The F Number On A Lens Means Something? Who Knew!

March 18, 2024 by Jenny List 58 Comments

The Raspberry Pi has provided experimenters with many channels of enquiry, and for me perhaps the furthest into uncharted waters it has led me has come through its camera interface. At a superficial level I can plug in one of the ready-made modules with a built-in tiny lens, but as I experiment with the naked sensors of the HD module and a deconstructed Chinese miniature sensor it’s taken me further into camera design than I’d expected.

I’m using them with extra lenses to make full-frame captures of vintage film cameras, in the first instance 8 mm movie cameras but as I experiment more, even 35 mm still cameras. As I’m now channeling the light-gathering ability of a relatively huge area of 1970s glass into a tiny sensor designed for a miniature lens, I’m discovering that maybe too much light is not a good thing. At this point instead of winging it I found it was maybe a good idea to learn a bit about lenses, and that’s how I started to understand what those F-numbers mean.

More Than The Ring You Twiddle To Get The Exposure Right

lose-up of the end of a lens, showing the F-number range — The F-number range of a 1990s Sigma consumer-grade zoom lens.

I’m not a photographer, instead I’m an engineer who likes tinkering with cameras and who takes photographs as part of her work but using the camera as a tool. Thus the f-stop ring has always been for me simply the thing you twiddle when you want to bring the exposure into range, and which has an effect on depth of field.

The numbers were always just numbers, until suddenly I had to understand them for my projects to work. So the first number I had to learn about was the F-number of the lens itself. It’s usually printed on the front next to the focal length and expressed as a ratio of the diameter of the light entrance to the lens focal length. Looking around my bench I see numbers ranging from 1:1 for a Canon 8mm camera to 1:2.8 for a 1950s Braun Paxette 35 mm camera, but it seems that around 1:1.2 is where most 8 mm cameras sit and 1:2 is around where I’m seeing 35 mm kit lenses. The F-stop ring controls an adjustable aperture, and the numbers correspond to that ratio. So that 1:2 kit lens is only 1:2 at the F2 setting, and becomes 1:16 at the F16 setting.

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A Solar-Powered Point-and-Shoot, Circa 1961

September 27, 2022 by Dan Maloney 23 Comments

Try to put yourself in the place of an engineer tasked with building a camera in 1961. Your specs include making it easy to operate, giving it automatic exposure control, and, oh yeah — you can’t use batteries. How on Earth do you accomplish that? With a very clever mechanism powered by light, as it turns out.

This one comes to us from [Alec Watson] over at Technology Connections on YouTube, which is a channel you really need to check out if you enjoy diving into the minutiae of the mundane. The camera in question is an Olympus Pen EES-2, which was the Japanese company’s attempt at making a mass-market 35-mm camera. To say that the camera is “solar-powered” is a bit of a stretch, as [Alec] admits — the film advance and shutter mechanism are strictly mechanical, relying on springs and things to power them. It’s all pretty standard camera stuff.

But the exposure controls are where this camera gets interesting. The lens is surrounded by a ring-shaped selenium photocell, the voltage output of which depends on the amount of light in the scene you’re photographing. That voltage drives a moving-coil meter, which waggles a needle back and forth. A series of levers and cams reads the position of the needle, which determines how far the lens aperture is allowed to open. A clever two-step cam allows the camera to use two different shutter speeds, and there’s even a mechanism to prevent exposure if there’s just not enough light. And what about that cool split-frame exposure system?

For a camera with no electronics per se, it does an impressive job of automating nearly everything. And [Alec] does a great job of making it interesting, too, as he has in the past with a deep-dive into toasters, copy protection circa 1980, and his take on jukebox heroics.

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F/stop Printer For Analog Printing Black And White Photos

July 5, 2012 by Mike Szczys 3 Comments

This beautifully crafted device is a timer used for getting the perfect exposure when making film prints of photos. But in addition to keeping time, it also does logarithmic calculations that are based on the f-stop values used for each exposure. It does this in 1/100th of a stop increments. While he was at it, [William] also decided to pack in a bunch of other features like dry down correction, and support for making test strips. This is a little hard to understand when discussed in the abstract, but just take a look at his video after the break where [William] walks us through an example exposure and all will become clear.

You can see from the construction page that the device is basically an Arduino shield. It provides a relay for controlling the exposure lamp, a keypad, rotary encoder, and character LCD. Slap it in a fancy case, connect it to the equipment you’re using, and you’ll be creating perfect prints in no time flat!

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