Tuesday, 29 January 2013

Light loss in macro photography

In macro photography the closer you get to the subject the more light is lost. It means that a certain f-number is no longer valid once the lens is extended from the camera to result in magnifications higher than about 1:10. Compensation for this light loss in macro photography is especially necessary when using extension tubes or macro bellows. The amount of light reaching the sensor (or film) is reduced by a considerable amount when the lens is moved away from the camera. Unfortunately there is nothing one can do about that except to increase the light because the camera may not be able to decrease the shutter speed or increase the ISO to an acceptable level.

The effective aperture value or f-value can easily be calculated. This shows how much light is lost due to the extension necessary for the macro image. This loss is especially noticeable at large magnifications over 1:1, where the lens must be moved out far from the camera, but it starts to be noticeable already at as low magnifications as 1:10.

As an example, using a 50mm lens at infinity you will need 150mm extension to get 3:1 magnification. At this high magnification you will need to increase exposure substantially since you'll lose a lot of light. The working distance will be 67mm, which is the distance between the lens and the subject.

Please note that the actual magnification a certain extension tube or bellows extension causes is not that simple. The internal lens extension and the lenses specific magnification factor of a specific lens must also be taken into considerations to get the exact magnification. In this example that is not taken into account.

Light loss is according the following simple formula:

Ef = Effective f-value (that is what you get after adding extension tubes or bellows)
If = Indicated f-value (that is shown on your lens or display, the one that is set on the lens)
F = Focal length of the lens
Ex = Extension (total length of the extension tube or bellows)

The formula is: Ef = If x (F + Ex)/F

As an example, with a 50mm lens and a 150mm extension tube you get 3:1 magnification and if the lens is set to f8.0 than:

Ef = 8 x (50 + 150) / 50

The result of the above is 32.

That is the effective f-value used for the exposure, meaning you'll need to increase the light 16 times compared to not using any extension at all. That’s why ring flash or some other light source near the subject is needed, or you have to increase the ISO sensitivity. The drawback of that is that you'll lose details, so it is better to increase light than to increase ISO. Of course, you can decrease the shutter speed also, but that is not always possible and in fact it is not recommended at all since it may result in motion blur caused by subject motion or camera shake.

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