Hutech IDAS-D2 Filter Test

The 2″ IDAS-D2 LPS filter for use with my William Optics Zenithstar 61 refractor telescope with a Canon 5D MkIV camera.

This is a quick play using the IDAS-D2 filter on the Canon 5D MkIV camera and a Canon 100mm macro lens to see what the colour cast was like in diffused daylight.

The filter bandwidth rejection ranges can be seen below;

Astro Hutech IDAS-D2 LPS Transmission graph
Image #1 with no filter, profiled in LR and WB set at 6500k  using the middle grey colour patch
Image #1 with no filter, colour profiled in LR and WB set at 6500k  using the middle grey colour patch
Image #2 with IDAS-D2 LPS filter, LR profile from image #1 used and WB kept at 6500k to show the natural colour cast of the filter
Image #2 with IDAS-D2 LPS filter, LR profile from image #1 used and WB kept at 6500k to show the natural colour cast of the filter
Image #3 with IDAS-D2 filter, LR profile from image #1 used and WB adjusted to 11000k automatically in LR using the middle grey colour patch, it failed to fully correct the colour cast
Image #3 with IDAS-D2 filter, LR profile from image #1 used and WB adjusted to 11000k automatically in LR using the middle grey colour patch, it failed to fully correct the colour cast

For info, I used a standard M58-M48 stepdown ring to mount the filter to the Canon macro lens and I think that configuration will work for normal DSLR night photography with the Canon lens without any significant vignetting.

In the ‘real’ world the IDAS-D2 filter is performing very well

The Veil Nebula – Canon 5DMkIV and William Optics Z61 refractor with Flat61A flattener on a SkyGuider Pro

Canon ISO invariance

A lot of Nikon camera sensors are ISO invariant and unexposed images taken at ISO 100 can be recovered in post processing without any significant image degradation.

Most of the earlier Canon sensors are ISO variant and suffer from a purple colour cast when any underexposed shadows are recovered in post processing.

The Canon 5D MkIV sensor, at first glance, is also ISO variant but after testing it appears like the 5D MkIV sensor in ISO invariant from ISO 400 upwards (at least to ISO 3200) which is where I stopped testing.

Baseline exposure @ ISO 3200, f/8.0 1/50 second correctly exposed
Underexposed by 1 stop @ ISO 1600, f/8.0 1/50 second and then increased by 1 stop in Lightroom
Underexposed by 2 stops @ ISO 800, f/8.0 1/50 second and then increased by 2 stops in Lightroom
Underexposed by 3 stops @ ISO 400, f/8.0 1/50 second and then increased by 3 stops in Lightroom
Underexposed by 4 stops @ ISO 200, f/8.0 1/50 second and then increased by 4 stops in Lightroom
Underexposed by 5 stops @ ISO 100, f/8.0 1/50 second and then increased by 5 stops in Lightroom

By comparing the ‘corrected’ images above it is clear that if the exposure is correct metered at ISO 3200 and then an image is shot at ISO 400 it is possible to comfortably increase the exposure in software by at least 3 stops without any image degradation.

There is obvious image degradation at ISO 200 and ISO 100 where the purple cast starts to appear in the dark colour square and image noise increases but ISO 400 seems to be the ideal ISO to work with if you want to retain highlight details and increase the dark regions thereby maximising the dynamic range of the camera.