Lens Vignetting in Embedded Cameras: How to Eliminate It

lens vignetting

Vignetting in photography might indicate that an image was shot or processed improperly. Unwanted lens vignetting may be inconvenient. However, there are situations when you may wish to maintain or even increase vignetting to an image.

In this post, we’ll look at lens vignetting, its types, recommendations, and how to get rid of it.

What is Lens Vignetting?

The term “vignette” was first used in the 18th century in English. Originally, the phrase refers to patterns on the edges of book pages. Later, early photographers used the phrase to describe an image that had lost definition around the edges. Vignetting is now defined as the darkening effect noticed around the margins of an image.

When an image’s brightness or saturation gradually decreases from its center to its four corners or edges, this is referred to as lens vignetting. It is caused by a variety of things, such as faults in the lens’ design, the focus point, or the aperture. The brightness decreases as one moves away from the center of the image.

What causes Lens Vignetting?

Images captured using telephoto, zoom, and lenses with a short focal length are particularly susceptible to lens vignetting. Because vignetting happens when the front of the lens prevents the camera from receiving light, it is significantly more obvious when the photograph is taken with a wide aperture.

Depending on the type of photo you’re photographing, vignetting may be better suited, no matter what the cause.

Types of Lens Vignetting

When taking photographs or examining images, vignetting can take many different types. There are optical, mechanical, and natural vignetting.

Whenever the lens barrel obstructs light, it causes optical vignetting. Additionally, it describes how light moves more slowly from a lens’s edge to a sensor.

Mechanical vignetting is a different sort of vignetting that can happen once something blocks the light coming from the sensor. This is frequently caused by lens accessories.

Natural vignetting is the type that is worth mentioning. The angle of light as it passes from the lens to the image sensor is referred to as this form of vignetting.

Optical Vignetting

Every lens inevitably experiences optical vignetting. The lens barrel ultimately prevents light from entering the lens in large-aperture lenses, preventing it from reaching the corners of the sensor. It can be quite prominent on some lenses while being hardly perceptible on others, based on the optical design and manufacture of the lens. However, vignetting still happens on the majority of contemporary lenses, particularly prime or fixed lenses with very large apertures.

Additionally, more elements cause the light intensity to decrease by the time it hits the sensor, like in a rectangle sensor, which will result in darker corners and a brighter center in your photos.

Mechanical Vignetting

Accessory vignetting is another name for mechanical vignetting. One of the most straightforward types of vignetting to comprehend is mechanical vignetting. Lens hoods are a common feature of wide-angle camera lenses. This lens hood aims to stop flares and ghosting in your photographs. Physical obstacles that restrict light from reaching the lens can include the lens barrel, a filter, a lens hood that is inadequately built or misaligned, or anything else in the way, which is the cause of this vignetting.

Mechanical vignetting generally appears as a prominent, circular darkening that is particularly noticeable in the corners of an image and disappears as the lens is stopped down (narrower aperture). The most straightforward vignetting technique is this one.

Pixel Vignetting

Pixel vignetting also affects digital cameras. It’s one more possible reason for the darkening of image edges. This sort of vignetting is unique to image sensors, unlike optical vignetting. Digital sensors’ flat surfaces mean that all of their pixels are identically constructed and oriented.

They have a curved lens and a flat, rectangular sensor. The central pixel of the sensor receives light at 90-degree angles, whereas the edges or corners get light at sloped angles. As a result, the sensors in the corners will see a little bit less pixel vignetting than those in the middle.

Artificial Vignetting

Vignetting is not necessarily a disadvantage or a problem. In certain instances, the vignetting effect can be attractive to the eye, directing the viewer’s focus from the edges of the frame to the center of the picture. In reality, while some photographers purposefully add vignetting or enhance its impact during post-processing, others like to leave optical vignetting in photographs without fixing it.

Furthermore, vignetting may provide a vintage sense to a shot. Whether to keep or introduce a vignette is a creative option; you can eliminate, improve, or construct a vignette.

Understanding Chief Ray Angle (CRA)

One of the crucial factors in choosing lenses and sensors is the chief ray angle (CRA). By casting shadows on the margins of the image, it degrades the visual quality. The design of the lens, not the sensor, determines the actual CRA of the lens, which is calculated from the light rays that travel through the lens. The CRA and microlens of the sensor are both connected to the effectiveness of the light incident on the CMOS or charge-coupled device (CCD) sensor.

Step-up Rings: Do They Cause Lens Vignetting?

A step-up ring is a device that allows you to utilize filters with a larger thread size than the lens. Although step-up rings are angled outwards, vignetting is still possible.

How to Reduce Lens Vignetting?

Vignetting is an unpleasant phenomenon that is difficult to completely eradicate. To get the appropriate image quality, however, vignetting correction techniques for embedded cameras must be used. There are several ways for removing lens vignetting. Some ways decrease vignetting in-camera, while others need post-production.

Let’s take a deeper look at these strategies in the following,

Examine for Lens Obstructions

Check the front of your camera lens first if you are seeing severe vignetting. Lens hoods, for example, might get dislodged or misaligned and appear in your shot. Vignetting can also be caused by stacking filters or utilizing mismatched accessories.

Tuning ISP

The image that is captured from the sensor is processed in large portions by the ISP. Each ISP employs a unique strategy to reduce lens shading through a sequence of steps. The specific registers in the ISP can be adjusted or validated while the image quality is tested in the appropriate software or manual hardware.

Numerical f-stop number

If the lens is stopped down by two f-stops, which raises the f-stop number numerically, natural vignetting can be prevented.

Usage of Higher Focal Length

Additionally, vignetting can occur when using lenses with low f/#s (the ratio of the focal length to the aperture size), short focal lengths, or lenses that need to attain better resolutions while spending less money. A higher focal length should only be used to eliminate mechanical vignetting.

Usage of Flat-Field Correction

The method for vignetting correction that is most frequently employed is a flat-field correction. This method evenly illuminates the flat surface with a single color. In this method, the lens cap is used to obtain the dark field reference frame and the light reference, which is used to calculate the flat-field correction.

Usage of Telecentric Lenses

As this telecentricity creates incredibly consistent image plane lighting, lens designers can manufacture lenses to just be image space telecentric to correct roll-off. The typical cos4θ fall-off in the lighting of the image plane from the optical axis to the field corner is eliminated because all chief rays have an angle of θ with respect to the image plane.

Wrapping Up

Vignetting is a ubiquitous feature in photography, whether you like it or not. Vignetting may be used in a variety of ways in an image. However, vignetting can be removed if it is undesirable. It’s essential to be cautious when using vignettes and think hard about whether your image needs one. We believe that this article provides you with the most important information that you may need for your embedded camera applications.

If you have any additional questions regarding how to resolve this problem in your embedded vision applications or are interested in incorporating embedded cameras into your products, please do not hesitate to Contact Us