General Optical Low Pass Filter swap project (to reduce moire and aliasing)

shaocaholica

shaocaholica

Well-known member
Thanks to USLatin for giving me the idea that this is possible with minor effort.


Project Status (I will post updates here):

The results are in and it didn't work :(

The single D30 OLPF was not strong enough to remove aliasing and moire. The double stacked D30 OLPF did not fit back into the camera. Oh well, it was still worth a try.


Project Description:

The proposed solution that this thread is about is to swap out the Optical Low Pass Filter (OLPF) with another one closer to 1080p to alleviate moire and aliasing artifacts.

So basically these HDSLRs capture motion off of a huge sensor with way more pixels than 1080p (1920x1080). On the 7D/T2i/550D, its about 9 times as many pixels as 1080p. In order to read the sensor fast enough and possibly to reduce rolling shutter, it is theorized that these cameras skip entire rows/lines of pixels in video mode. This makes sense as its a simple solution and still gives up HD video.

However, the downside is that by skipping information, you are in essence sub-sampling. That leads to artifacts called moire and aliasing on fine details in the image which contain information that has a higher spatial frequency than the sampling resolution (~1920x1080).

Now, the role of the OLPF (also know as an anti-aliasing filter) is to "blur" the image so that details beyond a certain resolution are filtered away. This filter sits in front of the camera's sensor so it function is independent of the lens used. The problem with the original factory OLPF in HDSLRs is that they are matched to the original full sensor resolution. In the case of the 7D/T2i/550D, its 18 megapixels. That resolution is much higher than the video acquisition resolution of 1080p/720p. In essence, the original OLPF does nothing in video mode. What we need is an OLPF that filters away any detail beyond 1080p.

The easiest to obtain candidate turns out to be the OLPF in the Canon D30 DSLR. Thats D30 not 30D. The D30 was Canon's first DSLR introduced in 2000 and its sensor is pretty much the same physical size as the 7D/T2i/550D. However, its resolution was only 2160x1440/3MP. Which means its OLPF will filter out all detail above 2160x1440 which is close to 1080p. Why not use a OLPF thats exactly matched to 1080p? Well, we can't find one and having one made is not financially viable when the cheaper D30 option exists.

So the swap itself is pretty simple*. Just take out the old OLPF and drop in the D30 OLPF. Here is an picture guide of this procedure but instead of another OLPF, this guy is installing an infrared filter, however, the process is identical:

http://ghonis2.ho8.com/rebelmod450d4.html

*not really

FAQ:

Q: Where can I get this done and how much does it cost!?!?!
A: I wouldn't get this done until its verified that it works. Let me be the guinea pig and let Barry verify the results. At that time, I will post detailed information on where you can get this done and how much it costs. Please don't start requesting this before we know if it works. All I can tell you right now is that it will cost somewhere between $400 - $500 for parts and labor at a competent shop. Check back here at this thread for updates.

Q: What other cameras will this work for?
A: Right now its limited to the 7D/T2i/550D. There are no suitable "donor" OLPFs available for the full frame 5DII and FourThirds GH1 (not that its a big problem anyway for the GH1).

Q: Whats the ETA on the results?
A: Thats up to Barry. The plan is to give the camera to a courier tomorrow who is going to take it to Austin, TX and hand deliver the camera to Barry Green at the SXSW festival on March 12th. From there he will run some tests on the camera against his unmodified 7D and T2i. I will let him post the results.

Q: How is this better than using Caprock filters and other lens filters/tricks?
A: This method is much more precise and is lens independent. If you swap a lens, you don't have to worry about which Caprock to use or if you need to put a fishnet on the back of it, etc, etc.

Q: So if the new OLPF filters away all detail above 2160x1440, what will happen do my 18MP still photos?
A: They are going to be blurry. Pretty much 2160x1440 worth of data in a 5184x3456 package. Will it be useless as a still camera? Thats up to you to decide but be warned the resolution will be severely limited.

Q: Why not just wait for Canon to fix this in their next HDSLR release?
A: Who knows when a proper fix will come? Plus I'm bored in the mean time.
 
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Thanks for starting a new thread. The only question I have for you is on the comment about losing light. I don't see this with my Caprock, which methods are you seeing reduces light into the lens?
 
This is awesome, I'm dying to see how this turns out... and GP, I think she or he's referring to the idea of putting a black mist filter or equivalent over the lens.
 
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grimepoch said:
Thanks for starting a new thread. The only question I have for you is on the comment about losing light. I don't see this with my Caprock, which methods are you seeing reduces light into the lens?
Click to expand...

That was just a guess. If the caprock acts as a synthetic aperture or whatever, theres no way light transmission would not diminish right? Unless the caprock is 99% transparent, it has to affect the light. Please correct me if I'm wrong.
 
As far as I can tell from what I see with it, it's a large clear piece of glass with small areas that diffuse light. The picture on the website is misleading, as I thought at first that it was actually blocking light before the filter arrived in the mail. At first I thought they sent a blank one till I turned it slightly and saw the pattern.

Instead of diffusing the whole image, it diffuses parts of it which is why selecting the right density of pattern is so important in regards to your lens size. Like Barry, I was having problems with this because you cannot see the Moire on camera and it is also a function of focus distance. Light wise though, I never lost light with the Caprock and I'd say with all of the other solutions it's not light you are worried about, but diffusion in the image. (For instance, picking the wrong caprock also removes the moire AND softens it too much). So basically, you get the same light affecting functionality as the OLPF.
 
I'll try but its getting dark out and I don't have much stuff that could cause moire close to me. I'll be attending Philip Blooms class tomorrow so maybe I'll have some footage by tomorrow evening.
 
Print out a resolution chart or something with very fine lines then shoot it with a sharp prime under good lighting that lets you stop down the lens into the sharpest aperature.

If this works, I'll definetly do this mod to a 7D or a t2i
 
steventimothy said:
If you touch the CMOS sensor with anything the camera is a boat anchor.
Click to expand...

Nope, not true. My T2i actually has its sensor damaged but its still usable for this test. The shop dropped the replacement OLPF on the bare sensor (original OLPF removed) and only a few pixels are "dead". The camera is no where near being a "boat anchor". I didn't have time to have them try to fix it with a replacement sensor so I'm using it as is for this project. I'll send it back to them after Barry is done so they can do the proper fix.
 
Good news: I've got the camera
Bad news: The LCD data connection must have got knocked out or something. I can't see squat on the LCD but HDMI/composite and USB out all work fine so I was able to shoot and focus.

Here's a quick test with what I have on hand in my apartment under tungsten lighting.

Nikon Manual 50/1.8 @ 2.8
ISO 400
24fps - 1/50

I used a geared head so I could rotate without changing focus. I picked these frames because they looked the worst. These are all crops but at 100%.

moire.jpg


Honestly, looking at the frame grabs and the motion, it doesn't look that great.
 
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Generally speaking, from these low res stills, it looks soft all around.

What do you think?

Softer than a 7D in the same conditions? I would vote yes. what are you using picture profile wise?

Any lighting you could toss at it?

Thanks for being the guinea pig X2. You should be remembered as a pioneer of this experiment. I don't think the OLPF is necessarily going to fix this scenario even if it is custom tailored. There is more at play - namely the way the sensor is being intermittently subsampled, rather than being read out then downsampled.

Here's a toast to the coming Digic V and whatever canon can do to improve read times and downsampling on the next generation of HDSLRs. I'm gonna stick to my 7D's until something with better colorspace and readout comes along.
 
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