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### AuthorTopic: HOYA Variable Density 3-400 filter - Attenuation profile  (Read 3845 times)

#### Guillermo Luijk

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##### HOYA Variable Density 3-400 filter - Attenuation profile
« on: March 05, 2017, 07:15:31 PM »

If someone might me interested in this variable density filter, I tested the attenuation produced at each of its positions to make exposure adjustment easier. As any filter based on polarizers, the attenuation curve becomes exponential when approaching the max. attenuation limit.

Hoya claims a 1,6EV-8,6EV range of attenuation, I measured 1,5EV-9,5EV, while HOYA claims reaching the max attenuation end is not recommended because cross-like dark will appear. The filter produces noticeable colour shifts, no idea how easy is to correct them with customised WB. I didn't check for sharpness.

Regards!
« Last Edit: March 05, 2017, 07:33:56 PM by Guillermo Luijk »
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#### Guillermo Luijk

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##### Re: HOYA Variable Density 3-400 filter - Attenuation profile
« Reply #1 on: March 11, 2017, 09:34:23 PM »

Tests at different filter positions show strong assymmetrical vignetting, and blue colour cast in the darkened areas for the highest attenuation positions:

The vignetting pattern changes quickly from one position of the filter to other, which makes imposible to obtain a standard correction map. I have done a numerical correction for the 13.5 position, and worked quite OK eliminating both the darkening so as most of the blue cast:

Vignetting and colour cast (obtained taking a picture of a flat surface), non corrected image and corrected image (colour banding was caused by rounding errors since the calculations were applied in 8-bit):

The R correction code, just four lines:

# Leemos imagen

# Normalizamos
for (i in 1:3) vignet[,,i]=vignet[,,i]/max(vignet[,,i])

# Aplicamos corrección a la escena y guardamos normalizando
# OJO: ningun píxel de vignet en ningún canal puede valer 0
writePNG(scene.out/max(scene.out), 'scene.out.png')

Regards

« Last Edit: March 12, 2017, 06:46:03 PM by Guillermo Luijk »
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#### Guillermo Luijk

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##### Correction and noise
« Reply #2 on: March 12, 2017, 06:43:11 PM »

It's interesting to know how much the exposure had to be lifted in postprocessing in order to estimate how much noise we could expect. The R channel needed the strongest push, up to 3,8EV. The heatmap shows shadow lifting on each area:

Regards
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#### davidgp

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##### Re: HOYA Variable Density 3-400 filter - Attenuation profile
« Reply #3 on: March 13, 2017, 03:22:40 AM »

Hi Guillermo,

I'm assuming the pattern, since it is the relative position of two polarizers, apart of changing from position to position, will also depend one he lens... Since after it is screwed to the lens, it will end in difference orientation from lens to lens...

Anyway... Interesting stuff... I was thinking getting one, but maybe I just should keep my collection of solid NDs.

Regards,

David