Introduction to Raw Files and Raw Conversion by Dave Ryan

By Dave Ryan, February, 2006

It’s hard to spend any time around photographers these days without hearing the terms raw and raw processing. Shooting raw and getting the most out of your raw images during conversion are some of the hottest topics among digital photographers. The problem is getting a consensus on how important it is to shoot in raw mode or exactly what sort of image editing should happen during the raw conversion. Every photographer is different and has needs specific to their particular photographic pursuits. A photojournalist shooting high volume on a tight timeline has different needs than a fine art photographer hand picking images for exhibition. Understanding these differences and being honest about your own goals and needs will help you understand whether raw shooting fits into your photographic toolkit and what sort of raw processing workflow makes sense for you. This tutorial can’t tell you what kind of photographer you are, but it can help you understand the issues and tradeoffs associated with working in raw.

Raw what is it?

Modern digital cameras offer a variety of ways to save your image files. The two most popular are Raw and JPEG. JPEG files are complete image files that can be universally opened, viewed and printed by any image editing or viewing software. Raw files on the other hand aren’t quite images yet. They contain all the information to create photos, but straight out of the camera they can’t be viewed without special conversion software. In fact when you view a freshly shot Raw image on your camera’s LCD you’re actually viewing a small JPEG that was created along with the Raw capture. Similarly when you check your in camera histogram while shooting Raw you’re actually viewing the histogram for a JPEG created from the Raw file. So if the Raw file isn’t actually a picture yet, what is it?

Raw files contain the actual digitized data from your camera’s image sensor. The data is a string of numerical brightness values straight from each red, green and blue photosite. And unlike the way film or our eyes see light these raw brightness values are linear values, in simplest terms just a tally of the number of photons that each photosite collected. If you could print this raw data you’d just see three very dark B&W images one representing the photosites sitting behind red filters, one for the greens and one for the blues.

The Raw file also contains all the settings that were in effect at the time your shutter was released. In other words all the shooting parameters that the camera auto selected or you dialed in including: white balance settings, sharpness, noise reduction, tone curves, exposure compensation, etc. But the important thing is that this information hasn’t been applied to the raw sensor data yet. It’s carried along as recommendations to your raw converter but you can choose to use that information as recorded or second guess those choices during raw conversion. This is one of the huge advantages of shooting in raw mode, the ability to second guess the camera settings you set in the field. So can you go back in time during raw conversion and change everything?

The settings that directly impact light traveling through your lens, exposing the photosites on your sensor, and creating a digitized brightness reading at the output of your camera’s analog to digital converter are fixed at the moment you release the shutter. So the aperture, shutter speed, ISO setting and overall exposure are decided in the field. You can’t second guess your depth of field during raw conversion, stop motion blur after the fact with a higher shutter speed or drop your ISO setting to get lower noise. Those decisions are hard coded when you release the shutter, but you’ve still got a lot of freedom to alter your field choices.

Much of the creative editing we undertake in the digital darkroom revolves around tasks such as removing color casts (adjusting white balance), increasing or reducing contrast, and fine tuning exposure. These are perfect candidates for correction during raw processing.

Raw files also have two other important advantages, their ability to capture a bit more tonal range than JPEGs and the ability to convert raw files in 16 bit mode. Like negatives vs. slide film raw files have a bit more exposure latitude than JPEG files. Depending on your camera and exposure settings its possible to recover a half stop or more highlight detail and a stop or so of shadow detail during raw conversion. So if your camera's histogram or blinkies tell you you've clipped a few of your highlights you might be able to save them during raw conversion(remember the in camera histogram is a JPEG histogram and doesn't show the extra exposure latitude of the raw file). Working in 16 bit mode gives you more freedom to adjust tones and colors without introducing two digital workflow nasties, posterization and banding.
Both of these editing induced problems involve pulling adjacent tones or colors apart to the point where you lose the subtle transition of the original image. You tend to see these problems in areas of smooth tonal transition like blue skies. If your image needs a lot of tone or color tweaking or you plan on working in Lab or a wide gamut RGB space then working in 16 bit mode or high bit as it's often called is a good idea.
JPEGs come out of the camera as 8 bit files so if you want to work in high bit mode you should shoot raw

It should be noted that Raw is not actually a specific file format like JPEG or TIFF. Instead Raw describes a file containing raw sensor data along with camera settings but raw files are not standardized between camera vendors. This means that Raw files from different camera vendors and even different cameras within a product line are not interchangeable. Nikon for instance uses their proprietary NEF format for Raw files and Canon has their CRW and CR2 formats. Take it further and the Nikon D200 requires a different version raw converter than the Nikon D70 or D2X. It’s a confusing hodgepodge and it often requires a trip to your raw converter’s software download page every time a new camera is introduced. There are initiatives to standardize raw files, but that’s another discussion….

So what about JPEGs?

It’s easy to conclude that Raw is the only format worth shooting since it gives so much flexibility for corrections after the fact. Personally JPEGs still play a big part in my photography. A well shot high resolution JPEG can produce a beautiful print straight from the camera or with some subtle adjustments in the digital darkroom. JPEG is a compressed file format so for the same image a JPEG is smaller than a raw file allowing you to capture more on each card, store more on your disk drives and shoot longer continuous bursts without filling your shooting buffer. I like to think of JPEGs as the digital equivalent to shooting slide film. You’ve got to get your captures correct in the camera, but once you do you’ve got an image that’s good to go with little or no tweaking. By contrast, every Raw file requires the extra step of conversion to some useful image format. If I’m shooting a large volume of images, especially if it includes high speed action or shooting casually or entirely for the web I shoot JPEG. If I’m shooting carefully structured portraits or scenics or shooting in difficult mixed light I shoot Raw. If I’m somewhere in between which is pretty frequent these days I shoot Raw+JPEG and make the decision which versions to keep during my image sorting.

Raw conversion software

So if Raw files don’t come out of the camera as useable images what needs to be done to them? That’s where raw conversion software comes in. Raw converters read the raw file, apply either the original camera settings or a version you choose to enter and convert the file into a standard image format such as JPEG, TIFF or PSD (Adobe’s proprietary photoshop format) that you can view, edit or print with common image editing programs. They come in many flavors and work either as standalone programs or plugins to programs like Photoshop(PS). Most can be run on individual images or run in batch mode to convert entire directories of raw files. Popular raw converters include: Nikon Capture (for Nikon shooters of course), Bibble, Capture One, Raw Shooter and Adobe Camera Raw(ACR). The bottom line is that if you shoot raw you’ll need a conversion utility like one of these to do anything with your images. Each of these has its own merits and shortcomings and serious shooters often use more than one of these to deal with specific problems. But they all share some common features and in the end strive for the same goal: to deliver the highest quality image from your raw file. I’ve used several of these but do most of my conversions in ACR these days. For one thing it’s bundled with Photoshop CS2 and Photoshop Elements but it’s also a very good converter that shares a lot of working conventions with PS which makes things easy. Regardless of which raw converter you choose the basic goals of conversion are the same but the keystrokes and certain special features will vary.

Raw workflow strategies

How you approach your raw workflow has a lot to do with the kind of shooter you are and how you balance your time behind the computer with your quality needs. On one end of the spectrum you’ve got the event shooter who captures hundreds to thousands of images on a busy day and needs to get them processed and up onto a sales website. Delays in processing translate to lost interest and lost sales but quality still counts. At the other extreme you’ve got the serious fine arts shooter who is willing to spend hours, days or longer on each image and strives to squeeze the last drop of quality out of each display print. Where the first photog might lean towards batch processing and doing as much image editing as possible with their raw conversion software the second is more likely to work each image individually and import their images into Photoshop or other full featured editor for most of their editing.

These represent extremes and with each project we might find our needs change. One day a high volume batch oriented workflow might make sense and the next we might prefer to slow down and really work an image. Or we might typically fall somewhere in the middle and need to modify our raw workflow to something that suits our particular needs. But these extremes of working style are useful for exploring the versatility of raw conversion and bring understanding to the sometimes conflicting advice we receive.

So where do your needs fall? Are you a high volume cowboy who shoots from the hip, needs consistently good results but can live without squeezing the utmost from every image? Or are you a digital Ansel Adams who’ll work and rework a print and won’t be satisfied until your print delivers the potential you saw in your minds eye? Or perhaps a more relevant question, who are you today for this particular project?

Raw Conversion For The Digital Artist

Given the time I open images individually in my raw converter and don’t even attempt to complete my image editing at this stage. I correct any gross color casts, make preliminary exposure corrections if necessary, perhaps choose a color space, but my goal can be summarized as: first do no harm. If I clip highlights, block up shadows or overly pump up the contrast at this stage I’ll seriously handicap myself when I open the image in PS. So I tread lightly at this stage and concentrate on those things that can be done more effectively during raw conversion and save the detailed work for PS with its powerful features such as adjustment layers, selections, masks and unique tools such as Shadow/Highlights. Two things I always address during raw conversion are white balance and gross exposure settings. Setting the highlights in particular is one thing that can be done more effectively during raw conversion than later on. This is related to raw’s linear representation of tone levels. The flip side is that I’m extra careful not to overdo the shadows slider at this point as I’ll get more finesse with this setting in PS. I keep a close eye on my histogram during raw conversion and make sure I’ve left a little breathing room that I can fine tune later. When working in fine art mode I don’t sharpen or apply custom curves during raw conversion. I just have more tools at my disposal for that kind of work in PS. I also don’t resize images when working in this mode as it limits my interpolation options and forces me to do my PS work on larger(translate slower) files.

Raw workflow for speed

If volume, speed and efficiency describe your photographic pursuits then you’ve got to cut some corners. You’ll also want to stay out of the time warp we call Photoshop if you can help it. Minutes have a habit of turning into hours as we experiment with selections, masks, adjustment layers and brushes. You don’t have time for this, not with another hundred images to get out this afternoon. So the strategy here is to do as much of your image editing as possible within your raw converter. If you’ve shot similar subjects under similar light you can even apply conversions from one image to an entire directory of images using your converter’s batch processing capabilities or utilizing the apply raw conversion features in Adobe Bridge. So when time is at stake I’ll try to do all my image sizing white balance, exposure, sharpening, noise reduction, contrast and saturation adjustments during the conversion stage. Hopefully the images will be good to go out of the converter and won’t need any further PS touchup.

Working with Adobe Camera Raw

So you might do more or less during raw processing depending on your needs, but what is it you do, and how do you do it? The following raw workflow describes the main image adjustments supported by ACR. Do you need to do all of them? Heck no, consider your needs as described above and use those tools that will help your image and makes sense for your workflow style. I’ll focus on the adjustments I do regularly, not every button, bell or whistle supported by ACR. If you want to know it all, get a copy of Bruce Fraser’s excellent book: Real World Camera raw with Adobe Photoshop CS2.

Opening Files in ACR

• DigitalPro4 is the hub of my digital workflow, I use it to load, inspect, edit, prioritize, file and rename my images. I also use it to launch external editors like PS and ACR. If I send a NEF (Nikon raw) file to PS CS2 it automatically launches the ACR converter. You can also launch ACR via Adobe Bridge or by double clicking a raw file (assuming your MAC or PC has associated that file type with PS) from your hard drive. You can also launch PS, go to file>open and select a raw file. One way or another, launch the ACR dialog with the image you’re interested in editing.

• When the ACR dialog first opens you’ll see your image with a large yellow caution rectangle. This just means your computer is busy processing the image. Wait until this vanishes before making adjustments.

ACR Preferences

• I disable the ACR auto adjustments by clicking Ctrl-U (Cmd-U Mac) on the first image I open and then clicking the right facing black arrow on the right hand side of the dialog box to select Save New Camera Raw Defaults. You only need to do this once and it’s saved for future use. I turn off the auto adjustments so I can see exactly what I shot without any corrections. If you’re a batch processing volume converter you may choose to leave the auto adjustments enabled.

• As long as we’re talking preferences I click that same black arrow and choose Preferences and set sharpening to Sharpen Preview Only and save that setting. That gives me a reasonably sharp ACR preview but doesn’t actually apply any sharpening to my image when it’s opened in PS. Again a volume converter will want to sharpen at this time to save a trip to PS or other sharpening tool.

• I generally work in Adobe RGB 1998 color space in PS so that’s my default ACR conversion space. If I’m struggling with clipped colors (usually reds) I often go to ProPhoto RGB to save as much info as possible and then convert in PS after using curves or other tools to salvage the clipped colors. If you work in sRGB or Colormatch RGB then that’s your ideal conversion space.

• With a fast computer or when I’m working in Prophoto RGB or I really blew the exposure and have to do some salvage work I operate in 16 bit mode. Otherwise I operate in 8 bit mode to save processing time. It’s similar to the JPEG argument, if I got it right (or close) in camera 8 bit is fine, if I’m going to do a lot of tone and color stretching I prefer to work in 16 bit mode. Volume converters, you know what you need but I bet its 8 bit more often than not.

• Unless I’m batch converting Web images I stay at the native image sizing and save any resizing for PS. Again the batch folks will size to fit their needs here.

• I always leave the preview, shadows and highlights boxes above the image window checked. This shows me the affect of any changes and lets me know right away if I’m clipping any highlights (pixels light up red) or shadows (pixels light up blue) it’s handy way to keep an eye on things.

Overall Settings

• I generally select Image Settings from the pull down Settings menu located just below the ACR histogram. That lets me see how the conversion works with my original camera settings.

• For batch processing or when working similar images I adjust one image then choose Previous Conversion for all subsequent images. This is a great trick for you panorama shooters out there. I’ll convert the brightest shot in my pano sequence first then convert the rest using Previous Conversion. That helps me to keep the exposure and color casts stable from image to image which really simplifies the stitching process.

Adjusting Exposure

• I like to work images by hitting the big problems first. If my colors look pretty good but my exposure needs work I start with exposure, if my WB is way off but my exposure look good I start with WB settings.

• The Exposure slider is very much like the white point slider in the PS levels dialog. You operate it a bit differently by adding or subtracting stops of light, but the result is the same, it brightens or darkens all tones by setting the brightest tones in your image. Go to far with this and you start blowing out highlights don’t go far enough and you end up with an unnecessarily muddy image. There’s a neat trick taken from the PS levels dialog that really helps here. Hold down the alt (Mac opt) key while sliding the exposure slider and the screen goes black until the first pixels clip and then they light up in the color that clips first. Usually reds will appear unless more than one channel is blown out. So unless my image has specular highlights that I’m willing to sacrifice I hold the alt key and slide the exposure up until the first red pixels appear then I back off a tad.

• One great feature in ACR is its ability to recover partially blown out highlights. If just one or two channels clip on the high end ACR does an amazingly good job of preserving detail based on the info in the other channels. This allows you to push the exposure slider till you get a bit of clipping as long as you don’t clip all channels. This is handy when backing off your exposure setting to avoid clipping gives you an overly dark image.

• The shadows slider works like the black point slider in levels and shares the alt(opt) key trick. I’m often willing to let some shadows go black, but generally only thin lines or scattered pixels. If there’s important shadow detail in an area I try to make sure none of it is lost here or I won’t get it back later.

• The Brightness slider is equivalent to the midpoint slider in levels and sets midtone brightness. If you push it up too far you can start to affect the exposure setting and you’ll start blowing out highlights. This is where having the highlights warning enabled comes in handy. You can also keep an eye on the histogram to to make sure you’re not overdoing it but with a calibrated monitor I usually just adjust brightness to taste. Brightness isn’t a cure all for underexposed images, if you push this control way up to salvage an image you’ll also dramatically increase noise in your shadows. It still pays to get the exposure right in the camera.

• You can push up both contrast and saturation during raw conversion but I generally save these moves for PS. One exception is when I’m having trouble holding both the highlights and shadows in an image, if I’ve got to salvage an image shot in high contrast light I’ll often back off on the contrast slider to hold as much shadow detail as I can. I can then use curves in PS to put contrast back where I need it. A volume converter could use these as desired and apply the conversion to an entire shoot in similar light. Both of these adjustments can lead to clipping either highlights or shadows so keep an eye on your warnings or your histogram if you start pushing these very far.

• That’s as far as I go if I plan on working the image in PS. Otherwise I might make curves adjustments, sharpening and noise adjustments. Occasionally I run into vignetting or chromatic aberrations that benefit from the lens adjustment tab but not very often.
   

Adjusting White Balance

• This is the other big raw adjustment and one you’ll probably use a lot. You have several tools at your disposal for WB adjustments.

• White Balance, you can use the pull down menu in the upper right of the ACR dialog to choose any of the settings you could have chosen in camera such as Cloudy, Daylight, Flash, etc. This is a great way to experiment with the way your camera renders colors with different WB settings

• Color Temp slider, If you don’t like the results at any of the preset camera settings you can simply slide the color temp slider to the left or right to suit your taste. Raising the color temp slider adds yellows to your image while reducing blues. Lowering it does the opposite.

• Tint slider, while the color slider adjusts the blue/yellow balance of your image, tint affects the balance of green vs. magenta. Raising tint adds magenta for a warmer image while lowering it boosts the greens.

• Color Eyedropper, this handy tool is great if you have something neutral in your image. You simply find something that should be neutral gray or white and click on it and the color temp and tint sliders are adjusted so that it becomes neutral adjusting all other colors at the same time. I use this if I have WB troubles and my image includes something that is white but still has detail. If you click on a blown out highlight all bets are off but with a detailed white this works nicely. You can still fine tune to taste using the temp sliders but the eyedropper gives you a nice starting point.

• WB settings and exposure settings are intertwined with one another so it’s not unusual to have to tweak your exposure settings after setting WB or vice versa. For instance if you set your exposure so that the red channel is almost but not quite clipping and then bump up the color temp slider the red channel will often begin to clip. So whether you tackle your image’s exposure or WB first it’s a good idea to double check both before finishing the conversion.

Exiting the ACR dialog

How you exit ACR just like what you do while in ACR depends on your workflow needs. To transfer your converted image straight into PS click Open. To apply the raw conversion changes to your image file without opening the file choose Done. To save a converted version of your file as a JPEG, Tiff, PSD or DNG(Adobe’s proprietary raw format) click the Save button. If you decided you really didn’t want to work this image after all and you don’t want to save your changes click Cancel.

Wrapping it up

I don’t believe in absolutes and reject the notion that there’s a good one size fits all shooting style or workflow. Just as your choice of camera body, lenses and subject matter reflect your interests and needs so should your choice of file formats and workflow. Do you need to shoot raw to be a serious or even professional photographer? Nope. Does shooting raw open doors and allow greater image control? You bet it does. I look at the vast array of cameras, lenses, accessories including computers and software as tools in our toolkit. I always recommend folks take some time to identify their goals and needs before accepting what everyone tells them they must have. From that standpoint shooting raw files and taking control of your image conversions are very powerful tools for your kit. But like any powerful tools they come at a price such as the need for larger memory cards, more powerful computers, more disk storage including backups, and maybe the biggest price, more time behind the computer. Is it worth it? Only you can answer that but whether or not raw becomes your shooting format of choice it’s a powerful tool that’s worth experimenting with and learning to use.

--Dave Ryan for nikondigital.org

About Dave Ryan: Dave is a photographer, photo instructor and mountain guide in Jackson Hole, Wyoming. He sells greeting cards as well as stock photographs of action sports, wildlife and landscapes. He views digital photography as the ideal merging of his love for photography and his career as an electronics engineer specializing in digital signal processing systems. He is a regular contributor to the forums here on nikondigital.org