F-ratio & Spectrum Shift, and NB1
All interference type of filters have the fundamental issue – the spectrum shift. This is the nature of coating against the angle of the light.
X number for IDAS filters is 14 degree. Current all filters have been designed to prevent the transmission loss or color shift based on this number.
If a filter is set in front of lens, it’s easy to figure out. Basically factors are the focal length and sensor size. In short, 50mm focal length or longer in APS-C sensor. (About 80mm or longer for full frame)
But behind-the-lens is much more complicated. Basically it depends on F-ratio. But strictly speaking it is determined by the combination of all factors – F-ratio, focal length, back focus distance, sensor size and size of rear lens (= or lens design in another word). So it’s really hard to clarify.
For example, even at same f-ratio, longer back focus or focal length is more tolerant than the shorter. Also larger rear lens can handle at faster f-ratio than smaller. Above 28mm F2.8 lens shows more angle than 50mm F2. So very complicated.
One example to start studying is
- Over 40mm back focus (real optical distance – not metal back)
- APS-C sensor
- telephoto lens including telescope
I can say the filter can handle at F2 in above conditions. Generally telescope’s BF is much longer than 40mm. So overall you don’t have to worry about this with telescope.
But if you are concerned about this, please contact your telescope manufacturer to clarify the actual light cone angle at the edge of full frame sensor for its specific telescope.
Anyway, the narrower bandpass is not always the winner. True transmission rate (as measured value) is an another important factor to determine the overall optical performance. Our goal is higher S/N – more photon of specific emission line while maintaining darker background at a particular exposure time.
IDAS NB1 has been carefully designed based on those factors.
NB1 is getting close to start shipping. Please stay tuned.
DigiBorg 