Understanding lenses – focal length vs F stop

Lesson 3 – basics of photography for beginners

In this tutorial we will try to understand the lenses, all relevant terminology in relation to them and explain the relationship  focal length vs f stop

In the Lesson no 2 –  understanding how to camera works and the basics of exposure, we have covered how the light enters the camera and how the image is captured. I would like to explore in more depth today what happens before the light goes into the camera and how the lenses are handling this task.

For a beginner all this talk about focal length, millimetres, F-stops, numbers and letters might be a little daunting at the beginning so I will try to make this as easy as possible, as it is very important to familiarise yourself with this terminology in order to learn photography.

I am going to define the role of the lens, few terminology explanations, few category and classifications of lenses, examples and a conclusion for you to take away and practice

Lens definition and specifications

Lens – is the optical device responsible for projecting the image in front of the camera to the sensor. The lens is defined by focal length , usually  expressed in millimetres, and the F- stop, or aperture, expressed usually from 1.2 to 22.

before we explain focal length we need to give you a 1 minute crash course in optics. The refraction is the phenomenon when the light changes direction when goes through a different transparent material, such as air – water or air to glass and vicevers. If the surface is not flat but curved, the light will be directed depending on the angle of the light. A lens is a designed glass element that has the ability to converge the light into on point.

The focal length defines the distance between the lens and the focal point.

The question is why is this relevant? well this focal length reflects basically how wide a lens is. I do not wish to get more technical than this as it might take a long time to go through the optics and that is not the purpose of my tutorial. Let’s just remember the following rule.

Small focal length reflects to wide angles, and that means you can capture more, a bigger image, a wider image. Big focal length means a narrow image, means a smaller picture.

Example – the widest lens is the fish eye which has the focal length of 8 mm and that means the angle of view is almost 180 degrees, that means you can see objects on the side that your normal eye cannot see unless you turn your head. usually the widest images come with optical deformation which can or cannot serve your intent

The narrow lenses start about 400 – 1200 mm, that means their focal length is 40 cm to 1 meter 20 away from the lens, and that means the image is very narrow. These are the telephoto lenses and their purpose is to capture images with the subject far away.

You do not need to go deeper than this as it might be daunting for people who did not study the optics, but let’s stick a the simple rule:

smaller numer – wide image

bigger number – narrow image

There is a wide variety of lenses out there, with variable focal lengths, called zoom lenses and they can go between various naumbers. You can play with the lenses and you should be able to learn all of this by associations without going into physics, but in time you will understand by intuition all of this focal length relationship.

F stop – is defined by aperture size, diameter of the whole of the lens

F stop is expressed in F numbers starting with 1.2 for the biggest aperture to 22 as the smallest aperture for a normal lens. There might be other lenses outside this range but it is irrelevant for our discussion. They F stop is basically how smaller the hole in comparison with one inch.

see here the technical description behind the f-stop

Just to make things simple, please remember one thing, F 1.2 is the biggest, and F22 is the smallest.

Going back to the previous lessons, large aperture = more light requiring faster shutter and that is why people call them fast lenses , small aperture = less light – need more time for exposure means slow shutter speed = slow lens. Lenses are not fast or slow as they do not move back and forth, this is just a naming convention for easy use of terminology

I think this is enough technical information. Once you get used to these numbers after spending a bot if time playing to whatever lens you might have, it will all come natural as a habit and you will know what this lens does what.

So in conclusion, we have explained briefly the relation ship focal length vs f stop,  please do not hesitate to send us any questions or queries and if you are a beginner, following the tutorials in order to help you become a competent photographer in a very short period of time

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