What is Crop Factor?
Crop factor is a term used to describe the relationship between the focal length of the lens and the size of the camera's image sensor. It helps us to compare the equivalent or effective focal length between different cameras that use different sizes of image sensors
The crop factor of a DSLR comes into play when a lens designed for a 35mm film camera or a "Full Frame DSLR" is used on a DSLR with a smaller image sensor such as an APS-C camera. Because the image sensor in the camera is smaller it effectively “crops” the image as a smaller area is exposed. Therefore when the image is printed that area of the photo is enlarged more than had the same lens been used on a 35mm or Full Frame DSLR.
The "crop factor" of a lens and camera combination effectively increases the focal length of the lens resulting in greater magnification of the subject.
For example a 50mm lens on a DSLR with a 1.5x crop factor has a field of view “equivalent” to using a 75mm lens on a 35mm SLR or a Full Frame DSLR. Nothing has changed on the lens, the sensor is simply capturing a smaller area of what the lens actually “sees” which gives us an image that appears to be magnified more or taken with a longer telephoto lens.
Determining the “Equivalent Focal Length” of a DSLR and Lens combination is as simple as multiplying the focal range of the lens times the crop factor or "multiplication factor" of the camera as it is sometimes called.
The crop/multiplication factor is also important when considering Point and Shoot digital cameras as well as Super Zoom cameras. This is because the image sensors on those cameras are much smaller than a 35mm camera so their multiplication factor is much higher.
Why Crop Factor Is Important?
Understanding the "Equivalent Focal Length" of the camera is important when comparing the zoom capabilities of different cameras.
Many Point and Shoot and Super Zoom cameras list their zoom capabilities in terms of magnification, such as 3X, 5X, 10X, 15X, or 20X zoom, similar to the designations used on camcorders for years. While these numbers are helpful in quickly identifying the camera with the greatest range of focal length, they really do not do much for us in comparing the overall effective focal range of the camera. This is why many manufacturers now list the 35mm equivalent in their full specifications.
When it comes to comparing cameras and knowing how much zoom capabilities a camera really has it is important to compare the "35mm Equivalent" specification instead of simply looking at the amount of zoom it has.
Here is an example of three different Sony Camera models to show this relationship. In order to have the effective focal range of the Super Zoom Sony HX1 you would need an APS-C DSLR like the Alpha 100 with a 19mm to 373mm lens which no one currently makes. Since the Alpha 900 is a Full Frame camera it would require a 28mm to 560mm lens to equal the full focal range of the Sony HX1.
|Camera||Actual Focal Length of Lens||Crop Factor||35mm Equivalent Focal Length|
|Sony HX1||5.0mm to 100mm||5.6||28mm to 560mm|
|Sony Alpha 100||18mm to 250mm||1.5||27mm to 375mm|
|Sony Alpha 900||18mm to 250mm||1||18mm to 250mm|
Examples of what Crop Factor Does
The relationship of image sensor size and field of view is easily seen in this photo. The circular area represents the area seen through the lens of the camera.
The black rectangle represents the image captured by a 35mm Film camera or a Full Frame DSLR and the blue rectangle is the image that would be captured by the same lens on a APS-C DSLR with a crop factor of 1.5.
See how this results in the center area being magnified more which means the effective focal range of the lens has increased.
The difference in how the photos taken with the two different cameras would print out is represented by these two photos.
In order to get the same angle of view that you have with the Full Frame DSLR you would need to use less zoom on an APS-C camera.
This can be helpful if you are needing to have the maximum zoom capabilities but at the same time would limit your ability to get really wide angle photos. Also we need to remember that the more magnification that takes place because of a smaller image sensor the more that digital noise is likely to become an issue.
When buying a Point and Shoot, Super Zoom or DSLR camera it is important to understand the "Equivalent Focal Length" of the camera and lens combination.
Different manufacturers use different image sensor sizes in their cameras. Some of the more popular multiplication factors by camera maker are listed in the table below. Of course several of these companies also make Full Frame DSLR's which have a crop factor of One.
Crop factor can work to your advantage or disadvantage depending on what type of pictures you take the most. If you do a lot of landscape or group shots that require a wider angle lens then a higher crop factor will limit your ability to get the widest possible angle of view. Yet on the other hand if you need the most zoom capability then a higher crop factor could work to your advantage as your effective focal length would be increased for the same lens.
Remember that the smaller the image sensor is the higher the crop factor will normally be. Because the smaller image sensor is effectively cropping the image the end result will be a photo where it looks like you have zoomed closer to the subject.
When comparing cameras be sure to look at the specifications for the 35mm equivalent of the lens. This will give you more uniform way of comparing a camera and lens combination or one point and shoot camera to another.