Objective Compensation under the Magnifying Glass

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How Tamron protects pictures from scattered light

Objective Compensation under the Magnifying Glass

No, this is neither about flashing chrome rims nor about the payment of outstanding invoices: Objective compensation provides even better light for your photos.

Less loss on the way to a good image

Optical compensation, also known as anti-reflective coating, is the refinement of the individual lenses in an objective. The light from the subject has to pass multiple glass surfaces equally before it reaches the sensor. This is a fundamental problem: An untreated glass surface reflects the light - scattered light and ghost images are the result. In addition, the scattered light in the objective reduces the contrast of the photo, leading to a general loss of light. On average, the reflectance is about 5 to 6 percent.

This optical reflection effect is comparable to the acoustic effect in an empty room. The bare walls throw back the sound uncontrollably. Recording studios therefore provide ceilings, walls and floors with special insulation. There is something similar in photography: The individual lenses are provided with compensation.

Practically, this means that the lenses are vapour-coated with light metal halides, so a softer surface is created, which reduces reflections.

Single or multiple coating

Several types of compensation are used in the manufacture of photographic objectives. The goal is always the same: To minimise the scattered light as much as possible and to reduce the reflection, without impairing the lens's high transmittance of light. Generally, the coating of the lenses must be as smooth and thin as possible.

The least expensive form of compensation is simple compensation or single-layer compensation. The name says it all: Each surface receives a single compensation layer. This procedure, which has existed since the 19th century, has, in the meantime, been replaced by more modern procedures, however.

Multiple layers with different refractive indices are applied in multiple or multi-layer compensation. These refractive indices are calculated through complex formulas and reduce the total scattered light even further. This procedure is complex and thus more expensive - for high quality objectives but now becoming standard.

Tamron uses BBAR compensation

For an extremely high quality image, Tamron has developed BBAR compensation technology (broadband, anti-reflection), which is used in all Tamron objectives. It not only reduces scattered light, but also guarantees the best possible colour balance. The lenses in newer objectives are provided with further developed BBAR compensation which provides still better light transmission for long as well as short wavelengths.

This reduction in reflection is achieved by extremely thin layers of magnesium fluoride which are applied to the glass surface. However, single layer compensation only affects a limited range of wavelengths and is not very effective. Only the application of layers having different thicknesses and different refractive indices shows an effective reduction in reflection over a wide range of wavelengths.

The following diagram (Diagram 01) shows, as an example, this combination for a particular glass. The reflectance is shown on the left and the wavelength of the light below. The reflections significantly decrease over the entire range of wavelengths with multiple compensation layers.

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