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What factors determine what exposure to give?

Updated: Apr 18

The main factors that should be considered when measuring light and setting exposure are:

1 - Lighting. The intensity and distance of the light source, including any light loss due to diffusers, acetates, etc., or atmospheric conditions between source and subject. The range of brightness (contrast) needs to be within the film’s tolerance range if full detail is to be recorded.

2 - Subject properties. How much your subject reflects the light – its tone, color, surface, from a black cat in a coal store to a milk bottle in the snow. The meter cannot tell what the subject is and so its readings need to be interpreted so light and dark subjects such as these are recorded well.

3 - Film speed. The ISO speed rating used, together with any alterations necessary due to film color sensitivity relative to the subject light source or if using extremely long exposure times; see reciprocity failure.

4 - Unusual imaging conditions. Light absorption due to lens filters and attachments, or an image made dimmer by extending the lens forward to focus close-ups.

On top of these come important interpretative or subjective considerations. For example, would it improve the picture to expose wholly for the brightest parts of the scene and make darker parts black; or expose for shadows and let light parts ‘burn out’? These judgments can only be made by you and are carried out by over-riding the camera’s settings.

The chosen exposure is delivered to the film by a combination of:

  • Intensity (image brightness): controlled by the lens aperture. Bear in mind that this choice will affect depth of field and to some extent definition.

  • Time: controlled by the shutter speed. This influences the way any movement of the subject or camera will reproduce, and the spontaneity of expression or action.

The subject is running in a flat plane across the camera lens, although depth of field will play a role, overall, with this image the figure should be in focus across the range of settings we use.

For this test the camera is again fixed on a tripod to reduce camera shake at slow shutter speeds; however, the effect of these different shutter speeds, unlike is for the still-life image, made apparent in the final photograph.

The faster shutter speeds manage to freeze frame the action in mid-flow, while the slower shutter speeds mean that the subject appears as a blur. In your normal day-to-day photography, it is unlikely that you will always have to deal with such extreme subjects but you will have to decide on a balance between time and intensity, making a decision as to whether depth of field or movement is the key concern.
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