What's causing this strange reflection?

I did a shoot the other day with a gal and she was wearing a skirt that seemed to cause some very strange reflections. I don't recall ever having a problem like this before when it comes to clothing. Is this a certain material causing this or could it be a certain detergent, starch, fabric softener, etc... I would really like to know so I can avoid this in the future. Also, is there anyway to fix this after the fact?

Close up example:

What you are seeing is a moiré pattern - an optical interference between the pattern in the fabric and, in this case, the pixels that make up the image. For more information, click on this link to an article on moiré.

SkipD wrote in post #11877809
What you are seeing is a moiré pattern - an optical interference between the pattern in the fabric and, in this case, the pixels that make up the image. For more information, click on this link to an article on moiré.

Thanks for the info....I didn't realize this was moire.

Anyways, that leads me to the question of how do I prevent this in the future as a photographer? Unfortunately, this wreaked havoc on many of this gals photos and I had no idea until I downloaded them to my computer well after the fact (luckily we shot in more than 1 outfit so we do have others). I would imagine there must be some type of guideline to prevent this (like to avoid certain fabrics). It's obviously not a huge problem (meaning that it happens very frequently with clothing) because I've photographed many different types of clothing and I can't recall ever having this happen, at least to this extent......so what is unique about this skirt that caused this?

edit....also, just to throw this back into the equation....is there a way to fix this in post? Did I just find a good reason of why film is still king

Moire is unfixable.

But - Sometimes the moire you see on your screen is caused by the interpolation needed to show a large image file (say 10 to 21 MP from the camera) correctly on your low resolution screen. I've seen moire in hair or fabrics on screen that disappears if I view the image at 1:1 or make a print.

If you make a print and see the moire, it was created on the sensor and it is not going to be something you can fix. The anti-aliasing filter on your camera should have prevented this though, so it is interesting if this is real moire.

Unfortunately, I don't have any pat answers for you as to how to prevent or repair the problem.

I've found very similar problems when trying to scan photographs from newspapers (typically old historical newspaper articles we've been trying to copy) with a digital scanner. The dot pattern in the newspaper photographs and the pixel sizing when printing the scanned image can be very combative with each other.

JeffreyG wrote in post #11878058
Moire is unfixable.

But - Sometimes the moire you see on your screen is caused by the interpolation needed to show a large image file (say 10 to 21 MP from the camera) correctly on your low resolution screen. I've seen moire in hair or fabrics on screen that disappears if I view the image at 1:1 or make a print.

If you make a print and see the moire, it was created on the sensor and it is not going to be something you can fix. The anti-aliasing filter on your camera should have prevented this though, so it is interesting if this is real moire.

Thanks for the info....I have also noticed it too when viewing images at smaller sizes and, as you noted, it usually goes away when magnified to a 1:1 ratio. Unfortunately, that's not the case this time.....even at 100% magnification, it remains. I'm guessing I'm probably stuck with it this time. I'll have one printed out just to see if and how it shows up in print.

What camera, by the way?

The 5D has always been rumored to have a weak AA filter for instance, so it is interesting to some extent to see which camera did this.

JeffreyG wrote in post #11878962
What camera, by the way?

The 5D has always been rumored to have a weak AA filter for instance, so it is interesting to some extent to see which camera did this.

EXIF says 1Ds MKII.

SkipD wrote in post #11877809
What you are seeing is a moiré pattern - an optical interference between the pattern in the fabric and, in this case, the pixels that make up the image. For more information, click on this link to an article on moiré.

Moire is not only indicative to DSLR cameras.. It was also a pain in the butt with film cameras..

Quad wrote in post #11881734
EXIF says 1Ds MKII.

Yep...thanks for pointing that out

easily fixed IMHO. Take off her skirt.....

Try downloading the Capture One 6 trial and processing the RAW files through it, it has rather flexible moire reduction capabilities since it was originally designed for medium format cameras, which don't even have AA filters.
All Raw converters use their own algorithms for demosaicing and some are generally more effective than others, in another converter you might not even get any moire, I've noticed this a long while ago when I was comparing conversion software.

Other than that, the only way to avoid this problem at the source is to use a diffusion filter to soften fine detail, or switch to a higher res camera that can resolve the pattern.

jra wrote in post #11877851
Anyways, that leads me to the question of how do I prevent this in the future as a photographer? Unfortunately, this wreaked havoc on many of this gals photos and I had no idea until I downloaded them to my computer well after the fact (luckily we shot in more than 1 outfit so we do have others).

Were all of them from the same distance? I've read that moiré should only show up at certain distances, but I haven't actually verified that.

Perhaps it was a problem this time because of the glossiness? You should be avoiding textures likes that, ideally, but that might not be practical.

Best way to get rid of it altogether is to get a camera with smaller pixels and/or stronger AA filter.

Thanks for all of the info guys It looks like it this moire is here to stay in these photos. I've done a little searching and from what I can tell, there's no sure fire way to fix it once it's there.....although I can "tone it down" by removing the color and possibly using the blur tool a bit. As toxic pointed out, it seems distance does play a factor. The few photos where I had moved closer had allowed the camera to render the detail well enough to avoid the moire. Here's an article that I found that I thought did a good job of explaining why it happens...>>> http://www.wfu.edu …ews/misc/DigPho​tog/alias/

jra wrote in post #11883944
http://www.wfu.edu/~ma​tthews/misc/DigPhotog/​alias/

That is an excellent article and it is now bookmarked. Thanks.

By the way, just for grins - if y'all want to use the really proper last character for the word moiré, instead of typing the "e" character, hold down the 'alt' key and type 130 on the numeric keypad. This works on PCs, but I'm not sure about Apple machines.