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Tips on How to Purchase a Lens

After the joy of adjusting your kit lens back and forth has worn off (let’s be honest, the joy never fades) maybe it’s time we start learning more about how important these glass cylinders are in photography. In this post I share my initial exploration on the subject of camera lenses whilst throwing in a few tips of how to start thinking about purchasing a lens. Hopefully this can help aid you in your own research into the world of camera lenses.

Types of lenses

There are two types of lenses:

  • Prime lenses – Single fixed focal length and one field of view. (The photographer has to physically move to get nearer or further from the subject)
  • Zoom lenses – Variable focal length, they can go from a wide field of view, to a narrow field of view. (The photographer can stay in the same position by using the lens’ zoom to get nearer or further from the subject)

Image sensor

The image sensor is located inside the camera. You are probably thinking, I thought this post was about lenses? Turns out, the lens’ focal length is affected depending on the size of the image sensor inside the camera.

  • Full Frame Camera – A full frame DSLR camera has an image sensor equivalent to 35mm film. Before digital photography, this was the standard size when printing images so that’s why it is considered the standard.
  • Cropped Camera – Most DSLR’s these days have image sensors that are smaller then 35mm film – meaning they have a cropped image sensor inside. In mechanical terms, DSLR’s with cropped image sensors physically crop a smaller rectangle out of the image taken by the camera.

Now for the interesting part – any lens you mount on a full frame camera will have the focal length specified on the lens – simple. Any lens you mount on a cropped camera will have the focal length specified on the lens multiplied by the image sensor crop factor.

Generally, cropped cameras by Canon have a crop factor of x 1.6 and Nikon have a crop factor of x 1.5. The appropriate multiplication factor will be specified in the camera’s description or instruction booklet which you are able to look up before shopping for lenses.

Example scenario:

  • Full Frame Camera – Image sensor is equivalent to 35mm film. A lens with a 50mm focal length will deliver a 50mm focal length.
  • Cropped Camera – Image sensor is smaller than 35mm film. It is a Canon camera so has a crop factor of x 1.6. A lens with a 50mm focal length x 1.6 = 80mm focal length.

With the above scenario, it can be useful to know the size of the image sensor inside the camera before purchasing lenses. If it turns out your camera has a cropped image sensor, the 50mm lens could turn out to be an 80mm telephoto lens.

Focal Length

Focal length is the measurement of the distance in millimetres from the middle of the lens to the point you are focused on. Focal length is not a measurement of the actual length of the lens body.

Lenses are split into different categories depending on their focal length:

  • Normal lenses have a field of view similar to that of the human eye. A 50mm Focal length lens is considered a Normal lens.
  • Telephoto lenses are longer than normal lenses. A 300mm Focal length is considered a Telephoto lens.
  • Wide angle lenses are shorter than normal lenses. A 28mm Focal length is considered a Wide angle lens.

Field of view

All lenses have a field of view which is determined by the focal length of the lens. Conceptually, when the focal length of a lens increases, the field of view becomes narrower meaning everything in view becomes magnified. As the focal length of a lens becomes shorter, the field of view becomes wider resulting in a broader scene.

Simply put:

  • When you zoom in you are narrowing the field of view and magnifying the scene.
  • When you zoom out you are widening the field of view and capturing more of the scene.

Here is a useful table of some examples to refer to when taking into account field of view and focal length. This is not an extensive range, just a few basic examples of how different focal lengths affect field of view.

Normal50mmMost accurate to the human field of view and supposedly good for portraiture
Telephoto85mmA narrow field of view and also supposedly good for shooting portraits
Wide28mmSuppose to be the best for landscape photography
Zoom35-55mmUsually a kit lens and has a variable field of view
Macro100mmSuppose to be best for shallow depth of field and therefore close up photography


The Aperture is the iris of the camera controlling how much light is passing through the lens. The smaller the number e.g. 1.2 the wider the aperture and the more light entering the lens. The bigger the number e.g. 16 the smaller the aperture and the less light entering the lens.

Every lens has an aperture range. Whether you want to create images with shallow depth of field or deep depth of field, your lens will determine your ability to do so.

  • Shallow Depth of Field – Subjects in the foreground are in focus and the background is out of focus (blurry).
  • Deep Depth of Field – Subjects in the foreground and background are sharp and in focus.

Each lens has a maximum aperture which is referred to as the lens’ speed. Having a fast lens means you can take advantage of wider apertures. An example of a fast lens is a 50mm with a maximum aperture of 1.4 – the lower the f stop number, the wider the aperture meaning the faster the lens. Having a fast lens can also allow you to use faster shutter speeds (especially useful for hand held shooting in low light situations) because of the ability to open the aperture wide enough to let light into the lens.


I like getting inspired by other photographer’s work. I scan cookbooks, magazines, blogs, stock photo websites and photographer’s online portfolios. I ask myself what I like about a particular image I have found. Is it the sense of space? Is it the bokeh? Is it the sharpness? This thought pattern has helped me to identify what I really want out of a lens. For example, maybe you just want to take super close up images of fruit therefore you might only need a macro lens – or maybe you want all of your images to have shallow depth of field, in which case you might need a fast lens to dial in a wide aperture.

Camera angles

I also like to think about camera angles, and by that I mean considering where I’m going to be standing when I’m taking photographs. Will I be directly over the top of a table or close up to the food? Maybe outside at food events? Maybe both! This has helped me to identify the focal length of the lens that’s best for my needs either Wide angle, Normal, Telephoto etc.

Second hand lenses

There are many retailers and individuals selling second hand or refurbished lenses on websites like Ebay – sometimes a lot cheaper than a brand new one. However, I’d recommend doing thorough research before making a purchase. Here are a few things I like to keep in mind when shopping online:

  • Seller – Check star ratings, previous customer reviews and the seller’s location.
  • Condition – Read the description carefully and ask any questions you feel need clarifying, such as when the lens was first purchased, any signs of mould etc.
  • Delivery cost – Check if it come in its original box and what delivery services are available e.g. next day, recorded etc.
  • Returns policy – It’s always good to know if they have a reliable returns policy and who is responsible for covering the cost of the return postage.


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