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A friend of mine was asking for advice on which lens she should buy the other day, to which I gave my standard reply of, “get a 50mm 1.8, unless you’re shooting on Nikon, in which case get a 35mm as the angle will work better on your crop sensor”, to which she replied, “what’s a crop sensor?”

The lack of knowledge about the type of camera surprised me but, looking back, I remember when I didn’t know either. This post is about setting it straight and helping you to make the right choice when it comes to buying a lens.


Step 1 – What is the Crop Factor?

The crop factor is common to most digital SLR cameras these days as they use smaller sensors than the more expensive cameras.

Professional cameras have a sensor the same size as a 35mm piece of film; any old lenses which were used on film bodies, using the same mount, would still work on digital SLRs.

The problem with this is that the technology was big and expensive, putting the price of cameras up.

To fix this, camera manufacturers started building new cameras with smaller sensors that would take the old lenses, as well as new lenses designed to mimic the same focal length.

Putting a lens meant for a full frame camera onto a body with a crop sensor produces the crop factor.

It’s called the crop factor because you’re effectively cropping the image. Imagine you’ve printed a photo out but, suddenly, the paper is half the size; you have to cut bits out, cropping the image.

This probably isn’t the world’s best analogy, so have a look at the diagram below (it was taken from my post on focal length so there may be some information on there that you’re not familiar with).

In both images, the lens is meant for a full frame camera. On the left, you can see the projection of that image as though it were projected onto a crop sensor – a lot of the image is missing at the sides.

The image on the right shows you what this is effectively doing – it’s taking a regular full frame sensor and bringing it closer to the lens so that part of the image is cropped out.

As you can see, you’re losing a lot of the image when you use a full frame lens; lets look at it from another angle.

A circular lens produces a circular image, then the sensor crops this depending on its size. Full frame sensors are all the same size, whereas crop sensor sizes tend to vary between manufacturers. Tor the sake of this diagram, know that it’s not to scale.

Step 2 – How Much Does it Crop?

There are two main sizes of crop (APS-C) sensors: those used by Nikon, Sony and Pentax, and the one that’s used by Canon.

The main difference is that Canon’s is slightly smaller and magnifies the image by 1.6 rather than 1.5.

What this means is that, if you put a 50mm lens designed for a full frame camera onto a crop sensor body, the focal length would effectively be 80mm, rather than 75mm.

These magnification differences don’t make a big difference at this end of the scale as the lenses are designed accordingly. All you need to know is which sensor you’re using so that you can work out the crop factor when you want to buy a full frame lens.

Step 3 – How do I Know if I’m using a Crop Sensor?

This is pretty easy to answer, simply ask yourself one question:

If you look for your camera body brand new at the moment, can you buy it for less that $2000?

If the answer is yes, you’re using a crop sensor. Full frame cameras are expensive and aren’t that common when you compare them to amount of crop cameras around. If you walked into a store and paid less than $2000 for it, you’re shooting on a crop sensor.

Another great way of telling is to look at the kit lens that came with your camera.

These lenses are designed to work with your camera body by providing a smaller lens projection; their focal lengths will be shorter than a full frame camera – staring at 18mm, rather than 24mm.

There will also be your camera brand’s marking to indicate which body it was made for.

Here are the most popular brands:

  • Canon — EF-S
  • Nikon — DX
  • Sony — DT
  • Pentax — DA
  • Sigma — DC
  • Tamron — Di-II

If, one day, you decide to upgrade to a full frame camera, these lenses will no longer work; buy carefully.

Step 4 – Why Buy Full Frame Lenses?

If you have only ever bought a prime lens, you’ll know why.

The lenses made for professional, full frame cameras are usually a lot better quality. You can still buy high quality lenses for crop sensor cameras but, if you really want to use the best lenses out there, you need to use full body lenses.

These lenses have been around long before digital cameras became popular and provide us with the speed and accuracy you would expect for the price.

Prime lenses in particular are really only made for full frame cameras.

50mm is a great focal length when you’re using a full frame camera but that’s not so good on a crop sensor because it’s unnaturally zoomed. If you’re a Nikon user and want to buy a f/1.8 lens, I would always recommend the 35mm over the 50mm for a much more natural appearance.

Some people are worried that the perspective may change, as perspective appears compressed at longer focal lengths but this isn’t something you need to worry about; it’s not the lens that compresses the image but the distance from the subject.

That being said, when you get into much wider angles, the barrel distortion at the edges is much more significant so, if you crop this out, it won’t change the perspective but you will notice the difference.

Step 5 -Buying The Right Lens For You

The first thing worth mentioning here is a realistic warning: if you’re going to replace your kit lens and you’re not planning on upgrading your body for a few years, buy a crop sensor lens.

The field of view that you lose in the shorter focal lengths is quite big – have a look at the photos below for comparison.

The first photo was taken at 18mm on a crop sensor and the second was shot at 24mm on a crop sensor. If you shoot wide often, or even at all, you’re going to want to keep the extra few millimeters.

I would thoroughly recommend prime lenses no matter which body they’re using. Just have a look at my gear post for some recommendations.

They’re sharper and better quality for the price than any alternatives such as an all-in-one wide angle to telephoto.

My best piece of advice is to consider carefully what you want to do with your photography – are you’re going to want to upgrade the body soon or shoot really wide angles? Then you’ll be able to choose the lens that’s right for you.

5 Steps To Understanding The Crop Factor

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I'm a self taught photographer from Brighton, England. I take a lot of photos and enjoy teaching my methods to anyone willing to learn- this is my blog, check out my video training & Google.

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