Tutorial: Setting Exposure on iPhones, DSLRs, and Camcorders
White Balance, Gain, and Iris
Next, you’ll set the white balance as described above, and then move on to gain. Briefly, gain is the electronic amplification of the signal, which makes the video brighter but can inject noise into the image. Whenever possible, you should select zero gain, as you can see best in Figure 1 as the - dB under the Bch set for white balance. In Figure 5 (below), you see gain set to low; in the camcorder’s menu system, I set this to 0 Gain.
Figure 5. Iris, gain, and white balance controls
Basically, you should use gain only as a last resort. If you can’t add lighting to a scene, or adjust the aperture to achieve your lighting target, then you should add gain. Typically, 3-6dB won’t be that noticeable, but if you go above 9dB or so, the image will start to appear visibly noisy.
The last adjustment is the iris control or aperture. Aperture (Figure 6, below) is the size of the hole that lets light into the camera and image sensor. It’s controlled by the lens diaphragm with settings referred to as F-Stops.
Figure 6. Aperture is controlled by the diaphragm within the camera (image from www.bhphotovideo.com).
Aperture controls both lighting and depth of field which, as explained on Photographylife.com, “is the distance between the closest and farthest objects in a photo that appears acceptably sharp.” The wider the aperture, the narrower the depth of field, which means that if you open the aperture wide to let in as much light as possible, you may impact how much of the content in the frame is in focus. Typically, this isn’t a concern with traditional camcorders because the image sensors usually aren’t large enough to produce a significant depth of field. If you’re shooting with a DSLR, however, you’ll probably need to double-check focus after setting aperture to ensure that the subject is sharp and in focus.
Though depth of field is important for movies and other artistic shoots, for most streaming producers, the primary reason that you adjust aperture is to achieve optimal lighting on the main focal point in the frame. This takes us to zebra stripes, which tell you when the lighting is optimal.
Zebra stripes are stripes in the camera that indicate a certain exposure level as measured by IRE, which stands for Institute of Radio Engineers. Briefly, 0 IRE is pure black, while 100 IRE is pure white. If the exposure in your frame goes below 0 IRE or above 100 IRE, you lose detail, which is typically called clipping, or more specifically, “crushing the blacks” or “blowing out the whites.”
When shooting speakers you target different IRE levels for different skin tones, 70 for most caucasian faces, 60 for Hispanics and others with olive-colored skin, and 45-55 for Blacks (see the excellent tutorial, Lighting Darker Skin Tones | 7 Cinematic Techniques). Most traditional camcorders come with two zebra stripe settings, 70 and 100. The first is for Caucasian faces; the second is to help you avoid exceeding 100 and blowing out the whites. In Figure 1, the zebra stripes are set to 70 IRE, and you see that this camera applies zebra stripes to all pixels with a value above 70 IRE.
Figure 7 (below) shows the zebra stripes on the Sony a6300, which allows me to create custom settings for the various target skin tones. You also see that the camera only displays zebra stripes to pixels within the target range, which is +/- 5 in the preset, as opposed to all pixels that exceed the specified IRE level. I find the Sony approach more useful. More to the point, if you’ll be filming many subjects with darker skin tones, be sure your camcorder has zebra stripes to support those skin tones or enables custom settings.
Figure 7. The Sony a6300 DSRL enables custom zebra settings and target range.
When would you use zebra stripes at 100 IRE? When exceeding 100 IRE would blow out the critical detail. When I shot my wife’s ballets, preserving the detail in the tutus was the order of the day, even if it resulted in faces that were too dark. For this reason, I set zebra stripes to 100 IRE and rode the iris to avoid losing detail in the tutus (Figure 8, below).
Figure 8. Setting zebra stripes to 100 IRE to avoid blowing out the detail in the tutus
With all this as a prologue, let’s review the critical camcorder-related steps to achieving good exposure.
1. Set the white balance
2. Set the shutter speed to 2x frame rate.
3. Set gain to zero
4. Choose the zebra stripe IRE level (and range).
5. Adjust the aperture to achieve proper exposure using the zebra stripes.
If you can’t achieve your target by adjusting the aperture, try adding lights. If that doesn’t work, you’ll have to start injecting gain, which should always be a last resort.
Working with DSLRs
In manual mode, most DSLRs let you set shutter speed, aperture, and ISO, which controls the image sensor’s sensitivity to light. ISO settings vary by the device; on the a6300 it ranges from a low of 100 to a high of 51200 (Figure 9, below). The lower the numbers, the less sensitivity to light, which means a darker image but minimal grain. Higher settings are more sensitive to light but can introduce both noise and grain into the image.
Figure 9. Adjusting ISO in the Sony a6300
The interplay between aperture and ISO sets up three potential suggestions:
• If a clear, sharp image is your primary goal, set ISO to a relatively low value and use aperture to control exposure. This is where I live most of the time and how I derived the settings used to produce Figure 10 (below).
• If your goal is to introduce film grain into your video, set the ISO to the desired value and control exposure via the aperture.
• If your goal is to control the depth of field, set the aperture at the desired value, and control exposure with ISO.
Figure 10. Final settings on the a6300
Figure 10 shows the finished exposure in the a6300. You see the shutter speed at 1/60, the aperture at F5.6, and the ISO at 100. For this shoot, I set zebra stripes at 70 IRE, and the stripes on my forehead and cheeks show these regions in that zone. Rule of thirds positioning isn’t bad, but that’s a different tutorial.