The MegaPixel Rating / Resolution of a Lens is related to the MTF and Sensor Pixel Pitch.

What is the Definition of Resolution? A Brief Intro. 

If you've been trying to understand what resolution actually means, you're in the right spot. Whether you are just interested or have a PhD in imaging quality / optics, this article has a take for you! The field of image quality metrics is extensive, and this is a small portion of it all!

Why is it 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.

Megapixels as Resolution

"Pixel Count Resolution" is used by the display industry and the general populace.

The pixel count resolution has been popularized by the cellphone industry in their Megapixel marketing war.

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 resolution of a camera is not actually 64MP

How to Express Pixel Count Resolution:

Include both the total number of pixels and the output image resolution.

Accurate Usage:

The camera has 64 MegaPixels and creates a 12 to 16 Megapixel image.

Inaccurate Usage:

The camera is 64MP.

The hill we will die on:  Marketing departments who say their camera is 40MP+ are mis-leading their consumers. Mobile phone cameras with >20MP rarely output images at the full pixel count; the sensors have advanced Bayer patterns (e.g. "Tetra-Bayer") to reduce HDR motion blur and/or to provide phase detection autofocus. 

SFR as Resolution

"System Resolution"  is used by Image Quality Engineers

The system resolution can characterize the quality 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 the change in contrast across a gradient in the image. 

How to Express Systen Resolution:

Include the line-widths per picture height, the field position, and the illumination.

Accurate Usage:

The SFR is 2000 LW/PH at 25° field angle under 1000lux 5000K illumination.

Inaccurate Usage:

The MTF of the image is 2000 LW/PH.

The hill we will die on: Image quality experts who use the term "MTF" interchangeably with SFR are technically inaccurate. By definition, the MTF is the Modulus of the Optical Transfer function, which does not include any inputs from the image processing pipeline.

MTF as Resolution

 "Optical Resolution" is used by Imaging Optics Engineers

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. This can be characterized by a Modulus of the Optical Transfer function (MTF) measurement or similar visual optical resolution test. 

Definition:

  • The ability of an optical system to resolve two points. 

How to Express Optical Resolution:

Include the MTF as a percentage, the frequency, the field angle, and the wavelength.

Accurate Usage:

The photopic MTF of the lens is 20%@200LP/mm at 50° field angle.

Inaccurate Usage:

The MTF of the lens is 20%.

The hill we will die on: MTF is not the most important type of resolution, despite what any optical engineer says. It is only one of the inputs to system resolution. 

Instantaneous FoV as Resolution

"Angular Resolution" is used by the Aerospace/Defense industries

The angular resolution characterizes the magnification of an optical system at a given field position. Computer vision engineers frequently invert the iFoV to pixels per degree to help quickly calculate the number of pixels across an object. 

Definition:

  •  The number degrees subtended by a pixel in the image. 

How to Express Angular Resolution:

Include both the angular resolution and the field angle.

Accurate Usage:

The imaging system has an angular resolution of 12px per degree at 25° field angle.

Inaccurate Usage:

The imaging system has an angular resolution of 12 pixels per degree.

The hill we will die on: You are one of 1000 people in the world that still regularly use 'resolution' in this context. 

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 even not 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. 

How to Express the Effective Resolution:

Include both the Pixel Count and the image sensor format size.

Accurate Usage:

The lens is suitable for a 12MP 1/2.3" sensor.

Incorrect Usage:

The lens is rated for 12MP so it's optical resolution is suitable for any 12MP sensor.

The hill we will die on: This misleading metric will forever remain used and abused by lens manufacturers. If you are a lens buyer: you have now been warned.

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.

For general lens effective resolution metrics:
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.

For effective resolution metrics of each Sensor Format Type:
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.