Why are 40mm pancake lenses so small, and why do lenses get larger when they’re much wider, longer, or faster?
Asked 8/13/2012
5 views
2 answers
0
Some 40mm f/2.8 lenses are exceptionally small and light compared with lenses that are much wider, much longer, or have larger maximum apertures. What optical or mechanical reasons make this focal length especially compact on many cameras? Is it mainly a physical limitation, or a result of lens design choices and trade-offs such as backfocus distance, mirror clearance, aperture size, and image quality correction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
7
It's to do with the backfocus distance, that is, the space between the back of the lens and the sensor. When this distance is greater than the focal length a simple symmetric lens design can be employed. The backfocus distance on a regular 35mm SLR is about 40mm.
To make a wide angle lens with a symmetric design the back of the lens would have to be closer to the sensor, which wouldn't leave space for the mirror. So it has to be further away with a reverse telephoto group stuck on to make up for it. This is known as a retrofocus design. Now you have two lenses in one, instantly increasing the size and weight. There are other factors too, such as the extra corrections required for wide rectilinear lenses due to the use of spherical lens elements.
The larger the aperture, the more glass is required, both for the aperture itself and for the necessary corrections. This also explains why longer f/2.8 lenses are bigger than the 40 f/2.8 pancake - the longer the focal length the larger the required glass elements are to maintain the same f stop. f/2.8 means the size of the aperture when viewed through the front of the lens is of diameter equal to the focal length divided by 2.8 i.e. a 50 f/2.8 lens must have a front element of at least 50/2.8 = 17.9mm. To make a 200 f/2.8 lens the front element must be 71.4mm
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
14y ago
0
Generated from our catalog & community — verify before relying on it.
A 40mm f/2.8 can be very compact because, on many SLR systems, that focal length is close to the camera’s required backfocus distance (roughly the space needed behind the lens for the mirror). That allows a relatively simple, compact design.
Wider lenses are harder on SLRs because the rear of the lens can’t sit as close to the sensor as a simple wide-angle design would like. To clear the mirror, designers use a retrofocus design, which adds extra optical groups and usually makes the lens larger and heavier.
Longer lenses also tend to grow because achieving a longer focal length generally requires more physical length and/or more glass.
A larger aperture also increases size: at the same focal length, a faster lens needs a larger entrance pupil, so the glass elements must be bigger.
Finally, modern lenses often include extra elements to correct aberrations introduced by the rest of the design. If you accept more compromises in sharpness, distortion, or other optical flaws, a lens can be made smaller. That’s why pancake lenses are possible, but they usually reflect careful design trade-offs as much as pure physics.
Recommended products
UniqueBot
AI14y ago
Your Answer
Related Questions
Why are maximum apertures like f/1.4, f/2.8, and f/4 so common in lenses?
Why isn’t there a 10-300mm zoom lens?
Should I use a UV or clear filter to protect my lens front element?
Do lenses with wider maximum apertures actually produce better photos?
Why are fast wide-angle prime lenses harder to make than fast normal lenses?