So I have an illustrious female associate who is rather talented with a camera. I bought myself a digital SLR and need to learn how to use it. She has kindly taken up the mantle of tutorship and begun to inform me as to the correct use of many dials and buttons the protrude from the infernal picture making device. My first lesson was on Depth of Field. Below is my definition cheat sheet and the assignment I was set along with my results.
Depth of field is the distance between the nearest and farthest object, in a photograph, that are acceptably sharp and can be considered in focus. The depth of field can be manipulated to be long or short at the behest of the photographer. The larger the depth of field (DoF), the larger the percentage of an image that is in focus, and thus; the smaller the DoF, the smaller the percentage. Depth of field can be modified using three tools. These tools will vary in usefulness on a situational basis, depending on lighting conditions, subject of photograph & desired exposure length.
Subject Distance is the distance measure from the subject of the photograph to the aperture of the camera. Increasing the subject distance will increase the DoF, and likewise, decreasing the subject distance will decrease the DoF.
Focal Length is the distance measure from the aperture of the camera to the film/sensor (image plane). Increasing the focal length will increase the DoF, and likewise, decreasing the focal length will decrease the DoF.
F-Stop / F-Number / Focal Ratio is a dimensionless number equal to the focal length divided by the effective Aperture. Increasing the F-number will increase the DoF, and likewise, decreasing the F-number will decrease the DoF.
Circle of Confusion is the diameter of the circle seen by an observer looking at a photograph that is indistinguishable from a point. The commonly accepted value is 0.25mm. This provides the boundary conditions for the near & far focal distances encompassing the subject field. DoF is defined as the difference between these focal distances.
Hyperfocal distance is the minimum subject distance at which the far focal distance extends to infinity. This means all objects beyond the near focal distance will be in focus. This is useful when shooting landscapes, with points of interest at many distances.
DoF Assignment: To place two objects of similar size at different Subject distances, then take a series of photographs varying the point of focus and DoF so that consecutively, Object A, Object B then Object A & B are in focus. Then take a photograph outdoors of a scene and utilise the hyperfocal distance to maintain focus for everything within the final image.
So as I set about this task I laid out two aftershave bottles around 30cm apart and decided to shoot as close to the first bottle as focus would allow. I was indoors and it was night time, so I only had the light of the energy saving bulb in my room and the flash of my Canon EOS 500D. This meant that I had limited options to vary the DoF utilising the subject distance.
Using exclusively the focal ratio and focal length I managed to achieve the task set. My results can be found here. I shot these freehand so the framing is far from perfect. If I were to repeat the project, I would use a tripod and daylight. I would also use visual rulers to allow me to measure the approximate depth of field for curiosities sake.As is typical of my style of learning and research, I find concepts that I understand from a scientific perspective that much easier to understand. Some may find it significantly easier to understand merely the actions and results without the causation.
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