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A friend of mine was asking me advice on which lens she should buy the other day and my standard reply was ‘get a 50mm 1.8, unless you’re shooting on Nikon, in which you should 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. So 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 something which is common with most digital SLR cameras these days as they use smaller sensors than much more expensive cameras. Professional cameras have a sensor the same size as a 35mm piece of film, so 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 and put the price of cameras up. To fix this, camera manufacturers started building new cameras with smaller sensors, which would take the old lenses, as well as new lenses designed to mimic the same focal length. When you put a lens meant for a full frame camera, onto a body with a crop sensor, it 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 parts out, which would crop 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, and on the left, you can see the projection of that image as if it were projected onto a crop sensor – a lot of the image is missed 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 it crops out part of the image.

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, and 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, but for the sake of this diagram, know that it’s not to scale.

Step 2 – How Much Does it Crop?

There are two main sizes from crop (APS-C) sensors; those used by Nikon, Sony and Pentax, and the one that’s used by Canon. The difference being that Canon’s is slightly smaller and magnifies the image by 1.6, rather then 1.5. What this means is that if you put a 50mm lens designed for full frame cameras onto a crop sensor body, the focal length would effectively be 80mm, whereas it would be 75mm, on the other sensor. These magnifications 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 for less that $2000? If the answer is yes, then 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, so if you walked into a store and paid less than $2000 for it, then 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 all designed to work with your camera body, by providing a smaller lens projection, so 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 on the camera 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, so buy carefully.

Step 4 – Why Buy Full Frame Lenses?

If you only ever bought a prime lens, then 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, then you need to use full body lenses. These lenses have been around way longer than digital cameras even become popular and provide us with speed, and accuracy that we 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, because it will appear to be much more natural.

Some people are worried that the perspective may be different as the perspective appears compressed at longer focal lengths, but this isn’t something you need to worry about as it’s not the lens that compresses the image, it’s 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 that’s 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 to people, 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 other alternative, 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, whether you’re going to want to upgrade the body soon, or shoot really wide angles, and then choose the lens that’s right for you.

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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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