Motorized Zoom Lens Sensor Format

The Image Circle Is Not Fixed

A zoom lens changes magnification by moving internal groups, and the illuminated circle those groups project changes with their positions. Where a fixed lens has one image circle that either covers the sensor or does not, a zoom lens has a range of them.

This is the single most common source of dark corners on zoom systems, because a demonstration at one magnification can show full coverage while another position does not.

Which End Usually Loses Coverage

Coverage tends to be tightest at the end where the field is largest, because that is the position at which the projection is most demanding on the optics.

The direction differs between designs, which is why the check has to be empirical rather than assumed. What transfers between designs is only the need to check both ends.

How to Check It

Image a uniform white target at the minimum magnification and again at the maximum, and compare the corner intensity with the centre at each.

A drop that is present at one end and absent at the other confirms the effect. A drop present at both may be illumination rather than coverage, which is a different problem with a different fix.

Doing this before the camera is ordered avoids discovering it during integration, when the remedy is either a different camera or a different lens.

Why Extension Tubes Make It Worse

Adding an external extension moves the lens beyond its design flange distance and enlarges the projected image. The enlargement increases with the extension length, so the image grows faster than the sensor area it must cover.

The result is that an extension added to reach a working distance can remove corner coverage that was previously adequate. The mechanical problem is solved and an optical one is created.

What to State in a Requirement

State the sensor format, the magnification range in which the sensor must be fully covered, and the largest field required.

Stating the range rather than a single magnification is what allows the supplier to check coverage across the travel instead of at one point. A requirement that names only the working magnification leaves the other positions unverified, and those are the ones that produce the intermittent dark-corner complaint.

The Camera Decision Order

Choose the sensor format from the field and resolution requirement, then confirm that the lens covers it over the whole zoom range, then confirm the mount.

Doing it in that order keeps the optics as a constraint on the camera rather than as a surprise after the camera is bought.

Frequently asked questions

Why do I get dark corners only at one magnification?

Because the image circle changes as the lens zooms. A sensor covered at one position can be outside the illuminated circle at another. Check corner intensity at both ends of the range.

Can I use a larger sensor if the corners are dark?

No. A larger sensor makes the problem worse, because more of its area falls outside the image circle. The fix is a different lens or a smaller sensor.

Do extension tubes reduce corner illumination?

They can. An extension moves the lens beyond its design flange distance, which enlarges the projected image and can push the circle inside the sensor corners.

How do I tell a coverage problem from an illumination problem?

A coverage problem appears as a symmetrical corner drop that changes with zoom position. An illumination problem usually varies with the source arrangement as well. Moving the illumination without changing the zoom separates the two.

Is the coverage the same at both ends of the zoom range?

Not necessarily, and the direction of the difference varies between designs. It has to be measured on the specific lens rather than assumed.

Should the lens or the camera be chosen first?

The sensor format is chosen from the field and resolution requirement, then the lens is checked for coverage across the whole zoom range. Choosing the camera last avoids buying a sensor the optics cannot fill.