Common lens questions we are often asked are, “What is a lens’s optical format (or size; we will use the two interchangeably) and how does it relate to specific image sensor sizes in industrial cameras? Along with, “Can any size machine vision camera be used on any lens format or do they need to be matched exactly?”
First lets review the sizes of the machine vision camera image sensors themselves (as seen below).
The image sensor size is typically put in terms of “inches”, but really has nothing to do with this and dates back to the “image tube” days. Without a big history lesson, a sensor that fit into an image tube with a 1” (inch) yoke was deemed a 1” image format . Today, we still use these terms and see commons sizes stated as 1/3”, ½”, 2/3” as seen in the image above. Note: The image size in ” does NOT calculate to mm and vice versa! It is nomenclature only.
However, what is important is to look at the diagonal across the given image sensor which is the “image circle”. (i.e The 1/3” format above has a diagonal of 6 mm. )
The size of the lens MUST be equal or greater than the size of the sensor ( circle size that covers the sensor) or you simply will not get the whole image!
The diagram above shows a 1/3″ format image sensor (6mm diagonal). In order to adequately cover the image sensor, you need a 1/3″ lens format or larger. On the left, we show a lens with a 1/4″ format, and it does not cover the sensor.
The end results from the improper mating of a smaller lens format than the image sensor format will be vignetting (dark corners where the lens does not cover the sensor) of the image.
What can I do when there is no specific lens format matching the image sensor format?
Lens manufacturers are continuing to design lenses to address the changing sensor market. However you will not always find a specific size format to match the lens. In these cases, you just need to ensure the lens format (image circle diameter) is larger than the sensor as mentioned in the above example.
An example is the newer 1/1.2” sensor sizes (IMX174, IMX249 ) which have a diagonal of 13.4mm. Although there are some lens manufacturers that designed a lens with the specific 1/1.2” format, there are not many. Referring to lens format diagram, the 1/1.2” format is between a 2/3” and 1” format. The 2/3” format has a image circle of 11 mm which will not fully cover the 1/1.2” format (13.4mm diagonal), and you will get vignetting of the image. The solution is to use the next size up which is a 1” format. This format is commonly found in many lens manufacturers, in turn providing many lens manufacturers to choose from.
Click here now for all lens sizes and manufacturers
In conclusion, you can use an image format on a lens on smaller image sensor size, but not the other way!.. You’ll have vignetting and lose part of your image!
What else do we need to consider in lens selection?
This blog post simply covers sensor formats. There is much more to consider in a lens selection such as resolution of the lens to resolve the pixels themselves, what focal length is needed etc.
Here are some further resources to help in the selection process. Additionally, 1st Vision has over 100 years of combined experience in industrial imaging in which you can contact us to aid in the section.
How to choose a lens –
Calculating resolution for a machine vision application –https://www.1stvision.com/machine-vision-solutions/2015/07/imaging-basics-calculating-resolution.html
Video Tutorial – Using the On-line lens focal length calculatorhttps://www.youtube.com/watch?feature=player_embedded&v=baF4lwl0LwM
1st Vision newly added our high quality 1” format lenses which provide an excellent price vs performance ratio – Read more here.
Images courtesy of Wikipedia
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There are at least three methods by which image sensor size categories are assigned. Since two of these methods produce category labels that look almost the same, confusion is common. Here, we try to sort this out.
Method 1 - Referenced to Image Tube Sizes
When the vidicon tube was invented in the early 1950s, the glass tubing used to make the first units had a nominal exterior diameter of one inch. These were referred to as "One-inch tubes". After allowing for the space taken up by the interior metal structures needed to control the electron beam, the portion of the photoconductor on the front glass accessible for scanning had a diameter of about 5/8 inch. Early television used a raster aspect ratio of 4:3 so the corresponding scanned area was 1/2 inch horizontal by 3/8 inch vertical - the canonic 3:4:5 right triangle. After a while, the image size assumed the name of the tube and a 5/8 inch diagonal raster became commonly called the 1-inch size. 5/8 inch is 15.875 mm and so 16 mm became the designated metric diagonal for 1-inch tubes.
Later, three more glass sizes became common; 2/3-inch and 1/2-inch for lower-cost cameras for scuurity and compact, rugged cameras for defense and 30 mm (converted in the reverse to 1.2 inches) for early Philips Plumbicon tubes, which needed some extra diameter to achieve resolution acceptable for broadcast television. The corresponding raster diagonals for these glass sizes were 11 mm, 8 mm and about 21.4 mm.
From these few numbers, all of the smaller imager sizes (and one larger size) were scaled. In some cases the scaling was approximate, partly because 11/16 is not quite 2/3 and partly because other sensor sizes were built and then squeezed into existing size categories. Now, there are very many actual raster (active area) sizes with small variations from the calculated nominals but the categories remain useful to group sensors for comparison. Table 1 below shows raster sizes commonly included in the size categories. Although most of the early imagers had a 4:3 aspect ratio, now sensors are found with 16:9 AR (HDTV), 3:2 AR (digital still camera) and 5:4 AR (computer graphics). There are also may special-purpose sensors with other aspect ratios. All of these are still classified by their image diagonal because that is the key parameter needed to assure proper coverage by lenses.
Table 1 - Dimensions Inch-Equivalent Sensors with 4:3 Aspect Ratio
For comparison, Table 2 shows dimensions for 2/3" sensors with other aspect ratios.
Table 2 - Raster Sizes for 2/3" Sensors of Various Aspect Ratios
Method 2 - Referenced to a 35 mm Film Frame Size
When lenses are used on digital cameras for photography, the users find it convenient to think of the camera field by considering what focal length lens is required with the small sensor in the camera to give the same field of view that they used to get with a 24 x 36 mm film frame. This focal length ratio is expressed in terms of a the focal length multiplier, the ratio of the size of the sensor in the camera in use to the 24 x 36 mm frame. If the sensor is 16 x 24 mm, for instance, the size ratio is 1/1.5 so to get the same field of view with the digital camera as the 35 mm film camer would have had, the lens focal length must be reduced by 1/1.5.
For convenience to the user, the sensor iteslf is also referred to as having a 1/1.5 size. This can cause confusion for engineers looking for sensors because for photographers, a 1/1.5 sensor is 16 x 20 mm but in video camera use a 1/1.5 sensor with a 3:2 aspect ratio is a 2/3" inch sensor with a raster size of 9.2 x 6.1 mm. This is not a mistake that anyone wants to make. Table 3 shows some common sensors sizes (all 3:2 aspect ratio) that are disugnated by the focal length multiplier.
Table 3 - Raster Sizes for Various Focal Length Multipliers
Method 3 - Referenced by Name
Some sensor sizes correspond to specific sizes used elsewhere. These sizes identified by the names of these references. Table 4 gives a few examples.
Table 4 - Raster Sizes Referenced by Name
Well, this is all really interesting and useful as a set of rules of thumb, but always check the actual size of any sensor you intend to use because the designators are often loosely applied.