Things are pretty chaotic in photography because you can choose from a plethora of sensor sizes and optics these days, and you can mount lenses made for any system in the middle of the vintage era on any other digital frame. This, in turn, leads to a lot of misunderstanding, especially when it comes to exactly what the angle of view of a lens is.

Digital, 35mm film was a reference format due to its mass adoption and popularity. If one used a 50mm lens on an SLR film camera, everyone knew exactly what it looked like in terms of field of view and the resulting image, so understanding and discussing different lenses and focal lengths was easy.

With the advent of smaller cameras that used a smaller sensor than 35mm film created a new problem – both field of view and captured images appeared narrower, because the corners of the image frame were getting “cropped”, or chopped off. To understand what happens in the camera with a smaller sensor, take a look at the below illustration:

Understanding camera sensor size and why it actually matters is one of the most important aspects of learning photography. We need to understand what each sensor is in terms of "equivalence".

I have used what is called a full frame camera for almost all of my images on this site. The camera sensor size is the most important factor in determining overall camera performance & image quality, given the optimal focus, f-stop, ISO, and shutter speed settings have already been obtained. All of this has an equivalence based upon sensor size. See also Maximum Aperture

Physical sensor sizes are shown in terms of width and height, usually in millimetres. A full frame standard sensor size is 36mm × 24 mm and is known as a full frame 35mm format camera.

The typical smartphone sensor is a 1/2.6″ to 1/2.5″ sensor, so about a 6x crop factor.

Let us assume you have an iPhone with a 4mm lens, and so, even at f/1.8, everything that’s basically not touching the front lens is in focus. Well, close. You will get a very deep depth field so it is hard to create BOKEH.

Similarly, the focal length multiplier relates the focal length of a lens used on a smaller format to a 35 mm lens producing an equivalent angle of view, and is equal to the crop factor. This means that a 50 mm lens used on a sensor with a 1.6X crop factor would produce the same field of view as a 1.6 x 50 = 80 mm lens on a 35 mm full frame sensor

So you can calculate this crop for yourself but the chart below gives some quick guidance for the mathematically challenged.

Whilst crop factors can get confusing for many photographers. The focal length of a lens is the physical property of a lens and it never changes irrespective of the camera sensor. So when you look at the above table, always keep in mind that the smaller sensor is not magically transforming your lens into a longer lens – it is just cropping a lot of the image.

Sensor Size vs Resolution

It is true that taking scissors and cutting the edges of the frame to yield a photo is similar to what a crop sensor does. However, there is one important factor that we should not forget about – sensor resolution. Since each digital camera sensor is comprised of millions of pixels, using a smaller sensor should translate to fewer pixels but it does not. If the sensor is made with physically smaller pixels, two sensors could potentially have the same resolution (in some cases, a crop sensor could actually have more pixels than a full-frame sensor).

For example, the Nikon D4 has 16 million pixels on its full-frame sensor measuring 36.0 x 23.9mm, while the Nikon D7000 also has 16 million pixels on its 23.6 x 15.6mm sensor. With such a drastic difference in sensor size but having the same number of pixels, the difference between the two is the physical size of each pixel. The Nikon D4s has much bigger pixels measuring 7.3µm, while the D7000 pixels are much smaller at 4.78µm, so those pixels are basically packed closer together. Since smaller pixels translate to more

Smaller sensor sizes do also have some advantages. Nearly all the lenses produced are sharpest at their centres, while quality degrades progressively toward to the edges. This means that a cropped sensor effectively discards the lowest quality portions of the image, which is quite useful when using low quality lenses (as these typically have the worst edge quality) and allows for cheaper lenses to be made for the camera. You will also get an in-focus image every time due to the DEPTH OF FIELD and there is no need therefore to focus. Of course one does lose the creative effects of these outer edge distortions, out of focus areas and the optical performance of wide angle lenses is rarely as good as longer focal length lenses.

Since a cropped sensor is forced to use a wider angle lens to produce the same angle of view as a larger sensor, this can degrade quality. Smaller sensors also enlarge the centre region of the lens more.

Let us explore this in a little more detail as on this sire we also have large format lenses used on a 35mm sensor. You maybe thinking why not make all lenses large?

If we explore the projected image on the sensor again.