I hear a lot of questions related to lenses and sensor size. What is the difference between Full Frame and APS-C (a.k.a. Crop Frame)? Can I use Full Frame lenses on APS-C sensors or vice-versa? If so, how does that affect my image? Is one size better than the other?  I’ve also heard and read some answers to these questions that either aren’t exactly accurate or are way too technical for most enthusiasts to enjoy. In this post I aim to simply answer these questions without getting too deep in the technical weeds.

The difference between Full Frame and APS-C

First off, what is this all about? It’s about the physical size of the sensor inside the camera. The sensor is what actually captures the light to create your image. We’re going to focus on the two most common DSLR and mirrorless sensor sizes: Full Frame and APS-C. Full frame is the same size as common film and APS-C is a third smaller. There are larger and smaller sensors sizes, but these are the two we will focus on.

Think of it this way. All of our lenses are round. They project a circular image onto your camera’s sensor. Lenses made to cover a Full Frame sensor are designed to project a bigger circle than those made to cover a smaller APS-C sensor. Because of this, Full Frame cameras and lenses tend to be larger and more expensive than APS-C cameras and lenses with the same megapixel count and focal length.
Why do more pros use Full Frame bodies? There are two reasons really. First, the larger sensors tend to produce images with less noise in low light situations. I won’t get into the technical reasons for that. It’s just true. Secondly, most pro photographers are not happy with having APS-C sensors crop into wide angle lens projections. We’ll revisit this problem in a moment.

Using Full Frame lenses on APS-C sensors:

Yes, you can absolutely use Full Frame lenses on APS-C cameras. In fact I regularly choose to do this in situations where I want to fill the frame with a distant subject. Why? The APS-C sensor crops the middle out of a Full Frame lens’s projection. Think of it like bringing an image you captured on a Full Frame camera home and then cropping out 1/3rd of it. The difference is that I’m not cropping away any pixels from the image. There is no loss of resolution. By placing my 400mm Full Frame prime lens on an APS-C body I get the same field of view as if I used a 600mm lens on Full Frame (400 x 1.5 = 600). This made it much easier for me to fill the frame with the wild condor below.  Going birding or on Safari? Think about bringing an APS-C body to use with your biggest lens.

All that extra reach using long lenses is wonderful, but using Wide Angle lenses on APS-C cameras is extremely frustrating. We photographers buy expensive wide angle lenses to project more of the world into our images at the same time. Cropping in to those projections defeats the purpose.

Look at the image above of this doorway in Istanbul. I captured this on a Full Frame sensor with a 14mm 2.8 lens. I could not have captured this view with a 14mm lens on an APS-C sensor. To capture the same field of view with APS-C would require about a 9mm lens projection (9 x 1.5 = 14.5). It’s possible for manufacturers to produce 9mm lenses that cover APS-C sensors, but the physics of a 9mm projection are very different from a 14mm projection. Objects in the distance will appear much smaller in relation to the foreground on the 9mm. This is the biggest reason that my main camera body has a Full Frame sensor. I love working with wide angle lenses.

Lets look at the opposite scenario. Can you mount an APS-C specific lens on a Full Frame body? The answer is usually yes, but… Lets put it this way. Just because you can, doesn’t mean you should. APS-C specific lenses are smaller and cheaper, but they don’t project a wide enough image circle to fully cover a full frame sensor. Many Full Frame cameras have a “crop-mode” that is automatically activated when you mount an APS-C lens on them. The camera essentially shuts down the outer 1/3 of it’s sensor and records an APS-C sized area that the lens is designed to cover. That sounds fine, but you are turning off a lot of resolution in this mode. For example my 36-megapixel Full Frame Nikon D810 drops to 15.4-megapixels in it’s APS-C mode (a.k.a. DX mode).
My choice as a wide angle junkie is to use Full Frame systems and occasionally use an APS-C body with my longer lenses when I want some extra reach. This gives me maximum flexibility and quality.

Mt Meru and it’s shadow at 14mm on Full Frame

Popular Misconception:

I often hear people mistakenly state that placing a Full Frame lens on an APS-C sensor (or vice-versa) will somehow affect the physics of the lens projection. Not so. Lenses simply project image circles. My Full Frame 50mm f1.8 Nikon lens has the same projection circle no matter what sensor or film it is mounted in front of. The aperture, depth of field and light gathered is unchanged. The 50mm perspective is unchanged, meaning that foreground and background objects will appear to be the same size as compared to one another in the image. The only difference that matters is that the APS-C sensor will crop into the middle of the frame and record a smaller rectangle from the center of the projection.

Conclusion:

I hope this helps demystify the choice between Full Frame and APS-C. Full Frame has performance advantages in low light and wide angle situations, and Full Frame lenses work well on APS-C cameras. APS-C tends to be smaller, lighter and cheaper than Full Frame and helps you fill the frame with distant subjects, but APS-C lenses don’t play well with Full Frame cameras.
If you have any questions about the topic feel free to leave them in the comments or use the Ask Hudson button.