To get the sharpest images possible with your lens, it’s best not to use the widest or smallest apertures. Most lenses are optimized in the f/5.6 – f/11 aperture range and give their best performance when stopped down a couple of f-stops from the widest aperture. If you have a 50mm f/1.8 lens, it’s best to use it around f/4 to f/8. Most consumer zoom lens have a sliding scale when it comes to their widest aperture. The Nikon 18-200VR lens I use has it’s widest aperture of f/3.5 when it’s at 18mm. At 200mm, it goes down to f/5.6. That’s what it means when you see a lens’ description like this: Nikon 70-300mm f/4.5-5.6G VR Zoom Lens where this lens starts at f/4.5 at 70mm and goes down to f/5.6 at 300mm. Usually, the faster the lens, meaning the lens starts at a very large aperture, the more expensive the lens is.

So, why not use the widest or smallest aperture? The problem with using small aperture sizes is that light waves are affected due to diffraction and though you have great depth of field, you lose out on sharpness. Large apertures are great for low-light, but unless you have an exceptional lens, its just too difficult to produce lenses that are razor sharp when wide open.

Everest in Disney’s Animal Kingdom. © Scott Thomas Photography 2008
Nikon D70/18-200VR, 1/200s, f/8, 200 ISO, -1.0 EV, 20mm focal length

These are good guidelines to start from. The best way to find out the best apertures for the lenses you own is to test them. Set up a small still life with various objects. Include something with text on it like a sign. Watches are also good as they are small and very detailed. Put your camera on a tripod and put it in aperture priority mode. Take a series of photos varying the aperture size with each one from the smallest f-stop to the largest. Download the pictures to your computer and view each photo at 100%. I found the sharpest aperture range for my 18-200VR lens to be around f/8 at 18mm and f/11 at 200mm doing similar testing when available light permits.

Further reading: Creative Uses of Aperture