Optical Imperfections Reference

Both digital and analogic photography have to process light through a lens and to the sensor or film stock before it translates into an image. However, in the 3D world, a camera is just a simulation, there is no physical or real procesing of light rays thorugh a lens or sensor. An image will have a set of optical imperfections due to the processes that rays have to go through, in 3D world, images are free of those imperfections.

Our eye is capable of telling a fake image when this expected imperfections are missing on a render. This is why it is important to know and apply all of the imperfections that a real image has to a 3D render, and gain an extra level of photorealism.

These are the optical imperfections expected in an image:

1. Chromatic aberration

2. Bloom

3. Light Wrap

4. Color Grading

5. Vignette

6. Lens Flare

7. Lens Distortion

8. Optical Ghosting

9. Motion Blur

10. Depth of Field

11. Dust and Scratches

12. Noise and Grain

Not all of these imperfections have to be present at the same time, but most of them should. Now I’ll show an example of each and a small description.

 

1. Chromatic Aberration

Also known as color fringing, happens when the optical lens splits the diferent light wavelenghts. As a result, the image can look blurred or noticeable colored edges (red, green, blue, yellow, purple, magenta).

 

2. Bloom

Bloom is most visible in areas where the images has been over-exposed. Light from an area expands and contaminates the nearest pixels, causing a glow like effect.

 

3. Light Wrap

On a backlighting situations, the overexposure causes light to contaminate the edges of foreground objects.

 

4. Color Grading

All digital sensors, film stock, image procesing and lens cause a certain look on the image. Color grading isnĀ“t an imperfection, but it is something to be aware of when compositing a CG render.

 

5. Vignette

The darkening of the corners of an image caused by the lens. Some artists use it as a creative control to focus attention on the center of the image.

 

6. Lens Flare

It is caused when high intensity light sources enter the lens reflecting and refracting producing visible artifacts, and lowering the contrast of the image. Although it is a defect that can distract attention from the main subject, lens flare can also be used as a creative aid to enhance the look of an image.

 

7. Lens Distortion

Not all of these imperfections have to be present at the same time, but most of them should. Now I’ll show an example of each and a small description.

 

8. Optical Ghosting

Highly related to lens flare, and the term is sometimes interchangeable. Optical ghosting produces less defined, soft and low contrast shapes when compared to lens flare. It is caused by light sources reflecting and refracting through the lens.

 

9. Motion Blur

Moving objects (relative to camera position) leave a trail on the image. It happens when at the time of exposure on the sensor or film stock an object moves through the image, resulting in a registry of its movement during that time. Images with motion blur feel natural and it is usually a characteristic we want our CG images to have.

 

10. Depth of Field

A camera can only focus its lens at a single point, this point extends through an area called depth of field, where everything outside this area will appear out of focus. It is altered by many factors (sensor size, lens aperture, focal length, focus distance), it is a characteristic we want our CG images to have.

 

11. Dust and Scratches

Camera equipment where light has to go through will get dirty or damaged, even on a micro scale. Some dust or a small scratch will be present at either the lens, the sensor/film stock, or the final projection film.

 

12. Noise and Grain

Grain in film stock and noise in digital sensors, on a higher or lower extent they are always present on an image. It is a defect that increases with the lack of light getting to the sensor, and with the stock chosen to accommodate this low light situation in the case of film stock. It should always be present in your CG images. Rendering techniques also produces noise due to low sampling, but it is not to be confused with a real camera’s (digital or analog) grain/noise, which looks natural and realistic to the audience.

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