Comparing sharpness in cameras with different pixel count

Introduction: The question

We frequently receive questions that go something like,

“How can you compare the sharpness of images taken with different cameras that have different resolutions (total pixel count) and physical pixel size (pitch or spacing)?”

The quick answer is that it depends on the application.

Are you interested in the sharpness of the image over the whole sensor (typical of most pictorial photography— landscape, family, pets, etc.)? We call these applications image-centric.
Do you need to measure details of specific objects (typical for medical imaging (we commonly work with endoscopes), machine vision, parts inspection, aerial reconnaissance, etc.)? We call these applications object-centric.

In other words, what exactly do you want to measure?

This page primarily addresses the comparison of object-centric images from different cameras,
where the objects can have very different pixel sizes.

The keys to appropriate comparison of different images are

the selection of spatial frequency units for MTF/SFR (sharpness) measurements, and
the selection of an appropriate summary metric (important since the most popular metric, MTF50, rewards software sharpening too strongly).

The table below is adapted from Sharpness – What is it, and how is it measured? We strongly recommend reviewing this page if you’re new to sharpness measurements.

Table 1. Summary of spatial frequency units with equations that refer to MTF in selected frequency units. Emphasis on comparing different images.

MTF Unit

Application

Equation

Cycles/Pixel (C/P)

Pixel-level measurement. Nyquist frequency f_Nyq is always 0.5 C/P.

For comparing how well pixels are utilized. Not an indicator of overall image sharpness.

Cycles/Distance

(cycles/mm or cycles/inch)

Cycles per physical distance on the sensor. Pixel spacing or pitch must be entered. Popular for comparing resolution in the old days of standard film formats (e.g., 24x36mm for 35mm film).

For comparing Imatest results with output of lens design programs, which typically use cycles/mm.

Line Widths/
Picture Height
(LW/PH)

Measure overall image sharpness. Line Widths is traditional for TV measurements.
Note that 1 Cycle = 1 Line Pair (LP) = 2 Line Widths (LW).

LW/PH and LP/PH are the best units for comparing the overall sharpness (on the image sensor) of cameras with different sensor sizes and pixel counts. Image-centric.

note: for cropped images enter the original picture height into the more settings dimensions input

Line Pairs/
Picture Height
(LP/PH)

Cycles/Angle:

Cycles/
milliradian

Cycles/Degree

Angular frequencies. Pixel spacing (pitch) must be entered. Focal length (FL) in mm is usually included in EXIF data in commercial image files. If it isn’t available it must be entered manually, typically in the EXIF parameters region at the bottom of the settings window. If pixel spacing or focal length is missing, units will default to Cycles/Pixel.

Cycles/Angle (degrees or milliradians) is useful for comparing the ability of cameras to capture objects at a distance. For example, for birding (formerly called “birdwatching”) it is a good measure of a camera’s ability to capture an image of a bird at a long distance, independent of sensor and pixel size, etc. It is highly dependent on lens quality and focal length. Object-centric.
It is also useful for comparing camera systems to the human eye, which has an MTF50 of roughly 20 Cycles/Degree (depending on the individual’s eyesight and illumination).

FL can be estimated from the simple lens equation, 1/FL=1/s1+1/s2, where s1 is the lens-to-chart distance, s2 is the lens-to-sensor distance, and magnification M=s2/s1. FL=s1/(1+1/|M|) = s2/(1+|M|)

This equation may not give an accurate value of M because lenses can deviate significantly from the simple lens equation.

Cycles/object distance:

Cycles/
object mm

Cycles/
object in

Cycles per distance on the object being photographed (what many people think of as the subject). Pixel spacing and magnification must be entered. Important when the system specification references the object being photographed.

Cycles/distance is useful for machine vision tasks, for example, where a surface is being inspected for fine cracks, and cracks of a certain width need to be detected. Object-centric.

Line Widths/
Crop Height

Line Pairs/
Crop Height

Primarily used for testing when the active chart height (rather than the total image height) is significant.

Not recommended for comparisons because the measurement is dependent on the (ROI) crop height.

Line Widths/
Feature Height (Px)

Line Pairs/
Feature Height (Px)

(formerly Line Widths or Line Pairs/N Pixels (PH))

When either of these is selected, a Feature Ht pixels box appears to the right of the MTF plot units (sometimes used for Magnification) that lets you enter a feature height in pixels, which could be the height of a monitor under test, a test chart, or the active field of view in an image that has an inactive area. The feature height in pixels must be measured individually in each camera image. Example below.

Useful for comparing the resolution of specific objects for cameras with different image or pixel sizes. Object-centric.

PH = Picture Height in pixels. FL(mm) = Lens focal length in mm. Pixel pitch = distance per pixel on the sensor = 1/(pixels per distance).
Note: Different units scale differently with image sensor and pixel size.

		Summary metrics: MTF50P and MTF area normalized) are recommended for comparing cameras.
Summary Metric	Description	Comments
MTF50 MTFnn	Spatial frequency where MTF is 50% (nn%) of the low (0) frequency MTF. MTF50 (nn = 50) is widely used because it corresponds to bandwidth (the half-power frequency) in electrical engineering.	The most common summary metric; correlates well with perceived sharpness. Increases with increasing software sharpening; may be misleading because it “rewards” excessive sharpening, which results in visible and possibly annoying “halos” at edges.
MTF50P MTFnnP	Spatial frequency where MTF is 50% (nn%) of the peak MTF.	Identical to MTF50 for low to moderate software sharpening, but lower than MTF50 when there is a software sharpening peak (maximum MTF > 1). Much less sensitive to software sharpening than MTF50 (as shown in a paper we presented at Electronic Imaging 2020). All in all, a better metric.
MTF area normalized	Area under an MTF curve (below the Nyquist frequency), normalized to its peak value (1 at f = 0 when there is little or no sharpening, but the peak may be » 1 for strong sharpening).	A particularly interesting new metric because it closely tracks MTF50 for little or no sharpening, but does not increase for strong oversharpening; i.e., it does not reward excessive sharpening. Still relatively unfamiliar. Described in Slanted-Edge MTF measurement consistency.
MTF10, MTF10P, MTF20, MTF20P	Spatial frequencies where MTF is 10 or 20% of the zero frequency or peak MTF	These numbers are of interest because they are comparable to the “vanishing resolution” (Rayleigh limit). Noise can strongly affect results at the 10% levels or lower. MTF20 (or MTF20P) in Line Widths per Picture Height (LW/PH) is closest to analog TV Lines. Details on measuring monitor TV lines are found here.