Sensor-size and Crop-factor

Table of contents:

Sensor-size and Crop-factor in Photography

Sensor is the most important, valuable and costly element of a camera. Most importantly, it affects the image qualiy, and the price of the camera; there are other aspects too.

Table of contents:

Camera sensors

It is a rectangular hardware inside a camera which receives light signals through the lens, and converts these to electrical signals; the latter being the source of a digital image file. This definition is applicable to the sensors in digital cameras only.

slow shutter-speed from running train

Sensor of a camera (source: internet)

There are a large varities of sensors of different sizes. For example, a fullframe sensor has a dimension of 36mm × 24mm; this ideal size (also known as 35mm format) was globally used in film-cameras in the earlier days. When digital era stepped into, manufacturers began to make sensors of lesser size, reducing the production cost and pricing, such as,

and many others. There are also medium format sensors, made by renowned companies like Pentax, Fuji which are larger than full-frame sensors.

Note: From now onwards whenever the term "full-frame camera" or "full-frame sensor" is used, it will indicate to a large sensor of dimension 36mm × 24mm.

Sensor and Crop-Factor

Crop-factor of a sensor is determined by comparing its length of the diagonal to the length of the diagonal of a full-frame sensor, typically 43.27mm.

Sensor-sizes and their crop-factors Sensor-size and crop-factor One inch A B C D 13.2mm 8.8 mm Micro 4⁄3rd E F G H 17.3mm 13 mm APSC I J K L 23.6mm 15.7 mm Full-frame M N O P 36 mm 24 mm

Sensor-TypeDiagonal length (in mm)Crop-factor
One-inch BD = √(BC² + CD²) = 15.86 43.27 : 15.86 = 2.73 ≅ 2.7
Micro 43 rd FH = √(FG² + GH²) = 21.64 43.27 : 21.64 = 1.999 ≅ 2
APSC JL = √(JK² + KL²) =   28.35 43.27 : 28.35 = 1.526 ≅ 1.5
Full-frame NP = √(NO² + OP²) = 43.27 43.27 : 43.27 = 1
Medium format = √(44² + 33²) = 55 43.27 : 55 = 0.79

The above table reveals the following facts:

When we take account of a sensor [other than full-frame], say APSC sensor, we compare its diagonal with the diagonal of a full-frame sensor. How many times is the diagonal of a full-frame sensor (43.27mm) larger than the diagonal of an APSC sensor (28.35mm)? 1.5 times. [43.27 ÷ 28.35 = 1.526].

Here comes the term "crop-factor". Crop-factor of an APSC sized camera is 1.5.
In the same way, crop-factor of a one-inch sensor camera is 43.27 ÷ 15.86 = 2.7; crop-factor of a Micro 43 rd sensor is 43.27 ÷ 21.65 = 1.99 ≅ 2.

Examples of full-frame cameras are Nikon d810, d750, z6, Canon 5d mark IV, Sony a7iii etc.

Nikon d7000, z50, Fuji x-T30, Canon 80d, Canon EOS M50, Sony a6600, etc. are APSC cameras.

Micro 43 rd cameras are Olympus OM-D E-M10 Mark III, Panasonic LUMIX G7 etc.

The one-inch-sensor (CX format in Nikon) cameras are Nikon v3, Sony RX-100, Canon G7x etc. These are large lists, you know.

The sensor-size of a full-frame camera is assumed to be the ideal size; all other sensor-sizes are compared to this size to find the crop-factor. Now, why the crop-factor of a camera is required?

Sensors and Lenses

A lens does not know what sensor is attached behind it. It does not know how much of the scenario the sensor can see. It allows the image to the sensor/film as per its own angle of view.

Because sensors are rectangular, they never see some portion of the circular image, which can be seen by a lens. Moreover, smaller the sensor, more wastage of the part of the image happens.

Note : Nikon, Sony, Fuji APSC sensors have the crop-factor of 1.5, but canon APSC has a crop-factor of 1.6, the sensor being slightly smaller than Nikon or Fuji APSC. Nikon calls their full-frame sensor as FX format, APSC sensor as DX format, and one-inch sensors as CX format.

Crop-factor depends on sensor-size Focal length, Sensor-size, Crop-factor Micro 4⁄3rd APSC Full-frame Image seen by the lense

In the above animation, the lens (not shown) is actually passing light in the form of a circular image (replica of its own shape). It does not care what sensor is located behind it. So the fullframe sensor sees part of the projected image; the APSC sensor sees lesser part of the image, and the micro 43 rd sensor sees the least part of it. The field of view in each three cases are different. Digital image files are created according to the data received by the sensor. So we end up with three different images of the same scenario by using the same lens and different cameras (sensor-sizes).

If the three images formed by the above sensors are viewed on a PC side by side, or they are printed on same sized canvas, the image formed by the micro 43 rd sensor (smallest here) will seem to have a longer focal length than the other two images. The image formed by the APSC sensor (middle) also will seem to have longer focal length than the full frame sensor image.

slow shutter-speed from running train

f/9, 1160 sec., ISO-500, FL-15 mm eqv.,
Nikon d7000, Sigma 10-20mm,
Kolkata, West Bengal, India 2016.

This is the image used in the above animation, if you wish to have a look.

Smaller sensors seem to produce longer focal length

If we know the crop-factor of our camera, we can assume the field of view in comparison to a full-frame camera. Suppose the above image in the animation has a 24mm lens. Now if we attach it to a full-frame camera, we get a field of view of a 24mm lens. But when we attach the same 24mm lens to an APSC camera, we get a field of view of 24mm × 1.5 = 36mm (crop-factor of an APSC sensor is 1.5 ). And when attached to a micro 43 rd camera, we get a field of view of a 24mm × 2 = 48mm. (crop-factor of a micro 43 rd sensor is 2).

On the other hand, when a 300mm lens is attached to a full-frame camera, we get the true field of view of a 300mm lens, the crop-factor being 1. Attaching the lens to an APSC camera, we get a field of view of 300mm × 1.5 = 450mm. But when attached to a micro 43 rd, we get a field of view of a 300mm × 2 = 600mm.

Equivalent focal length

So, it is clear that wide angle lenses no longer remain so wide when used with lower sensor formats. I shoot landscapes with a 16mm wide angle lens on a full-frame camera, but if this lens is attached to an APSC camera, it records a field of view as wide as 24mm (16mm × 1.5 = 24mm).

This 24mm on the APSC camera (instead of 16mm) is known as equivalent focal-length. The actual focal-length of 16mm never changes, as seen in the animation.

Until now, we were referring to the lenses meant for 35mm format cameras (FX Nikon, EF Canon, FE Sony lenses); they cover the full width and height of a full-frame camera, and of course, the lower size formats.

Since the circular image (in the above animation) is not fully utilized by the smaller sensors, manufacturers began to produce such smaller lenses for APSC formats, so that they cover the full width and height of an APSC sensor, but are too small to fill a full-frame sensor. They are meant for APSC cameras, and cost lesser, size reduced. Nikon call them DX, Canon name them as EF-S & EF-M. Sony call them as E mount. They are, generally, not suitable for full-frame sensors.

Manufacturers of micro 43 rd cameras are also doing the same thing. They are manufacturing lenses exclusively for micro 43 rd sensors, they won't work on APSC or full-frame sensors normally.

If I tell you that I have a micro 43rd 14-42mm lens, you should be able to imagine that the lens will get the field of view of 28mm to 84mm (as seen by a 28-84mm FX lens). I cannot shoot a photograph of true 14mm at the wide end with this lens. Thus the crop-factor of 2 helped us to calculate the equivalent focal-length.

As stated earlier, a 300mm lens attached to a micro 43 rd camera will produce field of view of 600mm. So why should we buy a costly and heavy 600mm FX lens and a full-frame camera? Things are not so easy here. With a 300mm on a micro 43 rd camera, we actually get a cropped view of an image produced by a 300mm FX lens on FX camera. There will be no actual optical zooming to 600mm on the micro 43 rd. The actual focal-length remains the same as 300mm. This cropping can be done in post-processing too; and resolution of the sensor also plays a major role in such cropping.

learnasyoulike.com Learn Visually

Want to leave a message for me?