Why no 40mm pancake (or similar)?

Started Feb 9, 2018 | Questions thread
EarthQuake Senior Member • Posts: 2,636
Re: Mount specs probably
1

uRebel Rob wrote:

Astrotripper wrote:

I don't know too much about optics design, but here's a little trend that might explain that.

Mount / Flange distance / Pancake lenses

MFT / 19.25 mm / 14, 17, 20 mm
EF-M / 18 mm / 22 mm
EF / 44 mm / 40 mm
EF-S / 44 mm / 24 mm
Sony E / 18 mm / 16, 20 mm
Leica M / 27.8 mm / 28, 35 mm

Pentax K / 45.4 mm / 21, 40, 43 mm

The 70mm/2.4 is pancake. And although rated as an ASPC, it works okay on FF too. On a cropped sensor, that's 105mm FFE.

Nikon F / 45.5 mm / 28, 45, 50 mm

And add the discontinued samsung NX

Samsung NX / 25mm / 16mm, 20mm, 30mm

Do you see the trend? There are no pancake lenses that have significantly longer focal length than the flange distance of the mount they are designed for.

... (except for the pentax 70mm) ...

That's probably not a coincidence.

But yeah, the easiest pancake is around the flange distance, but Pentax has both the 21mm and the 70mm, both pretty far outside the normal range. Canon's EF-S pancake "cheats" a little, as it allows the lens to go into the camera body (and why you can't use canon EF-S lenses on FF bodies).

Pentax is known for it's pancakes, and it's going to be hard to match/beat the ~ 0.5x to 1.5x the flange distance range that they have.

Obviously I omitted collapsible lenses that are only pancakes when collapsed.

So if you are hoping for a pancake short telephoto, I'm afraid you're out of luck.

Based on pentax's 70mm pancake FL being about 1.5x its flange distance, and applying it to m43's flange distance (and heavy rounding): 1.5 * 20 = 30mm. I think a m43 pancake 30mm lens is possible, but 40mm might be outside the flange distance's possibilities.

These two posts provide a good summary.

It's also worth noting that when we account for the flange distance, this exceptional Pentax 70mm lens has a real-world size of 51.7mm - just a hair shorter than the Panasonic 42.5mm 1.7 (55mm). Which is both encouraging and damning. Encouraging in that a lens this focal length can be made this small, but damning in that, if the same design was used for an M43 lens, it would not be a pancake.

Perhaps a more sensible way to look at it is distance from sensor to end of lens, as this is what really matters when comparing lenses from systems with different flange distances. With telephotos, part of the optical design is inside the camera body, which means a camera body that has a larger space between the sensor and mount allows the designer to make shorter telephoto lenses, assuming the same optical design for both.

So here we have the optical design length:

Pentax 70mm: 70.86mm

Panasonic 42.5mm: 74.25mm

To get a lens as small as the Pentax 70mm (not accounting for flange size), but in M43 mount, it would need a 51.61mm optical design length, which is likely a very significant design problem. This means the user would probably have to be willing to live with a high price due to an exotic design, or poor image quality due a compromised design that prioritizes length over all other variables, and possibly both.

Most people won't be excited about high prices and poor image quality, which is why I imagine such a lens is not on the market.

Of course, a 40mm pancake would be easier to make for M43 than a 70mm, though similar design restrictions apply, just to a lesser extent. A 40mm 2.8 with decent image quality and low-ish price is probably not out of the question. That said, how many people want to give up 1.5 stops of light to shave 10mm off the length of the lens? Probably not many, which I would guess is reason #2 why this lens doesn't exist.

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