Purple fringing and FX1

[Tim Ashbrooke]

Hi guys

I have an FX1E and noticed that on some high contrast edges in HDV mode, there's purple fringing. This is a phenomenon of high pixel count digital cameras. I am assuming that this is happening because of the high pixel count of the FX1's CCDs. Anyone had problems with this phenomenon?

cheers
Tim

[Steven White]

Most reports I've read attribute this to chromatic aberration in the lens.

[Jimmy McKenzie]

I have owned 2 cams lately, one the 8 megapixel Sony, the replacement is the Nikon DSLR. The Sony is infamous for it's built in additional colour. Even if you don't want it. The Larger chips and superior product blood lines of the Nikon avoid this malaise.

It seems that chromatic abberation is now part of the video camera line-up as well. Pity. Their electronics are so good.

If you call Sony to report the purple fringing they will state that this condition is normal. I've gone that route. Happily their products do retain an average re-sell value, so dumping the inferior camera was quite easy.

Your results may vary, but I have encountered this several times in the past and I am determined not to have history repeat itself at the expense of my images.

[Patrick Swinnea]

I've seen this in a few downloaded mt2 files. I did a search for "purple fringe" and it turns out it used to be a big problem with the DVX100, but if you turned Cinegamma "off" it went away. (Not sure if it's the same on the 100a or not).

If that was the case with the DVX there may be settings that will minimize this and/or post tricks to remove it (beyond avoiding high contrast shots). I have a documentary/comedy feature to shoot in September and so far for my budget I haven't found anything I like better than the FX1. My end goal is a film blow up and theatrical distribution (we can all dream, right? :)
So...
I just wonder how that purple will look blown up.


Anyone else have input on their experiences with this? I realize I may be making too much of it for a $3000 camera, but if there isn't a solid workaround I may end up renting the HVX200 and some P2 cards. (And I am in serious need of a new production camera, so renting makes me sad).

[John Beale]

I think there are two distinct mechanisms for this type of artifact and either one, or both, may be in play.

Chromatic abberation is caused by the lens. You will see it most on the edges of the frame and should not see it at all at the exact center of the frame. If you have, for example a black tree branch against a bright sky, if it's chromatic abberation you would expect to see (for example) a green fringe on one side of the branch and red on the other side. This is caused by the lens having slightly different focal lengths for the different wavelengths (red, green, blue). There is software available for still cameras that actually reduces chromatic abberation in software, by splitting the image into red, green, and blue layers and re-scaling them to counteract the lens effects.

"Purple Fringe" on the other hand could be caused some kind of non-uniform saturation in the CCD chips, if the red, green, and blue chips may have different behaviors near the full-well point. If so, this would only be present on overexposed regions, and should go away if you just reduce your exposure, but of course if it's a bright sky that may make your intended subjects too dark.

If the purple fringe shows up even in the center of the frame, and it shows up even if you don't have over exposed areas, then I believe it must be something else other than the two things mentioned above.

[Barry Green]

Purple Fringe on the DVX was, I believe, pretty well narrowed down to overexposing cinegamma footage, and came about because cinegamma has no "knee" circuitry.

I don't think the FX1 has a similar circumstance -- its gamma curves all have knee circuits, don't they?

Seems pretty easy to narrow down the chromatic aberration theory, as the website for the EidoMedia lens modification showed that there was quite a bit less purple fringe with the Nikon lenses attached, did it not? They took the same shots with the Nikon and with the regular FX1 lens, so you could compare what difference the glass makes. If the fringe is in both shots, you could trace it to the electronics, but if the fringe appears in only one shot, it's more likely the glass.

Here's their webpage of pictures:
http://www.eidomedia.com/hdv/test/immatest/test.htm

[Andrew J Hall]

I recently did some late afternoon landscape shots of trees on a skyline on a very windy day (Sony Z1p). The sky was 95-100% white and the leaves were dark against the sky. Looking at 2x zoom in the top right hand corner I could just make out a green / purple fringe but I mean 'just' - there is no way it would be visible with motion on a TV, in fact even on my PC monitor it is basically invisible at 100%.

Andrew

[John Jay]

Yep the FX/Z series is prone to lateral Chromatic Aberration under certain circumstances

see forex:

http://www.moronthrottle.com/images/HDVwithcannon.jpg

thanks to Sean for the devilish little arrows :)

In fact the anamorphic expansion from 1440 to 1920 compounds the problem horizontally

I have trawled the universe for a plugin for After Effects and surprisingly none exist - so developers take note

[Mike Tiffee]

Originally Posted by John Jay

Yep the FX/Z series is prone to lateral Chromatic Aberration under certain circumstances

see forex:

http://www.moronthrottle.com/images/HDVwithcannon.jpg

Wasn't that grab from a modified camera using a SLR lens? I thought I remember it from that.

The Sony WA lens adds some CA, here's a cropped 1:1 image of a FX-1 with the WA.
http://www.miketiffee.com/hdv/out_wide_zoomed_1to1.jpg

Here's the full image so you can get an idea of how much CA you're seeing- it's actually very small. Also, this is with the WA and with the camera zoomed all the way in.
http://www.miketiffee.com/hdv/out_wide_zoomed.jpg

[Patrick Swinnea]

Barry, thanks for clearing up the DVX issue. Also, thanks for the comparison link. I guess that means it's the glass :(

Mike, you wrote, "Also, this is with the WA and with the camera zoomed all the way in" in regards to your sample pic (thanks, btw).

So if I'm understanding correctly there is more/less fringe depending on the focal length? This makes sense in my head but I may be way off here. If this is the case, do you think there's an optimal focal length that provides the least amount of fringe?

[Hse Kha]

I recently got the Z1U and to my shock I also noticed this Chromatic Abberation. Very sad.

At first I thought "how can this Zeiss T* Lens possibly be responsible?" and thought that it must be a quirk of the CCD.

Anyway it is a shame that the lens can't measure up to the 1M pixel CCDs :(

[Patrick Swinnea]

Would this show up in SD mode? In the examples I've seen the fringe is there, but very small. In SD mode there might not be enough resolution to see it. Am I correct in thinking this?

[Boyd Ostroff]

FWIW, it is rumored that Sony makes the lenses and licenses the Zeiss name, and that Panasonic does the same with Leica...

[Hse Kha]

Yes, both Sony and Panasonic make their own "Zeiss" and "Leica" lenses. BUT one hopes that Zeiss and Leica design them!

And even failing that, at the very least one hopes that Zeiss and Leica approve the designs before they allow their prestigeous respective marques to be printed on those lenses! Sadly that does not seem to be the case... Presumably those German companies are more than happy with the fat royality cheques... :(

[Andrew J Hall]

I did some test shots of purple / green fringing with my Z1p today, you can see them at:

http://www.andrew-hall-artist.com/jo...ndex.php?p=146

Personally I don't find it a problem to the extent it is visible.

Andrew