Color corrections of our lenses

Photographic vs. Visual color correction

The color correction of Visual-tuned lenses is designed to give peak Strehl ratio where human eye is the most sensitive.

For this reason, these lenses deliver best possible image sharpness and contrast for visual observations. Technically speaking, they are tuned to reach peak Strehl ratio in a limited range of wavelengths (430 nm – 700 nm).

Our recommendation is for these Visual tuned lenses to be used with mirror diagonals. This type of lens gives excellent photographic results when used with IR/UV filters.

The color correction of Photographic-tuned lenses is designed for covering a sensibly wider range of wavelengths. These lenses will be better corrected for near IR and violet wavelengths. Depending on the type of the photographic sensor used and its sensitivity range, images can be taken without the need of an IR/UV filter.

The acceptable corrected wavelength range is usually 400 nm – 1000 nm. As photographic sensors are sensitive over this range, this type of color tuning nicely matches the needs of photographers.

It has to be noted that Strehl ratio becomes slightly lower in red and blue colors and experienced planetary observers might detect the difference under excellent seeing conditions. To get optimal Visual performance from these lenses, some glass needs to be added in the light path, and in our case, this can be represented by a prism diagonal (1.25″ or 2″ size). As prism diagonals typically have better optical quality, better light transmission and less light scatter than mirror diagonals, we encourage all amateur astronomers to use them.

Considering that APO telescopes tuned for “Photographic” color correction can be used both for photography and for visual “work” (with prism diagonal), we believe this tuning of color correction is optimal (Zeiss tuned the color correction of their APQ series of apochromats very similarly) and it shall be used as the ‘default option’ for our apochromat objectives.

We are happy to take special orders for visually tuned objective lenses, if the Customer prefers this type of color tuning.

Pal Gyulai
Optical Designer of refractive optics
CFF Telescopes