Understanding Depth of Field in Photography
The effective depth of field is also affected by the sensor size, to calculate the effective f-number in relation to full-frame, just multiply it by the crop factor. So if you have a 50mm f2 lens and you mount it on a camera with an APS-C sensor, you'll need to multiply both the focal length and f-number by 1.5 times to calculate the effective coverage and the effective depth of field - so it would act like a 75mm f3 lens in terms of effective coverage and depth of field on a full-frame system. If you mounted a 50mm f2 lens on a Micro Four Thirds body, you'd multiply both figures by two to end up with a 100mm f4 lens in terms of effective coverage and depth of field on a full-frame body. Note the exposure is not affected by the crop factor, so an f2 lens will have the same exposure as another f2 lens regardless of the sensor behind it. But the effective coverage and the effective depth of field are.
This is important as many owners of cropped-frame cameras make the mistake of only applying the crop factor to the coverage and assuming the depth of field will just be the same as a full-frame system. For example a 25mm f1.4 lens on a Micro Four Thirds body may have coverage equivalent to 50mm on full-frame, and share the same exposures as a true 50mm f1.4 lens, but in terms of relative depth of field, it's not equivalent to a 50mm f1.4; instead it's equivalent to a 50mm f2.8 on a full-frame body. This is why it's harder to achieve a really blurred effect on systems with smaller sensors, although easier to achieve a larger depth of field. If you wanted to match the coverage and the effective depth of field as a 50mm f1.4 lens on the Micro Four Thirds system, you'd need a 25mm f0.7 lens, although this would have the benefit of faster exposures than a true 50mm f1.4 lens.
Focus a third of the way into the scene to maximise depth of field.
Use this Depth of Field Calculator to determine depth of field under various conditions.
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