I noticed that many telephoto zooms, including the Tamron 150-600 mm and the Sigma 150-500 mm, feature a small maximum aperture of f/6.3 at the 400-mm and longer zoom settings. Before buying one of them, I need to know if that will cause a problem for autofocus? I thought that cameras could not focus if the lens had a maximum aperture smaller than f/5.6.
You’re right, J.T., some digital SLRs were designed to autofocus only with lenses that can maintain a maximum aperture of f/5.6 at all focal lengths. And yet, I have tested both of the lenses you mentioned—as well as some 18-300 mm f/3.5-6.3 and similar zooms—with several Nikon or Canon cameras. In all cases, autofocus was always maintained. It appears that the manufacturers are using technology that leads the camera to believe that the maximum aperture is f/5.6 instead of f/6.3. This does not have an adverse effect on the exposure, and it allows for autofocus at the long focal lengths.
Granted, the small maximum aperture at 400-mm and longer focal lengths transmits less light to the AF sensors. As a result, autofocus is not quite as quick as it is at shorter focal lengths where the wider maximum aperture transmits more light to the AF sensors. While testing the Tamron SP 150-600 mm f/5-6.3 VC USD at 500 mm and at 600 mm recently, I found that the most reliable autofocus was provided when I switched from the multi-point AF sensor to the single, central focus detection point. Occasionally, the camera hunted for focus, but once it was acquired, the photos were sharp.
We would all prefer to have a telephoto zoom with a very wide maximum aperture throughout the range, but that would require lens components of a substantially larger size. That would also increase the cost of manufacturing and the weight, of course. For example, a popular 200-400 mm f/4 lens weighs 3.36 kg and sells for about $6300, while the new Tamron 150-600 mm f/5-6.3 zoom tips the scales at 1.951 kg and can be had for $1300. The Sigma 150-500 mm f/5-6.3 lens weighs 1.9 kg and sells for about $1100. Granted, the pricey 200-400 mm lens is built like a tank and was designed to meet the needs of the most demanding professionals. Still, the Tamron and the Sigma products are fine performers, and I believe that either will satisfy many photo enthusiasts.