Why all prime lenses are not pancakes?

Why some fixed focal length lenses are pancakes (= thin) yet some other prime lenses are big?

:-O

This post and the rest of the thread should give you some answers:

https://www.dpreview.com/forums/post/58263688

mobi1 said:

Why some fixed focal length lenses are pancakes (= thin) yet some other prime lenses are big?

Click to expand...

Pancake lenses tend to be slow and have focal length fairly close to normal (the diagonal of the sensor). Shorter focal lengths need additional retrofocus elements to get the focus plane back to the sensor. Longer focal lengths need to be longer. Faster lenses tend to need more correction., which needs more elements.

For very basic optics, the focal length of a lens is going to be the distance from the center of a lens to the sensor. A pancake lens’ focal length is then going to be at best a value roughly equal to camera’s flange distance, unless it sticks inside of the camera, which may not be desirable or possible.


So on a Nikon F mount, you’d only able to get something like a 43mm lens or a bit longer in a pancake design.

It’s possible to design a lens that is optically longer than its physical length—also known as a telephoto, strictly speaking—or shorter than its flange distance—or retrofocal—but this requires extra optical elements, and so won’t be a pancake lens.

It’s entirely possible that future optical designs could be more compact than current designs, with high index of refraction materials such as diamond, or negative index of refraction materials if possible, or flat lenses that incorporate myriads of tiny optical antennas, which is still only an impractical laboratory curiosity.

It’s not a type of lens design, it’s just a nickname given to a very thin lens. And if you look at these lenses, they’re usually pretty slow, like f/2.8.

So you can choose small and slow-ish, or bigger and fast, but you can’t get small and fast.

Another newer factor is sensor size. You can get a much smaller AND faster lens if it supports only micro 4/3 or APS-C compared to full frame.

mobi1 said:

Why some fixed focal length lenses are pancakes (= thin) yet some other prime lenses are big?

:-O

Click to expand...

Same reason a 5.4L V-8 engine can't occupy the same physical space as the OEM engine in a Mini Cooper.

mobi1 said:

Why some fixed focal length lenses are pancakes (= thin) yet some other prime lenses are big?

Click to expand...

Compare the number of elements and groups between a top of the end prime and a pancake of the same focal length.

Pro 25mm lens:



Pancake 25mm lens:

I'm a big fan of small and slow. Just bought a secondhand copy of Pentax's DA 70mm f/2.4 Limited-- an interesting and unusual semi-pancake telephoto. It owes its small size to two if the factors you mention: a fairly large minimum aperture and size reduction to cover a smaller sensor.

Mark B. said:

mobi1 said:

Why some fixed focal length lenses are pancakes (= thin) yet some other prime lenses are big?

:-O

Click to expand...

Same reason a 5.4L V-8 engine can't occupy the same physical space as the OEM engine in a Mini Cooper.

Click to expand...

Challenge accepted!

mobi1 said:

Why some fixed focal length lenses are pancakes (= thin) yet some other prime lenses are big?

:-O

Click to expand...

probably becuz longer focal lengths are "thicker" than a pancake

www.flickr.com/photos/mmirrorless