Photo Sensors – Part 1

In this photography tutorial we will try to answer the question “What is really a photo sensor?” but without all the technicalities that such a subject would require.

I will begin by saying that the photo sensor has the same role in a camera as the retina has in our eyes: it “sees” the image projected to its surface converting it into electrical signals that are processed by the embedded computer of the camera (which acts here as its “brain”). It is interesting to remark how the cameras have evolved in such a way that, today, in the era of digital camera, we are probably closer than ever to what mother nature did with the human eye in millions of years of evolution.

There is a lot of literature describing the photo sensors in great detail, from the days of the chemical sensors (photo film, photo plates) to the current electronic digital sensors (that are a bit more diverse than one may think). Here are some good places to start:

In the following discussions I will refer to digital photo sensors simply as sensors.

From a photographer point of view, the most important characteristics of a photo sensor are:

  • Resolution is the number of pixels (pixel = single point of an image) usually expressed as a product W × H (W = width and H = height); the resolution characterizes the ability of the sensor to reproduce the fine details;
  • Frame size and form factor represent the geometry of the sensor (details will follow);
  • Sensitivity interval indicates the ability of the sensor to perform in different light conditions from full sunlight to darkness with reasonable good results;
  • Noise levels characterize the ability of the sensor to produce clean images even in less than optimal lighting conditions.

The ideal sensor would be small and it would have an almost infinite resolution; the sensitivity interval will be very large matching at least the sensitivity of the human eye and the noise level would be negligible at all sensitivities. Well… such a sensor does not exist yet! As we will see, the characteristics mentioned above are interdependent and, usually, they compete against each other; for example, a small sensor with a high resolution is noisy in low-light conditions.

Sensor Geometry

Historically, the frame size of most consumer film cameras was 36 × 24 mm – this being the so called “35 mm standard” (full frame) with origins in the motion picture industry from the beginning of the 20th century. Eventually, the format survived for about 100 years and also penetrated the digital camera technology (some professional DSLR cameras). Smaller size sensors are more common, especially in compact cameras.

The following table shows some of the most common sizes of sensors currently in use:

TypeWidth (mm)Height (mm)Diagonal (mm)Area (mm x mm)CF
35 mm36.0024.0043.27864.001.00

For a better comparison, I placed some common size sensor frames in a diagram:

Some common sensor sizes

From both the table and the diagram we see that there is a great variety of sensor sizes (actually, there are many more sizes less common that are not listed here).

An important derivative of these geometrical characteristics is the cropping factor, named sometimes incorrectly the focal length multiplier. The cropping factor (CF) represents the ratio between the standard 35 mm width (i.e. 36 mm) and the width of the sensor. Obviously, for a full frame sensor this factor is equal to 1. Why is this factor important? Let’s consider the following diagram:

Sensor size and the angle of view

For a given angle of view and a given lens, different sensors have to be placed at different distances from the lens (the smaller sensor is placed closer). This other situation is also true: for a given lens and a given distance from the sensor the angle of view changes in the sense that the smaller the sensor, the smaller the angle of view is. For compact cameras the phenomenon has less significance as we cannot change the lens (or the sensor). But for interchangeable lens cameras (most DSLRs), knowing how the angle of view changes becomes important.

The cropping factor gives us the ability to compare cameras and lenses using the 35 mm standard as a reference. For example, one camera is a Panasonic TZ5, equipped with a 1/2.3” sensor and a zoom lens with a focal range of 4.7 to 47.0 mm; another camera is a Canon 40D, equipped with an APS-C sensor and a zoom lens with a focal range of 18 to 200 mm (EF-S 18-200). Neither camera has a full frame sensor, so we will use the cropping factor to characterize both by translating the focal distances into the 35 mm standard world using this very simple calculation:

 f35 = f × CF

where  f35 is the 35 mm equivalent focal distance, f is the real focal distance (marked on the lens) and CF is the cropping factor.

  • TZ5 (CF = 5.88): f35 is in the range of 27.6 to 276.5 mm;
  • 40D (CF = 1.62): f35 is in the range of 29.2 to 324.3 mm.

As we can see the TZ5 goes a bit wider, while the 40D with EF-S 18-200 offers more on the longer side.

Sometimes the manufacturers are doing the calculations and provide the numbers already translated into the 35 mm standard. But in most cases the specifications use the real numbers and one must do the calculations in order to correctly compare cameras and lenses. That being said, I would rather want to see the angle values used more often; there wouldn’t be any conversion involved but it would definitely look less intuitive for some.

In the mean time, if you own a camera, please read its documentation or, if you lost its manual, go to Digital Photography Review web site: they have an excellent database of cameras with all the specifications. If the 35 mm numbers are not available, look for the sensor type, find it in my table and use the cropping factor to calculate these numbers – they will prove useful one day (and, eventually, in one of our future discussions).

The next post will focus on the main function of the photo sensor: capturing light. Keep an eye on these pages.


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