With SLR cameras and interchangeable lenses, you have a wide range of possibilities for setting the aperture of your lens, along with the lighting, sensor sensitivity and shutter speed. When I’m taking most of my photographs, I use my aperture-priority setting so that I don’t have to worry about the shutter speed, and I also try to shoot at the lowest possible ISO setting (usually in the 100 to 400 range) so thatI get good, sharp photographs. And, as mentioned in another post, I try to make sure that I shoot so that my shutter speed is at least as fast as the reciprocal of my lens’ focal length.
If I’m shooting for fun, then I’ll typically set my aperture at f/5.6 because then I usually don’t have to worry about my shutter speed being too slow (again, depending upon my lens focal length and other factors). So, this is my fun & carefree setting.
But, when I’m seriously shooting, for example, portraits, then I set my aperture at what I feel is my lens’ sharpest setting and I’m using a tripod. Since I’ve shot thousands of photographs with my camera and lenses, I have a good sense of what lenses to use in what situations. But, I also need to know how to use these lenses in these situations.
Your lens’ sharpest setting at the focus point will often be at the middle of the range of possible apertures.
The lens that I frequently use for portraits has an aperture range from f/1.4 to f/22. Therefore, the lens will be at its sharpest in the f/5.6 to f/11 range. If I want the background to be sharper, then I’ll tend towards f/11. Or, if the available light isn’t quite enough, then I’ll open up to f/5.6.
Photographers will frequently refer to this aperture range of maximum sharpness as the lens’ “sweet spot.”
With another of my favorite lenses, it’s aperture ranges from f/4 to f/32. When I’m seeking my sharpest photographs, then I’ll set the aperture on this lens in the f/8 to f/16 range. Also, depending upon the lighting conditions and other factors, I may choose this lens particular lens over my faster lens, just because I can get sharp photographs in brighter conditions with this lens than with my other great lens.
Remember that shooting at smaller apertures can sometimes detract from photo quality
As you’re likely aware, opening the aperture to let in more light can often leave the background out of focus. Conversely, the more that you stop down the aperture, the more that the whole scene will be in focus (although this can come at a cost).
I’ll explore “depth of field” and sharpness in a different posting, but for now, suffice it to say that while your photographs may very well be “sharp” when you stop down to your smallest aperture, you’llstart to run into issues involving how the lens diffracts light.
At these smallest apertures, most lenses suffer diffraction issues that cause the different wavelengths of light to be not quite coherent, so that you’ll see more “color fringing” at the edges of distinct objects, detracting from your photographic quality.
Faster lens usually have a faster sweet spot
If you’ve been following along, you might now have a better sense of why professional photographers often want to have as fast a lens as possible (with a wide aperture). Yes, these fast lenses work well with fast-moving objects (sports) and in low light. But, more importantly, with a fast lens, the lens’ “sweet spot” lets in more light than a slower lens (and thus, a faster shutter speed), so that it’s easier to get a good, sharp photograph at these middle aperture ranges in poorer lighting.
So, having a “fast” lens when it’s wide open is nice, but having a lens with a “fast sweet spot” is even nicer! So, try to buy the fastest lenses that you can afford for this reason.
When not working for Brilliant Prints (The United Kingdoms Best Trade Canvas Printing Company) Tim Barnes is a passionate wedding photographer and has shot more than 200 brides (…in a nice photographic way).