Friday, April 11, 2014

How Raw Works

There seems to be a lot of misunderstanding of what raw is and how raw developers work. Once you get involved with raw on a long term basis, it is easy to forget exactly what it is you are doing when working with raw. Maybe it is time to review it again.

This is a VERY simplified explanation. I'm sure much more detail could be added to it, but I think it is adequate for a high level explanation.

The image you see on the screen in your raw editor already is a bit mapped conversion. Your raw conversion program displays the the image according to the default values built into it. The extra information is there in the raw file, but not displayed on screen. If the conversion no longer needs certain data to meet your needs, it is deleted from the on-screen representation, but not from the raw file itself.

As you make changes, your raw editor goes into the raw file and displays any additional information it needs to create the image as you want to see it. But at no time does the image you see on screen represent the ENTIRE set of data contained in the raw file. I suspect we would not recognize it as an image at all if it did.

As an ex DBA, I tend to think of raw images as a sort of mini database for the creation of a single image; not as the image itself. As you work with the image you read that database looking for the information you need to create the image.

Once you have altered the on-screen representation to your satisfaction, and you tell it to export or save to a tif file, what gets converted to tif is that onscreen representation. And this leaves behind any unused data in the raw file when the tif is created. And you would not see any loss in a good conversion compared to the onscreen representation.

This is why I claim that any conversion from raw to tiff is inherently 'lossy'. The raw development process is one of deciding WHAT raw data to use and what to leave behind. 

It is also why you always want to save your raw image files if you can. You never know when an alternate development and conversion will be useful.


  1. Nice explanation. I'm wondering then, are RAW converters inconsequential if we end up tweaking or modifying the image anyways?

    1. Not at all are they inconsequential! Raw is incredibly useful in maintaining our editing options.

      When we shoot jpgs, we get ONE interpretation of how that photo should be exposed and presented. Oh sure, we have the ability to edit it later, but our starting point from which we begin, is always the same.

      But with raw, we have a MUCH wider set of starting points with which to start editing from. True, we end up ultimately editing the bit mapped version anyway, but we are no longer restricted to that single interpretation as defined by the out of camera jpg. With raw, if we think, "man, this photo is really a bit over-exposed!" we no longer have to figure out a way to work around that over-exposure. Instead, we can go back to the raw image, and create a more appropriate exposure with which to work!

      That sort of flexibility is priceless in my eyes!

  2. Dear Glen Barrington:
    your were the first one to post an answer to my RAW beginner's question on the dpreview forum. I wanted to write back to you directly and thank you for your response and the link to your blog. I found very useful information on it. Looks like as retired computer engineers we might think alike. I did not like the posts on dpreveiw as it felt like a bunch of immature posters were out to prove what they knew and tried to impress each other. Not very helpful for a beginner. I have given up on dpreview posts but will continue to read reviews on their website. I have bookmarked your blog site and will follow it. I am not a Facebook and Google + guy, so I will just read your contributions. Thanks again for taking the time to reply to my post, I appreciate it.