How Big Does A Lens Have To Be

The EXIF data on the RAW image of this photo tells me that I photographed this owl in 2011 with a Nikon D7000 and a lens of 300mm focal length. The D7000 is a crop-sensor camera, and to compare like with like for different cameras, there is a convention to describe focal length in terms of the ‘full-frame’ equivalent.

This means that a 300mm lens has a focal length of 450mm full-frame equivalent.

Photographers would describe a lens with that focal length as a telephoto lens, meaning it is built for taking shots where the subject is at a distance.

It’s not a long telephoto by ‘serious’ standards. Photographers at an Olympic event or shooting wildlife might shoot with a 600mm lens.

Angle Of View

Long lenses have a narrow angle of view. Wide angle lenses have a wide angle of view. If that’s not clear, imagine rolling up a newspaper and looking down the tube. That’s a narrow angle of view.

There are online calculators that will tell you what a lens will cover at different distances based on the angle of view of a lens of that focal length.

From looking at the photograph I can estimate the approximate side to side distance across the frame. From that, and using an online calculator, I calculated the distance at which I probably took the shot was 25 feet (8 metres) from the owl.

Not very far at all, really.

Big Enough?

There are two ways that a lens has to be ‘big enough’.

First, the basics. Lenses are circular, but sensors are rectangular. So the lens has to make an image circle big enough to cover the long side of the sensor.

It has to cover right to the corners of the sensor otherwise the sensor isn’t going to record anything in the corners.

It’s the same with lenses covering the whole of the film in film cameras, of course. But I am dealing with digital cameras here.

Not all cameras have the same size sensors. Point and shoot cameras have smaller sensors that big professional cameras. And to state the obvious, the bigger the sensor, the bigger the diameter of the lens has to be to cover the sensor.

When I say ‘lens’ I mean the collection of glass elements that make up the lens. They are necessary to enable the lens to create a focused image circle at different distances between the camera and the subject.

We are probably all familiar with turning the dial on the lens to focus it. Or if not, then you may have heard the autofocus winding in an out. Either way, the lens is changing the distances between the elements that make up the lens.

Film Cameras

The film for 35mm film cameras is usually 36x24mm, (a 3:2 ratio) and that is the size that in digital cameras is described as full frame.

The first digital cameras were made by companies that already made film cameras, so when they thought of the size of the sensor, they followed that same 3:2 ratio.

There is no reason they needed to use this aspect ratio, other perhaps than that the first digital camera bodies were based on film cameras.

Perhaps they thought that there was some inherent quality in this ratio that appeals to the hard wiring in our brains.

Maybe, but there is enough disagreement among photographers about the ideal aspect ratio that I do not think any particular ratio is hard-wired into us.

In fact, if anything, I think we are drawn to the pleasure of ‘out of the ordinary’ aspect ratios, such as long panoramas.

Four By Three (4:3)

In fact, when Panasonic and Olympus introduced micro-four-thirds cameras with small sensors, they broke with the 3:2 tradition and used 4:3 aspect ratios for their sensors.

Because the long side of the 4:3 aspect ratio is slightly shorter relative to the short side of the rectangle compared to 3:2 aspect ratio, the diameter of the lenses could be a even smaller and still have an image circle that covers the sensor.

And that’s why micro-four-thirds cameras are smaller than others – partly by using smaller sensors and partly by using a 4:3 aspect ratio that allowed them to use smaller lenses.

Purely from the point of view of economy of resources, a square is the best shape for a sensor because it matches the image circle best. Many portraits are shot in square format, and maybe in a parallel universe that is the shape that would have caught on.

Or perhaps not, I prefer the first of these two images (the second is a crop from the first) because rectangles are more dynamic than squares. But it’s in the eye of the beholder, and you may think differently.

Long Enough

Now to the second way that a lens has to be big enough, and that is the length of the lens. This is a simple matter of the geometry of lenses. In order to ‘home in’ on a faraway subject, the lens elements inside the lens body have to have enough distance relative to each other to produce a sharp image at a distance.

Therefore, telephoto lenses are longer than wide angle lenses, all other things being equal.

A Last Word

This is a Fuji X-T2, one of the cameras I have, and it has twenty-four-million megapixels. The sensor is 23.6 mm × 15.6 mm, or what is called an APS-C size sensor. It is two-thirds the size of a frame of film. You can see the sensor in this photo of the camera shown without a lens.

What Are Sensors

Sensors are an array of micro-lenses cemented onto a board. Each of the micro-lenses reacts to the wavelength and strength of the light falling onto it. Together they build up a picture.

In a twenty-four-million megapixel camera there are twenty-four million micro-lenses cemented onto the sensor.

That’s twenty-four million micro-lenses. Amazing what we humans can do technologically, yes?

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