Digital artefacts/diffraction?

[Jimbo]

Hi all,

Here's a photo I took this morning: http://i48.photobucket.com/albums/f2...shbag/test.jpg

It's a blend of 2 bracketed RAW images, one for the sky and one for everything else. Details are:

Canon 300D
24-105L lens
ISO100
f/16
28mm
Sky: 2.5sec
Below horizon: 6sec

Now, you may notice that above the hills on the other side of the bay there appears to be a high bank of cloud. There was no cloud. This seems to be some kind of artefact, but I'm hoping someone can tell me what the hell is going on - I've noticed this in other images.

Is it a result of diffraction limit? Or is there something wrong with my aged 300D?

[Tannin]

Hi Jimbo. Looks like a JPG artifact to me - i.e., trouble mapping the source colours to the (smaller number of) output colours. It is certainly nothing at all to do with diffraction. Do you see the same thing when you save your shot as a TIF?

(It's worth noting, by the way, that there is no such thing as a "diffraction limit". None at all. Every picture, every lens, has some loss of quality due to diffraction. All that happens is that, as you stop down, you get a bit more and a bit more. The increase in diffraction is continuous. The only way in which it makes sense to say "diffraction" and "limit" in the same sentence is if you are talking about something completely different: this is where you have a lens of extremely high quality and people say it is "diffraction limited". This simply means that the lens is so good that the optics are close enough to perfect as makes no difference, and thus that the primary limit to your picture quality is diffraction - and diffraction is something that all lenses have in equal measure at any given aperture. Diffraction has nothing to do with lens quality, it is a product of your aperture, and that alone.)

[ricktas]

Hey Jim

You can link the photo in from photobucket rather than just posting the link. That way the photo will show in your thread. Instructions on linking are here, scroll down the thread as there is a photobucket example and its easy, just a cut-n-paste and you end up with the photo in your thread, rather than the link, like this:

[Jimbo]

Rick - thanks, I tried doing that the other day, but must've got the code wrong or something.

Tannin - the same artefact is evident if I open the RAW file directly in Photoshop. Similar artefacts are evident in the fog above the cenotaph in this image:

[ving]

ahh... i had no idea what you were talking aobut at first... dust bunnies? is that what you are talking about? if in the first pic you are talking about the reddy/pinky tinge, those are crepuscular rays... um, from the sun.

i see no artifacts.

[Jimbo]

ving,

you may be right - after all, I've never cleaned the sensor in the last 6 years (never previously seen the need to). Here a full rez crops of what I'm referring to:

The line going across that looks like the edge of a cloud (not the crepuscular rays):


Strange pixellations in the fog:





Hopefully the interweb thingy doesn't mess with those images so you can see what I'm referring to.

[ricktas]

I cant see what you are referring to on my monitor?

[Jimbo]

Hmm, weird, because I can definitely see it on mine. Maybe it's a gamut issue with my monitor..

[JM Tran]

I see nothing in the circled photos either?

what type of monitor are u using, and is it calibrated?

[Jimbo]

Well, I think we've identified the problem as my macbook pro's screen - just got a CRT monitor from the shed and I can't see it either! And no, it's not calibrated - I've always meant to but never got around to getting a calibration tool.

[ricktas]

laptop screens are notorious for being inaccurate.

A calibration tool is a must if you are serious about photo editing and colour accuracy, in my opinion. But at least we now know the issue is screen not your camera, so thats a bonus

[ving]

ving,

i can see it easily in this... try less contrast if you are adjusting contrast, more colour depth (ei 16 bit or 32 bit instead of 8), or try compressing the picture less....

i have found that when i get tihis doing one of the above (not always the same one) sometimes fixes it.

can but try

[mikew09]

Same Jimbo, I don't see anything. Only dust bunnies in the other shot.