Moire' effect on Cameras - DPChallenge Forums

03/03/2005 01:24:29 PM · #1

Hey I was wondering if there was anybody that could in detail explain a little to me about what moire is and some examples i searched on google to no avail thanks alot
particularly when photographing clothing example a plaid shirt thanks
Leon

Message edited by author 2005-03-03 13:25:05.

Azrifel

Fujifilm X100VI

03/03/2005 01:29:11 PM · #2

I have one photo that shows it very clearly. I'll look it up but still need to process it.

Ok:

100% crop from a basic jpeg (I shoot NEF + basic jpeg) saved at 80% with irfanview, so it is horribly soft.
Check the building in the middle. It has orange and purble bands running at a diagonal across the brick wall. This is very mild moire.

Message edited by author 2005-03-03 13:37:53.

w24x192

Canon PowerShot S50

03/03/2005 01:34:55 PM · #3

Look here for MoirÃ© article

Lots of other cool stuff there, too.

03/03/2005 01:39:25 PM · #4

thank you i looked at the link it clealy shows the moire effect but it doesnt particularly explain from what i read so far what causes it and how you can avoid it

03/03/2005 01:40:14 PM · #5

Originally posted by Azrifel:

I have one photo that shows it very clearly. I'll look it up but still need to process it.

Ok:

100% crop from a basic jpeg (I shoot NEF + basic jpeg) saved at 80% with irfanview, so it is horribly soft.
Check the building in the middle. It has orange and purble bands running at a diagonal across the brick wall. This is very mild moire.

Oh yea i can really see that how do you get rid of that? PS?

Azrifel

Fujifilm X100VI

03/03/2005 02:05:24 PM · #6

Originally posted by LEONJR:

Oh yea i can really see that how do you get rid of that? PS?

I use the moirÃ© correction tool of Nikon Capture, it takes away the color pattern. But Capture only works with Nikons.
Not that it happens a lot.

Basically moirÃ© ruins the detail and is hard to correct. The easiest way to avoid / correct it with shooting fabrics/clothing is to shoot from another angle or distance. The pattern of the fabric should not get the chance to become 1:1 with the photodiode layout of the sensor.

03/03/2005 02:23:02 PM · #7

Originally posted by Azrifel:

Originally posted by LEONJR:
Oh yea i can really see that how do you get rid of that? PS?

I use the moirÃ© correction tool of Nikon Capture, it takes away the color pattern. But Capture only works with Nikons.
Not that it happens a lot.

Basically moirÃ© ruins the detail and is hard to correct. The easiest way to avoid / correct it with shooting fabrics/clothing is to shoot from another angle or distance. The pattern of the fabric should not get the chance to become 1:1 with the photodiode layout of the sensor.

Iam sorry when you say 1:1 what exactly do you mean ?

Azrifel

Fujifilm X100VI

03/03/2005 02:33:17 PM · #8

Originally posted by LEONJR:

Iam sorry when you say 1:1 what exactly do you mean?

Take the brick wall from my exemple. The bricks have a very specific layout. Now I point my lens at the wall. The lens compresses the bricks to get them to fit on my sensor. Depending on my focal length, the distance, the angle etc, one brick can cover for example:
100 pixels
50 pixels
10 pixels
5 pixels

It can even cover 1 pixel, or 1 pixel in height and two wide.
Somewhere near this point, depending on the anti-aliasing filter of your camera's CCD and usually at a slight diagonal, you will get a situation where the grid of bricks interferes with the grid of pixels. I can't explain it good enough, but that's when you get moirÃ©. Hence the 1:1.

With fabrics it is the wires and woven patterns that can cause this.

radiman

Canon EOS-300D Rebel

03/03/2005 02:37:06 PM · #9

I thought that a moire' pattern only pertained to the pattern that occured from multi color process printing. I guess that's all I have heard it referenced to.

03/03/2005 02:37:26 PM · #10

OHHHH gottcha thanks for the info

03/03/2005 03:32:32 PM · #11

The bird had pretty intricate feathers but the distortions here are what I believe is moirÃ©. I always thought it was a lens issue; some are more susceptible than others.

03/03/2005 03:42:01 PM · #12

what makes you feel like theres something wrong with the bird it looks like a wonderful picture to me ?

03/03/2005 08:21:03 PM · #13

Sorry it took me so long to answer, had to leave for awhile. I do not think there is anything really wrong with the photo it is just that I was told it showed moire by soemone when I first posted it. As I'm not real familiar with moire I posted it as a possible example. I think the reference was to the colors and disortions in the feather patterning. Because it was intricate it may not have reproduced well.

I'm still trying to figure out just what moire is. LOL

03/03/2005 08:38:43 PM · #14

Moire is really not that hard to define, but it is hard to conceptualize. The best example from everyday life that I can think of is two screens one just in front of the other. If you rotate one screen slightly, you'll see a pattern of broad dark and light bands appear. That's because of the way the wires of one screen overlay the wires and openings of the other.
Moire in a camera is just like that. If a light-dark repeating pattern is projected (by the lens) onto the sensor, and the pattern repeats at almost, but not quite the same frequency as the pixels, you'll get moire. The funky color effects happen because not all colors are sensed at all pixel positions (because of the color filter array). The order of color filters in any given row is GRGR... the next row alternates GBGB.. every other pixel senses green, but only every fourth senses red or blue. The pattern is offset in each row, and the resulting two-dimensional filter patern is called a Bayer Pattern.
In most cameras, a filter is used over the top of the sensor that limits the resolution of the resulting image, and is referred to as an "AA" or anti-aliasing filter. This filter helps to eliminate "jaggies" on diagonal lines, and also greatly reduces Moire.

Message edited by author 2005-03-03 20:40:48.

03/03/2005 08:52:02 PM · #15

Thanks Fritz. That helps and I definately see it in the brick building photo below. But I do not see it in my parrot pic, at least not in the same way. However, someone here said they thought it showed moire a month or so ago. Does the parrot pic display moire?

03/03/2005 08:54:52 PM · #16

Originally posted by jbsmithana:

Yes, it does, but possibly not for the same reason. the camera may very well have resolved the feathers accurately, but downsampling can introduce moire where non was present before. A 100% crop of the worst-affected areas would tell for sure.

Canon EF 17-40mm f/4.0L USM

03/03/2005 09:02:11 PM · #17

Thanks. I'll go back and take a look at a crop of the origianl.

dahkota

Canon EOS-7D

03/03/2005 09:03:27 PM · #18

The parrot pic shows moire mostly on the head and cheek. the pattern you see is not the actual pattern in the feathers, it is an effect created by the light pattern. A feather is not solid; it is thin 'strands' so to speak, stuck together. When the light hits it in certain ways, a moire pattern emerges because the light is alternating with shadow. If you look at the bottom right of the photograph, the feathers do not show the moire pattern but appear flat. The light isn't bouncing off the feather in the same way.

A good place to observe moire is often on TV. If shirts with tight checkerboard patterns or thin alternating stripes are worn, the shirt looks like it is moving or vibrating. Esentially, moire is an optical illusion.

Not sure if this helps...

Dahkota

Canon EF 17-40mm f/4.0L USM

03/03/2005 09:12:41 PM · #19

Here is the 100% crop. I think I'm getting it now. Looks like it was not from processing but in the original.

dahkota - thanks for the additional info. It helps as I have seen that on the news shows often. that would match up with the bird feather patterning issue.

edit: I went back and looked at both versions. Looks like my post processing did make it worse. I'll have to watch that in the future. Thanks for all the help.

Message edited by author 2005-03-03 21:20:17.

dahkota

Canon EOS-7D

03/03/2005 09:24:49 PM · #20

//www.exploratorium.edu/snacks/moire_patterns.html

Here is a very basic explanation. While your camera is not your eye, many of the same principles apply.

"Moire patterns magnify differences between two repetitive patterns. If two patterns are exactly lined up, then no moire pattern appears. The slightest misalignment of two patterns will create a large-scale, easily visible moire pattern. As the misalignment increases, the lines of the moire pattern will appear thinner and closer together."

This is the part (simplified) kirbic was refering to in his post. Essentially, your sensor has a pattern and the subject has a pattern. If the pattern doesn't match but they are close (a brick for a pixel for example), moire will be created.

Its a little different in film photography (grain rather than pixel) but the effect is still the same.

Dahkota

Canon EF-S 55-250mm f/4-5.6 IS

03/03/2005 09:28:46 PM · #21

Originally posted by jbsmithana:

Here is the 100% crop. I think I'm getting it now. Looks like it was not from processing but in the original...

edit: I went back and looked at both versions. Looks like my post processing did make it worse. I'll have to watch that in the future. Thanks for all the help.

the original looks like the feathers are very well resolved, no moire at all. As you've surmised, the resampling is what resulted in teh moire pattern. I've seen this on bird pics of my own, but never to that degree. Moire caused by resampling is definitely something to watch for, and something we don't often think about.

Canon EOS-60D

03/03/2005 09:30:18 PM · #22

This example of moire is freaky but mesmerizing. Just to give you something new to look at. ;o)

03/03/2005 09:33:40 PM · #23

Originally posted by laurielblack:

This example of moire is freaky but mesmerizing. Just to give you something new to look at. ;o)

Hah! Perfect example Laurie! But you didn't explain the underlying mathematics, I'm so disappointed ;-)

Message edited by author 2005-03-03 21:34:16.