[rrstuff]

There is a Pentax M, DA ltd, Canon 40mm STM, Voigtlander 40mm F2, Konica-Hexanon 40, Nikkor Ai-P 45mm and probably more.

Why are most pancakes around this focal length?

[SpartanD63]

I'm not entirely certain, but perhaps it's because it's close to the 50mm "standard" and slightly wider lenses allow for slightly smaller designs for the same aperture. (The front element of a 40mm f/2 only has to be 20mm whereas a 50mm f/2 requires a front element of 25mm, which is slightly bigger) Beyond that, I can't think of any reason other than that slightly wider lenses might sell better to people who value the size? (Not sure if that's the case.)

Anyone who knows more, please feel free to correct me.

[Adam]

There is a Pentax M, DA ltd, Canon 40mm STM, Voigtlander 40mm F2, Konica-Hexanon 40, Nikkor Ai-P 45mm and probably more.

Why are most pancakes around this focal length?

Because you don't need much glass to get that focal length and decent image quality. The actual distance from the sensor to the lens is around 4-5cm.

[maltfalc]

look up the definition of "focal length", then look up the distance from the lens mount to the sensor. the answer should become obvious.

[hcc]

There are also the Pentax DA15mm Ltd, DA21mm Ltd, DA70mm Ltd. All are about the same size as the DA40mm Ltd, although not as small as the DA40mm XS.

My 5 cents...

[Uluru]

It depends on design, final desired quality and the purpose.
A macro lens in that range will be more complex to make and thus thicker. That is why DA35 with f2.8 is thicker than DA40 with same f2.8.
It also depends whether the lens has a motor in it or not. DA40 is so small because sit doesn't have a motor, but uses camera's own screw drive motor.
It also depends which exact lens design and formula will be used and how many corrections are needed for desired image output, how much glass of what exact quality, etc.

[Na Horuk]

The 40mm is pancake because, as stated, of the register distance, the distance between the final lens element and the sensor. Other lenses need more elements (or space between elements) to project their focal length on the sensor
But this also depends on the design and the priorities of the engineers: some have the priority to make the lens as compact as possible, others have no regard for size (like the new Zeiss Otus 50mm f1.4 or the latest Sigma 50mm f1.4, compared to the tiny Pentax 50mm f1.4)

[lytrytyr]

There is a Pentax M, DA ltd, Canon 40mm STM, Voigtlander 40mm F2, Konica-Hexanon 40, Nikkor Ai-P 45mm and probably more.

Why are most pancakes around this focal length?

That's not going to be an easy question to answer convincingly,
because there are also several pancakes around 20mm:
DA 21 (like @hcc mentioned), M20/4,
various versions of the Voigtlaender Color-Skopar, etc.

[ogl]

There is a Pentax M, DA ltd, Canon 40mm STM, Voigtlander 40mm F2, Konica-Hexanon 40, Nikkor Ai-P 45mm and probably more.

Why are most pancakes around this focal length?

40-43 mm is the best focal length for human eye.
Image diagonal of FF is 43 mm.

Cameras vs. The Human Eye