jpg artifacts?

I use a Nikon D200 and has not used my Capture NX yet, but rely on jpg (Yes, I know I have to pull myself

together and start shooting raw!). Although I always use "JPG Fine", i.e. lowest compression, artifacts show up

now and then. Although the pictures are resized, the curved streaks are just as visible in the full-resolution

originals.

D200 settings are standard except +2 sharpening.

 

In "sky", you will note the curved streak across the sky just above the star (Venus, actually). In some

instances, the streaks are a different color than the background - see "lights" where the artifacts are green -

on a brown background!

 

Any suggestions?<div></div>

lights<div></div>

I seem to be unable to see, what you see. The light has a doublet towards the lower right edge, due to reflection inside the optics. Then, there is a brighter ring along the inside edge of the light. I am not sure, but this may be due to sharpening.

the lights are deliberately out of focus. You have to look above the lights. There, you will note two curved dark bands, one over each candle.

<p>I'm not seeing it. But I suspect what you're talking about is not a JPEG artifact, but rather posterization due to limited bit depth.</p>

 

<p>JPEG breaks the image down into 8x8 blocks and processes each block individually. So JPEG artifacts can basically be lumped into two categories: artifacts that happen within a block, and visible block edges (which occur because adjacent pixels on either side of an edge are processed as parts of separate blocks, which can result in substantial differences being introduced between their values). If you're seeing an artifact which is larger than eight pixels and is not along a line marking the edges of these 8x8 blocks, then either it's two or more adjacent blocks which (more or less coincidentally) happen to have similar artifacts, or it's not a JPEG artifact at all. And the camera's highest-quality compression setting makes visible artifacts rather unlikely.</p>

 

<p>Now, if you take a scene that has areas where the colour and/or brightness changes only very gradually, and you produce a limited-bit-depth image file, you can get posterization: instead of smooth transitions between gradually-changing tones, you get areas of essentially constant tone with abrupt transitions between them. A dark, but not quite totally black, sky is a good example. There simply aren't enough bits per pixel to represent a smooth transition.</p>

 

<p>If you take the highest-quality JPEG image from the camera and use it as is, you're usually OK, but if you then start doing tonal adjustments to it in an image editor (curves, levels, stuff like that), you will usually end up making the abrupt transitions more apparent. The cure for this is greater bit depth. JPEG can't offer this; with the exception of a little-known (and essentially never-used in the general imaging community) 12-bit option, all JPEGs use 8 bits per channel. RAW files are virtually always deeper than 8 bits; I don't know your camera's specs, but most DSLRs of that vintage provide RAW files with a depth of 12 bits, giving 16x as many tones as an 8-bit file, and many newer models offer 14-bit RAW files. Shoot RAW, do your editing at 16 bits* as much as possible, and you will likely find that this problem goes away.</p>

 

<p>*: why 16 bits, when the original file only had 12? Because it's much less efficient to work with data structures which are packed into partial bytes. There's no technical reason why photo editors couldn't have a 12-bit mode to go along with their 8-bit and 16-bit modes, but the 12-bit mode would be slower than either 8- or 16-bit modes. So photo editors don't have 12-bit modes.</p>

thanks Steve, this makes sense.

Now to the spooky part: this text is written om my wifes laptop - and my pictures show NO artifacts on her

computer! No wonder you guys cant see anything wrong! Still, they are not limited to my own PC - the "sky" pic

still displayed the halo in a print e-mailed from my pc! Should I look inside my pc instead? Totally confused...

You were probably viewing the images on a 6-bit LCD monitor.

It is a 3 yrs old LG with DVI interface. But how does it slip through to prints? Same problem at the print hardware?

Hi

 

looks like optical aberrations to me ... I'm not sure if it helps you, but I posted this some time ago to assist a fellow with understanding digital artifacts (as opposed to optical issues) what pixelization vs jpg artifacting was.

 

http://www.photo.net/digital-darkroom-forum/00PC75