03-30-2015, 10:46 PM
Designing a 16-70 f/2.8-4 may be easier, it may not. It depends how you arrange the groups of lenses and the general form of them. If for example a group begins with a negative element, you would not want it behind the aperture stop in a wide-angle lens because the beam will vary in position enormously as you move away from the center of the lens. This will force the lens to have enormously large diameter lenses to avoid vignetting. Thus, a "P" group behind the stop is the most advisable form for a WA zoom.
Increasing the aperture while maintaining quality is difficult. Whether it is more or less difficult than designing a constant aperture zoom - I cannot say absolutely. What I can say is that f/2.8 is not so ridiculously difficult an aperture to reach, nor is it exceptionally sensitive. F/2 and faster is where things start to become difficult and the difficulty/cost rises rapidly.
Up to f/2.8 or so is fairly "easy," depending on the FoV of the lens. Faster than that and you run into big vignetting issues and in general limit the design even more.
Increasing the aperture while maintaining quality is difficult. Whether it is more or less difficult than designing a constant aperture zoom - I cannot say absolutely. What I can say is that f/2.8 is not so ridiculously difficult an aperture to reach, nor is it exceptionally sensitive. F/2 and faster is where things start to become difficult and the difficulty/cost rises rapidly.
Up to f/2.8 or so is fairly "easy," depending on the FoV of the lens. Faster than that and you run into big vignetting issues and in general limit the design even more.