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DIFFERENTIAL FOCUS Calibration

Some DSLRs provide the ability to make micro adjustments in the autofocus system. To make these adjustments, the photographer conducts tests to determine the actual location where the camera focuses with each lens. Based on the results of these tests adjustments can be made to move the location of focus to match the camera sensor. It can be challenging to determine the location of best focus with some lenses. The process described here provides the user with a sensitive test that is easy to conduct.
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  • This Figure shows how the resolution of a camera’s optical system changes as you move to either side of best focus. The Resolution of a low f/# setting like f/2.8 will drop very quickly as you move in either direction away from best focus (indicated by 0). If the f/# is increased to something like f/5.6, the peak resolution will be lower due to the diffraction limit, but the drop off will be more gradual. This is illustrated by photographers that increase f/# in order to increase their depth of field.

    This Figure shows how the resolution of a camera’s optical system changes as you move to either side of best focus. The Resolution of a low f/# setting like f/2.8 will drop very quickly as you move in either direction away from best focus (indicated by 0). If the f/# is increased to something like f/5.6, the peak resolution will be lower due to the diffraction limit, but the drop off will be more gradual. This is illustrated by photographers that increase f/# in order to increase their depth of field.

  • This Figure illustrates the difficulty in trying to determine the location of best focus when looking for maximum resolution. There is always some tolerance in determining when focus has changed and that is shown in this Figure by the quantity “Resolution Change”. This is the smallest amount of change that the human eye can see. Another way to look at this is that the slope of the focus curve is very near zero around best focus and is zero at best focus. So the closer you get to best focus, the less “leverage” you have to determine the location of best focus.

    This Figure illustrates the difficulty in trying to determine the location of best focus when looking for maximum resolution. There is always some tolerance in determining when focus has changed and that is shown in this Figure by the quantity “Resolution Change”. This is the smallest amount of change that the human eye can see. Another way to look at this is that the slope of the focus curve is very near zero around best focus and is zero at best focus. So the closer you get to best focus, the less “leverage” you have to determine the location of best focus.

  • An alternate technique used to determine best focus (frequently used in industry) can be referred to as “DIFFERENTIAL FOCUS”. This technique uses two resolution targets on either side of best focus in a region where the through focus curve has a significant slope. In this Figure, two resolution charts are placed equal distance in front and behind the best focus location. Resolution at these locations is lower than the maximum resolution at best focus. However, the rate of change in resolution is much greater than at best focus and this change goes in the opposite direction for the front and rear charts as the focal plane is adjusted. For example, the resolution on the front chart increases as the focal plane is adjusted toward the camera and the rear chart resolution decreases. The goal is to see the same resolution on both front and rear charts. In other words, the absolute resolution is not important but the DIFFERENCE between the resolution of the front and rear charts should be zero. The focus error is much smaller with this technique.

    An alternate technique used to determine best focus (frequently used in industry) can be referred to as “DIFFERENTIAL FOCUS”. This technique uses two resolution targets on either side of best focus in a region where the through focus curve has a significant slope. In this Figure, two resolution charts are placed equal distance in front and behind the best focus location. Resolution at these locations is lower than the maximum resolution at best focus. However, the rate of change in resolution is much greater than at best focus and this change goes in the opposite direction for the front and rear charts as the focal plane is adjusted. For example, the resolution on the front chart increases as the focal plane is adjusted toward the camera and the rear chart resolution decreases. The goal is to see the same resolution on both front and rear charts. In other words, the absolute resolution is not important but the DIFFERENCE between the resolution of the front and rear charts should be zero. The focus error is much smaller with this technique.

  • This Figure shows the simple setup for using Differential Focus to calibrate the micro focus adjustment in your camera. The Focus Target provides something for your camera to focus on. The two resolution charts are setup equal distance in front and behind the Focus Target. In general, it is best to position your camera between 25 and 50 focal lengths from the Focus Target. The longer the distance and the higher the effective f/#, the farther the resolution charts should be from the Focus Target (so long as they are both the same distance). A good starting distance “D” for the resolution charts is about 10 to 15 inches when working with 300mm to 400mm lenses.

    This Figure shows the simple setup for using Differential Focus to calibrate the micro focus adjustment in your camera. The Focus Target provides something for your camera to focus on. The two resolution charts are setup equal distance in front and behind the Focus Target. In general, it is best to position your camera between 25 and 50 focal lengths from the Focus Target. The longer the distance and the higher the effective f/#, the farther the resolution charts should be from the Focus Target (so long as they are both the same distance). A good starting distance “D” for the resolution charts is about 10 to 15 inches when working with 300mm to 400mm lenses.

  • This Figure shows the setup as viewed from the camera. It’s important that the camera focuses on the Focus Target and not the Resolution Charts. It’s recommended that you use a tripod to support the camera and lens and other techniques common with high resolution photography. An additional advantage in using the Differential Focus technique is that you are able to see the results on the camera’s viewing screen without having to download images to a computer. The object is to have the same reading on both front and rear resolution charts rather than looking for the maximum resolution at the best focus location. Use the image magnification on the camera LCD viewer when reading the chart values. Since you’re not looking for maximum resolution, you should be able to see the results on the camera viewing screen. The chart numbers are a close approximation to line pairs per millimeter (lp/mm).

    This Figure shows the setup as viewed from the camera. It’s important that the camera focuses on the Focus Target and not the Resolution Charts. It’s recommended that you use a tripod to support the camera and lens and other techniques common with high resolution photography. An additional advantage in using the Differential Focus technique is that you are able to see the results on the camera’s viewing screen without having to download images to a computer. The object is to have the same reading on both front and rear resolution charts rather than looking for the maximum resolution at the best focus location. Use the image magnification on the camera LCD viewer when reading the chart values. Since you’re not looking for maximum resolution, you should be able to see the results on the camera viewing screen. The chart numbers are a close approximation to line pairs per millimeter (lp/mm).

  • Purchase this image and cut along the red lines. (The water mark won’t show on the print.) Attach the target and charts to vertical flat surfaces as previously shown. The rear chart should be mounted high enough so it can be seen just above the front chart. Take care to make the target, charts and camera image plane parallel to each other. Keep the resolution charts close to the center of the image when aiming your camera, but make sure the camera focuses on the Focus Target. Set the camera to Aperture Priority and adjust f/# to the lowest value and take your first photo. Adjust the ISO so the shutter speed is at or above 1/400 sec. View the image on the camera back and enlarge it so the highest frequency Resolution Chart number can be determined for both the front and rear charts. If the front chart is resolved higher than the rear chart, then the focus is in front of the Focus Target. Adjust the camera Micro Focus and take additional photos until both charts are resolved to the same frequency number. Check with your camera’s manual for details on making the Micro Focus adjustment.

    Purchase this image and cut along the red lines. (The water mark won’t show on the print.) Attach the target and charts to vertical flat surfaces as previously shown. The rear chart should be mounted high enough so it can be seen just above the front chart. Take care to make the target, charts and camera image plane parallel to each other. Keep the resolution charts close to the center of the image when aiming your camera, but make sure the camera focuses on the Focus Target. Set the camera to Aperture Priority and adjust f/# to the lowest value and take your first photo. Adjust the ISO so the shutter speed is at or above 1/400 sec. View the image on the camera back and enlarge it so the highest frequency Resolution Chart number can be determined for both the front and rear charts. If the front chart is resolved higher than the rear chart, then the focus is in front of the Focus Target. Adjust the camera Micro Focus and take additional photos until both charts are resolved to the same frequency number. Check with your camera’s manual for details on making the Micro Focus adjustment.

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    An alternate technique used to determine best focus (frequently used in industry) can be referred to as “DIFFERENTIAL FOCUS”. This technique uses two resolution targets on either side of best focus in a region where the through focus curve has a significant slope. In this Figure, two resolution charts are placed equal distance in front and behind the best focus location. Resolution at these locations is lower than the maximum resolution at best focus. However, the rate of change in resolution is much greater than at best focus and this change goes in the opposite direction for the front and rear charts as the focal plane is adjusted. For example, the resolution on the front chart increases as the focal plane is adjusted toward the camera and the rear chart resolution decreases. The goal is to see the same resolution on both front and rear charts. In other words, the absolute resolution is not important but the DIFFERENCE between the resolution of the front and rear charts should be zero. The focus error is much smaller with this technique.
    This Figure shows the simple setup for using Differential Focus to calibrate the micro focus adjustment in your camera. The Focus Target provides something for your camera to focus on. The two resolution charts are setup equal distance in front and behind the Focus Target. In general, it is best to position your camera between 25 and 50 focal lengths from the Focus Target. The longer the distance and the higher the effective f/#, the farther the resolution charts should be from the Focus Target (so long as they are both the same distance). A good starting distance “D” for the resolution charts is about 10 to 15 inches when working with 300mm to 400mm lenses.
    This Figure shows the setup as viewed from the camera. It’s important that the camera focuses on the Focus Target and not the Resolution Charts. It’s recommended that you use a tripod to support the camera and lens and other techniques common with high resolution photography. An additional advantage in using the Differential Focus technique is that you are able to see the results on the camera’s viewing screen without having to download images to a computer. The object is to have the same reading on both front and rear resolution charts rather than looking for the maximum resolution at the best focus location. Use the image magnification on the camera LCD viewer when reading the chart values. Since you’re not looking for maximum resolution, you should be able to see the results on the camera viewing screen. The chart numbers are a close approximation to line pairs per millimeter (lp/mm).