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Your camera's ISO settings may not be accurate
My suspicions began a few years ago, after I had purchased a Canon 20D. My pictures were coming out just a tad dark, even when I used manual metering and went with the “sunny 16” exposure rule (shutter speed is the same as the ISO at f/16 on a sunny day).
|The histograms, which indicate proper exposure, seemed to be shifted ever so slightly to the left, indicating underexposure. Something was not quite right.
Then DxOMark.com went live, and my suspicions were confirmed. Each of DxOMark's objective lab tests of over 50 digital cameras, which are now freely available online, includes the camera's actual, measured (as opposed to the manufacturer's claimed) ISO sensitivity. It turns out my camera's measured ISO sensitivity was off by about a third of a stop at all ISOs!
Check the charts: Here are two extreme examples of two cameras' ISO sensitivity results. The closer the circle is to the line, the closer the measured ISO is to the manufacturer's ISO. In the top example, the measured ISO is very accurate, one of the best results. In the bottom chart, measured ISO is off by nearly a full stop at all ISO settings, one of the worst performances. (Charts courtesy DxOMark.com.)
As I studied DxOMark's test results I was surprised to learn that my camera turned in a relatively good performance. As I looked at other camera test results, it became clear that measured sensitivity at the sensor is a bit lower (sometimes, higher) than the specs indicate. In some cases, the difference between the claimed and measured ISOs are as much as a half stop or more, but a third to a fifth-stop difference is typical. Only a handful of cameras consistently come within a few ISO points of the manufacturer's stated ISO in DxOMark's tests.
Cost, age don't matter
The other big surprise was that there wasn't much correlation between the cost of the camera and the accuracy of the measured ISO accuracy. There are starter DSLRs with better than average accuracy and pro cameras that showed only average results, as well as starter cameras that did poorly and pro models that did well. Camera age made no difference, either: Cameras produced in 2004 in some cases fared better than those introduced in 2008. Some did worse. There was no pattern.
Should you be alarmed? Nah.
Now for a reality check: While this information may be surprising, it's not really a big deal, because it is so easy to overcome. In the real world (as opposed to the lab) most cameras' variation in measured ISO from the manufacturer's ISO will have a minimal affect on the image. You only really start to see it if the gap is 1/3 of a stop or larger, and you can resolve this in-camera by adjusting your exposure compensation.
After the fact, if you shoot RAW and then run the image through Adobe Raw converter, the software may automatically fix the exposure. If not, you can simply adjust the exposure while in your conversion software. When working with JPEGs, simply refer to the histogram, and adjust exposure accordingly. (It's good to do that anyway.)
The difference between the official ISO and the measured one is usually fairly consistent when changing ISOs. If you find a third-stop difference at ISO 100 (ISO 80), you'll probably also have a third-stop difference at ISO 800 (that would be around ISO 640). Some models, however, are not consistent. That's why I've listed each ISO performance below, by camera.
A bit of historic (read: film) perspective
If you think this is a compelling argument to return to film, sorry. That's not a valid excuse. Those treasured mechanical cameras, when it came to exposure accuracy, were generally worse. While the actual shutter speeds of a camera fresh out of the factory may have been fairly accurate (within 1/3-1/2 a stop), with time and use the coils and springs that run the shutter would loosten and shutter speeds would slow down. That's why semi-annual cleaning, lubrication and general tune-ups at the shop were recommended.
Film speed could vary slightly from batch to batch (but the difference was usually very slight, especially with slide film). For print film, this difference was hardly noticeable because of the variations inherent in the print process.
What if your camera wasn't tested?
As of this writing, DxOMark has tested 54 cameras--most of them DSLRs. Since they will only test cameras that capture RAW images, most compact cameras will not be tested. A quick way to test your camera's exposure accuracy is to photograph a blue sky using manual metering, if your camera has that option. Check the histogram and if it is slightly too the right, compensate using exposure compensation by 1/3 stop and check again. When you get a nice even curve that begins at one edge and ends at the other, your exposure is accurate. Set your camera's EV preferences to reflect your results. This is a down-and-dirty way of doing it but for snapshooters, it should be enough.
When ordering prints, many labs (including AdoramaPix) give you the option to let the person running the printer to make adjustments for the best possible print. This is a good idea if you order prints direct from the camera or memory card with no alterations and want to make sure any wacky ISO sensitivity difference doesn't show up on the print. An experienced printer can compensate for slight exposure variances. At AdoramaPix, this service is free.
Find out how your camera did!
1. Go to www.dxomark.com
Our thanks to DxOMark.com for allowing us to use and interpret their data.