There is a bang-up bargain of confusion surrounding crop factor, and information technology is specially difficult to explain, but let’s requite information technology a try, shall nosotros?
Earlier we swoop in, let me dispel two vicious rumors related to ingather cistron that are circulating through the photography (Internet) world today:
- Crop gene does Non affect a lens’s focal length.
- Ingather factor does NOT affect a lens’southward aperture.
Before you lot scroll to the bottom of the screen to leave a comment to the contrary, let me explain why I am stating these facts…
The focal length of a lens, expressed in millimeters, is the distance along the lens’due south optically central axis (starting time at the rear nodal bespeak) to the image plane in the camera (often illustrated past a “Φ” on the meridian plate of a camera body) when the lens is focused at infinity. The image plane in the camera is where you will discover your digital sensor or picture show plate.
Therefore, a 50mm lens tin measure 50mm from the point where light rays begin to leave the lens in the aforementioned management as they entered the lens until they arrive on the image plane. Some “pancake” lenses and mirror lenses have optical tricks to shorten them, but in general, the focal length is that physical measurement.
A zoom lens tin can modify the concrete focal length of a lens. Sometimes this motion is contained within the lens—the lens body does not physically change length—and other times the lens does modify its size.
However, regardless of what kind of camera or sensor you place behind the lens, the focal length will not change just because you accept a larger or smaller sensor or frame of picture. I will explain after how sensor size (or film size) changes the
focal length—not the
focal length of the lens.
is the size of the opening in the lens. Some lenses have fixed apertures that cannot exist changed, just most photographic lenses have variable apertures to command the amount of calorie-free entering the lens. This opening is regulated by a diaphragm comprising blades that can be adjusted to vary the size of the pigsty (aperture) through which the lite passes.
In photography, discontinuity is expressed as a ratio of the focal length to the diameter of the discontinuity opening. The ratio is commonly referred to equally an f/number, f/finish, focal ratio, f/ratio, or relative aperture.
This ratio is based on physical measurements and is completely independent of the size of the camera’s sensor or the size of the film you are shooting. Sensor size has an effect on depth of field, but not because information technology changes aperture. Aperture is independent of motion-picture show frame or sensor size.
The first thing to know near
is that, as with all “factors,” we need to accept a base reference from which to work. In the photography earth, this reference is a piece of 135 motion-picture show. In the digital photography world, “total-frame” sensors are the aforementioned size as this movie; a film frame with a width of 35mm. Cameras of this photography format are collectively known every bit “35mm cameras.”
A 35mm movie strip measures 35mm across
One source of crop factor defoliation is the utilise of “35mm” when discussing the reference. The value in this case is used non as a focal-length measurement, only as a measurement of the dimensions the frame of moving-picture show. The film epitome area measures 24 10 36mm, but the strip is 35mm wide. So, when you call up of “35mm” when information technology is used in reference to motion picture or the size of a camera sensor, know that you are
referring to lens focal length. You can mount a lens of whatever focal length, even a 35mm lens, on a 35mm camera. The focal length is the focal length. Film and sensor dimensions are different.
For years, the 35mm photographic camera has been, by far, the world’s near pop camera format. Because of this, for those of us who grew up in the globe of 35mm cameras, when we call back of the field of view given by a lens of a certain focal length, we tin can visualize what the photograph should look similar. In the 35mm camera world, a lens with a focal length of around 50mm volition provide a “normal” view with its human-middle-similar field of view. Lenses with shorter focal lengths will provide a wider view and lenses with longer focal lengths will provide narrower or telephoto views.
Life was simple dorsum when almost everyone was shooting 35mm cameras and 35mm film. Sure, there were those making magic with medium format and large format cameras, and at that place were point-and-shoot cameras that took specially made smaller films. My first camera, handed down from my grandmother, was a Kodak Instamatic thirty, with its thirteen x 17mm 110 picture. Back then, no one really paid attention to “ingather factor,” even though information technology existed. I’d bet well-nigh photographers didn’t know the dimensions of their 110 film, nor did they know the focal length of the tiny lenses! You just looked through the camera and took the picture show information technology gave yous.
Then, digital photography arrived. In its early days, nearly sensors were smaller than 35mm film, and a virtual tin of worms was opened. Why? Because the sensors were smaller than 35mm film, the images seen through a lens of whatever particular focal length had a dissimilar field of view than that of the same lens on a 35mm film camera. Suddenly, a 50mm lens no longer had a “normal” field of view; it was a chip more of a telephoto.
The cropped sensor “sees” a narrower field of view
If yous never shot 35mm picture, this was no big deal because your listen’s eye did not have a 35mm film reference for different lenses. Simply photographers inbound digital imaging decided that they needed to know the “35mm equivalent” field of view of various lenses when attached to a camera with a digital sensor smaller than 35mm motion-picture show. The reality of it is, “ingather gene” serves to translate a measurement into a linguistic communication in which many of today’s photographers were never fluent to begin with. And, considering of this, many of yous out at that place have been very confused and frustrated by the mention of crop factor. Hopefully this article will finish your defoliation!
A round lens produces a circular prototype circle—not rectangular. The sensor, or pic, at the back of the camera captures a rectangular portion of this image circumvolve. When we use 35mm film every bit a standard, any camera with a sensor smaller than a frame of 35mm film will cover a smaller portion of the paradigm circumvolve produced by a given lens and will thereby modify the field of view of that lens. This is the “crop” part of the crop factor.
However, because traditionally, the field of view produced by a given lens has been described not equally a measurement of degrees, but by the focal length (kind of the “proper name”) of the lens, we demand to translate the cropped field of view into an equivalent lens focal length.
For case, if you adhere a 50mm lens to a camera with a smaller-than-35mm movie sensor, yous volition have to multiply the focal length of that 50mm lens by a factor derived from the size differential of the sensor to calculate the 35mm equivalent focal length. This will so give y’all the ways to effigy out the lens’south field of view based on that new equivalent focal length. This is the “factor” part of crop cistron.
This multiplication factor is the ratio of the size of the digital sensor to the dimensions of the 35mm film negative.
Formula: The diagonal of a rectangle tin be determined by a2+ b2
Full Frame: 24mm2
576 +1296 = 1872
Square root of 1872 = 43.3mm
Full-Frame or 35mm Diagonal / Crop Sensor Diagonal = Crop Gene
So, if you lot accept a camera with an APS-C-sized sensor (circa 15.6 10 23.5mm or 14.8 ten 22.2 on Catechism), plug in the numbers and you lot will become a crop factor of 1.5x (or i.6x for Canon).
Then, to find the equivalent focal length of the new field of view afforded by the smaller APS-C sensor, multiply the truthful focal length of the lens by 1.5x to go the 35mm equivalent focal length of the lens. A 50mm lens on a camera with a 1.5x crop gene APS-C sensor gives a field of view equivalent to that of a 75mm lens on a total-frame or 35mm picture camera.
Think, the bodily focal length of the lens is unchanged, every bit is its discontinuity.
In our example, if you weren’t familiar with a 50mm lens’s field of view in the first place, this doesn’t really matter. But if you were familiar with the 50mm lens’s field of view, y’all will know that this same lens, when placed in front of the smaller sensor, has a narrower field of view than your normal vision has.
A comparison of relative sensor sizes.
If you accept a zoom lens on a smaller-than-full-frame camera, you tin figure out the effective focal-length equivalent by multiplying both focal length numbers by the crop cistron. For case, a lxx-200mm lens becomes a virtual 105-300mm lens on a i.5x APS-C sensor.
Cameras with sensors or films larger than a 35mm frame will have sub-ane crop factors. For instance, a medium-format Pentax 645Z’s sensor measures 33 ten 44mm. This gives it a crop factor of 0.78x. A 50mm lens on this Pentax camera gives an equivalent field of view of a 39mm lens.
Full-frame versus The Rest
The crop factor discussion inevitably leads us to the full-frame versus smaller-sensor fence. For my take, click hither.
And then every bit not to bulldoze downward the well-trodden path here, in summary, full-frame cameras are ideal for landscape images because there is no crop gene and wide-angle lenses maintain their wide-angle field of view. Smaller-sensor cameras give lenses a virtual telephoto effect that is ideal for some sports, wildlife, and macro work. Both formats have advantages and disadvantages.
Another matter to mention: there are “regular” lenses and there are lenses specifically designed to operate on smaller-sensor cameras. These pocket-sized-sensor lenses may not work well on their full-frame cousins. On a 35mm moving-picture show or full-frame digital camera, you lot may experience heavy vignetting. If the small-scale-sensor lens does piece of work on a total-frame digital camera, the camera might simulate the smaller sensor the lens was designed for and automatically provide the crop factor field of view. A regular lens will piece of work happily on a total-frame digital, 35mm film, or smaller-sensor camera. The ingather factor will only utilize to a lens if information technology is used on a small-sensor camera. Today, some manufacturers refer to their “regular” lenses as “full-frame lenses” to emphasize that they are not designed specifically for smaller-sensor cameras. Only, earlier digital photography, all 35mm format lenses were “full-frame.”
The Final Word
Crop factor is really quite simple. The disruptive thing is that, as I said before, it exists to translate an angular measurement (degrees of field of view) almost into a linear measurement (millimeters of lens focal length) so that old-school 35mm photographers tin figure out the existent field of view of a lens based on an equivalent focal length resulting from using sensors smaller than 35mm film. Get it? Got information technology. Good!
I suppose that is useful in many ways, but I accept seen many frustrated photographers over the years try to understand and explain this concept. Sprinkle in some bogus information on the Web near magically changing focal lengths and apertures, and everything has become a mess!
I promise this has cleared things up for those who are new to photography or who were confused a few minutes ago. If non, I stand by to take your questions! And, in instance you were wondering, the Instamatic 110 movie camera has a crop cistron of 2x.
For more information almost the theory behind crop factor, be certain to watch this engrossing video.