exposure metering techniques
exposure compensation | metering modes | variable aperture lenses

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Zoom lenses come in two flavours:

- constant aperture zooms, where the aperture stays constant as you zoom, (even though in reality the aperture can vary by as much as 1/3rd stop), and

- variable aperture zooms where the aperture changes as you zoom in and out, eg f3.5 to f5.6. This may affect how you should meter for a specific scene. But whether you will notice this change in aperture on your camera’s lightmeter, depends on how the aperture setting is controlled by that specific camera.

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There are basically two different ways that the aperture settings are controlled by various camera models :

Where the desired aperture is mechanically set via the aperture ring around the body of the lens … this info is then mechanically transferred to the camera body and then used in conjunction with the shutter speed / ISO setting / metering pattern, etc, to get to a meter reading.

In this case, the variable aperture of a lens will show up during the entire aperture range. ie, f4 becomes f5.6 … f8 becomes f11 … f16 becomes f22 … as you zoom to the longer focal length. In this case you will see the meter reading in your camera change as you zoom in and out.

Where the desired aperture is electronically set on the camera body via a thumbwheel … this info is then used to calculate the exposure and to set the lens to the specific aperture needed.

In this case, for a variable aperture lens, the aperture will ONLY vary when the maximum aperture is set at the widest length, and you then zoom in. For any other aperture that you set via the camera thumbwheel, you will get the aperture you requested. ie, f3.5 will change to f4.5 .. but f5.6 will remain f5.6 … f8 will remain f8 … f11 will remain f11. Therefore you won’t see the exposure info change in the camera’s viewfinder, except if you zoomed from wide to tele at wide open aperture.

Using Nikon equipment as an example to show the difference here:

With older Nikon bodies, like the Nikon F90x for example, the aperture has to be set on the lens itself. Therefore a variable aperture lens is a real pain to use in the studio, because as you zoom,the aperture will change for any aperture setting.

With the modern generation of Nikon cameras, like the D100 or F80, the aperture is set on the camera body, and the variable aperture is much less of a issue, and the lens for all purposes acts like a constant aperture lens except for the maximum aperture.

Metering with a variable aperture lens where the aperture read-out changes as you zoom in and out , is more complex than with a lens where the aperture stays constant.

This means that when you zoom away from your original composition that you metered for, you would need to alter the settings to keep to that original correct meter reading – because the effective aperture would have changed while zooming.

In that instance, you need to meter at the actual focal length (ie zoom setting) that you’re going to use.

For example: say you have a 28-105 mm zoom. The aperture on this type of lens normally varies from f4 to f5.6 as you zoom. If you now take a meter reading at 105mm (@ f5.6) and set an appropriate shutter speed in manual mode, you will need to change the aperture if you zoom back to 28mm because the aperture will have changed to f4 as you zoom. That one stop difference in exposure will affect your final image if you don’t change the aperture back to f5.6 – which was the aperture metered for.