Purple bleeding/artefacts in the video image on the D90

[Video]

Hi, wondered if anyone found this to be common but using only the factory settings on my D90 and in M mode I get quite a bit of purple bleeding/artefacts (see pic attached) in parts of the video image when using the D Movie mode at 720p with Nikon's supplied 18-105mm kit lens.

Is this normal?

 

[yoclay]

That's chromatic aberration in the trees, with moire on the center house. The glass windowpane is probably exaggerating the issue by refracting the light in odd directions.

 

[Video]

Thanks for confirming what it is. I definitely get the chromatic aberration quite a lot even when I'm outside and not shooting through glass.

Is it due to the quality of the Nikon kit 18-105 lens? Will better prime lens get rid of it?

 

[damonb]

Chromatic aberration usually occurs in areas that are overexposed. In the case of your example, it's everywhere your overexposed slushy white sky and an object meet. When shooting objects against sky, you need to close the aperture down, f16+ say, and then you will start to see detail (better exposure) even in the sky in your image. I agree that shooting through a window cannot help. The kit lens is not the problem. You should be able to get a good image with good exposure with your kit lens, but you must practise with the settings the camera allows you to manipulate. Check out the stickies for tips on manipulating video settings. And don't panic! The D90 is ace.

 

[Video]

Thanks for the reply. Unfortunately the lens I was using isn't fully manual so as I understand it the smallest aperture available is f8 so will look into either getting a fully manual 50mm and/or some ND filters.

Any recommendations on these?

Cheers!

 

[beckspace]

can I dare to say that this a video issue?

Hope that no one here shoots me in the face

I think it's the 105-18mm chromatic aberration too, but somehow the video function exaggerate the effect. If you change the D90 to Vivid +3, those bleedings will become purple macrodots, and if you take a picture at the same setting, the purple bleed will be almost invisible

My wild guess is the downsample of the 12 megapixel to 720 resolution. It jumps lines while reading out the photosites and then compress the image horizontally. This is the only thing different (besides the shutter) between video and photo functions, and the purple bleeding macrodots at Vivid +3 always have a square shape, so it has to be it

 

[damonb]

I'm not sure you're right about the lens beck.

The D90 has inbuilt chromatic aberration correction and its efficacy has been tested with the kit lens, e.g.:

http://www.imaging-resource.com/PRODS/D90/D90A4.HTM

Specifically : "Chromatic aberration in the corners of camera JPEGs taken with the Nikon D90's kit lens is extremely low, and hardly noticeable at either wide angle or full telephoto."

So it's not the still camera and not the lens. As for video, it still can't be the lens. But it could be the video function, as you suggest. I still think manipulation of settings is the best bet to avoid it as much as possible, even if the issue is downsampling.

 

[damonb]

Thanks for the reply. Unfortunately the lens I was using isn't fully manual so as I understand it the smallest aperture available is f8 so will look into either getting a fully manual 50mm and/or some ND filters.

Any recommendations on these?

Cheers!

The smallest aperture on your lens is f3.5, not f8. But don't worry so much about the lens or filters just yet. Why not electronically wind the aperture open to something close to f3.5 and set a shutter speed of around 40. Try setting your ISO to 400, set the picture control to Standard, and set the white balance according to your surroundings (note the kind of light in your surroundings) - at night in your house you could set it to the lowest level under "choose color temp" which is 2500k - and find an acceptable exposure by turning on LV and aiming the camera at an area you think might be a 'mid' level light. Then lock exposure (see manual for how to do this) and begin the video - focus on something, an object, anything. And watch that bad boy video pop open.

If you want to get a prime lens, by all means get a 50mm f1.8 Nikkor - that is the best bargain you will ever own. Buy it new, it's that cheap! It is not fully manual, which means it'll snap to focus on the D90 body when AF is engaged, but it has an aperture ring so you can manually adjust it if you wish - the best of both worlds. I defy anyone to tell me there's a sharper 50mm lens on the market made by anyone. Enjoy.

 

[mattsand]

i think it's actually moiree on the trees too. i use secondary color correction to make it blend in, and some of my warp sharp filter helps too.

 

[beckspace]

I have an example that made me never use Vivid +3 again in D90 movies. I saw this purple bleed in the first week I bought it

I never tested this setting with other lenses, well, I never used this setting again

those two images were shot at the same place at the same moment, you can see that something goes really wrong with the video, but not with photo mode

I think that the these colorful purple (and blue, green, red, orange, the rainbow colors) dots are the light diffraction created by the lens (the stars at the photo image), since each dot is the sun being reflected in a dark background creating a lot of contrast, the sensor (jumping lines) reads the diffraction as solid colors in the downsampling. that's my theory

 

[mattsand]

no it's just moiree. the reflections are white and registers as such by the lens, but so small and/or sharp that they somtimes hit only the red/blue pixel and not the green next to it. when the image is pixel skipped that kind of aliasing increases since the distance between the blue/red and the green pixel is longer.

 

[beckspace]

no it's just moiree.

Matt, please, stop answering like a Nikon engineer. We all know that you didn't designed the D90

I sent this image to Thom Hogan and he said "You’re likely seeing lateral chromatic aberration that is now being “enhanced” by your camera setting. Get out of Vivid"

He agrees with me that it comes from the lens and the video mode in those settings exaggerate it. My image just like the first one was shot with the lens kit and showed the same effect

And besides Moiré by definition is "when two grids are overlaid at an angle" which can happen with trees, but surely, by definition, not with tiny dots as you just said. The only thing that creates a pattern is the lens diffraction, then we can say that it creates a colorful moire effect around the white dots. That it has to do with the sensor filtration, skipped sampling, imaging process, yes, but no one besides a Nikon engineer can exactly tell what it is. and you know... they don't talk. We can only theorize about

 

[mattsand]

Matt, please, stop answering like a Nikon engineer. We all know that you didn't designed the D90

please stop answering like a moron. why would i post something if i didn't know it to be true? i know a lot of people do around here and i hate them as much as you do, but i thought i had a reputation of being an asshole but a trustworthy one. feel free to call me a prick but don't question the facts.

 

[mattsand]

by the way, aside from the fact that it's pretty hilarious to have words defined for you by someone who says i "didn't designed" something, a moiree pattern is caused by the interaction of two patterns, not necessarily two grids. the two patterns in your case are the water reflections and the cmos sensor. the reason there's no moiree in the stills is because the image has been low pass filtered before hitting the sensor, which thus in effect doesn't have a pattern anymore.

 

[damonb]

Dude, I don't see where you were called a prick here. Being right does not require you to become belligerent. Why not have a reputation as trustworthy? Why does it have to have the "asshole" part as you put it? All that aggression takes way too much energy.
If you want to educate people, you won't teach them with abuse. And if you don't want to educate people, why would you post in this thread?

I have a question for you: I also do not see this problem in stills. Is this a problem the D90 is saddled with in video because nothing could be done about it any which way, or is this a problem that is a designed limitation, in your opinion?