If a crop sensor is used, it will look like the focal length is much longer. Photographers that need long telephoto lenses can benefit from this. But instead of using a crop sensor, it is also possible to crop a full frame sensor to have the same field of view. Let’s take a look at the differences.

It isn’t really that difficult. A small sensor will record only a part of the projected image of the lens you are using. A bigger sensor will record a larger part of that same projected image. A full frame sensor with the dimensions of 24 x 36 mm will have a larger area compared to a 1.5x crop sensor that measures 23 x 15 mm. But when the recorded image is viewed at the same size on a screen, the image of the crop sensor will result in a magnification of 1.5 times that of the full frame sensor.

This is why a lens on a camera with a sensor that is smaller than a full frame sensor will act as if it has a longer focal length. A 24mm the lens will act like a 36mm lens, a 50mm lens will become a 75mm lens, and a 400mm lens will imitate a 600mm lens. This is when the sensor has a 1.5x crop. If a micro 4/3 sensor is used, with a crop factor of 2x, the focal lengths will be 50mm, 100mm, and 800mm compared to its full frame cousin.

Use Crop Sensors When You Need a Long Focal Length

If you need long lenses for your photography, a crop sensor seems to be ideal. Because of its smaller sensor, the lenses will appear to be much longer. There are even cameras, like the Nikon Coolpix P1000, with such small sensors that its lens will act like a 3,000mm full frame equivalent. Imagine what a 3,000mm lens on a full frame would look like.

But let up ignore these superzooms with fixed lenses and look at the lens interchangeable cameras like the DSLR and its mirrorless siblings. On a 1.5x crop camera, a 400mm lens will act like a 600mm lens in a 400mm package. Put the lens on a 2x crop camera, like the micro 4/3, and it will be an 800mm lens in a 400mm package. It has a huge benefit due to its size and weight and how easily it can fit into a camera bag.

Crop a Full Frame Sensor to Imitate a Long Focal Length

Since a crop sensor only records a part of the projection circle, it is also possible to imitate this by cropping the image of a full frame sensor. You can end up with the same image as from a crop sensor. This way, we can enlarge our subject the same way compared to a crop sensor but in the post-processing part of the workflow.

Here is the catch. When cropping a full frame image to have a larger magnification, we throw away resolution. We lose pixels. If you want to crop 1.5x to imitate the image from a smaller sensor, you lose between 30% and 40% percent of the resolution. With the high pixel counts of modern sensors, that doesn’t have to be a problem whatsoever. You still end up with enough pixels to make large prints, if necessary.

Is a Crop From a Full Frame Sensor Better Concerning Resolution or Not?

Most crop sensors have resolutions that are somewhere between 18 million and 24 million pixels. Full frame sensors have somewhere between 24 million and 30 million pixels. Newer cameras, both crop and full frame, can have even a higher resolution.

Let’s do a little math. I am going to compare a 20-megapixel Canon EOS 7D Mark II with a 30-megapixel Canon EOS 5D Mark IV as an example. For the Canon EOS 7D Mark II, the surface area is 336 square millimeters. The pixel density is 59,500 pixels per square millimeter. The Canon EOS 5D has a surface area of 864 square millimeters, resulting in a pixel density of 34,800 pixels per square millimeter.

If we use a 400mm lens on both a Canon EOS 7D Mark II and a Canon EOS 5D Mark IV, and we crop the image of the latter, resulting in the same focal length or subject magnification if you will, the Canon EOS 7D Mark II has a much higher resolution. Therefore, the crop camera would record much more detail compared to the full frame camera with a post-processing crop.

What About Image Quality?

At first sight, you would think a photo from a crop sensor will show much more detail because of the higher pixel count. But there is a major downside to a lot of pixels per square millimeter. It can result in higher noise levels, especially when the ISO level is cranked up. A high noise level will result in a loss in detail. 

Which Is the Better Choice?

It all comes down to this question: what is the best thing to do? Should you choose a crop camera to benefit from the gain in focal length, or should you go for the full frame camera and use a post-processing crop?

Before I answer this question, you have to take the high resolution of full frame sensors into account. I took my Canon EOS 5D Mark IV as an example, but if you have a Sony A7R IV or a Nikon Z 7, the number of pixels per square millimeter will reach up to the same levels as the Canon EOS 7D Mark II. On the other hand, the increased resolution of the new Canon EOS 90D balances the differences again. You could calculate the difference yourself if you want.

From that point of view, I would definitely recommend choosing a full frame sensor and cropping the image yourself. It will also give the flexibility of using the large field of view when using wide-angle lenses and the possibility to play with a nice shallow depth of field more easily. 

From the other point of view, the smaller sensors will allow smaller cameras and lenses compared with the full frame. The micro 4/3 sensor, like in the Panasonic Lumix DC-G9, makes it so much easier to travel with long focal length lenses. And I guess that can be a real benefit for a lot of photographers. And regarding the increased noise levels compared to full frame sensors, I wouldn’t worry about that too much, unless perhaps when you need the highest ISO levels possible.

If you had decided at this moment between a crop sensor or to crop a full frame image, what would you choose and why? I would love to read your experience and opinion on this subject. I think it would also be a great help for those photographers that have to make a choice in the near future.