DSLRs and moiré

Laurence wrote on 2/7/2012, 2:37 PM

This is meant to be a discussion of multi-megapixel cameras (including DSLRs and the Sony VG10 and 20) and the horrible problem of moiré.

I have a Nikon D5100 and in spite of the warnings against this camera, I like it very much. The one thing I absolutely HATE however is the moiré problems that this and other cameras like it have.

I just ordered a Caprock anti-moiré filter that should work very well if I mount it on my 35mm prime lens and use it for interviews where people ignored my suggestions to wear solid colors. The particular 35mm prime I have has no image stabilization so this will be a tripod only solution.

My other problem shooting with the DSLR is wide shots where I want to highlight buildings rather than some person in front of them. I do a lot of business ads and a nice establishing wide shot of their facility is something that I use quite a bit. I have been getting away with using stills for the wide shots, but I want to be able to have movement in front (cars, people, trees waving in the wind, ripples in any water nearby etc.)

Thus I have been experimenting with how to minimize moiré. What I have found is that nothing I do in post seems to have any effect. Various blurs, chroma blurs, etc. only seem to add lack of clarity. They have no effect whatsoever on the problem.

The one thing that does seem to help is throwing the image out of focus a little.

Here is what seems to work best so far (please, help me improve upon this if you have any ideas or knowledge):

1 Frame your shot with an aperture of about 10.
2. Zoom in all the way optically, then use the focus assist zoom to go in as close to some offending area as possible (bricks, roof tiles, shingles, etc.).
3. Go all the way to infinity. Back off slightly until the offending pattern is in focus. Go back towards infinity until the pattern blurs slighty.
4: Leave the digital zoom, reframe your shot and start shooting.

This seems to work quite well and the balance between clarity and noticeable moiré is quite nice.

Here are a few observations on this:

An f-stop of less than 10 makes the focus adjustment too delicate and the chances are that everything will look too soft.

An f-stop of more than 10 can work and add a little sharpness, but the moiré starts to be quite noticeable even focused at infinity.

Going a little shy of focus rather than blurring towards infinity has a problem in that moiré will kind of shimmer in and out throughout the zoom range. Going a little past focus towards infinity seems to keep the moire problems at bay throughout the zoom range.

The slight softness in focus is quite noticeable at full zoom and hardly even there framed wide. This is really most useful for wide shots.

Different cameras (especially with different sensor sizes and zoom ranges) very likely have different optimal f-stop ranges.

This is just the result of a couple of days experimentation. It is quite likely that there is already a work-flow that outdoes this. If so, I would love to be pointed in that direction.

Comments

Laurence wrote on 2/7/2012, 3:14 PM

Here is a sample of what I am talking about:

http://vimeo.com/36370225

R0cky wrote on 2/7/2012, 4:11 PM

Thanks for this post Laurence. I just ordered a VG20 and one of the Caprock filters....

I hope to love this camera.

rocky

Laurence wrote on 2/7/2012, 4:16 PM

If I was ordering again I would go with a Panasonic GH2.

Laurence wrote on 2/7/2012, 4:27 PM

I am still doing tests. One approach that seems to work quite nicely is to go all the way wide at f10, put the lens in manual mode, and turn the focus all the way to infinity. If I zoom in at all it is soft, but as long as I am full wide this seems to work. Dial in a smaller aperture and you can see the moiré again.

On the D5100, the "landscape" preset is at f10, so if I go to the landscape preset, slide the focus in manual and go all the way wide, I will get a reasonably sharp image with no moiré. If I want to zoom in a little, I need to either walk closer or do my other trick.

In the next generation of cameras I'm sure all this will go out the window.

Laurence wrote on 2/7/2012, 4:33 PM

I find the moiré on tight patterns indoors to be much less of a problem than outdoor patterns like shingles, bricks, siding, etc. I think this is because the ISO is higher and it blurs the tight patterns a little. I was at an event not too long ago and I went out of my way to shoot every tight pattern I could see (available indoor light) and only noticed the slightest amount of moiré. Outdoors though, lookout!

amendegw wrote on 2/7/2012, 4:46 PM

Any chance of putting a few seconds of the original footage of that house in a spot for download? I've got a couple of ideas, but don't want to post anything until I have a chance to test.

...Jerry

PeterDuke wrote on 2/7/2012, 5:05 PM

Moire is aliasing due to undersampling something with fine detail. Once aliasing has occurred it is nigh on impossible to unscramble the egg. It is therefore imperative to reduce detail prior to sampling (or downsampling).

If you were converting HD with sufficient sampling to SD with insufficient sampling, you could low-pass filter (blur) before conversion and therefore do it in post, but if you are doing the sampling in the camera at the final resolution, you have no option but to do it optically before sampling.

JasonATL wrote on 2/7/2012, 5:07 PM

Laurence, I like your ideas regarding focusing. Your video demonstrates the effect quite well and the benefits you can achieve. I'm curious to hear how the filter works for you.

I agree that there is almost nothing to be done in post. I've managed to reduce the multicolored shimmering in some cases by keying to the colors of the moire shimmer (saturated blues and reds, usually for me) and desaturating them. This uses the Sony Color Corrector Secondary to key and desaturate. This helps some (especially on a roof that is, itself, desaturated), but I don't think it is as effective as what you show in terms of retaining the actual detail.

Fortunately, my Canon EOS T3i/600D does have a nice solution that totally removes moire at little apparent cost other (and resolution remains as high as non-crop). Thus, I don't have to worry about the moire of a shot like you showed, assuming I have a wide enough lens and long enough focal distance. For your 35mm lens on your camera, I could frame the same shot with my 11-16mm with no moire. I don't add this to "rub it in" or to claim Canon is better than Nikon. Rather, I offer this so that others might be aware that on some DSLRs, there is a built-in workaround that is a built-in feature of the camera.

I see that the new Nikon announced today specifically addresses moire, which shows (I hope) that the major DSLR manufactures are beginning to get the message that this is the major drawback of these cameras compared to traditional video cameras.

Again, thanks for sharing this tip!

fldave wrote on 2/7/2012, 5:14 PM

Panasonic GH2 is what I am very interested in purchasing, since it appears to not have this issue.

Laurence wrote on 2/7/2012, 5:14 PM

Yeah, that's what I'm finding, and the only thing that a DSLR has on it that is anything like a lowpass filter is the focus adjustment. What I'm finding is that if you blur out the shingles/bricks/etc. too much, the whole picture is blurry. If you blur it until just the edges of the shingles blur when you are zoomed way in (with both optical and digital zoom) it's not really enough. You have to sort of fudge it, and the amounts vary by how far away you are. If you have too big an f-stop, the shallow depth of field means that you are throwing the whole building out of focus. To small an f-stop and the small details will give moiré patterns even at infinity. F-10 or f-11 seems about right on my setup, but even then it takes some trial and error to get it right. I have a shoot on Thursday that needs a wide-shot of a building that has vertical bars in the siding and horizontal joints in the roof. It is right by a highway and a still would be obvious. I will probably bring a point and shoot or a GoPro for the wide shot. It is so frustrating that getting a decent wide-shot of a building is so difficult with this type of camera!

paul_w wrote on 2/7/2012, 5:17 PM

Agree with reducing moire at source as much as possible. Most DSLRs have a 'sharpening' adjustment. Would simply reducing the amount of sharpening help? I know this is a known fix (or at least a help) with Canon dslrs. As part of the profile setup. And this is applied in real time to the image processing between sensor and codec during record.

Paul.

Laurence wrote on 2/7/2012, 5:17 PM

>Panasonic GH2 is what I am very interested in purchasing, since it appears to not have this issue.

From what I've read and seen in videos online, the GH2 has much less of this issue. It does have some aliasing and moiré, but nowhere near as much as the Canons, Nikons and Sonys.

Laurence wrote on 2/7/2012, 5:40 PM

On the Nikon I have the sharpening turned down almost all the way. I've experimented with this and no sharpening looks about the same moiré-wise as no sharpening, and yet it seems a little lacking in clarity. The Nikon only lets you adjust aperture, not ISO or shutter speed on video, but you can adjust profiles that include things like saturation, sharpening and exposure response. I have a pretty neutral profile set (hardly any sharpening, and no enhancing of the color) because it seems to color correct better. I'm really happy with the look once it is graded. My only complaint really is the moiré, and that is really only with the establishing wide outdoor shots.

I ordered a Caprock 1.0 filter today. They have a money back guarantee where if you send it back, all you have to pay is the shipping.

This Barry Green article is what inspired me to try the Caprock. That and these videos:

http://vimeo.com/7523222

https://vimeo.com/12668818

https://vimeo.com/16070464

https://vimeo.com/8990500

Laurence wrote on 2/7/2012, 5:50 PM

The Barry Green article was for tests done with an APS-C sensor on the Canon 7D. I talked to a Caprock tech and he said that for 7D sized sensors, the values that people seemed to like are 1.0 and 1.4 whereas for the full frame 5D, 2.0 and 2.8 where the popular ones. I ordered the 1.0 because according to the Barry Green article, that was the proper value for an APS-C sized sensor and the 35mm prime lens that I already have. I like this lens for interviews and that should get me around interviews with patterned clothes (which scare me to death right now).

farss wrote on 2/7/2012, 6:01 PM

Closing the iris down a lot means diffraction is softening the image.
The Caprock filters appear good but as you've found are kind of restrictive.
You could try something like a Black Promist filter but I'm reluctant to make any recommendation as you'd need to test, test, test and with the various strength filters against aperature. Those filters are not cheap and you could be spending a lot for nothing.
Another old trick to soften an image is to stretch a piece of pantyhose over the rear element of the lens and put it back on the camera. That seems pretty easy to do with a PL mount lens but could be very problematic with DSC lenses with all those electrical contacts. Even if you can manage to do this the effect might not be to your liking. Some quite old lenses also had the ability to fit a filter into the optical path behind the iris but they are now rare.
You might try a lower resolution lens, some of the old low acuity optics is effectively going to provide low pass filtering but then you're going to need adaptors and probably ave back focus issues.

Sorry, all I can provide is some random "out there" ideas and TBH if any of them were a solution I'd be surprised if others haven't already tried them and its all over the web...or not if it was a bust. Still you never know.

Bob.

Laurence wrote on 2/7/2012, 6:14 PM

I've read discussions of using softening filters and things like UV filters with clear nail polish coatings. These were ideas put up by people who hadn't seemed to have actually tried them though. Reports back from people who experimented said that none of these ideas worked. The only successes were from either the Caprock filters or this device for the Canon 5D from Mosaic Engineering. Three problems with the Mosaic Engineering device are:

1/ It only works with the Canon 5D.
2/ It throws off your backfocus so forget zooming way in to set your focus.
3/ It will lower your still photo resolution so you lose the ability to go back and forth between stills and video.

farss wrote on 2/7/2012, 7:19 PM

"Reports back from people who experimented said that none of these ideas worked"

They will work, the laws of optics are quite clear on that. Getting enough softening to kill the high frequencies causing the aliasing depends on a number of factors. Once you get that happening though other things happen as well. The end result is going to be something between the same as defocussing or something more dramatic that causes the highlights to bloom.

The mosaic filter seems like it is an OLPF and that is the ideal solution because such filters have a sharp cutoff. The "fog" or "blur" filters have a gentle roll off and may have other artifacts that are desirable or not.

You are hard up against the laws of physics, there's no way around them.
Red seem to have tried to bend them with their Scarlet by changing how much of the sensor is scanned. That of course impacts FOV and I'd be more inclined to just have a still camera and a video camera on hand.

Bob.

PeterDuke wrote on 2/7/2012, 8:02 PM

Sunex is another brand of optical low pass filters to consider

http://www.optics-online.com/lpf.asp

Edit

I just found this doc. which says that the filter should be placed between the lens and the sensor, so not much use:

http://www.optics-online.com/doc/files/Optical%20Low%20Pass%20Filters%20Theory%20and%20Practice.pdf

Further thought

Fix the OLPF to a teleconverter adapter, with or without its lens.

Laurence wrote on 2/8/2012, 9:44 AM

Here's another approach that seems to work pretty well:

The smaller the f-stop (higher the number), the closer it gets to infinity being in focus everywhere. Because of this, an f-stop number like 22 will give you everything in focus with the lens manually focused to infinity. Thus, if you leave your camera focus at infinity and go gradually larger with the f-stop, you introduce a small controllable amount of defocus at the distances you would typically use for wide shots.

What I'm finding is that for a quick wide shot of something with a moiré inducing pattern, you can just call up an f-stop of somewhere between f:10 and f:22 (with f:10 being the greatest defocus and f:16 being the least) and get a pretty decent wideshot of a building. The advantage of this approach is that it's quick, predictable, and gives you pretty good results.

Laurence wrote on 2/8/2012, 11:40 AM

I'm liking the approach of setting infinite focus and then gradually increasing the aperture size to control defocus.

Here is a video I just put up which shows the effects of infinite focus at various apertures on moiré. Unfortunately it is downrezzed to 720p and the subtleties are hard to see, but you can download the original from Vimeo.

https://vimeo.com/36425051

Laurence wrote on 2/10/2012, 6:44 PM

I just wanted to mention here that I tried out a softening filter today. Just a cheap one designed for giving a dreamy look for portraits. Anyway, way too soft, and I realized something that will keep me away this approach permanently: it is impossible to focus with the softening filter in place. Autofocus doesn't work at all and manual focusing is too difficult. Also, it doesn't do anything that a bit of slight manual soft focus doesn't do as well or better.

I am coming to realize that one of the reasons why people prefer manual focus is that it isn't as sharp. I 'push to auto-focus after moving the focus box to my subject and find that gives me consistent great focus. I see now that if I did it manually, I would probably get close to as good a look with less moire. I just can't see the image well enough in the viewfinder, especially outside (even with a lens shade), and my progressive bi-focals mean that my eyes aren't exactly in focus anyway. Not at all like focusing through my camcorder viewfinder with peaking on and the eyepiece adjusted for my diopter.

Anyway, I'm giving up on softening filters. The Caprock anti-moire filter should be coming tomorrow or Monday. Hopefully I will have better luck with that.

On the other hand, on my shoot yesterday, I did the wideshots over several times with my "f-stop and infinity" technique and I got footage that looked sharp and free from moiré. I am realizing that on my cheap kit Nikon lens, that I think that the lens extends slightly beyond infinity and that what I'm really doing is more accurately described as "using the bet of lens play beyond infinity to soften focus predictably".

My current plan is to use the Caprock on a prime lens especially for interviews with problem clothing and to use the slight soft focus trick to avoid moire in architectural patterns and surfaces in outdoor wide shots.

Laurence wrote on 2/10/2012, 6:55 PM

I am also realizing that there are plenty of times when the lack of proper low pass filtering actually looks better than it would otherwise. The detail may be false, but if it looks good, who cares. So where I am is using sharp focus on closeups (where the extra fake detail looks very pleasing) and softening a bit by shooting infinity (or maybe a little past it) for wide shots where the slightly soft focus acts kind of like an analog low pass filter.