Interference Filter and Spectrum Shift

One of common questions for IDAS filters is that a filter is allowed to set in front of wide-angle lens or not. I’d like to describe more details here. All of IDAS filters are so-called “interference type”, which is sensitive to incoming light angle. More angle, more shifting to the shorter wavelength direction (toward to blue side).

frontIllustrates a filter is set in front (there is the issue for the light angle to a filter in case of wide-angle lens)

ANGLE-HEUIB-II-Fshows the spectrum shift in HEUIB-II by an angle

25 degree is out of Ha emission line

Generally 80mm focal length in full frame (50mm in APS-C) allows to use IDAS filter in front of lens. The shorter has the issue. This is why IDAS, the pioneer of body-mounted filter, have created the solution to avoid the spectrum shift issue with wide-angle lens. Following show how light angle goes to a filter set behind a lens.

28mmF2.8LENS-Angle0-28-3828mm F2.8

50mmF2LENS-Angle0-16-2350mm F2


 As illustrated the above, 50mm F2 is still within Ha line entire full frame sensor while 28mm F2.8 is slightly out of range at each corner of full frame. This is why IDAS carefully determines bandpass width to allow you to use fast speed lens without attenuating by spectrum shift. Keep in your mind, narrower bandpass, more possible issue by several factors such as F-ratio, focal length(depending on type of lens design) and size of sensor. It’s deeply complex each other. IDAS always designs filters based on various factors of users’ optical system.

We plan several new IDAS filters – body-mounted filters for full frame cameras. Probably filters for Sony Alpha series will be first available.


And also completely new-concept narrowband filters for one-shot color cameras is being prepared too. Please keep tune here.


~ by tedishikawa on November 5, 2015.

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