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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A camera's field of view is determined by the focal length of the lens and the size of the image sensor capture area. A short focal length lens reduces the size of the image projected onto the sensor, resulting in more of the scene being captured in a given sensor area, thereby increasing the field of view (see figure 1c). Increasing the image sensor area will also increase the field of view, simply by allowing a larger area of the projected image to be captured (see figure 1b).
In digital SLR cameras, the term 'focal length multiplier' is often used to describe the size of the capture area of an electronic image sensor, relative to the traditional 35 mm film capture area, which is 24 mm x 36 mm. For example, a digital camera with a focal length multiplier of 1.7X has an actual imager size of 14.1 mm x 21.1 mm. If a digital camera with a 1.7X focal length multiplier is used with a 50 mm focal length lens, it will have the same field of view as a 35 mm film camera with an 85 mm focal length lens. For a photographer making the transition from film to digital, this can be a problem because all of his familiar lenses will effectively have the focal length multiplied by 1.7X. If the photographer does not already have lenses that can achieve the desired field of view with the smaller sensor field, new lenses must be purchased, adding to the cost of entry into digital photography.
Recognizing this problem, some camera manufacturers do offer digital SLR cameras with 24 mm x 36 mm field size, however the larger sensors are more expensive. Figure 2 shows how sensor cost increases with diagonal dimension. The equation used is the Poisson relationship, which is the most commonly used model for silicon die cost. The Poisson relationship describes the expected number of defects in a chip of a given size, based on some average number of defects per unit area on the wafer. The plot in figure 2 shows that die cost increases rapidly for very large die area approaching the 35 mm image sensor format size (defect assumptions being held constant).
The alternative to using a camera with a larger, more expensive sensor, is purchasing a lens with a shorter focal length (assuming one exists in the lens family). In this case the issue is lens performance of the long focal length lens with the large sensor (as in figure 1b), vs. performance of the short focal length lens with the smaller sensor ( as in figure 1c). It is possible to estimate this trade-off from MTF (Modulation Transfer Function, a measure of how contrast falls off as spatial frequency increases) curves for the long and short focal length lenses. Figure 3a shows that the contrast in the corner of the field with the long focal length lens used with the large sensor, is worse than for the shorter focal length lens used with the smaller sensor, as depicted in figure 3b. This example assumes that pixel pitch is constant, so that inter-pixel contrast at a given spatial frequency (lpm) is the same.
Another incentive for increasing sensor area is resolution. For a given pixel size, a larger image sensor area means more pixels, and therefore more image detail (pixel size trade-offs wil be discussed in "Digital Photography Essentials # 2, Pixel Size"). Figure 4 shows a list of common optical format sizes and associated applications. The largest format size shown in figure 4 is the 24 mm x 36 mm size used in "35 mm" film cameras. It is interesting to consider whether the popular 35 mm optical format will survive the transition to digital, or whether a new format, such as the 4/3 inch format, will emerge to dominate in the age of digital photography. A smaller format would allow for lighter and cheaper lenses, with the same field of view, depth of field, and other optical characteristics as the historical 35 mm family of lenses.
common image sensor sizes