What is sensor equivalence?
There are a surprising number of different camera formats (sensor sizes) to choose from now compared to a few years ago. In order to compare various camera formats we typically use the standard 35mm film size (36 x 24mm) as our basis of comparison since that has been the standard for so many years. We know we will get a certain field-of-view (FOV) and depth-of-field (DOF) with various focal length lenses for this format so we use a lens based on these parameters to get the picture we want. But if you consider using a lens with the same focal length on a camera with a smaller sensor, the FOV and DOF parameters will be quite different. So, we make the comparison with an equivalence, like a sensor crop factor of 1.5 (or 1.6 depending on the camera) for an APS-C sensor. For example, we can say that a 100mm lens on an APS-C camera will give an equivalent FOV as a 150mm (or 160mm) lens on a full-frame camera. Similarly, if we want the same FOV as a 100mm lens on a full-frame camera, we know that the equivalent lens on an APS-C camera would have to be about 67mm.
If you are considering buying another camera with a different sensor size than the one you have, this concept of equivalence is useful since you will know what focal length lenses will give you a similar FOV and DOF as the camera you currently have. For example, the FOV and DOF of a 50mm f/2.8 lens full-frame camera will be the equivalent FOV and DOF as a 25 mm f/1.4 lens on a micro 4/3 camera. The table above should help illustrate this equivalence for a number of popular sensor sizes. The difference in DOF is often an important consideration for photographers interested in creating dreamy out of focus backgrounds. There are lenses made for micro 4/3 format cameras with apertures as large as f/0.95 in order to produce out of focus areas comparable to those obtained with full-frame cameras.
Keep in mind that loss of image quality due to diffraction is based on the exit pupil of the lens not the lens f-stop and this is why very small sensor cameras (with very small lenses) experience diffraction at much lower f-stops than large sensor cameras. Diffraction becomes an issue around f/2.0 to f/3.0 in camera-phones vs. f/16 to f/22 for full-frame cameras.
Note that some the equivalences shown in the table for the smallest sensors are theoretical since their design does not provide an equivalent focal length. The iPhone 5 camera, for example, has a fixed lens with a focal length of only 4.1mm so it is obvious that it will not provide the same FOV as a 50mm on a full-frame camera. If you know the focal length of the lens and the sensor crop factor, you can compute the equivalent focal length. So, for the iPhone it is 4.1mm x 7.1 = 29.1mm.
Cell phone cameras have made vast improvements in the last few years and you could argue that they are now competing with or surpassing some digital still cameras. Some claim the resolving power of these very small, but carefully engineered, molded, (usually) plastic lenses compare equally to or better than the best 35mm lenses. To be sure, cell phone cameras have a number of limitations due to their small sized sensors, but their portability can’t be beat and the pictures are quite good. A complete review of this camera-phone (and others) can be read at www.dpreview.com.
Finally, if you are thinking of lightening your load with a smaller camera, there are now many choices to consider. Remember, in terms of image quality, sensor size is the most important feature to consider and that higher megapixel count does not always equate to higher image quality. I hope you find the concept of equivalency helpful in comparing cameras with different sensor sizes and in selecting a camera to fit your needs. The choice is yours – bigger sensor, bigger weight and size. Smaller sensor – smaller weight and size.