Optimal viewing scale

Definition

The optimal viewing scale is determined by the highest resolution
at which an image is of excellent quality, and has no visible defects.

Scale affects defect visibility

Wrong focus, graininess, processing artefacts can detract from an image's appeal. Viewing an image at a smaller size or scale factor, can make degradations less visible or annoying, improving the image's perceived quality.

A down-scaled version (20%) of an original photo, showing excellent quality and no visible defects. This is the optimal viewing scale if it is the largest resolution that is also of excellent quality.

A crop from the original 6000x4000 px photo (100%). The image appears to be of low quality, blurry, and grainy.

Quality defects reduce image clarity and fidelity

Examples of individual types of defects. The images are heavily degraded on purpose to make it obvious. In natural photos we usually find combinations of defects, which have varying degrees of visibility. In the figure below, the optimal viewing scale of the distorted images is very small.

A high quality image (left-most) is degraded in several ways (right).

Finding the optimal scale

From left to right: an image is downsized to find its optimal scale. The zoom value is shown at the top in blue. The left image is partially visible to make it fit into view.

Left image: a high-resolution, but overall out of focus, and low-quality image is downscaled to a lower resolution.

Middle: at the optimal viewing scale, the image shows no quality degradation, and essential image details are sharp and clear. The essential details are in the foreground, thus it is acceptable for the background to be out of focus.

Right: further downsizing does not improve the quality of the image.

At the optimal viewing scale:

The image is of excellent quality.
It is well focused, clear and of high fidelity.
Exception: it is acceptable for parts of a photo to be out of focus, if this is used for artistic effect, for instance in macro, tilt-shift or other well-captured narrow depth-of-field shots.
The image is NOT showing any noise, artefacts, or other defects.
It is best suited to be printed or displayed.
e.g. wall prints, magazine cover, desktop background, etc.

In the examples below ...

Each row shows crops taken from the same source image. Each crop is extracted after previously downscaling the source image by a different amount. You can often notice strong degradations at the original size (left-most), whereas the lowest resolution is of excellent quality (right-most image).
The OPTIMAL SCALE is often somewhere in between.

Note: it is best to view each image as is, do not click to enlarge.

The image is at its optimal scale at the original size (100% zoom). This macro-shot is well focused; the blurs are used for artistic effect, and to enhance the perception of depth.

Artistic intent

The optimal scale of the image on the left is 100%. Even though there are many out-of-focus areas, these are used for improving the feeling of depth. This effect is often encountered in macro-photography (close-ups). The focus is correct, as the frontal plane is in focus, this includes the droplets and flower petals nearest to the camera.

Participating in the experiment

You will annotate images using a browser-based interface, called Zovi. This allows browsing a collection of images, zooming to select the optimal scale for each, and saving the selection.

Zovi UI controls

Zoom: use the zoom slider at the top of the screen
Pan: click and drag the image
Next image: press the green arrow button or the right arrow key.

Zovi annotation interface for selecting the optimal scale / zoom.

Steps to complete the work

Open Zovi in the Chrome web-browser
Select the optimal scale for each image
Move to the next image and repeat
Submit your results at the end

2. Select the optimal scale

explore the image to find any visible defects
zoom out until the defects are no longer visible
repeat, if necessary, until no other defects are found

It takes about 15 seconds to find the optimal scale/zoom for each image.
Thank you for participating in our study!