Zoom lenses come in two flavors: a fixed aperture such as f/2.8 or f/4, or a variable aperture where the maximum aperture decreases as the lens is zoomed. Most lower-priced zooms are the latter, as the design allows manufacturers to keep the cost down, reducing bulk, and adding image stabilization.
While fixed aperture lenses are often associated with being the top self where image quality is concerned, higher-end variable aperture lenses do have their benefits. If you were to take the Tamron 70-200mm f/2.8 G2 and add the 1.4x teleconverter your effective focal length and aperture would be 98-280 f/4 with reduced AF performance and increased weight. Whereas a lighter lens like the new Tamron 100-400mm f/4.5-6.3 Di VC USD would be beneficial for those who need long reach without hindered AF.
Just announced, the Tamron 100-400mm f/4.5-6.3 Di VC USD (Model A035) is a new ultra-telephoto for full frame cameras. The new lens features 3 LD (Low Dispersion) lens elements for greater aberration reduction and Tamron’s original eBAND Coating for superior anti-reflection performance.
The A035 comes in at 39.3 oz, making it the lightest lenses in its class thanks to magnesium alloy in key areas of the lens barrel, which improves weight reduction, strength, and portability.
The Model A035 is fully compatible with Tamron’s 1.4X teleconverter and the Tamron TAP-in Console, which enables lens customizations, focus adjustments, and VC-mechanism adjustments among others. Rounding out the Model A035 is Tamron’s excellent vibration compensation, Dual MPU (Micro Processing Units), and an optional Arca Swiss compatible tripod mount.
Tamron’s 100-400mm f/4.5-6.3 Di VC USD will come in Canon EF and Nikon F mounts. The expected launch date at the end of 2017 with no price announced as of yet. You can sign up to be notified when more details are announced here.