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.
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.
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,
● APSC sensors (23.6mm×15.7mm),
● MICRO 4⁄3 rd sensors (17.3mm×13mm),
● One inch sensors (13.2mm×8.8mm)
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-Type
Diagonal length (in mm)
Crop-factor
One-inch
BD = √(BC² + CD²) = 15.86
43.27 : 15.86 = 2.73 ≅ 2.7
Micro 4⁄3 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:
One inch sensor is 2.7 times smaller than full-frame
Micro 4⁄3 rd is 2 times smaller than full-frame
APSC is 1.5 times smaller than full-frame
Full-frame is the ideal size; its crop-factor is 1
Medium format (44mm×33mm) is larger; its crop-factor is 0.79
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 4⁄3 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 4⁄3 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.
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 4⁄3 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 4⁄3 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.
f/9, 1⁄160 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 4⁄3 rd camera, we get a
field of view of a 24mm × 2 = 48mm.
(crop-factor of a micro 4⁄3 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 4⁄3 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 4⁄3 rd cameras are also doing the same
thing. They are manufacturing lenses exclusively for micro 4⁄3 rd
sensors, they won't work on APSC or full-frame sensors normally.
If I tell you that I have a micro 4⁄3rd 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 4⁄3 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 4⁄3 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
4⁄3 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.
