Hi, name’s Ladi. I’m a photographer and an art lover. I noticed a lot of people struggle with two main topics as photographers; difference between cropped and full frame sensors & what constitutes a fast lens.
Some photographers are often confused on these topic and others are oblivious to such.
Firstly, the camera sensor has multiple sizes. From the sensors in camera phones that are very tiny to the so called 35mm sensor often referred to as full frame sensor.
So, basically a full frame sensor has the same size as a 35mm film i.e. 36mm by 24mm. So, when a lens is used on a full frame digital sensor it will have the same perspective if it was used on a 35mm film camera i.e. analog camera. Now, a cropped frame is relatively smaller, this means that the image produced by the same lens on a cropped frame will be different from that made on an analog camera. The difference in perspective is often described with its crop factor. The crop factor of any camera can easily be found online and we don’t need to delve into the mathematics here. Basically the crop factor of a camera/sensor is how much bigger a full frame camera is. For example, the Canon 60D has a crop factor of 1.6 and a Canon 5D is a full frame, therefore the 5D‘s sensor is 1.6 times bigger than that of the 60D.
This crop factor affects two main qualities that concerns the average photographer. Focal length and depth of field.
The crop factor affects the focal length in a way that gives the cropped frame an advantage. An image is sharpest at its centre and dullest at the edges. The cropped frame chops off most of this edge and leaves a sharper but smaller image. For example a 50mm lens on a cropped frame will give you an image taken with an 80mm lens on a full frame (50mm x1.6 = 80mm). Meaning you will have to move closer to your object when using the same 50mm on a full frame to get the same image size you will get on a cropped frame.
This translates to the depth of field, where the full frame produces a shallower depth of field i.e. gives more blurred out backgrounds compared to the same lens attached to a cropped frame camera. So in portraits, full frame has the advantage of blurred backgrounds but cropped frames produce sharper landscape pictures. Because the cropped frame has a larger depth of field its landscape pictures will be relatively sharper.
For in depth analysis on this topic you can refer to this web article; http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm
Now to fast lenses. If you have noticed a lens with the following 1:3.5-5.6 or of a similar range. It means the maximum aperture of that lens varies with the focal length. The above example is for an 18-55mm zoom lens, it means at 18mm the aperture can open up to F3.5 and at 55mm to a maximum of F5.6. This means that as the focal length increases the shutter speed has to drop to achieve the same brightness/light intensity. So a fast lens usually has the same maximum aperture regardless of a change in focal length. For example 1:2.8 24-70mm, means that what ever focal length the lens is at between 24mm to 70mm you can still use the same shutter speed to achieve the brightness F2.4 will produce. Also, for the 18-55mm, the sharpness of the image drops with increase of focal length while that of the 24-70mm remains constant. So therefore a fast lens with a fixed maximum aperture is often recommended but they come at a heavier price tag.