Lens Metrics

What do the metrics mean?

Information about Metrics

FoCal IQ provides a lot of metrics - measurements of various aspects of the equipment.

The following sections give a brief explanation of the meaning of each metric.

Note: You can quickly jump to the appropriate section from the details pages by clicking on the help icon .

Typical Focus Adjustment Magnitude

This is a measure of the size of the typical total amount (magnitude) of focus adjustment a lens requires across all camera bodies.

This ignores the direction of adjustment, so if two users were to apply settings of +6 and -6 respectively then the magnitude would be 6. We calculate this value from individual results so the magnitude can have a slightly different value to the typical value (with direction).

The magnitude of the focus adjustment is good to show an indication of how much a lens is likely to need calibrating, ignoring the direction. Any value greater than about 2 or 3 indicates that the lenses are coming out the factory with a requirement for user calibration, and it would be a very good idea to calibrate your lens if you haven't done so!

Focus Adjustment Variation

The focus adjustment variation shows the range of focus adjustment values in which around 70% of users fall.

A small value (e.g. 2 or 3 focus adjustment units) indicates that most copies of the lens need a similar amount of calibration (not necessarily no calibration). A large value (e.g. 10 or more) indicate that there is a significant spread of focus adjustment required.

Adjusted Focus Quality Improvement

This value indicates the typical percentage quality improvement between an uncalibrated lens and a calibrated lens.

A high value indicates that calibration is more essential for this lens.

Parfocality

(Zoom Lenses Only) A parfocal zoom lens is one which does not require any adjustment in focus position as you zoom through the range. So if you focus the lens when wide open (e.g. 24mm) and then zoom to the telephoto end (e.g. 70mm), the image will still be in focus.

Parfocality is a theoretical ideal, and even the most expensive lenses require a small amount of shift as you change zoom. However, lower quality lenses or those with a compromised lens design may exhibit a large amount of defocusing between the ends of the zoom range.

We express parfocality in focus calibration units (manufacturer specific) - so that's AF Microadjustment units on Canon, AF Fine Tune on Nikon/Sony etc. The value is derived from the Wide/Telephoto Difference histogram data.

An ideal lens will have a parfocality value of 0, meaning the same calibration value is required at both ends of the lens. In reality, a value of 1 or 2 units is acceptable - indeed typical - for a high quality lens. As this value increases, it indicates that there is a larger discrepancy between the calibration requirement at either end of the zoom range and suggests that other aspects of the lens design may also not be optimal.

Parfocality Variation

Like the Focus Adjustment Variation, this value indicates how similar the parfocality is across the whole set of lenses tested.

A small value indicates that all lenses are very similar which is a good thing. Consistency - even among issues like focus differences across the zoom range - suggests that lenses are coming out the factory in a very similar, uniform state.

Typical Focus Adjustment Value

This is the typical value that is set on cameras to calibrate the focus.

The word typical is used here to indicate the median value, which is the center of everyone's settings.

The AF Microadjustment / AF Fine Tune histogram chart can give a visual representation of this value.

A value close to 0 is ideal, meaning that most users are not requiring much calibration. A value away from zero suggests that this lens should be calibrated to get the best from it.

Calibration Quality Variation

The Calibration Quality Variation is an indication of how much the typical, calibrated, wide open quality varies among users.

It is expressed as a percentage, where 0% indicates that all users have exactly the same quality value (this would never happen!).

Peak Aperture Range

This is the range of aperture values that approximately 70% os users fall within. This is expressed as a fractional aperture range, meaning the values may not be selectable on a camera but represents the actual aperture values for the range.

Typical Peak Aperture

The typical peak aperture is the aperture where the largest amount of users achieve the highest image quality. It is derived from the aperture distribution, usually the highest bar on the Peak Quality Aperture Distribution chart.

Aperture Stops (Wide to Peak)

This is the number of aperture stops between wide open aperture and the peak aperture. If a lens is sharpest wide-open, this value will be 0. As the peak image quality aperture moves away from the wide open aperture this value increases.

As an example, if the wide aperture is f/2.8 and the peak quality aperture is f/5.6, there are 2 stops between the wide and peak apertures.

Aperture at Peak Quality

This value is the aperture of the best image quality, derived from the combined aperture profile data.

This provides a slightly different view of the best aperture from Typical Peak Aperture, although they should be very similar.

Wide Quality Change (vs best IQ)

This is the percentage quality change from the peak image quality to the quality when the lens is wide open.

If the lens is sharpest wide open, this value will be 0%. Otherwise it will be a negative value, indicating a drop in quality as the lens aperture is opened from the best quality aperture.

Peak Quality Variation

The peak quality variation is the range of the measured peak quality, expressed as a fraction of the typical peak quality.

A lens that has very little variation in peak quality will have a low value, and as this number increases it indicates more variation in the best quality achievable across copies of this lens.

High Quality Apertures (to f/8)

This is a count of the number of apertures between wide open and f/8 that achieve a quality with approximately one-half of the depth of field from the peak quality.

A higher count indicates that more of the lens aperture range is available for very high quality captures.

Narrow apertures (with an f-number greater than f/8) are ignored due to the effect of diffraction.

Wide Aperture

This is simply the widest aperture available for the lens at the tested focal length.

Astigmatism (at best IQ)

The astigmatism at the aperture with the best image quality.

Astigmatism (wide)

The astigmatism at the wide aperture when the lens it optimally calibrated.

Median Astigmatism

The median (average) astigmatism across the whole of the aperture range.

Wide Astigmatism Change (vs best IQ)

The percentage astigmatism change between the value at the aperture of best image quality and the value when the lens is wide open.

Red Quality (at best IQ)

The red quality at the aperture with the best image quality.

Red Quality (wide)

The red quality at the wide aperture when the lens it optimally calibrated.

Median Red Quality

The median (average) red quality across the whole of the aperture range.

Wide Red Quality Change (vs best IQ)

The percentage red quality change between the value at the aperture of best image quality and the value when the lens is wide open.

Blue Quality (at best IQ)

The blue quality at the aperture with the best image quality.

Blue Quality (wide)

The blue quality at the wide aperture when the lens it optimally calibrated.

Median Blue Quality

The median (average) blue quality across the whole of the aperture range.

Wide Blue Quality Change (vs best IQ)

The percentage blue quality change between the value at the aperture of best image quality and the value when the lens is wide open.

Aperture Change Image Motion

This is the average movement in the image position (in µm) across the aperture range.

Focus Change Image Motion

This is the average movement in the image position (in µm) across the focus range.

Calibrated Focus Error

The Calibrated Focus Error is the combined measured focus error across the range of focus calibration measurements, adjusted to focus adjustment units.

A value of 0 is best, indicating that all autofocus operations would focus on exactly the same point.

Vignetting

Vignetting is the darkening of the corners of an image and is shown as the EV drop between to the wide-open corner brightness level.

Affected Stop Count

This is the number of aperture stops affected by vignetting - counting from the widest aperture through to the peak aperture. Narrow apertures are ignored.