Tech Talk: Rating Your Sensor

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Today we’re going to introduce rating and EI.

Your camera has a range of ISOs you can choose from, most likely running from ISO100 to ISO3200 in third stop increments. Camera and film manufacturers used the International Standards Organization methods to set the speed, or ISO, of your sensor or film. They generally are set to favor overexposure, as underexposure is harder to correct. Therefore, in test conditions ISO 100 performs at ISO 80-100 to get you exposed at the general target for the contrast and exposure standards set by the organization. This used to be referred to as a film’s “box speed.” For digital work, I’ve come to think of it as “nominal.” Therefore, when I say I shot it at ISO 125, that’s what I was set and exposed for.

The ISO recognized that most scenes deviated from test conditions, as did developing or processing. A film may be rated at ISO 100, when developed in a certain developer for a specific time, but not for any other combination. Film shooters would “rate” their film at different speeds for their methods, including exposure and development controls. For instance, some would shoot FP4+, which is ISO125 in Ilford’s ID11 developer, rated at EI80 developed in Kodak D-76 for their preferred method of exposing and developing a standard shot.

EI, or “exposure index,” is what you rate your film at for your methods, and usually refers to speed rating assigned that’s different from the film's actual speed. Generally, when you shoot with your digital SLR, you shoot maybe at ISO 100, and edit it in your manufacturer’s provided application and get decent results. You are shooting and making your shot at nominal speed, or box speed with film.

You may find that in high contrast scenes at nominal speed, you are getting shadows that are clipped to black and whites that are blown and have no detail. In that case, it’s time to change your EI, or rate your speed different. To do this correctly, you should test your camera, and we’re going to cover the procedures to do that for YOUR best results. Since this test is based on your equipment, metering, and editing, someone else will get different results using the same equipment. This test is based on your style and methods, which others may not follow. Keep that in mind when “rating” your gear, you can recommend your “rating” to others with the same camera, they will most likely appreciate the information, but each “rating” is subjective, and will work best in your hands.

This testing becomes especially important when shooting at extremes of lighting. In very low light, like that in most old buildings, noise is a real concern when trying to increase your image brightness during editing, and in bright daylight lit landscapes skies and clouds can become just blotches of white without any detail.

The procedures for setting your EI involved a gray card, meter, and your camera. You should be set up for this in light conditions that match your intended shooting conditions to effectively use this method.

Step 1, set up your gray card in lighting that matches your shooting conditions. Step 2, set your desired ISO reading on your camera, and select RAW mode. Step 3; set your camera to f/8 or f/11 and AV mode. Step 4, meter your gray card, use spot metering and fill as much of the frame as possible with the gray card. Note the shutter speed. Step 4, set your camera to manual mode, and enter the shutter speed metered in step3, which should give you an 18% gray image. Step 5: compose your test shot. You can pull back some, including other elements. I have a test screen that includes black and white pieces and collapses nicely into a camera bag, and prefer to use that to check contrast ranges while doing this test. Step 6, take the following series of shots- at the metered reading obtained previously, -1/3 stop, -2/3 stop, -1 stop, +1/3 stop, +2/3 stop, and +1 stop. Step 7, load the images onto your computer. Then open your image editing application. Step 8, Open each image in turn, and find the one that is MOST CLOSELY identical to the gray card in exposure for your set up. Make sure to perform absolutely no adjustments to the images, as we are looking for the most closely identical image. Take note of the settings used for this shot. Step 9, this is your EI for that type of scene with that ISO selected. For example if –1/3 stop produced the best rendition for ISO100, then your EI would be 125 or EI125. You will effectively be rating your ISO100 shots at 125, and removing 1/3 stop on all your shots in those conditions. To accomplish this, when shooting at ISO100 simply remove 1/3 stop exposure, either by increasing your shutter speed 1/3 stop or closing down your aperture 1/3 stop.

Rating your sensor or speed before shooting can save a lot of time editing and the heartbreak of a shot that doesn’t live up to your expectations. It’s not perfect, and varies based up taste, style, and personal preference, but it will bring you closer to achieving your vision. It’s definitely something to try in conjunction with this week’s exercise on contrast and contrast control. Combine it with M+ and M- exposure placement for even better results. By exposing properly, you can keep your contrast in the desired range, and make it easier to achieve your vision in your final processing stage.

For those interested in more information about film speed, Wikipedia has a great article. It includes formulae and details about how box speed and nominal are attained.

I’m looking forward to seeing your results and hearing from you. Let me know what you think and how you’re using your camera! Share your thoughts here or at the Hohenfels Volks Facebook page. Of course, commenting on both Facebook and here is always appreciated, too! Don't forget, we're on Google+, too!