HA-ETTR: An Easier Way To Expose To The Right Using The Camera's Highlight Alert
Highlight Alert-Expose To The Right (HA-ETTR) is a variation on ETTR that gives the photographer an easier way to determine the correct exposure for the best quality image.
The limited dynamic range of a digital camera means that getting the correct exposure is critical to getting the best quality image. The correct exposure is the one where the maximum amount of tonal details are captured without overexposing critical highlights that must retain details. Until now the Expose To The Right Technique (ETTR), as explained by Michael Reichmann, has been the best way to get the correct exposure. HA-ETTR makes using the ETTR technique easier.
Light metering built into cameras is easily fooled. Light backgrounds, dark backgrounds, or back lit subjects are all examples of common situations that will result in badly exposured images if you rely on the camera for exposure control.
Judging the exposure by looking at the LCD on the back of your camera is a technique that simply doesn't work . The image shown is from a JPG image generated within the camera and will look different with different camera settings such as WB or Style (Vivid, Portrait, Scenic, etc).
Critically, the image will also look different as the brightness of the ambient light varies.
Expose To The Right (ETTR)
Only the ETTR technique will consistently produce images with the maximum amount of shadow tonal details possible without overexposing critical highlights that must retain details.
ETTR gets its name from the fact that the exposure is adjusted so that the histogram of the image displayed on the camera LCD is placed close to, but not butted up against, the right edge of the histogram graph. This gives the highlights, which are on the right side of the histogram, the maximum exposure that will allow them to retain details without being overexposed.
The technique and all the reasons why it will give you the best images is fully explained by Michael Reichmann in these two articles.
In practice the camera metered exposure is simply a place to start the process of determining the ETTR exposure. You make test shots while varying the exposure away from the camera metered exposure until the histogram is positioned correctly to the right.
The histogram displayed on the camera LCD is for the JPG image generated within the camera, not from the RAW data. Since WB and Style effects the internally generated JPG and associated histogram, setting these correctly to match what will be your final image is critical for good ETTR exposure determination.
The Problem With ETTR
In the second and later article Michael Reichmann mentions that some highlights such as specular ones can, and in fact should be overexposed in order to get the best exposure for the rest of the image. This means that in those cases the histogram must be butted up against the right edge of the histogram graph so that specular or non-critical highlights are overexposed, but critical highlights that must retain details must not cross that right edge or they will be overexposed.
If the critical highlight is large it will produce a peak in the histogram that is easily identifiable vs. the spectral or non-critical highlights. This makes it possible adjust the exposure to place the critical highlight peak close to but not butted up against the right side while still allowing the spectral and non-critical hightlights to be overexposed.
With a small critical highlight, the peak for that highlight may be too small to be picked out in the histogram. You are reduced to making a judgement call of the exposure, or exposing for the specular or non-critical highlights and increasing the exposure in post processing.
This weakness of the standard ETTR technique is overcome in the variation of ETTR that I have developed - a simplified ETTR technique that uses the Highlight Alert (HA) instead of the histogram to determne the ETTR exposure.
Like the histogram, the HA is also from an internally generated JPG so different camera settings such as WB and Style (Vivid, Portrait, Scenic, etc) will also effect it. Be sure to set these to reflect the settings of your final image after post processing.
The HA-ETTR method has several of advantages over the regular histogram ETTR method.
1: Positioning the histogram to the right is very difficult when spectral or non-critical highlights are present and can become a judgement call. With HA-ETTR you are either at the right exposure or you are not - there is no judgement call to be made.
2: With HA-ETTR you review the image full sized on the camera LCD, as opposed a smaller thumbnail image plus the histogram and data. Reviewing a full sized image makes judging composition and lighting ratios easier.
3: When Highlight Alert is used, any highlight that is overexposed will blink during image review, which makes it easy to find them. This is very important since even small but critical highlights can be easily found and differentiated from non-critical ones that it is okay to overexpose.
Determining the correct HA-ETTR exposure is similar to the way you do it with the standard ETTR technique. Start with the camera metered exposure then make test shots and look for an exposure that just causes the critical highlight you have picked out to blink.
The exposure that caused the critical highlight to just start to blink minus 1/3 stop turned out to be the correct HA-ETTR for RAW files with my Canon 7D (see Note #1).
For JPG's, because of their reduced dynamic range, I found that I needed to reduce the HA-ETTR exposure by 2/3 stop to prevent any possibility of the highlights losing texture. This preserves the highlights at the cost of a small loss of shadow details.
With subjects lacking a critical highlight you can temporarily introduce one in the form of a white card (see below), but this is really not necessary. Just treat the subject like a low contrast subject.
For low contrast subjects, follow the regular HA-ETTR procedure, increasing the exposure until some part of the image just starts to blink, reduce the exposure by 1/3 stop, and capture the image. As with regular ETTR, normalize the image in post processing to obtain the final image.
EA-ETTR is an easier method of determining the ETTR exposure. Enable the Highlight Alert, find the minimum exposure that causes a critical hightlight that must retain details to blink, then reduce the exposure by 1/3 stop for perfect RAW exposures.
I wanted a controlled target that would let me show how HA-ETTR works. I settled on using my X-Rite ColorChecker Passport as my standard and cliped it in front of my DIY White Card (see Note #3 below) and my 18% Gray Card. Lighting was one 300 Ws studio strobe in a 40"x40 softbox located about 6' away from the target, at the same height as the target, and aimed at the target. The studio strobe power and distance were adjusted until the HA just started to blink at ISO 100, 1/200 sec, and f/8.
The images taken of the back of my camera show a thumbnail image, the histogram,and data.
The nomimal and measured values for the two ColorChecker Passport gray scales can be found in this test article by Robin D. Myers.
The bottom row of the upper Photo Enhancement chart is a Clipping Scale. The far right white square is named, "Light Clipping 4". The white square on the bottom row on the lower ColorChecker Classic chart is named, "White".
ColorChecker Passport White Squares
The images below are presented at approximately the resolution of the 7D's LCD. The actual camera LCD is about 2.5" wide and 1.7" high so you can reduce the size of these images on your monitor to about those dimensions to make the viewing a bit more realistic.
The highlight is large in this first image and it is easy to see that the highlight peak in the histogram it is butted up against tHe right edge, indicating overexposure. The second image shows what the LCD looks like when the HA causes the overexposed highlights in the image to blink.
Here is what the LCD image looks like when the Highlight Alert blinks.
Here are the values from Robin D. Myers and the averages of the RGB measurments made in Lightroom.
Reducing the exposure from f/8 by 2/3 stop to f/10 gives measured values close to the nominal and Robin D. Myers values of the ColorChecker Passport, but in my opinion the image is actually underexposed by about 1/3 stop for RAW files. This is why I recommend finding the exposure that causes the HA to blink then reducing the exposure by 1/3 stop. The LIghtroom histogram backs this up. Note that the highlight clipping warning is turned on but there are no overexposed areas showing in this f/9 image.
Here is an image of only my ColorChecker Passport showing against a light gray background. This shows what the camera LCD display looks like with only a small highlight. Note that the peak for the single LIght Clipping 4 white square is lost in the "grass" at the right edge of the histogram and impossible to see, yet it is easy to see the white squares are blinking.
It is even easier to see the blinking highlights when you forget about displaying the histogram and display the image in the full LCD window.
For the best quality portrait prints the red channel of the large facial highlights (not small spectral ones) should have a value of about 88% to 95% in Lightroom, 220 to 240 in Photoshop. The exact value depends upon the printer as well as the subject, but the total range is only about 1/3 stop.
These facial highlight values fall neatly into the ability of the HA-ETTR technique to determine the exposure. Using HA-ETTR is a good way to calibrate the response of your flash meter for portraiture.
You can always temporarily place a white card in a image to give you a standard reflectance value for your HA-ETTR readings.
Plastic white cards have the advantage that they can be cleaned with soap and water or even sanded down again if they get dirty. Here is just one commercially available example, the Adorama Gray Card Exposure Aid, Pack Of(3). As stated above, you must calibrate the HA-ETTR technique with your camera for the specific white card you use.
I found a 1/16" thick 14"x14" square of translucent white acrylic at an art supply store for about $1 US. I used fine sandpaper to sand down the front and back of the sheet to remove the shine and give it dull non-direct reflective surfaces. I scored one edge with a box cutter and snapped off a 4"x14" strip which I then split into two 4"x7" pieces. These small white cards are kept in my camera backpack for location use. The remaining 10"x14" piece is for studio use.
When I tested this DIY white card against the ColorChecker Passport I got almost identical RGB values for the DIY card and the Light Clipping 4 square. My DIY white card has proven successful as a white target for HA-ETTR.
If you meter on this DIY white card it will produce a gray image, thus it also serves as a gray card for setting a camera custom white balance or for setting the WB in post processing.
Sep 21, 2014
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