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?

All About Lens Apertures

Jillian from Jills Scene wondered whether I would do some how-to posts on photography basics like aperture settings, exposure, shutter speed, and how to get the best out of a zoom lens.

So here is the first of the how-tos – and it’s all about apertures.

Lenses, Apertures, Irises, Blades, and everything else

Built into a lens is a set of overlapping blades that can be opened up or closed down to make the ‘hole’ or iris in the middle of the lens bigger or smaller.


Even in the earliest days of photography, camera makers had worked out a simple way for photographers to be able to change the aperture in the lens.

The photographer would make the aperture bigger or smaller by twisting a ring on the outside of the lens. Twist one way to make the blades open up so the aperture gets bigger and twist the other way to make it smaller.


And it is such a good system that it is still used today.

True, the lenses in many modern cameras are powered by tiny motors so the photographer can just press a button to change the aperture electro-mechanically.

But it’s still the same principle – move the blades in or out to make iris in the lens bigger or smaller to let more or less light in.

By the way, one thing that separates a good lens from a so-so one is how many blades there are in it.

More blades means the more nearly circular the aperture is – and that means the clearer the image is that is focussed onto the film or digital sensor.

Imagine a lens with just three blades. The aperture would be like a curved triangle and light would spill and scatter and reflect off the edges of the blades.

Correct Exposure

You look at a scene and press the shutter. The shutter opens and lets the light through the lens and captures the scene on a piece of film or a digital sensor in the camera.

To get the exposure right, the camera needs just enough light and not too much for the scene being photographed.

Exposure is a combination of the size of the aperture and shutter speed. You can adjust the shutter speed and aperture by making one bigger and the other one smaller and it will give the same exposure.

For example, a one second exposure and an aperture open a certain amount is the same as two seconds of exposure and the aperture open half as much.

And that leads to the next clever thing about lens design that’s been around almost since the beginning of photography, and that’s f-stops.

What The f-Stop

If the blades on a lens opened and closed continuously with no intervals, then photographers would have no indication of how much more open or closed a lens was as they turned the dial.

So right from the early days of photography, lens makers designed lenses with intervals, or click stops. As you turn the dial, the blades open one click at a time. And each click opens or closes the aperture by a specific amount. Some lenses have clicks for every full ‘stop’. That means that each click opens or closes the lens by a factor of two.

Here’s an example. Let’s say the lens is wide open at its maximum aperture. And let’s say it has six ‘clicks’ to get to its minimum aperture.

Then the amount of light getting in after one click will be half the amount with the lens wide open. Another click and it’s halved again. Another click, halved again – and so on.

And that makes it very handy for adjusting the shutter speed and the aperture to keep the same exposure because you can double the shutter speed and halve the aperture. Or halve the shutter speed and double the aperture.

And so on.

The question is, when and why do photographers want to use these different combinations of shutter speed and aperture.

That deservers a post of its own to fully explain, but let’s just highlight a few of the more obvious facts.

In low light, photographers need every bit of light entering the lens that they can get, so that means a large aperture.

Of course, they could choose a slower shutter speed, but maybe there isn’t a slow enough shutter speed in the camera. And there’s another problem, which is that with a slow shutter speed there is more risk of a blurred image because of camera shake.

Camera shake just means that the photographer can’t hold the camera steady while the shutter is open.

And in bright light there might simply be too much light entering the lens if it is at maximum aperture.

Of course, you could choose a faster shutter speed. But if you have reached the fastest shutter speed (say 1/8000th of a second) and that’s the fastest shutter speed in the camera then you need to stop down the lens to let in less light.

Next time I’ll talk about more about f-stops and aperture, and about depth of field. Knowing about depth of field is a huge help in planning photographs and having them come out like you imagine.