Tue 27 Jan, 2009
Tags: Graphics, Photography, Software, Tutorial
In this installment of Woodworkers And Digital Photography we will focus on taking the picture, that is capturing the RAW file. Much of what we have discussed will come into play here. The emphasis is on getting the correct exposure and composition. In Part 6 we will learn to use Adobe Photoshop Lightroom to develop the RAW file and in Part 7 we will finish up by using Adobe Photoshop CS3 to create the portfolio print.
My Shop Does Not A Studio Make
If your shop is anything like mine, it is not conducive to photography. Certainly a photographer would not model a studio after my shop. My walls are construction plywood. I have numerous windows on three sides and often (weather permitting) I have a garage door open to the shop. The ceilings are white. The lighting is fluorescent mixed with daylight streaming in the sides. Reflected light from the ceiling is mostly fluorescent. From the walls it is a combination of fluorescent and daylight, but because of the makeup of the walls it is mostly yellow to brown. I don’t use either flash or studio lights. The former would cast harsh shadows and the latter take to long to setup and take down.
Use Low ISOs To Capture A Noise Free Image
The light level produced by the lighting described in the previous paragraph is low. To obtain correct exposure I must use a rather long exposure time, a large aperture (low f-stop) or a high ISO setting. We didn’t discuss ISO in detail, but suffice it to say that at higher ISO setting more noise enters the RAW file and becomes visible as randomly colored pixels. These noise pixels show up mostly in the darker areas of the picture since that is where the light level is lowest which produces the lowest signal/noise ratio. Hence, we will always use ISO 100 or 200, maybe on a rare occasion 400, but no higher. Note the noise in the face and chest area of the picture at right which was shot at ISO 1600.
Stick With f-stops Between f/8 And f/11 For Good Depth Of Field
That leaves us with a large aperture or a long shutter speed. We know that smaller apertures produce larger DOF which is what we want. So we might decide to use the smallest aperture possible. But there is another problem. Below about f/11 the aperture is so small that it tends to produce diffraction related anomalies. We will use between f/8 and f/11 which will yield the desired DOF and will not destroy the quality of the image. We start by selecting the aperture priority mode (Av on the mode dial) set it to the desired f/stop. Next we set the desired ISO. Now we are ready to mount the camera on a tripod. Though we haven’t discussed a tripod since I listed the necessary equipment in Part 1, it should be obvious by now that we will end up using a long shutter time making the tripod necessary to avoid camera shake that might be produced by hand holding.
If you have a good zoom lens and the situation allows, use the lowest focal length. This will produce the largest DOF. Sometimes I have a problem with fixed work stations getting in the way and I end up having to use the telephoto (higher focal length) range to get my subject in the picture. This occurs mostly with large pieces.
Take A Test Picture And Adjust Exposure
Now is the time to adjust the tripod for height, focus the picture and set the camera to timed exposure. We can use either manual or auto focusing, both should work just fine. It may be useful to limit the auto focusing and exposure points to just one, whether the center point or one that is most appropriate for the composition. Next we depress the shutter button half way to get an exposure reading, set our feet, take a deep breath and completely depress the shutter button. We do not move until after the picture is recorded and the file is transferred to the storage device. Usually a red light is available on the camera to let us know when this has occurred.
The Histogram Is A Useful Tool To Assure Correct Exposure – But Only A Tool
At this point we have a test shot which we can view on the camera’s LCD. We don’t remove the camera from the tripod or alter the camera or tripod’s position to do this. We simply choose the preview button. At this point the picture is not the most important thing to preview, rather we want to look at the histogram. The Canon 40D has two styles of histogram preview; one displays a small image with three separate red, green and blue histograms (above left) while the other includes a gray scale brightness histogram (right).
Histograms are simple. Don’t let them intimidate you. The x axis represents the intensity level from darkest on the left to brightest on the right, or quantitatively 0 on the left and 255 on the right. This provides 256 buckets. To create the histogram each pixel in the picture is placed in the bucket that correctly represents its tone or brightness level. In a bright or high key picture there will be many pixels in buckets that crowd the right of the histogram. In a dark or low key picture there will be many pixels in buckets that crowd the left of the histogram. In an evenly lit scene with highlights and shadows there is likely to be a single hump curve with most pixels in buckets near the middle of the histogram. Still some histograms will have two or more humps. The amplitude of the histogram tells you how many pixels fall in each bucket, the y axis. There is no one correct histogram shape. But there are some things that one should look for in a histogram – most importantly is exposure and how much data was captured.
In the above histograms ignore the picture itself, it is just one I took as an example. Also, when taking this snapshot I used the automatic program mode (P on the Canon) so the camera chose the shutter speed and the f-stop and focusing was automatic. Ignore all of this. What I want you to do is click on each image to enlarge it and notice that the picture is underexposed. You can tell this because the histogram is too far to the left. Notice the histogram has five zones and only three and a half of the lowest zones are used. This shows up in both the RGB and brightness histograms. Not only is this picture underexposed but we are not capturing as much image information as the camera is capable of. In fact we are probably capturing less than half the information available. If this were a picture with lots of detail and a full range of tones we would sorely miss this information when we attempt to develop the RAW file. More on that later.
The brightness histogram is a sort of amalgam of the red, green and blue histograms. The algorithm for producing the brightness histogram from the RGB histogram is not trivial and varies from camera manufacturer to manufacturer. But you can see the general relationship is consistent. At left is another histogram, this one is of the very same picture but produced by Adobe Photoshop Lightroom. This histogram also shows that the picture is underexposed though there are no vertical lines to give you an idea how much.
Calibrate Your Camera’s Histogram
If you have never done it, you should take several pictures with your camera, under various types and levels of light, and compare your camera’s histogram to the that of Photoshop Lightroom. If there is significant difference make a mental note of what it is and how much. Photoshop’s histogram is likely far more reliable than your camera’s, so in the future adjust your camera’s histogram mentally to correspond with Photoshop. For example, if my camera produced the histogram we see here, one and one half zones underexposed, but when viewed in Photoshop produced a histogram that showed correct exposure (all the way to the right without blowout), I would then mentally adjust all my camera’s histograms about one and one half zones to the right. Of course this is an extreme example to make the point and your camera is not likely to be that far off. Nor should you base a correction on the result of just one picture, but a number under various conditions. By the way, notice that Lightroom displays secondary color histograms, as well as red, green and blue histograms.
Adjust Exposure Until It Is Correct And You Are Capturing The Available Data
What we have learned from this test shot is that we have to adjust the exposure, increase it in this case. I would try increasing it a full stop and take another test shot. Since we don’t want to change either ISO or f-stop we can use the camera’s exposure compensation setting to accomplish this. Alternatively the camera’s bracketing capability could be used. I usually do this until I get the histogram I like. I must be sure not to go too far, or I will blow out the brightest pixels, that is render them as pure white with no detail.
Next I take another test shot, this time with a gray card in the picture as shown right above. Click on the picture to see an unadjusted enlargement. Notice there is a little too much yellow and magenta. I can use the fact that the upper right hand corner of the card is 18% gray, the bottom two are pure black and pure white and the upper left is a neutral gray. When I process it in Lightroom I can correct this color imbalance which is called white balance correction. The nice thing about shooting RAW is that I can do this in the computer, I don’t have to worry about it at capture time in the camera. Besides, Lightroom is far better at this than the software in the camera.
Take The Picture
Finally, if I were shooting for my portfolio, I would remove the gray card from the picture and take the actual shot. The picture at left is the same composition, but I left the gray card in so you can see the color after corrected in Lightroom for white balance. Nothing else has been done. Notice the grays, black and white in the card are now correct. Though it may not be obvious to you, since you don’t work in my shop, I can tell that the other colors are also correct. The advantages of a gray card are many, but this is especially true in mixed lighting conditions – it can save a lot of work and trial-and-error in the development process. I actually have done this enough that I have characterized my shop and have a white balance template in Lightroom called SRWW Shop. Now I can take a photo without a gray card test shot and simply apply SRWW Shop to it when I load my RAW files into Lightroom. We will discuss this white balance technique more in the next installment.
Once More On The Advantages Of Shooting RAW
One last subject I want to discuss before ending this installment is the advantage of shooting RAW. We have discussed this before, I know, but now I want to show you an example. The picture at right is a jpeg just as it would look coming out of the camera. Most DSLRs have the ability to shoot both RAW and JPEG at the same time. This picture is the jpeg image. I adjusted for exposure and color corrected the RAW image in Lightroom in 16 bit per pixel mode and then converted it to a jpeg file. The jpeg image was adjusted for exposure and color corrected in Photoshop CS3 in jpeg 8 bit per pixel mode and saved.
Now let’s compare the resulting histograms. The RAW processed jpeg histogram is at left. The JPEG processed jpeg histogram is at right. Both histograms were produced by Photoshop CS3. Note their general shape is the same, but the JPEG processed JPEG histogram has missing data or more accurately missing tones, which can manifest itself as banding, the abrupt steps in tones instead of smooth undetectable transitions. This is caused by the camera throwing away large amounts of information when it converted, in camera, from RAW to JPEG. The picture at left was taken from the camera as a RAW file, processed in a RAW format, and then after all exposure and color correction adjustments were made, converted to JPEG. Since this picture retained all its information through its processing the resulting JPEG has many more tones and isn’t likely to show banding. This is a graphic demonstration of why capturing and keeping as much information as possible until the very end is critical. That is one major benefit of RAW. The other major benefit of RAW is that you can always go back to the RAW file and re-develop your picture, perhaps differently to create a different interpretation.
So far we have captured a picture in RAW format that is correctly exposed. In the next installment we will develop the picture in Adobe Photoshop Lightroom. See you in Woodworkers And Digital Photography – Part 6.
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