Understanding DSLR Lenses | Focal Length, Aperture, Full Frame vs Crop

If you’re learning about DSLR photography or filmmaking, i’m sure you’ve come across a lot of information during your research. One of the more elusive things to understanding DSLRs, is figuring out the lenses that go with them! On the positive side, there are a lot to choose from and on the negative side—there are a lot to choose from.

Two questions I am asked all of the time is  “Which lens should I use?” and “What lens is compatible with my camera?” I think the best way to answer these questions is to break down the basics of DSLR lenses. Once you understand the basics, I think it will help you determine what lenses work best for your situation and for your camera.

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Understanding DSLR Lenses

Before we start talking about anything as it relates to DSLR lenses, We need to understand 3 basic things. Full frame versus crop sensor, focal length, and aperture. The focal length and aperture information is always present within the lens title. Whether the lens is designed to work with full frame or crop sensor cameras is sometimes not listed depending on where you look, so it may require more research.

Once you understand these three concepts, you will understand how to shop for lenses better.

Full Frame Versus Crop Sensor

When it comes to DSLR cameras, there are full frame cameras and crop sensor cameras. These concepts are related to the sensor size of the camera. Full frame cameras have  a sensor that is approximately 36 x 24mm. It varies slightly depending on the brand, but any DSLR camera with a sensor size around 36 x 24mm is considered full frame.

Crop sensor cameras have significantly smaller sensors than full frame, and they range from approximately 22.3 x 14.9 mm depending on the brand. Because of this smaller sensor they have a development known as “crop factor”. Crop factor relates to focal length, more on that in the next section.

For now, we want to talk about what DSLR lenses are compatible based on full frame/crop sensor. Full frame DSLR cameras are compatible with only full frame lenses in most cases. Crop sensor cameras are compatible with both full frame and crop sensor lenses.

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I believe, in an effort to make more cost effective lenses, companies started making DSLR lenses that are designed to only work with crop sensor cameras. The easiest way to find out if a lens is designed to work with crop sensor cameras is to look at the lens title or description.

Each brand has a specific “lingo” with letters and abbreviations. I have a post coming up soon that will break down all of that information in more detail so be on the look out for that but for now i’ll give you a quick run down.

  • For Canon,  EF-S is the abbreviation for crop sensor. EF is the abbreviation for full frame, but they are compatible with both.
  • For Nikon and Tokina, DX is the abbreviation for crop sensor FX is the abbreviation for full frame, but they are compatible with both.
  • For Sigma, DC is the is the abbreviation for crop sensor. DG is the abbreviation for full frame, but they are compatible with both.
  • For Tamron, Di-II is the abbreviation for crop sensor. DI is the abbreviation for full frame, but they are compatible with both.

Sony is an exception to the rule. They have lenses that are designed to work with crop sensor cameras, but can still be used on full frame cameras. You will just have to change a setting in your full frame Sony camera to avoid vignetting (an undesirable distortion to the image). There will also be an adjustment to the “true” focal length. See below.

I’ve made an info graphic below to give you an idea of which lenses are designed to work with crop sensor cameras and full frame cameras.

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Focal Length

The definition of focal length is the distance between the center of a lens and it’s focus. It tells us (via millimeter) how much of a frame is in view. It is the basic identification of a lens and can be easily found in the lens title because it is a range of numbers or a single number followed by mm. So you will see a 24-70mm, or 35mm. That is the focal length.

If you are interested, Nikon has a really technical post about focal length. But if your eyes are glossing over, you can continue reading to learn the more practical uses!

I mentioned in the last section that crop sensor cameras have what is known as a “crop factor”. This means that any lens you put onto a crop sensor camera, will be magnified by a certain number.

The “magnification number” will vary from brand to brand (as usual), but to give you an idea, Canon crop sensor cameras have a crop factor is 1.6x. Sony crop sensor cameras have a crop factor is 1.5x.

In order to determine the “true” focal length on a crop sensor camera, you will have to take the numbers before mm and multiply it by the crop factor applicable to you. So a Canon 24-70mm lens will be more like 38.4 – 112mm after the crop factor.  This inevitably means that if you are trying to shoot wide on a crop sensor camera, you will need to compensate by finding a lens is wider than usual.

If you are just starting out and looking for a lens, I suggest going with what I call and “all-purpose” lens. This is a lens ranging from 24-100 on full frame and 18-85 on crop sensor. Lenses that fall within this zoom range will give you the most bang for your buck and will cover you in most scenarios. I would also recommend avoiding “kit” lenses and opting for a professional grade lens with an aperture of at least 2.8 or less if you can.

Kit lenses are generally offered in package deals with DSLR cameras. They are pretty cheap to purchase alone, and usually have high aperture ranges (more on aperture in the next section). The glass and build of these lenses usually isn’t as good as professional grade. And even though they have longer focal lengths (like the Canon EF-S  18-135mm for example) the quality of the image wont be as nice as a Canon EF 24-70mm.

I recommend looking for a lens that has the most zoom range possible while maintaining a lower aperture.


The technical definition of aperture is a space through which light passes in an optical or photographic instrument, especially the variable opening by which light enters a camera.

Basically, it’s how much light is coming through the lens. Every lens has an aperture range or a stagnant aperture that can be set at any focal length. You can find it in the lens title easily by looking at the number behind f/. So in Canon lenses, you will see 24-70mm f/2.8 or Canon 18-135 f/3.5-5.6. The 2.8 and 3.4-5.6 is the aperture.

When you see a range of numbers, like 3.5-5.6, it means that when you zoom in the aperture will automatically drop. Once the aperture has been lowered you will not have the ability to change it. So if you are shooting a wide shot at a focal length of 18, the lowest aperture you can have is 3.5. But if you decide to zoom in to 135mm, it will automatically change the aperture to 5.6. That’s another reason why I recommend going for a higher quality lens over the kit lenses if you can help it. If you are going to use a kit lens, you’ll just need to make sure you have tons of light to compensate for the aperture limitations.

In terms of aperture, the lower the number, the more light will come through. The higher the number, the less light will come through. In the last section, I mentioned that if you are looking for a starter lens, you should look for one that has the most zoom range possible while maintaining a lower aperture. This is because lower aperture will give you the ability to produce stronger bokeh , and typically these lenses are better quality all around.

In Closing

I hope this article helps you decipher the crazy world of DSLR lenses. I have another post coming soon that will break down the lens titles in more detail. I’ll be discussing the abbreviations and lens “lingo”. Be on the look out for that!

Do you have a favorite lens that you use? I’d love to hear your feedback in the comments and if you have any questions sound off  below!

Olivia J

Olivia J is a video producer, writer, and director specializing in corporate video, events, documentaries and film. To see some of her work visit oliviaj.me