OLPF-mod/fish-net/back-filter for Moire?

[Dan_Kanes]

That sounds reasonable, yes... the big thing would be determining the appropriate strength, and 1080p would be the way wrong strength. You'd want something more like 720p to match up with what the camera actually resolves.

This doesn't really make sense, it's almost like reverse trolling since this is pretty much Your Forum Barry. I understand what you are saying technically, but what is your theory on why it doesn't resolve as highly as it could? is it just "bad design?"

Did you consider that perhaps the mismatched OLPF is part of what is causing the camera's inability to resolve above 720? That and the line skipping of course.

Naturally, what we're all talking about here is conjecture anyway since I don't think anyone is going to go ahead and do anything.

Of course, like someone else said this will be a moot point when the cams start downsampling an entire readout of the sensor instead of lineskipping.

Would love to see the results of someone having a D30 OLPF installed on a 7D.

 

[Barry_Green]

This doesn't really make sense, it's almost like reverse trolling since this is pretty much Your Forum Barry. I understand what you are saying technically, but what is your theory on why it doesn't resolve as highly as it could? is it just "bad design?"

The DSLR, by its very design, works oppositely from pretty much any other video camera ever. What works for others (using sharper lenses, keeping the lens in the sweet spot, etc) actually makes the video look worse on a DSLR, because of the way the sensor does point-sampling instead of a full sample. The sharper and better the lens, the more aliased false detail gets through and contaminates the image. So you have to combat that by reducing the amount of detail that gets through -- either by blurring the image before it makes it through (caprock anti-moire filter, or employing diffraction, or just de-focusing) or you have to deal with it on the back end (an appropriate-strength OLPF).

Did you consider that perhaps the mismatched OLPF is part of what is causing the camera's inability to resolve above 720? That and the line skipping of course.

Nobody but the Canon engineers really know for sure, so this is conjecture based on reverse engineering done by people who have studied the results, so take it with an appropriate grain of salt.

The SLR samples about 1/10 of its sensor to create its image. By some estimates it reads one pixel out of nine. It doesn't bin all 9 together, as that would result in a monochrome image. But it samples pretty much one pixel out of each group of nine, and that makes it very problematical because it's not getting a true representation of what all those nine pixels should be; instead it's getting a high-res sample of what that individual point is. As a result, its sampling is inaccurate and contaminated with false detail.

It samples about 1600x90 off its sensor and ends up demosaic'ing that down to a 720p-ish image with all sorts of aliasing. Look at a resolution chart and you'll see, it ends up being in the neighborhood of 720p.

The low-pass filter would be key to eliminating the false detail that causes the aliasing. If the low-pass filter passes more detail than the sensor can handle, it will by default NOT reduce aliasing, it will instead CAUSE it. You want the OLPF to be tightly and closely matched to the sensor's true resolving capability. For a 7D/5D, that would mean somewhere around a 720p-capable OLPF.

The OLPF basically takes that nine-pixel area (a 3x3 square) and blurs it to where it's all one big nine-pixel blob. Which the sensor would then sample properly, it would get an appropriate representation of the detail of what that 3x3 square actually holds, and you'd see a nice appropriately-resolved image that doesn't have false detail or aliasing.

But, some people would complain because it wouldn't look as "HD" as what they already have. Even though that "HD" look is fake, because it's caused by false detail and image contamination, rather than an accurate representation of what's really there.

Read the "Aliasing" article to understand this whole subject more; it's really rather a fascinating subject.

 

[USLatin]

I don't know if there is an OLPF filter in the 7D but I suppose there must be. If there is one, then it is designed for their stills mode, 18MP, not for their video mode. Replacing it with one that would give best results for video would also limit your stills to 720p resolution, which would suck major. However, if the primary use of your 7D were video, then screw the stills mode, try to get clean video. Not gonna happen though... I think the best thing would be to finally get a nice MB and a great set of filters that would later also work with your Red or whatever you ended up with.

 

[Barry_Green]

Yes there is one, and yes it's designed for the 18mp mode, and that's exactly why the aliasing happens in video mode.

And yes, if you replaced it with one that was appropriate for video mode, then yes it would absolutely blur and ruin your stills, and that's why that won't be happening in DSLRs.

What's good for stills is bad for video. What's good for video is bad for stills.

 

[USLatin]

Unless you figure out a way to make the recording of the full resolution a possibility at manageable data rates, like Red.

 

[Barry_Green]

Right, which means compromising on the price tag.

And even with Red, it's still compromising stills for video, a 3K Scarlet will be capable of a max of about 6mp stills, which I think most folks would agree is very much compromised as compared to the reigning 18-megapixel standard of the 7D or even the 21mpix of the 5D.

You have to get up to a 6K camera before the stills from a Red camera will compete, sharpness-wise, against the stills from a dedicated stills camera.

 

[USLatin]

Right. A 5k S35 Epic would give very good stills though, better yet with a FF35 Mostro down the line. Enough for most professional photography uses. But the price remains, and if what you really need is a stills camera then why look elsewhere.

 

[shaocaholica]

I just called lifepixel IR conversions in WA state and they are pretty interested in a OLPF swap to see what happens. Apparently they have a large stock of D30 OLPFs from IR conversions. I'm waiting to hear a call/email back later today about the technical details.

Ballpark conversion price would be ~$400 including the new OLPF. Assuming it works.

 

[Barry_Green]

Well that's some resourcefulness right there...

 

[USLatin]

But with 2,160 x 1,440 effective pixels the OLPFs from old D30's would let too much detail pass through. There would still be moire in areas with detail in the range between 720p and "1440p". The filter would have to be much stronger. You would need a much older one, one for a camera that had a resolution closer to around 1280 x 720. Right Barry?

 

[shaocaholica]

I don't have a 7D to send them for the initial test. Anyone who's interested in sending in their 7D can contact me and I'll get you my contact there. I'm 99.9% sure that they can reverse the process if it doesn't work. You can ask them personally.

Otherwise, I'll donate my T2i once they come in.

 

[Dan_Kanes]

Nice work!

I don't have a 7D to send them for the initial test. Anyone who's interested in sending in their 7D can contact me and I'll get you my contact there. I'm 99.9% sure that they can reverse the process if it doesn't work. You can ask them personally.

Otherwise, I'll donate my T2i once they come in.

 

[Barry_Green]

But with 2,160 x 1,440 effective pixels the OLPFs from old D30's would let too much detail pass through. There would still be moire in areas with detail in the range between 720p and "1440p". The filter would have to be much stronger. You would need a much older one, one for a camera that had a resolution closer to around 1280 x 720. Right Barry?

I'm gonna run a test between a 7D and a 720p camera to see how comparable they are. If the 7D sees a lot more than the 720p camera, in terms of actual resolved detail, then maybe that D30 OLPF would be okay.

One thing's for sure, it'll be a heck of a lot better than nothing!

 

[shaocaholica]

One thing's for sure, it'll be a heck of a lot better than nothing!

Thats the point. The D30 OLPF comes close in specs and is readily available as a drop-in more or less. Once its proven than the D30 OLPF provides an improvement in image quality, then we can try to source something closer to ideal. But I have a feeling the ideal 1080p or 720p OLPF will be hard to find or have made with a reasonable price. It also needs to have IR-cut properties as well since thats part of the original OLPF stack. If you know a source, speak up.

 

[USLatin]

One thing's for sure, it'll be a heck of a lot better than nothing!

Heck yes.

If you know a source, speak up.

I sure don't. I also imagine finding something ideal would be impossible. I suppose the only way to find the perfect match would be to have one manufactured, and that would be hard to get done.

Once its proven than the D30 OLPF provides an improvement in image quality, then we can try to source something closer to ideal.

This would definitely be the way to go about it, even if nothing can be dug up.

I am excited to see what comes back from Barry's tests and what happens with a modified 7D, using a D30 OLPF. The window of frequencies giving moire would be much smaller. Being that it would be the frequencies just above the clean image means that it would often pop up. Defocusing an image would deal with moire beginning at higher frequencies as those are the first to blend together. That means that the degree of difficulty with throwing focus just off the subject but keeping it as in focus as possible would be just the same as without modification. However, a properly focused image with a D30 OLPF should be FAR cleaner if I am thinking this through correctly. The area presenting moire would be smaller, thinner.

 

[shaocaholica]

I just called Sunex in CA which supplies OLPF and they say they can make a precise OLPF to spec with a minimum order of 100 pcs. @ $100 a piece. However, at 1080p/S35 spec, the thickness of the filter is quite large ~6mm. The say you can half that by only filtering in one direction which might also work if 6mm is too high.

 

[Barry_Green]

Well, aliasing is far far far far far worse in the vertical direction than it is in the horizontal; I doubt a strictly vertical filter would perform well but if you had to choose only one direction, vertical would be the way to go.

 

[Dan_Kanes]

I just called Sunex in CA which supplies OLPF and they say they can make a precise OLPF to spec with a minimum order of 100 pcs. @ $100 a piece. However, at 1080p/S35 spec, the thickness of the filter is quite large ~6mm. The say you can half that by only filtering in one direction which might also work if 6mm is too high.

Would they do 1 or 2 test samples (maybe one at 720 lines and one at 1080 lines) so that we don't have to fork out a total of $10,000 before we know if it will work well or not?

Props for doing the footwork on this. I will SO TOTALLY do a rebel with one of these OLPFs and a PL mount if it does indeed work.

 

[shaocaholica]

Would they do 1 or 2 test samples (maybe one at 720 lines and one at 1080 lines) so that we don't have to fork out a total of $10,000 before we know if it will work well or not?

Props for doing the footwork on this. I will SO TOTALLY do a rebel with one of these OLPFs and a PL mount if it does indeed work.

I already asked about samples but since this spec is so far off normal they don't have samples on hand and it has to be a custom job with a minimum order of 100. So if anyone decides to do this they better know the precise strength and how to sell the other 99 pieces.

 

[DrBlaz]

very interesting.

May be this is a dumb idea, but what would happen cutting a soften filter and putting it in the back of the lens?