For the Beginner: Focus Notes

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flyinglentris
flyinglentris Regular Member • Posts: 275
For the Beginner: Focus Notes

Focus Notes in a Nutshell:

AF (Automatic Focusing) is achieved by switching the lens Focus Mode Switch to the AF position. Focus is done automatically under the control of the camera's digital interface to the lens. AF Points or AF Areas are chosen to assist focusing a subject or it is done fully automatically by pressing the Shutter Release Button half-way.

M (Manual Focusing) is achieved by switching the lens Focus Mode Switch to the Manual position and then turning the Focus Ring to obtain subject focus.

AE Lock is an automatic camera feature which locks focus and exposure values while allowing the photographer to tend to other camera ops before actually fully depressing the shutter release to take the shot. Pressing the shutter release half-way will similarly set and lock focus for a short interval.

Focusing Distance Range Selection, available on certain lenses, can be switched to effect autofocusing time. Setting the switch to the farther focal point setting allows the autofocus mechanism to skip the distance from the closer point to the farther point, thus saving some time finding the focal point. However, if the subject is within the shorter range, make sure the switch is set appropriately.

DoF (Depth of Field) focusing can be done by using the Distance Scale and Distance Index, the Depth of Field Scale which may be on certain lenses, by calculation, by chart or in certain cameras, by a special DEP shooting mode. The Distance scale does not indicate DoF by itself, only the primary subject focal distance. It must be applied to further calculations to determine DoF.

NOTE: When using macro lenses, extension tubes or close-up filters, standard DoF calculators will not produce accurate results. Magnification affects the focal properties of the lens.

DoF is dependent upon both the subject focal point distance and the aperture setting used for shooting. The subject focal point determines the subject's sharpest focus and the Hyperfocal Point when a specific aperture is applied. The subject focal point and hyperfocal point are NOT the same point. The Hyperfocal distance is defined as the closest distance to the lens that it can be focused while keeping objects at infinity in acceptable focus. Note that the hyperfocal point may still be used to determine DoFs which do not extend to infinity. The aperture setting determines the depth or distance between two points, one before and nearest to the primary subject focus point and The Hyperfocal Point and one behind and furthest from it. Within this range, focus is deemed acceptable.

Therefore to determine and setup depth of field ...
1) Bring the subject into focus
2) Read the distance scale to determine the subject focus distance.
3) Using a calculator, chart or if available, the Depth of Field Scale, determine the aperture needed to achieve the desired DoF.
4) Set the Aperture
5) Recompose and Refocus
6) Take the shot.

DoF may be shallow with the subject focal point close to the camera or it may be deep with the subject focus far from the camera. In shallow DoF, the distance of acceptable focus is short, resulting most often with the background out of focus and blurred. In deep DoF, the zone of acceptable focus is long and may extend behind the subject towards infinity.

The property of Consecutive Depths of Field allows for selecting alternate Hyperfocal Points such that focus will achieve smaller DoFs which move closer to the lens. A lens focused near Hyperfocal Distance H will focus from H/2 to infinity. When focused near H/2, it will have acceptable focus from H/3 to H. Near H/3, it focus from H/4 to H/3, and so forth. It is not typical to apply consecutive depths of field.

DoF is not simply determined by using the Distance Scale and Distance Index. The scale is usually marked in both meters and feet. Keep in mind that this scale only indicates the distance to the point of sharpest focus, the subject focal point. By itself, it does not indicate depth of field. Further, the distances indicated are typically marked in intervals of half feet and half meters. Accuracy in determining the actual focal point distance must be interpolated from the scale. A DoF calculator or DoF chart are required to determine the Hyperfocal Point and closest and farthest points of acceptable focus.

NOTE: The camera sensor and lens used affect the determination of the Hyperfocal Point and DoF. If the calculator or chart used do not incorporate the sensor type and lens type in the calculation, simply using the Subject focal distance may result in blurring at the background edge toward infinity. In those cases it is best to increase the distance a bit to achieve full DoF to infinity.

On older lenses and some new lenses, the Depth of Field Scale and its Index may be used to assist in the determination of DoF. Perhaps, because calculation of the Hyperfocal Point and DoF are affected by the type of camera sensor, this scale has all but disappeared from newer lenses. Be cautioned that use of Depth of Field Scales may not be accurate, depending upon the camera used.

To use a DoF Scale ...
1) Bring the subject into focus
2) Read the distance scale to determine the subject focus distance.
3) Manually adjust the lens' aperture ring or zoom ring
4) Read the DoF from the DoF Scale between the aperture marks lined up with distance marks on the Distance Scale.
5) Depending upon the lens' maximum aperture, to achieve focus to infinity, the distance from the subject may need to be physically adjusted by moving the camera.
6) Recompose and Refocus
7) Take the shot.

The Infrared Index on Distance Scales is used to indicate a focal distance adjusted for monochrome infrared film. Note the word 'film'. On some lenses, there are multiple Infrared Compensation Indices.

Depth of Field Calculators are the preferred means of determining Hyperfocal Point and DoF. Software DoF Apps may be installed on cell phones and easily carried into the field or any place that a shoot may occur. These DoF calculators take as input, the camera type, the lens type, aperture setting and subject focal distance. Calculation is immediate, producing minimum focal distance, subject focal distance, farthest focal distance and Hyperfocal Point.

Charts are less preferred since they should be based upon camera type and are extra items to be brought with to shoots. Charts or tables cross-reference aperture with subject distance. Multiple charts are required, one for Total DoF, Hyperfocal Point and Minimum and Farthest points of acceptable focus. If the Depth of Field Calculator used generates charts based upon camera and lens, charts and tables are much more acceptable in active usage and provide one of the fastest ways to choose setup conditions for a shoot.

DEP Shooting Mode is an automatic feature of some cameras allowing the photographer to setup DoF by choosing nearest and farthest acceptable points of focus. Certainly, the photographer has to be very cautious about choosing and entering those points. The camera applies calculations for the aperture, shutter speed, Hyperfocal Point and it would seem, the subject focal point. There is a sacrifice of control over the camera and lens which may not be desirable. DEP or A-DEP shooting mode is not suitable for accurate DoF calculations and is a feature likely meant to accommodate amateur and personal use photographers.

The C/H indicator is used in Fish Eye Zoom and Wide Angle Zoom lenses mounted on cameras with APS-C or APS-H sensors to indicate that when focused at infinity a photo may be taken without suffering vignetting.

Final NOTE: Camera shake can affect final outcomes of a shoot by producing blur. This is not so much a focus problem, but is solved by being steady when hand held, using a tripod and/or built-in lens image stabilization.

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