Aperture Control on Macro Lens 微距鏡之光圈控制

The marked focal length and the marked maximum aperture of the lens are usually defined based on focus at infinity.  When you are focusing at very close distance, the actual aperture is affected too.  Depending on the focusing distance, the effective aperture may drop 2 stops or more below the indicated value.  You must not ignore this phenomenon when you are taking the photos.  For example, when you set the aperture value to F16, you may expect it is F16 but the actual aperture may be F32.

Picture of Lens Aperture

While TTL automatic exposure measures the light entered, the exposure of your picture is usually correct as long as you stick to automatic exposure.  If you use settings from a light meter and you don’t consider the aperture value compensation, your pictures are likely underexposed.  In some other cases, user unaware of the issue tend to use an aperture that is usually too small.  As a result, the image may be blurred due to slow shutter or diffraction.

Lens without Automatic Aperture Value Compensation

Most of the macro lens do not support automatic aperture value compensation.  In their user guide, they usually include a table advising you how to calculate the effective aperture value.  Here are some examples of aperture value compensation tables:

Canon EF100mm F2.8L Macro IS USMAperture compensation table for Canon 100L Macro Lens

Sony 100mm F2.8 Macro (SAL100M28)Aperture compensation table for Sony 100mm Macro Lens

Leica DG MACRO-ELMARIT 45mm/F2.8 ASPH./MEGA O.I.SAperture compensation table for Panasonic 45mm Macro Lens

Based on the table and the magnification ratio/shooting distance, you may calculate the effective aperture value.

e.g. 1-stop compensation
If you want to use F8, you need to set the aperture value to F5.6
If you want to use F11, you need to set the aperture value to F8
If you want to use F16, you need to set the aperture value to F11

e.g. 1 1/3-stop compensation
If you want to use F8, you need to set the aperture value to F5.0
If you want to use F11, you need to set the aperture value to F7.1
If you want to use F16, you need to set the aperture value to F10

e.g. 1 2/3-stop compensation
If you want to use F8, you need to set the aperture value to F4.5
If you want to use F11, you need to set the aperture value to F6.3
If you want to use F16, you need to set the aperture value to F9.0

e.g. 2-stop compensation
If you want to use F8, you need to set the aperture value to F4.0
If you want to use F11, you need to set the aperture value to F5.6
If you want to use F16, you need to set the aperture value to F8.0

Lens with Automatic Aperture Value Compensation

Some lens has built-in automatic compensation of effective aperture.  They automatically do the compensation and always show the effective aperture.  When you set the aperture to F5.6, they are really using F5.6 (well, roughly).  If you focus to infinity and set the aperture all the way to the max.  When you focus to closer objects, you will notice that indicated F number changes accordingly.

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