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Full Frame vs APS-C Crop Sensor
There is a great deal of confusion when it comes to comparing crop sensor camera lenses to full frame camera lenses. Often the specification of a crop sensor lens may be stated as 10 mm with a comparable 35 mm focal length of 15 mm. This may give the user the impression that they will have a 15 mm focal length. This is incorrect. The focal length doesn't change. Only the field of view changes. Therefore, it should be stated that the field of view is the same as a 15 mm focal length lens.
A full frame camera projects an image onto a sensor or film that is 36 mm x 24 mm in size. An APS-C camera projects an image that is 23.5 x 15.6 mm in size. An APS-C sensor costs less than a full frame sensor. APS-C lenses are smaller and lighter in weight. Larger sensors are often used for larger size prints.
What is of primary concern is the field of view that is produced by these two sensors. As seen in the diagram below, the field of view that is seen by the full frame sensor is wider than the field of view seen by the APS-C or "crop" sensor. The focal length remains the same, which is often confused. The only aspect that changes is the field of view, not the focal length.
Lens and the images projected onto a sensor are round in shape, whereas the sensors are rectangular in shape. The diameter of the circle needs to be larger than the diagonal of the rectangular sensor. A full frame 35 mm lens must have an image circle larger than 43.27 mm. An APS-C camera lens needs to have an image circle larger than 27.04 mm. If an APS-C lens is used on a full frame camera, the image circle would not be large enough to cover the corners of the sensor. If a full frame camera lens is used on an APS-C camera, it will cover the corners of the APS-C sensor.
What makes this concept confusing is that when photos are added to a photo software program, they are often enlarged by the software to a specific size. Most people don't pay attention to the enlargement size in the software. Photos are then presented online using the same frame size, which gives the appearance that they are enlarged. But, the photos are not enlarged from the camera, only from the photo software. In this article, the actual photos are presented in actual size format.
When a DX lens specification says, for example, a 100 mm DX lens is equivalent to a 150 mm FX lens, it is referring to the field of view, not the magnification. It should be stated as the field of view of a 100 mm DX lens is equivalent to the field of view of a 150 mm FX lens.
To obtain the field of view obtained with the FX camera and 70 mm lens, using a DX camera, change the lens (or zoom out) from 70 mm to 46.6 mm. In order words, a photo taken with a DX camera and 46.6 mm lens would look exactly the same size as the photo taken with an 70 mm FX camera lens. The resolution of an FX camera will be greater than that of a DX camera because the full frame sensor is larger and contains more pixels per size. This is important when printing larger prints. It probably won't make much difference when displaying photos on the web or printing smaller sizes such as 8" x 10".
Camera resolution between the two is usually about the same with the same lenses. See Resolution for a comparison.
Why buy an FX camera?
For printing very large prints or for larger focal length telephoto lenses. As of June 2018, the largest DX lens made is 400 mm whereas FX lenses reach 800 mm. Some FX lenses will fit onto a DX body. However, this will often cause vignetting.
The APS-C (crop) sensor is smaller and less expensive to produce.
The focal length remains the same. Only the angle of view changes.
One can use a full frame lens on a crop sensor camera. But, not a crop sensor lens on a full frame sensor camera.
Actual Size: Full Frame Sensor vs Crop (APS-C) Sensor. The same lens was used on both cameras. Notice that the tree and other things are the same size in both photographs. Only the field of view is different.