DSLR Lens Calibration
Introduction
Out-of-focus images are always frustrating, and DSLR shooters know that un-calibrated lenses tend to produce such annoyances from time to time. But fear not, lens calibration is easy and ensures that your DSLR gets it right – every single time.
Why Calibration Is Necessary
A DSLR has an image sensor and a separate autofocus sensor located at the bottom of the camera. This design combined with small manufacturing tolerances in lenses can result in small inaccuracies that lead to either front- or backfocused images.
The fact that every lens renders focus differently can additionally impact the margin of error negatively.
Calibration
Every lens has to be calibrated separately with each camera body.
Necessary Equipment
DSLR + lens
Lens calibration tool (e.g. Spyder LensCal)
Two tripods
Spyder LensCal on a light stand - easy to level
Set-Up
The DSLR and the calibration tool have to be aligned at the same level. Usually, the camera and lens should be calibrated at the intended shooting distance (this is also recommended by Nikon).
This is more practical than calibrating at 20 times the focal length of the given lens, which is often recommended as a general rule of thumb.
Calibration Process
Set the camera to aperture priority or manual and select the widest possible aperture. Also, select the native ISO.
The autofocus system should be set to single point (always select the center point) and S-AF.
Take the first test shot by carefully focusing on the center of the calibration target when looking through the viewfinder.
Examine the digital image in order to determine if or how much the focus is off. Access the cameras settings menu and apply correctional factors.
Refer to the camera manual on how to move the focus either towards or away from the camera.
Start out with a larger correctional factor.
Take another test shoot and repeat the process as often as needed, always aiming to reduce the adjustment increments from each iteration to the next.
Setting the AF fine-tune value on my Nikon D800
After the result seem satisfactory, move the calibration target back and forth and take additional test shots. Apply correction if needed. Don’t move the calibration target outside the intended shooting range and try to achieve the best compromise.
Ideally, the depth of field should extend further behind the target than in the front, as this would be the correct near-far distribution of depth of field.
Keep in mind that due to focus shift at different distances and apertures, a 100% accuracy is unobtainable.
Go on to test the new calibration settings in real world scenarios before using the camera-lens combination in a commercial shoot.
Focus Shift
There are two main issues which cause focus shift.
The first one stems from focusing the lens on different distances. This is why a lens can only be calibrated to be accurate at a certain range of distances. In newer lenses, the focus shift effect is usually negligible.
The second issue is that changing the aperture of a lens changes its focus slightly. Mitigate this effect by calibrating the lens at its working aperture.
My Nikon D800 with the Nikkor 105 f/2 DC ready for calibration - a lens notorious for calibration issues (at least in my book)
Conclusion
Success loves preparation – time spent calibrating a lens is time well spent as out-of-focus images cannot be saved in post production.
