Sensor size – I dont get it?

Ever had that thought when looking at different cameras and trying to figure out what its all about. You are not alone. Most of us have had  some trouble understanding how it works, when we first moved to digital cameras, especially those converting from film to digital. The Panasonic FZ1000 with its “1 inch sensor”  is causing some confusion yet again. The truth is its a simple thing to grasp & quite complicated as well.

Below is a comparison chart of the most commonly used camera sensors. You will note that each sensor has a crop factor indicated on the left hand side. We will come to that later. As you can see the CX (Nikon ) one inch sensor , ( the same size as both the Sony RX10 and FZ1000 ) isn’t really an inch in size.

This list from Dpreview shows all the relavent size information in one list.

As you can see from the list ( you will need to scroll down a bit ) the diagonal size for a 1″ inch sensor is 16.00 mm (15.8 actual). So where does the one inch come from?

Without delving into the mists of time and bringing up all sorts of math, the size comes from the standardisation of vacuum tubes as used in old  television systems where a 25mm tube contained a sensor with 16 mm diagonal.

The graphic to the right shows what’s going on. The circle shows a one inch sensor as in the Fz1000 or similar. The graphic shows the vertical, horizontal and diagonal sizes. The aspect ration of most sensors is 4:3, . This is how the term “1 inch sensor” is historically derived. The “1/2 inch sensor” used in the HS20 thru to HS50 is exactly half of the 1″ sensor size and the corresponding circle will be smaller. Simply put a 1″ sensor is a rectangle that fits inside a (16.0 mm ) diameter circle.

Kinda archaic but there you have it.

Why is the circle important for sensor description?

Not only does the circle provide a standard form from  which to derive our sensor sizes, it plays an even more important part optically. Cameras with a sensor as we know require a lens to provide light evenly across the sensor. This  of course is the same with all cameras film or digital. It’s called the light or image circle, and is the minimum light  area needed to provide light for an image. This has further ramifications for would be camera owners. The larger the circle required for the sensor the larger the image area from a lens has to be and the larger & heavier the lens will be.

The following Nikon Graphic from  digital camera world is a good example of why certain lenses work with only certain types of camera. In this case the FX and DX series Nikons use different sensors sizes and require different lenses to provide the correct image circle for full coverage.

The same is true of the Micro Four Thirds sensors which are 1/3 larger than a 1″ sensor and therefore require a larger image circle.

The crop factor.

The crop factor is determined by the height of the sensor in relation to height of a 35 mm film frame which is 24 mm in height.

. For example the 1″ sensor has a height of 8.8 mm. Simple maths tells us that 24/8.8 = 2.72 for our crop factor. So what you ask. Well take the Fz1000 for example. The stated lens focal length of the lens  is 9.1 mm to 146 mm or 16x zoom.

If we want to relate this to what the focal length would be if using film or a 35 mm full frame camera, we need to employ the math again. We know our sensor has a crop factor of 2.72, this then means that 9.1 x 2.27=24.75 (25mm) and 146 x 2.72=397.12 (400 mm).That’s a pretty handy zoom range. Essentially you are using a small telescope with a 400 mm focal length, making distant objects a good deal closer visually and this is pretty much what a 400 mm telephoto lens is.This is called focal length equivalent or equivalence depending upon the author and description.

This image of a Nikon 80-400 mm telephoto lens mounted on a Fuji X-Pro1 gives some idea of the size and weight of a more standard lens. This is where the bridgecamera wins hands down – No Weight!!

Note that the camera makers tend to round out a little on their stated  focal lengths. Its easier to say 25 mm or 400 mm than the actual answer, and to be fair 1 or 2 mm isn’t going to make any difference to your images at all.

And that very basically is how sensor size, and the relationship between lens , crop factors & focal lengths works.


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