If not, what's the difference? A photographic comparison as an example would be great (assuming they are different).
1As many of the answers note, that depends on your sensor size... can you clarify what camera you're planning to use it on?– ReidOct 22, 2010 at 22:28
@Reid: I'm planning to buy and use a Canon EOS Rebel T2i whose sensor size, from what I understood reading the answers, is not full frame.– SrikanthOct 24, 2010 at 10:59
As a general rule of thumb, in the modern DSLR world that is primarily dominated by Canon and Nikon, focal lengths are stated at their "when used on 35mm/full-frame body" values. Focal length is focal length, and doesn't physically change when the imaging medium changes in size, however different sizes of film or sensor do change the effective angle of view that is actually captured. Focal length is a rather useless value, all things being equal, as it doesn't really tell you much of anything about what the lens will do when used on a particular camera. The angle of view is a considerably more useful value, but that can be difficult to calculate given the factors involved. A simpler solution is to determine the angle of view "bucket" that your lens falls into for the size of sensor you are using.
On Focal Length, Angle of View, and Sensor Size
The angle of view of a lens of a particular focal length is dependent on the size of the imaging medium. On on a 35mm sensor or film (full-frame sensor, such as what you get with a Canon 1D or 5D, the Nikon D3's), an 18mm DSLR lens is a wide-angle lens. Full-frame bodies are expensive, and most DSLR's come in a variant of APS-C, or cropped-sensor, sizes. Most APS-C sensors are around 22-23mm. The smaller sensor captures less of the image circle projected by the lens, which effectively reduces the angle of view. While the lens is an 18mm focal length lens, on a sensor that is smaller than 35mm, the lens "behaves" as though it is a longer focal length. As such, the focal length ranges that determine if a lens is wide, normal, or telephoto change with the size of the sensor.
Common Fields of View @ 35mm
There are some common fields of view, assuming a full frame (35mm) sensor, that can be used to generally group focal lengths. There are slightly differing schools of thought on this, however here is a table from a well-known source, DPReview.com:
Focal Length Angle of View Bucket ---------------------------------------- < 20mm Super Wide Angle 24mm - 35mm Wide Angle 50mm Normal Lens 80mm - 300mm Tele > 300mm Super Tele
(From dpreview.com: Focal Length)
According to Wikipedia, a "wide-angle is a focal length substantially shorter than the focal length of a normal lens." A normal focal length is one which closely matches the diagonal of the image medium. In a full-frame 35mm sensor body, the sensor is 36x24mm, which is a diagonal of 43.3mm. The closest common "normal" focal length used by most manufatrurers is 50mm, which is pretty close to the 43.3mm diagonal of a full-frame sensor. (Similarly, in large-format photography, the film size is 4"x5", or 101.6x127mm, which has a diagonal of 163mm. With 4x5 cameras, a "normal" focal length lens is usually around 150mm.) While the term substantially in the Wikipedia article is not well-defined, the Wikipedia article cites 35mm and less as "wide-angle" and "ultra-wide-angle". Similarly, focal lengths substantially longer than a normal lens are "telephoto" through "super-telephoto" lenses.
Crop Factor and Angle of View
Since the field of view buckets that a lens falls into are dependent on focal length, and effective focal length is dependent on sensor size, one most first determine the effective focal length on a sensor smaller than 35mm. An easy rule of thumb can be used to calculate the effective focal length: Divide the diagonal of a 35mm sensor by the diagonal of the actual sensor, and multiply the resulting number (the "crop factor") by the focal length.
cropFactor = fullFrameDiagonal / croppedSensorDiagonal effectiveFocalLength = actualFocalLength * cropFactor
This assumes you know the diagonals of your camera sensor. The diagonal can be computed fairly easily using the Pythagorean Theorem if you know the width and height of the sensor. Pythagoras' theorem quite simply states:
In any right triangle, the area of the square whose side is the hypotenuse (the side opposite the right angle) is equal to the sum of the areas of the squares whose sides are the two legs (the two sides that meet at a right angle).
This translates into the formula:
a^2 + b^2 = c^2
A sensor, cut in half along its diagonal, is a right triangle, and the length of the diagonal (c) can be computed as such:
diagonal = sqrt(width^2 + height^2)
If you know the dimensions of your sensor, and are not afraid of a little math, you can computer the effective focal length of any lens on any camera body, and once the effective focal length is known, determine whether it is ultra-wide, wide, normal, tele, or super-tele. As it is unlikely most people will know their sensor dimensions, here is convenient a table of sensor sizes, diagonals, and crop factors:
Sensor Crop Factor Diagonal Width Height ------------------------------------------------------------------------- 4x5 Large Format Film 0.27 162.6mm 127mm x 101.6mm Digital Medium Format 0.64 67.1mm 53.7mm x 40.3mm Full-Frame 1.0 43.3mm 36mm x 24mm Canon APS-H 1.26 (1.3) 34.5mm 28.7mm x 19.1mm Pentax/Sony/Nikon DX 1.52 (1.5) 28.4mm 23.7mm x 15.6mm Canon APS-C 1.62 (1.6) 26.7mm 22.2mm x 14.8mm Sigma Foveon 1.74 (1.7) 24.9mm 20.7mm x 13.8mm Four Thirds 2.0 21.6mm 17.3mm x 13.0mm
As an example, to demonstrate and prove the concept. The Canon APS-C sensors have a crop factor of 1.6x (most commonly stated, more accurately, 1.62x). This is calculated as follows:
cropFactor = 43.3mm / 26.7mm = 1.6217228...
APS-C Effective Focal Lengths
Now that you know the crop factor of common sensor sizes, you can compute the effective focal length of any lens you may use. Assuming the 18mm focal length of the original topic, its effective focal length on a Canon APS-C sensor (i.e. 550D, 60D, 7D) would be:
effectiveFocalLength = 1.6 * 18mm = 28.8mm, or 29mm
You can compute the focal range of a zoom lens just as easily. Given the Nikon 14-24mm lens:
shortFocalLength = 1.5 * 14mm = 21mm longFocalLength = 1.5 * 24mm = 36mm
Full Frame vs. APS-C Angle of View Buckets
The most common sensor sizes for many of the most common DSLR cameras are Full Frame and APS-C. We have a table of angle of view buckets for full frame, so it is useful to compute a table that defines the angle of view buckets for APS-C. I've used a crop factor of 1.55 to cover Canon APS-C and Pentax/Sony/Nikon APS-C:
Focal Length Angle of View Bucket ---------------------------------------- < 12mm Super Wide Angle 15mm - 23mm Wide Angle 32mm Normal Lens 51mm - 200mm Tele > 200mm Super Tele
Given the two AoV bucket tables, we can arrive at a table of common lenses and their AoV's for both Full Frame and APS-C sensors:
FF Focal Length FF AoV APS-C Focal Length APS-C AoV -------------------------------------------------------------------------------- 8-15mm Super-Wide | 12.4mm-24mm Super-Wide to Wide 10-22mm Super-Wide | 15.5mm-34mm Wide to Normal 14-24mm Super-Wide to Wide | 22mm-37mm Normal 16-35mm Super-Wide to Wide | 25mm-54mm Normal to Tele 24-105mm Wide to Tele | 37mm-163mm Normal to Tele 70-200mm Tele | 108mm-310mm Tele to Supertele 100-400mm Tele to Supertele | 155mm-620mm Tele to Supertele -------------------------------------------------------------------------------- 14mm Super-Wide | 22mm Wide 24mm Wide | 37mm Normal 35mm Wide | 54mm Tele 50mm Normal | 78mm Tele 100mm Tele | 155mm Tele 135mm Tele | 209mm Supertele 300mm Supertele | 465mm Supertele
Visual Examples of Focal Length and Angle of View
I'll try to take some example photographs when it is light again to demonstrate the differences between super-wide, wide, normal, tele, and super-tele. (Its too dark right now to get any decent shots outdoors, and indoors I don't really have enough space to get telephoto/supertelephoto shots.)
Thanks for the detailed and wonderful answer. I'm planning to buy and use a Canon EOS Rebel T2i, which has an APS-C sensor. One of its kit lens has focal length range from 18 to 135mm. So I guess from the tables above that it supports the higher end of the wide angle spectrum. The other wide angle lens from Canon for this camera (10 - 22mm) that I liked is as costly as the camera itself. So I'll just settle for the kit lens for now.– SrikanthOct 24, 2010 at 11:13
@Artknish: Glad to be of service. :) Regarding the 18-135mm, it would barely qualify as "Wide" and would generally fit the "Normal to Supertele" range on the T2i. With APS-C, it is really tough to get wide angle. The EF-S 10-22mm is a good lens, and pretty much the only thing that really offers a "wide angle" of view on an APS-C body. It is the effective equivalent of the EF 16-35mm L lens (10 * 1.6 = 16mm, 22 * 1.6 = 35.2mm). The bummer about the EF-S mount is that it only works for APS-C bodies, and wouldn't be compatible with, say, a 5D if you chose to upgrade in the future.– jristaOct 24, 2010 at 18:08
I think so, on a full-frame body that is. It really depends on the sensor / crop factor. For most APS-C cameras, your 16mm looks like a 26mm on a full frame... and that is still wide actually :) One of the "bibles" for lens reviews, The Digital Picture, would even call the Canon 16-35mm f/2.8 an ultra-wide angle. I have such lens, and at 16mm it is very wide, with a lot of (expected) optical distortions.
Here is a shot of my office at home (below). Believe me, it's not that "narrow", but I wanted to have all walls in the frame. Only possible at 16mm.
If 16mm is ultra wide, then what's a 6mm? Mega-wide? Uber-wide? I think we need to come up with a new term! Oct 22, 2010 at 20:08
@Matt Grum, a 6mm would generally be a point-and-shoot type of camera.– DebajitOct 22, 2010 at 21:01
yeah but you can get a 6mm lens for a 35mm SLR! Oct 22, 2010 at 21:40
I suspect that the OP doesn't have a FF camera; probably should ask for clarification before answering as if s/he did.– ReidOct 22, 2010 at 22:29
@reid: I addressed that. Even if he is on a crop factor, a 16 would be considered very reasonably wide. Oct 22, 2010 at 22:57
Generally, 18mm is considered wide-angle, but it depends on the size of the sensor. A DSLR camera with an 18mm lens WOULD take wide-angle shots, but if you have a compact digital camera or a cameraphone, where the sensor can be really tiny, an 18mm lens would look more like a 50mm lens or longer does on a DSLR.
The Micro Four Thirds system recently introduced has interchangeable lenses but a much smaller sensor than DSLRs, so the standard 14-42mm lens that comes with the Olmypus E-PL1, for example, would give a similar field of view to a 28-84mm lens on a 35mm camera (Full frame).
2yup, depending on sensor/film size 18mm is either a super-telephoto, telephoto, normal, wide, or ultra wide... Oct 22, 2010 at 20:09
No "generally" about it. @Matt's correct - depends on the sensor size.– ReidOct 22, 2010 at 22:29
Yes, which is why I said 'Depends on the size of the sensor'. Did you read beyond the first word of my answer?– user456Oct 23, 2010 at 7:17
My experience, using the Nikon 18-70mm on my D50, is that 18mm is fairly wide for a DX camera. I only need my 10-20mm for extremely wide shots, but the 18-70 gets them most of the time. An 18mm lens on a full frame camera would be pretty wide.