Magenta edge tinging at high ISO and a possible solution...

Started Aug 17, 2009 | Discussions thread
photogerald Senior Member • Posts: 1,798
My results with 0.9.0.0!

Firstly, I wanted to thank Dominique (the OP) for coming up with the idea of RAW border correction, and of course GordonBGood for providing us with a tool to perform this correction with ease.

The test which I am going to present here is a bit different from the others - I never shoot at ISO 3200 or higher, instead, I wanted to see what the RAW border correction could do for my images.

For my test I have selected an image that was taken on my recent trip to HK. It was shot at ISO 1000 and is representative of the type of image that I would take at what I consider to be "high" ISO. I realize that this same image could have been taken at ISO 100 (and with better results), but I don't like lugging around a tripod while on vacation.

The RAW file (PEF) was first run through GordonBGood's RAW Border Correcter tool (v0.9.0.0). Both the "corrected" and original RAW files were processed using Pentax Photo Lab 3.61 with default settings and all noise reduction disabled. A series of exposure adjustments were made in Photo Lab (+1ev, +2ev, +3ev) for each. The resulting JPGs were then resized to 2000 pixel width using IrfanView, before being uploaded to PBase. The images shown here have been further resized by PBase, however if you would like to see the larger 2000 pixel versions, just change "large.jpg" to "original.jpg" in the image URLs (if anyone would like to see the full-sized originals, just let me know).

Here is the "before" image (no exposure adjustment):

This image looks pretty good as-is. However, once you compare it directly with the "after" image (with Raw Border Correcter applied), you'll see that even it benefits from the correction (you might need to download both images and switch back and forth between them to see the difference):

Next, let's look at what happens when we apply a +1EV adjustment (the original image might look better with a bit of an exposure boost anyways):

Here the colour cast in the dark area along the bottom should be apparent if you're using a calibrated monitor. If it isn't, just compare it with the corrected version:

At +2EV things start to get ugly:

Fortunately, K20D RAW Border Correcter saves the day:

Still not convinced? How about +3EV:

Surely there's no hope, right?

Wrong!

Holy smokes, Batman! This is ISO 1000 pushed 3 stops! Ignoring the overexposed areas, this would be equivalent to ISO 8000 right? This image is actually quite usable juding by the original (unfortunately, the sharpening applied during resizing has exaggerated the noise in the images shown here).

What's a bit surprising to me after looking at all these shots is the effect that the tool has on the non-black areas (sky and buildings). I know you mentioned earlier that the correction shouldn't affect the colour balance, but the buildings do appear to be slightly changed. Or does this merely indicate that the buildings were also affected by the magenta cast? This isn't a complaint, by the way - I actually prefer this look. I'm just curious to know if I'm interpreting the results correctly.

Anyways, overall, it's as if a layer of grit (magenta-coloured!) has been removed from the images.

Taking this idea further, I decided to try it with the following ISO 400 image:

Let's jump straight to +3EV:

And the corrected version:

The difference is subtle, but it's there (look closely at original.jpg). Admittedly, this isn't a very good example as the dark areas aren't really that dark. That said, I've run into colour casts with photos taken at this ISO under artificial lighting which required some large white balance corrections, and I would expect these to benefit more from the RAW Border Correcter tool. I plan to look into this some more.

So, to wrap up this rather long post, I'll summarize here my thoughts regarding GordonBGood's K20D Raw Border Correcter tool:

-this tool is a must have for images taken at ISO 3200 and 6400, no doubt about it (look at the examples from the other posters)

-this tool will even benefit images taken at moderately-high ISOs (such as ISO 800 and 1600), and is highly recommend especially if the images require any sort of PP

-it remains to be seen how much of a benefit there will be below ISO 800, but I have a strong feeling it will prove useful whenever large exposure or white balance adjustments need to be made (in PP)

I would strongly encourage every K20D shooter to give this tool a try, even those like me who don't usually shoot above 1600 ISO. I'm confident you'll find this to be a must-have. And let us know your findings.

Thanks again to everyone who has contributed to this idea. I can't tell you how excited I am after trying this tool - it feels like as if I am shooting with a new camera (K20D Super)!

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