Making sense of the STM lenses and the 650d AF

Started Jun 9, 2012 | Discussions thread
meland
Senior MemberPosts: 2,976
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Re: not getting it, but not stopping you
In reply to caterpillar, Jun 12, 2012

caterpillar wrote:

Remember too, that 35-55mm or so, the design of lenses is pretty straightforward. In fact, the basic lens design of the 50mm hardly has changed the past maybe 75 years. Maybe just new coatings, baffles, and added aspheric here or there, but the basic format is still the same. Ergo, the same for the 40mm.

Such designs are also easier to manufacture. Even if they changed the design to make it smaller, again, it's not that expensive to make because it requires less glass, and there is very little effort to correct aberrations. This is why they can make 50mm f1.8 quite cheaply. Making that 40mm f2.8 FTM, would not probably make it larger so as to lose its pancake size. I can only surmise that Canon just made one to see how the market will respond and then from then on they will make an f2.0 or better version. Or they could be just putting a lot of distance to their 35 f1.4L (or an imminent 35 f1.4L mk2). Again, I don't think the size would go big even if it were f2.0.

May I try and explain. The Pancake 40/2.8 is basically a Tessar design. Unfortunately the Tessar is limited to f2.8 if you want to have acceptable optical quality. So if you want to make a 40mm f2.0 then it probably would have to be a different design entirely, i.e. A Double Gaussian - like the 50/1.8 and then it's back to the size (length) of the 50/1.8 and it's no longer a pancake.

The pancake 40mm has a specific objective and that is to be small. If you want it to be f2.0 that is a different objective entirely.

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