"The lens is already designed to have an f/2.8 aperture at 200mm. Keeping that same aperture size works out close to f/1 at 70mm. Making a constant aperture L lens variable aperture shouldn't increase the size or weight at all. It seems like more of a marketing issue than a technological one. But, then I could be missing something."
Yes, I believe you are. Consider the examples I offered. Remember, the 70-200mm f2.8 lens is a 70mm f2.8, also. This means that the front element would become useless. You will need a piece of glass that can produce f1.4 at 70mm. Obiviously the one currently in the 70-200 doesn't.
A simple way to explain f-ratio is, let's suppose you have a 70mm f2.8 lens. If you put a 35mm circle behind it, it becomes a 70mm f4. If you put a 17 1/2mm circle behind it, it becomes a 70mm f5.6 and so on and so on. This is tricky because the 70mm f2.8 always remains a 70mm f2.8. You are just halving the amount of light the sensor sees. But you get the idea involved by this? But now what if we want a 70mm lens that is faster lens? We can't simply put a circle behind it. Can we?
It will take a brand new completely different design. That design is going to be costly. It is going to be huge. It is going to be heavy. It may not even be possible with todays technology.
The creation of the EF 85mm f1.2 L lens front element, for instance, is a daunting, extremely difficult process. So much so most other manufactuers can't do it. And that would be something on the order of what you are asking.
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