# A thin camera and a thin lens with a large glass element

After seeing these 3D designs, I was wondering if a camera that is thin could possibly have such a large piece of glass in the lens (given the lens piece doesn't expand when powered on).

As far as I understand, a large glass will produce a brighter image, but will also increase the distance between the lens and the film plane.

Can a small camera possibly have a large lens in the front?

• Apr 10, 2018 at 21:37
• Do you mean: a large glass will…?
– Stan
Apr 10, 2018 at 23:24
• I suspect those mock-ups are from the new "function follows form" school of product development (I won't call it industrial design), so it's probably a moot point, but... how do you know the lens doesn't move forward when turned on? Of course, since it's imaginary anyway, maybe it has a curved sensor? :D Apr 11, 2018 at 1:29
• @junkyardsparkle if the lens does indeed expand in this designer's mind, then my question doesn't make sense. For the sake of argument, I'm using these designs to ask the impact of glass size in lens to the film plane/lens distance :) Apr 11, 2018 at 18:56
• Got it. Still might be fun to hear some curved-sensor answers, though. ;) Apr 12, 2018 at 6:24

## 4 Answers

As a rough "rule of thumb," the lens diameter can be up to twice the measurement from the lens to the film plane.

This "simplistic answer" is based on the theoretical limit of the "speed" of a "normal" photographic lens to be less than ƒ/0.5 (that is, denominator must be at least 0.5).

A more detailed and precise answer would involve discussion of the materials involved and their geometry/configuration.

Can a small camera possibly have a large lens in the front?

There are really "large" lenses. This search is not the biggest one, but the look of it is great.

https://www.google.com/search?q=200-500mm+f/2.8

But you are referring to a large diameter compared to the depth of it.

Yes, there is a limit. The image on the left is a diagram of an f1 lens, where the green diameter and the red focal length is the same.

The second image has the focal length reduced by half, so it would be an f0.5 lens. There is a point where the light simply does not bend due to refraction anymore, so yes it has a limit.

That limit depends on the material used. I assume there could be several hacks to overcome this, for example using an insect-like array of lenses, similar to radiotelescopes arrays.

The limitation is givven by the Index of Refraction. For more information you can explore this: https://en.wikipedia.org/wiki/Refractive_index

Regarding the image, it is not that extreme. The image is not exactly isometric but it looks close so here are some lines "measuring the example"

So that example looks like a lens close to f1

• Thanks, great answer! The link with the telephoto lenses is slightly out of the scope of my question, since I was asking if a large glass lens could realistically be very thin (pancake versus long cylinder-type lens). The part of your answer where you say "There is a point where the light simply does not bend due to refraction anymore, so yes it has a limit." is what I'd love for you to expand on if you can! Apr 11, 2018 at 19:01

I think this depends on your definition of 'small camera'.

The Hasselblad SWC (Ken's got the pictures) is, I think, a reasonable example of a big lens on a small camera.

Also, the Canon 7 with the 50mm f/0.95 (Shown here on a Leica M6)

Coming from the SLR world - both of these cameras are tiny and sport some very large glass. So, the answer to your question is simply: Yes.

• Thanks for these examples! In the Hasselblad SWC, the lens cylinder is definitely not a flat/pancake style shape though. It's true that the Canon .95 has a huge diameter of glass in the front and isn't very long as a lens. Apr 11, 2018 at 18:58
• @MicroMachine - no worries. Wasn't really sure what you were looking to learn by the question and figured some real world examples might be it. So - you're looking for more pancake lenses with really large front elements? Apr 11, 2018 at 19:48
• Yes! Thanks for helping putting it in fewer words than I could - pancake lens, thin camera - but large glass lens (in diameter) at the front Apr 11, 2018 at 20:13
• @MicroMachine - I think the problem is, it's really hard to build a large aperture lens without corrective elements. Look at any lens diagram for a 50mm larger than f/2.8. Smaller max apertures mean optics can be simplified, but this also cuts down on the needed size. So, trying to find a simple, large aperture lens is almost a contradiction. Check this one out for simple design: global.canon/en/c-museum/product/s21.html Apr 11, 2018 at 22:22
• But if you use a large piece of glass, and corrective elements, can the resulting lens still be more wide in diameter than deep? I'm not an expert in optics so I don't know what rules apply when "corrective elements" are involved Apr 12, 2018 at 0:26

The image brightness presented by a lens intertwines its working diameter with its working focal length. The focal length measurement is the foremost piece of data that tells us the power of the lens. A long focal length lens magnifies and delivers a narrow angle of view. Conversely, a short focal length lens produces a tiny image and delivers a wide-angle view.

Now the lens functions like a funnel, the greater its working diameter, the more light captured thus the brighter the image it projects. We want nice bright images projected so we prize lenses that have a large working diameter. In modern times, both film and electronic image sensors have and are gaining more and more sensitivity to light. In other words, a modern camera need not have a lens with a super lens diameter to do the deed.

That being said, the working diameter (aperture)and the focal length (power) are intertwined. Each time the focal length is doubled, image brightness falls off. The falls off is abundant, double the focal length and the falloff is 4X (1/4 of the original brilliance).

What you are missing: Todays cameras are very small. This is because sensor technology has improved to the point whereby we can make tiny image sensors that do the job. Now a tiny image sensor only requires a short lens (by comparison with a larger camera). Short lens yield bright images so the need for a large working diameter is negated. As an example a fast (bright) lens fitted to a 35mm film camera will have a focal length of 50mm and a maximum working diameter of 36mm. A miniature camera with a 10mm focal length, mounted with a 7mm diameter lens, will do the same job, both project an image that has the same brightness.

This is a ratio thing!