 would improve the appearance of the blossom, I think that the difference between the two for the geranium itself is very little; what is noticeable, however, is that the background changes significantly. The larger aperture (f/4) blurs the background much more, creating a more pleasing background and certainly making the bloom stand out in the image.
Of course, not all flower photographs benefit from wide f-stops. Wild geraniums are fairly flat flowers, so they work better with limited depths-of-field than more three-dimensional blossoms, such as irises and lilies. Nonetheless, the takeaway is to always think and always experiment. Don’t take only one shot. Instead, take any number of shots, varying your aperture settings while keeping the focus and exposure setting consistent. Once back at your computer, examine your work to decide what works best for each particular close-up image.