Image material and comparability

Image suitability

The suitability of a questioned image cannot be disposed of by a single figure. Technical resolution is indeed a relevant marker, but more recent studies expressly show that no clear, generally valid threshold for expert usability can be derived from it. Images of similar size may differ greatly in usefulness depending on contrast, sharpness, artefacts, camera characteristics, covering, and the feature constellation.

For practice this means: a measurement image is not suitable simply because it is digitally large. Nor is it unsuitable simply because at first glance it appears technically limited. What matters is whether the structures important for the evidential question are recognisable and can in fact be compared meaningfully in a later juxtaposition. Particular examination is required of sharpness, contrast, illumination, perspective, compression, covering, and the number of actually visible features.

The examination of image suitability is therefore not a formal preliminary act, but a professional step in its own right. It helps determine whether a full assessment is sensible or whether narrow methodological limits already become apparent at this stage.

Comparability

Material is comparable only where the same, or at least corresponding, feature regions are actually visible on both sides. Differences in head position, gaze direction, focal length, illumination, sharpness, compression, or covering may alter individual regions to such an extent that an apparent finding becomes nothing more than a technical artefact. Comparability therefore means more than the mere presence of two images of a person.

Forensic practice must therefore distinguish carefully between recognisability and comparability. A feature may be visible on both sides and still not be comparable in a methodologically sound way. Especially in low-grade source material, there is otherwise a risk that differences caused only by imaging conditions are interpreted as personal characteristics, or that actual differences are smoothed away.

That is why comparability requires a sober examination of which regions can really be juxtaposed. Where this is not the case, the limitation itself forms part of the conclusion. The absence of comparability is not a defect of the opinion, but a factual property of the material.

Image suitability and the limits of technical thresholds

The suitability of an image for a morphological identity opinion cannot be reduced to a single technical threshold. Resolution and facial image size are important factors, but they do not by themselves determine usability. Even at lower resolution, material may still be assessable in an individual case, whereas images with apparently favourable technical values may remain only of limited use because of low contrast, covering, shadowing, perspective, or artefact-related disturbances.

For expert assessment, several considerations must therefore always be examined together: the number of recognisable features, their recognisability, their individual character, and the technical and recording-related conditions of the image. A serious opinion addresses these influences expressly and states the resulting limits of the conclusion.

Typical disruptive factors and artefacts

Typical disruptive factors include, among other things, focal length, object distance, illumination, motion blur, pixel artefacts, facial expression, age-related change, covering, disease- or lifestyle-related changes of the soft tissue relief, and other alterations of appearance. Such influences may create apparent dissimilarities, but they may also conceal true differences.

For that reason, each observed feature must be examined as to whether it is genuinely morphologically reliable or better explained technically. Artefacts are not a side issue, but part of the core expert task.

Documentation

Documentation concerns not only the later opinion, but already the material chain. Important are original files, metadata, known exports, post-processing steps, where applicable sequence information, and the exact designation of the comparison images used. Only in this way does it remain traceable on what basis the later conclusion rests.

This is especially important where questioned material has already passed through several stations before the instruction: police export, messenger forwarding, screenshot, printout, or scan. Every such step may alter quality, crop, colour impression, or the visibility of individual regions. Without clean documentation, it later becomes difficult to distinguish original properties of the image from later alterations.

Documentation is therefore not an appendix but part of the professional basis. It secures the distinction between source material, technical visualisation, and expert interpretation, and thereby also the traceability of the later evidential assessment.

Comparison images and 3D

Where a question makes it appear useful, standardised comparison images or supplementary 3D acquisitions may be helpful. Their purpose lies in better control of perspective, surface course, and comparison position. They are, however, no end in themselves. Not every file requires this effort, and not every discrepancy can subsequently be compensated for by additional recordings.

In practice, comparison images are especially helpful where the available material is limited in perspective or quality, but the relevant feature regions can in principle still be documented more suitably. Supplementary 3D data may also be useful where contours, depth relations, or asymmetries need to be examined more closely. Decisive remains that such material serves the evidential question rather than merely enlarging the file.

Even where comparison images or 3D data exist, the substantive assessment remains an expert task. The additional material may improve the basis of comparison, but it does not replace the methodological weighing of what really follows from it.

File formats

Various output formats may be provided for supplementary 3D data. They serve controlled further processing and documentation, not the replacement of the expert assessment.