With consumer digital cameras, the term resolution of is often incorrectly interpreted as the number of addressable photoelements. Quantitatively, resolution refers to the ability of a camera to optically capture finely spaced detail, and it is usually reported as a single valued metric. Resolution is closely related to spatial frequency response (SFR), a multi-valued metric that measures contrast loss as a function of spatial frequency, but the two are distinct, since resolution is the highest spatial frequency that a camera can usefully capture under cited conditions.
These two qualities are addressed in ISO 12233:2023—Photography – Electronic still picture imaging – Resolution and spatial frequency responses, which specifies methods for measuring the resolution and the SFR of electronic still-picture cameras. It also defines terminology and test charts relevant to the test procedures covered in the document.
What Is Spatial Frequency Response (SFR)?
SFR is a numerical description of how contrast is changed by a camera as a function of the spatial frequencies that describe the contrast. Because it can also serve as an umbrella function for deriving other metrics, such as sharpness and acutance, SFR is highly beneficial for engineering, diagnostic, and image evaluation purposes.
What Are the SFR Measurements in ISO 12233:2023?
ISO 12233:2023 primarily describes two SFR measurements, edge-based spatial frequency response and sine-based spatial frequency response, which was added to the 2017 edition of the standard.
The choice of which method to use for measuring SFR is at the discretion of the standard user, but several factors should be influential in making this decision. Edges are common features in naturally occurring scenes, acting as visual acuity cues for judging image quality. Edges, in the eyes of almost any viewer, are visually important, making them prone to image processing in many consumer digital cameras. Because of the significance of edges in assessing contrast differences, the edge-based spatial frequency response measurement can be useful for many ISO 12233:2023 users.
Alternatively, sine wave features, as stated in ISO 12233:2023, are “intuitively satisfying.” Because sine waves transition more slowly than edges, they are not prone to being identified as edges in embedded camera processors. This presents an important advantage for the sine wave starburst test pattern: the ambiguity that image processing imposes on the SFR can be largely avoided by their use.
However, the standard also notes that “all experience suggests that there is no single SFR for today’s digital cameras,” suggesting that comparing edge-based and sine wave-based SFR results under the same capture conditions could be a good tool for determining spatial image processing in digital cameras.
The exact procedures for carrying out each of these methods are addressed in the ISO 12233:2023 standard.
What Are the Changes to ISO 12233:2023?
ISO 12233:2023 is the third edition of the standard for resolution and spatial frequency responses in electronic photography, having several noteworthy changes. Specifically, in this revision:
- The e-SFR test chart was modified by replacing the “slanted square” features with four-cycle “slanted star” features to enable diagonal measurements in addition to horizontal and vertical measurements.
- The e-SFR algorithm was modified by using a Tukey window, by using a 5th-order polynomial equation to fit the edge, and by correcting for the edge-angle sampling.
- Clause 6, “Edge-based spatial frequency response (e-SFR),” and Annex D, “Edge spatial frequency response (e-SFR) algorithm,” were updated to clarify the steps in the e-SFR algorithm.
- In Annex C, “Edge SFR test chart,” the reflectances of the surround and the light and dark patches were clarified.
- New Annex J, “Non-uniform illumination compensation for some applications.”
- New Annex K, “Derivation of correction functions.”
- New Annex L, “Acutance calculation.”
- New Annex M, “Matlab function for computing e-SFR.”
ISO 12233:2023—Photography – Electronic still picture imaging – Resolution and spatial frequency responses is available on the ANSI Webstore.