The Resolution of Lenses and Images.
What is the Definition of Resolution? And Why Does it Matter?
Resolution defines the sharpness of an image and can indicate advanced camera functions.
There are four types of resolution:
- Pixel Count Resolution
- System Resolution
- Lens Resolution
- Angular Resolution
All types of resolution are related and jointly impact computer vision.
Why Can Resolution Be So Confusing?
You're not alone! Even the experts interchangeably use the term "Resolution" when it actually refers to four different technical metrics. We ran across this misunderstanding between Optical Engineers, Image Quality Engineers, Sensor Engineering, and Computer Vision Engineers during a 2018 conversation within the IEEE P2020 working group.
The resolution of a sensor is also closely related to the CMOS sensor format type (e.g. 1/3", 1/2", etc..) which we discuss in a separate article.
Megapixels as Resolution
Pixel Count Resolution
The pixel count resolution has been popularized by the mobile phone industry, but is frequently misleading for 50MP+ cameras as the final output/viewed image rarely has 50MP+ resolution.
Definition:- The total number of light capturing photodiode wells in a sensor.
- The total number of light emitting elements for displays and projectors.
- The total number of pixels in a digital image.
The camera has 64 MegaPixels and creates a 12 to 16 Megapixel image.
The camera is 64MP.
SFR as Resolution
What is System Resolution and Why Does it Matter?
The system resolution can characterize the quality of any digital image. It is frequently used in digital camera production lines to ensure the lens, lens-to-sensor alignment, and image processing pipeline meet a minimum quality threshold. It can also be used to benchmark different cameras and image processing/compression methods. This metric includes inputs from the Lens, the Lens-to-sensor alignment, the Sensor, the ADC, and the Analog+Digital Image Processing Pipeline.
Definition:- The Spatial Frequency Response (SFR) measures contrast as a function of spatial frequency, derived from the edge-spread function of a slanted edge (ISO 12233).
Follow ISO12233 or read Imatest's website.
How to Express System Resolution:Include the line-widths per picture height, the field position, and the illumination.
The SFR is 2000 LW/PH at 25° field angle under 1000lux 5000K illumination.
The MTF of the image is 2000 LW/PH.
What is The Impact of SFR on Computer Vision?
From: Vasiljevic, et. Al. "Examining the Impact of Blur on Recognition by Convolutional Networks"
MTF as Resolution
"Optical Resolution" is used by Imaging Optics Engineers and is an Input to System Resolution
The optical resolution characterizes the 'sharpness' of an optical system. The MTF applies to both focal and afocal optical systems, including microscopes, imaging lenses, binoculars, and projection optics.
Definition:- The Modulation Transfer Function (MTF): the contrast a lens preserves at a stated spatial frequency and field position.
- Two-point criteria such as the Rayleigh criterion answer a different question, the closest separation at which two point sources stay distinguishable, and they do not convert into an MTF percentage.
Include the MTF as a percentage, the frequency, the field angle, and the wavelength.
The photopic MTF of the lens is 20%@200LP/mm at 50° field angle.
The MTF of the lens is 20%.
Instantaneous FoV as Resolution
"Angular Resolution" is used by the Aerospace/Defense industries and Computer Vision Engineers
The Instantaneous Field of View (IFOV) is the angle one pixel covers in object space, so it is quoted per pixel. Pixels per degree is its reciprocal, a sampling density, and on a lens with distortion both figures change with field angle. Neither one is the resolving power of the optics, which is an MTF or Rayleigh question. Pixels per unit distance across an object is a related spatial-sampling figure that only follows once the object range and geometry are fixed.
Definition:- IFOV: the angle in object space subtended by one pixel, in degrees, arcminutes, or milliradians per pixel.
- Pixels per degree: the reciprocal of the IFOV expressed in degrees. It is a sampling density, not an angle.
Use Our Advanced Field of View Calculator which includes angular resolution for distortion profiles.
How to Express Angular Resolution:State which quantity you mean, degrees per pixel or pixels per degree, and give the field angle where it was measured.
The IFOV is 0.083° per pixel at 25° field angle, a sampling density of 12 pixels per degree.
The IFOV is 12 pixels per degree.
What is The Impact of Angular Resolution on Computer Vision?
Angular resolution limits the smallest object a network can reliably detect, not the largest. The impact on performance is direct. Dollar et. Al shows the direct result on average miss rate of pedestrian detectors. With less than 50 pixels, even state of the art detectors can have a 50% miss rate on detection. This increases rapidly as scale decreases.
From: Dollár, et. al. "Pedestrian Detection: An Evaluation of the State of the Art"
Combined Megapixels and MTF as Resolution
"Effective Resolution" is used by Lens Supplier marketing teams
The effective resolution is a quick method to filter down a lens selection when you are creating a new camera. The optical resolution (MTF) of lenses can be challenging to understand without extensive experience in camera hardware. Many experienced camera engineers do not even know the tricks which manufacturers can play with MTF charts/testing, so effective resolution can be a shortcut directly to system-level resolution.
Definition:- The sensor pixel count resolution for which a lens' optical resolution is supposed to be suitable.
Include both the Pixel Count and the image sensor format size.
The lens is suitable for a 12MP 1/2.3" sensor.
The lens is rated for 12MP so its optical resolution is suitable for any 12MP sensor.
This misleading metric will forever remain used and abused by lens manufacturers. If you are a lens buyer: you have now been warned.
Reference and related links
- Dollár, et. al. "Pedestrian Detection: An Evaluation of the State of the Art"
- Vasiljevic, et. Al. "Examining the Impact of Blur on Recognition by Convolutional Networks"
So, How do we at Commonlands use resolution when talking about our lenses?
As a lens supplier, we provide an objective Effective Resolution score based on empirical System Resolution measurements.
What matters for performance is the final image quality.
- We perform through-focus ISO12233:2014 measurements using a number of sensors that have different pixel pitch.
- Then, we find the maximum image circle.
- After applying the maximum sensor size and aspect ratio, we calculate the total number of pixels.
- We take the same pixel pitch measurement from the general lens effective resolution.
- Then, we calculate the number of pixels that would fit on a 4:3 aspect ratio sensor of the specific Format Type.
M12 Lenses Rated for Strong Effective Resolution
Four M12 lenses from the Commonlands catalog. Each is rated for effective resolution using the through-focus ISO12233:2014 process described above.
Trying to Determine Your Camera Requirements?
Use our free web-based AoV Calculator to determine your system's Field of View requirements. Then, use the M12 Lens calculator to match your requirements with the available lenses. Our Depth of Field Calculator also provides the hyperfocal distance and depth of field for every sensor and lens combination.
We also have a couple of other calculators that many engineers find interesting.
Here are a Few M12 Lenses. Search all 50+ Using Our M12 Lens Calculator.
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191°@6.4mm Fisheye Lens
CIL281-F1.8-M12A650
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190°@6.8mm Fisheye M12 Lens
CIL282-F1.8-M12B660
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Low Distortion 3.25mm M12 Lens
CIL034-F2.3-M12A660
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Wide-Angle 4mm M12 Lens
CIL339-F1.6-M12B640
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Low Distortion 6mm M12 Lens
CIL059-F1.7-M12B650
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