The View Up Here

Random scribblings about kites, photography, machining, and anything else

DPP, DLO, CA, OMG! WTF?!

Posted by Tom Benedict on 30/12/2013

Hapuna Point Sunset

It all started with this idea, see… I wanted to photograph a sunset.

Ok, to be fair the sunset thing started way before. I was photographing sunsets with film, for crying out loud. Digital is just an extension of that. But the sunsets here in Hawaii really lend themselves to a particular technique: long duration or long exposure photography. With film the biggest thing you had to look out for with long exposures was reciprocity failure, or the tendency for film to stray from the “1 stop aperture = 1 stop exposure time = 1 stop speed” rule. With digital the biggest bugaboo is noise. And as it turns out some of the smaller effects I’ve been ignoring start to dominate, too.

Overall Image Quality

One of the first things I had to deal with is the fact that I’m still using the EF-S 18-55mm kit lens that came with the camera. I have other lenses, mostly from my days of film. Unfortunately of those only two are worth mentioning: my EF 100mm f/2.8 Macro and my EF 50mm f/1.4. The rest aren’t sharp enough for digital work. Neither is wide enough to do what I wanted, so I was stuck using the kit lens.

From earlier tests I knew at 18mm the EF-S 18-55mm lens is soft in the corners and suffers from significant chromatic aberration. There’s not much you can do about the CA without resorting to software, but even a soft lens will benefit from stopping down. Stop down too much, however, and diffraction around the aperture will cause the image to become fuzzy. So it’s a balancing act: too wide open or too stopped down, and the image is soft. Somewhere in the middle is a sweet spot that’s different for each lens. I tested my EF-S 18-55mm lens and found it had a sweet spot around f/10. So that’s what I used for the above photo.

There’s another advantage to my using this lens that comes up later during post-processing. More on that later.

Hot Pixels

My camera suffers from hot pixels. Most cameras do. For short exposures you’d never even notice, but when you start making exposures of a second or two seconds or two minutes those hot pixels really start to show up. In the past I’ve removed them using Photoshop’s rubber stamp tool. But the longer exposures I was doing for the sunsets brought out dozens, no hundreds of the things. I had to find another way.

There’s a neat program called Pixel Fixer that helps remove hot pixels. Here’s how it works: You take a high ISO, long(ish) exposure with the lens cap on, and feed that to Pixel Fixer. You set an upper threshold for what constitutes “dark”, and any pixels that stray above that value are flagged as “hot”. Pixel Fixer doesn’t like to deal with more than about 280 or so, so you have to tweak your cutoff. But once it’s mapped them, it can remove hot pixels from any RAW image, generating a new RAW image.

It can actually do more than just removing hot pixels, as it turns out. You can also use it to subtract dark current from a long exposure, and to median combine multiple images. In each case it starts with RAW images and produces a new RAW image. So it plays nicely with RAW workflow.

Chromatic Aberration

Canon DSLRs all come with a copy of Digital Photo Professional, or DPP. It’s a very capable RAW processor that lets you do all sorts of stuff including white balancing, noise reduction, etc. As of 2012 DPP includes a function called Digital Lens Optimization, or DLO. When used on a photo made by a supported camera and a supported lens, it lets you remove most of the characteristics of the lens from the image – namely diffraction and chromatic aberration. Both the T2i and the EF-S 18-55mm are supported by DLO, so I gave it a try.

DLO works great, but there’s one big catch to using it: When used on an image with any amount of noise, noise-like artifacts show up in the output image. Long duration digital images always include noise. So when I first tried to use DLO on this photo, the results looked horrible.

The solution was right there on Canon’s web site: turn off any form of sharpening before running DLO on the photo. Then go back and apply a judicious amount of sharpening on the DLO-processed image.

DPP will do two forms of sharpening. One is called “sharpening”, and seems to generate the noise-like artifacts almost as if they were a feature. The other, unsharp-masking, tends to create weaker noise-like artifacts, but creates stronger artifacts at edges and sharp color transitions. For this one I used very weak unsharp-masking.

All this time I’ve been using the term “noise-like” when talking about the artifacts DLO generates. To the eye it looks like noise. But while trying to get rid of it in my images I found that when I applied DLO to a set of images from the same photo session, the artifacts showed up in exactly the same place in each image! It’s not random at all. Which is a real bummer, because there are easy tools for dealing with random noise in an image. One of the more effective ones is to make multiple images and take the median. (Hey! That’s something that Pixel Fixer does!) But median filtering emphasizes features that are common to all the images. In the case of the noise-like artifacts, that’s exactly what it did. The noise got worse. I still don’t have a real fix for this, except to be careful with the sharpening after DLO has been run.

Results

I’m pleased with the results. The photo shows no apparent hot pixels, is quite sharp, and shows no chromatic aberration anywhere in the frame. Not bad for a kit lens. Yeah, I’d still like to get some L glass in the 16-20mm range. But for now this is pretty darned cool.

– Tom

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