Optical Resolution is the ability of an eye, camera, lens, optical instrument, sensor, display or complete imaging system to distinguish fine spatial detail within an object, scene, image or view.
In its broadest perspective sense, Optical Resolution concerns how much structural information can actually be distinguished at a particular scale, distance, Field of View and set of observing or imaging conditions.
Resolution is therefore not determined by one factor alone. It may depend upon:
- optical quality;
- aperture;
- wavelength;
- focus;
- aberrations;
- magnification;
- projection scale;
- sensor or detector sampling;
- pixel dimensions;
- display size;
- viewing distance;
- contrast;
- illumination;
- atmospheric conditions;
- motion; and
- the resolving capacity of the final observer.
The fundamental question is:
What is the smallest spatial structure that can still be distinguished through this Perspective System under these particular conditions?
Optical Resolution within Perspective
Perspective concerns the transformation of spatial information from Object or Target Space into one or more Image or Perspective Spaces.
Optical Resolution determines how much of that spatial information survives the imaging process as distinguishable structure.
Thus:
Physical Structure → Optical / Perspective Process → Resolved Image Structure
A Perspective Image may reproduce the general geometry of an object correctly while failing to preserve its finest structural detail.
Resolution therefore concerns not merely whether an image exists, but how much information about the target can be distinguished within that image.
Resolution Terms at a Glance
Several related meanings of resolution need to be distinguished.
Optical Resolution
The ability of an optical or imaging system to resolve detail in the object being imaged.
Visual Resolution / Visual Acuity
The ability of an observer or visual system to distinguish fine spatial detail.
Spatial Resolution
The amount or fineness of spatial detail distinguishable within an image or spatial field.
Angular Resolution
The ability to distinguish spatial detail according to the angular separation subtended at the eye, camera or optical system.
Projection Scale Resolution
The smallest level of Object-Space structural detail or size interval discernible within a Perspective Image at a particular projection scale.
Sensor or Detector Resolution
The resolving limitation associated with the sampling or detecting element of an imaging system.
Pixel or Display Resolution
The number and arrangement of discrete image samples available to record or display an image.
Temporal Resolution
The fineness with which change through time is recorded or displayed, for example through frame rate.
Resolving Power
The capacity of an optical or imaging system to separate fine details that are close together.
These meanings are related but should not be treated as exact synonyms.
Optical Resolution of a Complete Imaging System
An Optical Image is rarely determined by a lens alone.
A complete imaging system may include:
- the spatial target;
- the surrounding environment;
- one or more optical elements;
- an aperture;
- a sensor or detector;
- image processing;
- storage or transmission;
- a display; and
- a final human or technical observer.
Each component may impose its own resolution limitation.
The effective resolution of the complete system therefore depends upon the combined performance of the entire Optical Image Chain.
A very high-resolution sensor, for example, cannot recover spatial detail that was never resolved by the preceding optical system.
Resolution and the Optical Image Chain
Resolution can change at successive stages of an Optical Image Chain.
For example:
Object Space → Lens → Sensor → Digital Image → Processing → Display → Eye
The system may lose detail through:
- optical blur;
- diffraction;
- aberrations;
- limited detector sampling;
- compression;
- resampling;
- display limitations;
- viewing distance; or
- the visual acuity of the observer.
The final visible resolution is therefore determined by the complete sequence rather than by the nominal specification of only one component.
Visual Resolution
Visual Resolution refers to the smallest discernible detail within a visual image or view.
The Dictionary also relates Visual Resolution to Projection Scale Resolution and notes that it may be assessed through structural decomposition or angular resolution.
The important point is that an object may remain physically present while some or all of its detail can no longer be distinguished.
Thus:
Physical detail may exist without being visually resolved.
Visual Acuity
Visual Acuity is the capacity of an observer or imaging system to resolve fine spatial detail.
For the human eye, acuity depends upon several interacting conditions, including:
- focus;
- pupil or aperture size;
- retinal sampling;
- illumination;
- contrast;
- wavelength;
- Field of View;
- object distance; and
- the physiological condition of the visual system.
For an artificial optical system, closely related variables include lens quality, aperture, aberrations, detector performance and sampling.
Visual Acuity should therefore not be treated simply as a fixed property independent of the conditions of observation.
Perspective of Visual Acuity
Perspective of Visual Acuity concerns changes in image sharpness, clarity and visible detail under different perspective and optical conditions.
An object may appear sharply resolved within one depth region and blurred in another.
Resolution may also change according to:
- magnification;
- lighting level;
- Field of View;
- aperture;
- focus adjustment;
- object depth; and
- the particular optical instrument employed.
This forms part of the broader Gradient of Acuity Perspective, in which clarity, detail and resolution vary across a Perspective Image or View.
Spatial Resolution
Spatial Resolution concerns the fineness of spatial structure that can be distinguished within an image.
It may refer to such things as the ability to resolve:
- small points;
- fine lines;
- closely spaced edges;
- surface textures;
- small openings;
- boundary irregularities; or
- other local image structures.
Spatial Resolution should not be confused merely with the physical size of the displayed image.
A larger image may reveal more detail only when the original imaging system has actually recorded sufficient structural information.
Angular Resolution
Angular Resolution expresses resolving ability in terms of the angular separation of spatial details as seen from an eye, camera or other optical centre.
It is particularly important because many Perspective Phenomena depend upon visual angle.
As an object recedes, a fixed physical detail normally subtends a smaller angle at the observer.
Eventually, separate features may subtend angles too small to remain distinguishable.
Thus:
greater distance → smaller visual angle → reduced ability to resolve fixed-size detail
Angular Resolution therefore connects Optical Resolution directly with Viewpoint, distance and Diminution of Size Perspective.
Projection Scale
Projection Scale describes the relationship between a dimension in Object Space and its corresponding dimension within Image Space.
For example, a 10-metre object represented by a 10-centimetre image has a particular image-to-object magnification and Projection Scale.
Projection Scale can vary across a Perspective Image, particularly with depth in central projection.
Resolution must therefore be considered in relation to the local scale at which Object-Space structure has been projected into the image.
Projection Scale Resolution
Projection Scale Resolution is the smallest level of structural detail—or Object-Space size interval—that can be discerned within a Perspective Image or View at a particular Projection Scale.
This is a particularly important concept within perspective.
If an object is imaged at a larger effective Projection Scale, smaller structural details may become visible.
For example, the apparently smooth outline of a pebble may reveal progressively smaller irregularities when viewed at increased magnification.
Thus:
increased useful Projection Scale → potentially finer Projection Scale Resolution → additional visible structure
This improvement continues only until another limitation of the imaging system is reached.
Projection Scale Resolution and Object-Space Units
Projection Scale Resolution is especially useful because it can be expressed in terms of the smallest Object-Space feature represented detectably in the image.
Two images may possess the same pixel dimensions while representing radically different areas of Object Space.
A microscopic image and an astronomical image could therefore have identical digital pixel dimensions but entirely different Object-Space resolutions.
The useful question is not merely:
How many pixels does the image contain?
but:
What smallest feature in the target does each image sample meaningfully represent?
Sensor and Detector Resolution
Sensor or Detector Resolution concerns the ability of the detecting system to record spatial detail delivered to it.
A digital sensor contains a finite number of sampling elements.
If an object boundary is represented by too few samples, small structural irregularities may:
- disappear;
- merge;
- become simplified;
- become aliased; or
- fail to be recorded at all.
Detector resolution therefore places a limit upon how much of the optically formed image can be retained in recorded form.
Increasing detector sampling can improve the recorded resolution only if useful optical information is available for the additional samples to capture.
Image Resolution
Image Resolution is a general expression for the amount of distinguishable spatial detail contained within an image.
In digital imaging it is often discussed using pixel dimensions, but pixel count alone does not determine complete Optical Resolution.
Image quality can also depend upon:
- lens performance;
- focus;
- sensor quality;
- sampling;
- compression;
- processing;
- display size; and
- viewing conditions.
A high-pixel-count image can therefore still contain poor resolved detail if the original optical image was blurred, defocused or otherwise limited.
Pixel Resolution
Pixel Resolution concerns the discrete digital sampling of an image.
Modern display and video formats are often described by the number of pixels in each dimension, for example:
- 1920 × 1080;
- 3840 × 2160;
- 7680 × 4320.
These are commonly described as Full HD, 4K UHD and 8K UHD respectively.
Such figures describe digital sampling dimensions rather than the complete Optical Resolution of the original scene-to-viewer system.
Two 4K images can therefore contain very different levels of actual resolved scene detail.
Display Resolution
Display Resolution is the number of distinct display pixels available in each image dimension.
It defines the digital sampling capability of the display surface.
However, the visible detail delivered to an observer also depends upon:
- screen size;
- pixel density;
- viewing distance;
- display optics;
- contrast;
- the resolution of the source image; and
- the observer’s Visual Acuity.
A high-resolution display cannot restore genuine scene structure that was absent from the captured or generated source image.
Spatial Resolution and Temporal Resolution
Spatial Resolution and Temporal Resolution describe different image properties.
Spatial Resolution concerns fine detail within an individual image or frame.
Temporal Resolution concerns how finely change through time is sampled.
For moving images:
Spatial Resolution → detail within each frame
Temporal Resolution → frequency of successive frames
A higher frame rate increases Temporal Resolution but does not automatically increase the Spatial Resolution of each frame.
Resolving Power
Resolving Power is the capacity of an optical or imaging system to distinguish closely neighbouring spatial details as separate.
It describes a capability rather than merely a nominal image dimension.
An instrument’s useful Resolving Power can be limited by:
- diffraction;
- optical aberrations;
- focus;
- aperture;
- wavelength;
- detector sampling;
- contrast;
- atmospheric conditions;
- motion; and
- signal or image processing.
Beyond the effective resolution limit, simply making the image larger will no longer reveal genuine additional structure.
The Diffraction Limit
Even a theoretically perfect optical system cannot resolve infinitely fine detail.
Because light behaves as a wave, a finite aperture produces diffraction.
This establishes a fundamental physical limit to the resolution obtainable through an optical system.
The limiting resolution depends particularly upon:
- wavelength; and
- aperture size.
An optical system may therefore reach a point at which improved detector sampling or greater image enlargement no longer produces corresponding gains in genuine resolved information.
Airy Disk
The Airy Disk is the diffraction-limited spot formed when a point source is imaged through a perfect circular aperture.
A theoretically perfect point in Object Space therefore does not normally produce an infinitely small geometrical point in a real optical image.
Instead, diffraction spreads the light into a finite pattern.
The scale of this diffraction pattern establishes a fundamental limitation on the separation of neighbouring image details.
The Airy Disk therefore links geometrical point projection with the physical limits of Optical Resolution.
Rayleigh Criterion
The Rayleigh Criterion provides a conventional criterion for deciding when two diffraction-limited point images can be regarded as separately resolved.
In the Dictionary formulation, the limit occurs when the first diffraction minimum of one point source coincides with the maximum of another.
The principle demonstrates that:
Optical Resolution ultimately has a wavelength-dependent physical limit.
Resolution therefore cannot be improved indefinitely merely by adding pixels, enlarging the image or increasing magnification.
Magnification is not Resolution
Magnification and Resolution should not be confused.
Magnification makes an image or represented detail larger.
Resolution determines whether additional structure is actually distinguishable.
Thus:
Magnification → increases image scale
Resolution → determines available distinguishable detail
An enlarged low-resolution image may simply show the same unresolved information at a larger size.
Useful magnification reveals additional detail only while the preceding optical and detector system contains that information.
Focal Length is not Angular Resolving Power
A longer focal length normally produces a larger image scale for a given image format and narrows the Field of View.
However, increased focal length does not by itself increase the fundamental angular resolving power of an optical system.
Diffraction-limited angular resolution depends principally upon factors such as:
- aperture diameter; and
- wavelength.
This distinction is important because a large image is not necessarily a better-resolved image.
Optical Zoom and Digital Zoom
Optical Zoom changes the magnification of the captured optical image by altering the optical system.
If the lens and sensor have sufficient performance, this can preserve or reveal useful detail because the subject occupies a larger region of the detector.
Digital Zoom instead crops and enlarges already recorded image information.
It cannot restore genuine detail that was never captured.
Thus:
Optical Zoom → changes optical image scale before or during capture
Digital Zoom → enlarges already sampled image information
Digital enlargement can improve visibility of existing pixels but does not inherently improve the original Optical Resolution.
Focus and Resolution
Fine detail can be resolved only when the optical image is sufficiently well focused.
Defocus spreads the image of each Object-Space point over a larger Image-Space region and reduces local sharpness.
Resolution therefore depends partly upon:
- focus position;
- object distance;
- focal length;
- aperture; and
- the acceptable standard of image sharpness.
However, focus is not itself resolution. A perfectly focused image can still be resolution-limited by diffraction, aberration, detector sampling or other factors.
Depth of Field is not Visual Acuity
Depth of Field is the range of Object-Space distances reproduced with a selected standard of acceptable sharpness.
Visual Acuity or Optical Resolution concerns the ability to distinguish fine detail within the image.
The concepts are therefore related but different.
A wide Depth of Field does not necessarily imply very high Optical Resolution.
For example, reducing an aperture can increase Depth of Field by reducing defocus, but an excessively small aperture can eventually reduce fine-detail resolution through diffraction.
Optical Aberrations
Real lenses are also affected by Optical Aberrations that may reduce resolution.
Relevant aberrations can include:
- spherical aberration;
- coma;
- astigmatism;
- field curvature;
- distortion; and
- chromatic aberration.
These can alter the concentration, shape or position of image detail and reduce the effective sharpness obtainable from the optical system.
The useful Optical Resolution of a real instrument is therefore commonly below the ideal diffraction limit unless its aberrations are sufficiently controlled.
Contrast and Resolution
Resolution is also influenced by contrast.
A fine detail may possess sufficient geometrical size to be theoretically resolved but remain invisible because its contrast relative to the surrounding image is too low.
This becomes particularly important at long viewing distances, where atmospheric scattering and transmission can reduce edge contrast.
Thus:
geometrically present detail ≠ necessarily optically visible detail
This distinction connects Optical Resolution with Atmospheric Perspective and Diminution of Contrast Perspective.
Atmosphere and Optical Resolution
The environment between an object and observer can form part of the effective optical system.
Atmospheric conditions can reduce visible resolution through:
- scattering;
- absorption;
- haze;
- turbulence;
- reduced contrast; and
- other transmission effects.
The Dictionary therefore explicitly notes that the environment in which imaging occurs can contribute to the Optical Resolution of the overall system.
Resolution is consequently not always an intrinsic property of the camera, eye or lens alone.
Diminution of Form and Resolution
Diminution of Form concerns the loss of visible structural or outline detail in a Perspective Image or View.
Volume 1 distinguishes several possible causes:
- loss of outline detail through atmospheric contrast blurring;
- loss of detail through reduced Projection Scale Resolution or detector sampling;
- loss of visible detail through Visual Acuity and optical limitations;
- limitations arising from Shape Sufficiency and the scale at which Form is being modelled or represented.
Thus, an object can appear progressively simpler with distance even though its Physical Form remains unchanged.
Optical Resolution and Apparent Form
The resolution at which an object is observed can affect its Apparent Form.
At low resolution:
- small protrusions may disappear;
- separate details may merge;
- holes may close visually;
- irregular edges may appear smoother;
- fine textures may disappear; and
- complex boundaries may become simplified.
At higher useful resolution, additional structural details can become apparent.
The same Physical Form can therefore produce different Apparent Forms at different scales and resolutions.
The Scale–Shape–Size Problem
Optical Resolution is fundamental to the Scale–Shape–Size Problem.
As Projection Scale and Resolution change, different levels of structural Form become visible.
Consequently:
Shape and measured Size are partly functions of the scale and resolution at which a Form is observed or represented.
A boundary that appears smooth at one scale may appear irregular at another.
A measurement made using coarse structural intervals may therefore differ from one made after finer detail becomes visible.
This does not mean that the physical object changes. It means that different levels of its structure become available to observation and measurement.
The Shape Sufficiency Problem
Resolution is also related to the Shape Sufficiency Problem.
Perspective analysis frequently represents physical reality using simplified geometrical Forms that are considered sufficiently accurate at a chosen scale.
A surface may be modelled as flat, a boundary as straight or a set of lines as parallel because smaller irregularities are irrelevant or unresolved at the working scale.
At greater magnification and higher resolution, new structural information may appear and the earlier geometrical description may no longer be sufficient.
Thus an ultimate analytical limit is not determined by imaging sharpness alone, but also by what geometrical model remains meaningful at the newly resolved scale.
Resolution and Multi-Scale Perspective
Multi-Scale Perspective connects views and images produced at different:
- magnifications;
- Fields of View;
- Projection Scales; and
- Spatial Resolutions.
A macroscopic image, microscopic image, telescopic view and nanoscopic image may all represent the same broader physical reality while revealing completely different levels of structure.
To compare such images meaningfully, the scale and resolution conditions belonging to each image must be recorded and understood.
Microscopes and Telescopes
Microscopes and telescopes can reveal spatial structures that are otherwise unresolved.
They do so by changing the angular or optical magnification with which target structures are presented to the detector or observer.
The newly visible detail can support finer observation and measurement.
However, magnification cannot continue to produce genuinely new information indefinitely.
Eventually, Optical Resolution is limited by such factors as:
- aperture;
- wavelength;
- diffraction;
- aberration;
- detector sampling;
- signal quality; and
- the structure of the target itself.
Digital Enlargement cannot create original detail
A key distinction must be maintained between revealing recorded detail and creating a larger representation of insufficiently sampled information.
Digital enlargement can:
- make existing pixels easier to inspect;
- interpolate between samples;
- apply sharpening;
- estimate missing structure; or
- generate new image content computationally.
But enlargement alone cannot recover genuine original spatial information that was never resolved and recorded by the imaging system.
Any subsequently inferred or generated structure should therefore be distinguished from directly resolved source information.
Optical Vanishing Limit
Optical Resolution also establishes limits of visibility with increasing distance.
An Optical Vanishing Limit occurs when a specified object, line, pattern or detail can no longer be distinguished because its image has become:
- too small;
- too faint;
- too blurred;
- too low in contrast; or
- otherwise insufficiently resolved.
The limit may occur as a:
- point;
- line;
- boundary;
- region;
- distance; or
- gradual transition.
This is an optical rather than purely geometrical form of vanishing.
Optical Resolution and Geometrical Vanishing
Geometrical Vanishing and Optical Vanishing should be distinguished.
A geometrical Vanishing Point represents the projective limit of a spatial direction.
Optical vanishing occurs when an object or detail becomes visually unresolved.
Thus:
Geometrical Vanishing → projective limit
Optical Vanishing → resolution or visibility limit
An object can therefore be geometrically represented in a Perspective Image while being too small or indistinct to remain optically visible.
Optical Vanishing Sphere
The Optical Vanishing Sphere represents the maximum distance at which a specified object or detail remains detectable or resolvable under stated conditions.
Its extent depends upon such factors as:
- object size;
- contrast;
- illumination;
- atmospheric transmission;
- observer acuity;
- optical magnification;
- sensor resolution; and
- image processing.
There is therefore no single universal Optical Vanishing Sphere applying equally to every object or every Viewing Direction.
Large, bright, high-contrast objects may remain visible much farther away than small or low-contrast details.
Resolution is not Image Quality
Resolution is only one facet of overall image quality.
A high-resolution image can still be poor in other respects, including:
- contrast;
- colour accuracy;
- dynamic range;
- noise;
- distortion;
- motion rendering;
- tonal reproduction; or
- geometrical accuracy.
Conversely, a lower-resolution image may still communicate the important Forms and spatial relationships required for a particular task.
The necessary resolution therefore depends partly upon the intended Perspective Function or Goal.
Why Optical Resolution matters
Optical Resolution is fundamental to perspective because every image or view has a finite capacity to communicate spatial detail.
Understanding resolution helps distinguish:
- physical detail from visible detail;
- magnification from genuine resolving power;
- Image Resolution from complete Optical Resolution;
- pixel dimensions from Object-Space detail;
- Spatial Resolution from Temporal Resolution;
- Visual Acuity from Depth of Field;
- Projection Scale from Projection Scale Resolution;
- optical information from digital enlargement;
- geometrical vanishing from optical vanishing;
- Physical Form from resolution-dependent Apparent Form; and
- increased image size from increased structural information.
Optical Resolution is therefore the ability of an eye, instrument or complete imaging system to distinguish fine spatial detail, governed by the interaction of projection scale, optics, aperture, wavelength, focus, aberrations, detector sampling, contrast, environmental conditions, display characteristics and the resolving ability of the final observer.
Related Perspective Topics
Perspective of Visual Acuity →
Projection Scale Resolution →
Scale–Shape–Size Problem →
Shape Sufficiency Problem →
Optical Image Chain →
Vanishing Structures →