Over the last few years I have tried more RAW converters than I care to enumerate but I always seem to end up back at Adobe’s Camera Raw (ACR) despite the occasional minor irritations and one persistently annoying one.

The primary gripe I’ve had with ACR is that it is impossible to turn off all noise reduction - a default level of noise reduction is applied even when the noise reduction sliders are set to 0. One would think that an advanced RAW converter would let advanced users take control of the whole process but apparently Adobe thinks they always know better and have therefore decided not to provide that option. To their credit, however, after the high numbers of complaints about ACR 4.1, since ACR 4.2 they have reduced the default amount of noise reduction applied in particular to high ISO images. I have not tried Lightroom 2.x but I assume it is the same (so someone please tell me if I’m wrong.)

Night photography presents special challenges to digital photographers who must carefully balance noise and detail through a combination of tools and techniques such as exposing to the right, averaging exposures, using post processing noise reduction (Noise Ninja, Neat Image, and others), etc. Having full control over the process is critical when seeking the highest quality in the final result - online or in a print. In some cases the default noise reduction applied by ACR can smear important details or generally produce a very “digital” looking image and whatever issues exist after the RAW conversion are amplified in post processing.

Despite that major drawback, in general I have found that for most images the output quality from ACR is very high and it certainly fits well in to my [for now] all-Adobe workflow. All other things being roughly equal I have to stick with the tools that make my life the easiest. Still, every once in a while I decide to try to find something better…

Raw Therapee (RT) is a free, fully featured RAW converter which uses DCRAW for decoding (like many other RAW processing applications) but has implemented their own demosaicing and image processing algorithms. I’m not going to write about features like file browsing, highlight recovery, and sharpening methods because I’m mostly interested in the quality of the output before all those other things are applied (with the exception of Adobe of course) which hopefully shows how the particular demosaicing algorithms handle each image. RT provides the option to use three different demosaicing algorithms - VNG-4, HPHD, and EAHD. HPHD is the default and at first glance seemed to be the best so I used it for this test.

The image I’m going to use is one of the three images, each ISO 400, that were averaged together to create the final image in this post. One important thing to mention is that my current camera (hurry up with that 5D Mark II Canon!), the Canon Rebel XT (350D) is a couple generations old and the “quality” of the noise has improved considerably in newer generations and surely the converters are able to produce higher quality output with higher quality input…or at least one would hope!

Processing parameters were matched as closely as possible - all the exposure and histogram related settings were set to defaults in ACR and I did my best to match them in RT where only minimal changes were required and certainly nothing that would meaningfully affect output quality. Luminance noise reduction and sharpening were turned off as completely as each tool allows (remember my primary gripe with ACR.) If there’s any question as to how the settings were configured, here are some screenshots. You can see the only thing I needed to do was to adjust the luminance curve in RT slightly to match the default ACR conversion.

Raw Therapee:

ACR (corrected - the previous image showed sharpening of 2 which is incorrect):

The ACR image was opened directly from ACR in to Photoshop and the RT image was saved as a 16 bit TIF and then opened in Photoshop. I layered the images on top of each other and adjusted slightly to make sure they were correctly aligned.

Here are crops from the horizon from both RT and ACR. I added a gradient map layer to convert to black and white (the method I most commonly use these days) and curves layer to increase the contrast to show the differences more clearly.

A couple of things are significant - the first is the hot pixel in the RT crop which ACR removes quickly by default and RT does’t attempt to remove. My camera produces quite a few hot pixels in a long exposure so removing them manually would be a hassle. The second is the default noise reduction that ACR applies. If you look closely you can see some of that “digital” look showing through. As I’ve said, this is something I would rather avoid from the outset by being able to turn the noise reduction all the way off. No sharpening has been applied to the image at this point but there aren’t really any details here to sharpen so the only result would be additional emphasis on the noise in the case of the RT crop and more artifacts and digital look for the ACR crop.

And just for reference, here it is sharpened (USM amount 300, radius 0.7) and as you can see there is a reason it’s generally best to constrain sharpening to areas of an image that would benefit from it.

This is a crop from the rocky area. The same gradient map was used to convert the image to black and white but a different curves layer was used to lighten area of detail.

Again the default noise reduction applied by ACR is very obvious (keep in mind this is a 100% crop) as well as the appearance of more detail in the RT crop. This is a perfect example of a case where I would like to be able to disable noise reduction or even better be able to apply it selectively in the RAW converter. The next crop is the sharpened detail in the rocks.

There is no doubt that the RT crop appears to show more detail than the ACR crop - and as stated previously I believe that is a result of the default noise reduction in ACR. Again, this is a 100% crop in Photoshop so any print would not show this level of difference however I still think it’s significant.

So as you can see for this particular image Raw Therapee works very well and in my opinion provides better results overall than ACR as long as the sharpening is applied carefully and selectively. This is not the case for all images though and soon (this took much longer than I originally thought) I will post another in-city image for which ACR seems to excel while RT struggles…stay tuned.

Brian

This entry was posted on Wednesday, November 5th, 2008 at 7:43 pm and is filed under Blog, Technique & Equipment. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.