By Oliver Mews
Photos are not always as sharp as they appear to be. One last glance through your viewfinder, ensuring that the AF points are positioned to perfectly capture your subject of choice. You carefully press the shutter release button. The camera’s pleasant humming noise quickly falls silent again, and your viewfinder displays a sharp photo. …At least that’s what it looks like. Of course, you cannot verify that the sharpness our DSLRs market was delivered.
We lack the optical tools to support our projects, such as the cross-section that was a common feature in analogue SLRs. Our camera displays are not helpful either. Quite often, we find out that our focus can be improved when we take a closer look at our photos on our monitors, where they are displayed in a size that cannot be compared to that of a viewfinder.
The AF sensor field in image 1 is targeting the second pointer but the focus is slightly behind (back focus)
But what happened, exactly? When working with an open aperture f1.4/f2.8, there is hardly any depth of field, which means that the photographer’s position has a massive impact on the focal point. A natural body movement that goes unnoticed can make the difference between the perfect image and a useless piece of pixel mess. Likewise, moving the camera toward the desired picture detail when using the centre AF point bears the very same danger. If you really want to get it right, make sure to use the AF point relevant to the image section you want to concentrate on, and don’t go down the path of focusing elsewhere and moving your camera accordingly. Thus, you’ll avoid any potential imprecisions. In addition, this workflow saves a lot of time, as there is no need to choose the relevant frame over and over again when taking several photos of the same subject.
Portrait shot by Dietmar Temps in a blog post released in SpyderBLOG 6/2017. The activated AF sensor should be the closest to the preferred focus area.
Even when the photographer does everything correct when measuring the autofocus, more than 50% of camera-lens combinations come up with inaccuracies, and therefore, autofocus variances. These inaccuracies when focusing are due to fractional tolerances during the manufacturing process, as well as wear and tear and environmental factors, such as temperature fluctuations. A camera manufacturer’s service technician can easily readjust them. As an alternative to this service, most DSLRs nowadays have a correcting option called AF fine-tuning.
List of cameras with AF fine-tuning option
|Canon||50D, 70D, 6D, 7D, 5DS, 5DS R, 7DMkII, 5DMkII, 5DMkIII, 1DMkIII, 1DMkIV, 1DsMkIII, 1Dx, 1DxMkII, 1Dc|
|Nikon||D7000, D7100, D7200, D300, D300s, D600, D610, D700, D750, D800, D800E, D810, D3, D3s, D3x, D4, D5|
|Pentax||K5-II, K5-IIs, K-20D, K-30, K-5, K-7D, K-2000/K-m, K200D, 645D, K-x, K-3|
|Sony||A850, A900, A77, a7R II, a7S II, a7 II, a7 R, a7 S, a7, a99, a68|
|Olympus||E-30, E-620, E-5|
Some DSLR cameras from the mid-price category are equipped with this function, however, you should be sure to check manufacturers’ websites.
Both front and back focus can be corrected by the value required, using digital sliders. A test image is necessary for evaluation purposes. For example, the page of a book lying on a table photographed at an angle from above can be used as an indicator. It is important to consider that in this case the AF measuring point takes a measurement on a sloping plane, which results in a measuring point that is not easy to calculate. A better option is to make use of a high-contrast object that is standing upright, such as the one in SpyderLENSCAL by Datacolor. There is a ruler with a measuring scale, placed at an angle of 45°, which makes reading the center of the focus very easy. Make sure to take your shot from the same height as your subject, with a frontal focus and ideally use a tripod. The SpyderLENSCAL’s small level will help you to adjust your position. It comes with a ¼-inch tripod mount, but a long table or even the floor will do to arrange a set-up where the camera and measuring tool can face each other.
The calibration process is simple: go for an open aperture, the lowest possible ISO, a high shutter speed, and make sure you have enough available light. Take your test photo – check it – make your correction – take another test photo – readjust – take the next test photo – done!
Coming straight off the manufacturer’s conveyer belt, the focal range or balance of a camera is not in the center, but at 2/3 back focus and 1/3 front focus. Photographers are given the opportunity to redefine their focal range by using this AF calibration process. When focusing on the eyes, a 1/3 back and 2/3 front focus, in combination with an open aperture, result in tack sharp nasal tips and blurry ears. Thus, photographers will produce significantly fewer photos that go straight into the bin.
Portrait shot by Dietmar Temps in a blog post released in SpyderBLOG 6/2017. Open f-stop/35 mm lens with a small focus depth. Focus on the left eye.