How Do I Focus on One Thing and Meter off Something Else?

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October 15, 2015 at 10:30 am  •  Posted in Q&A, Tips & Techniques by  •  0 Comments

I read a useful article about controlling exposure with careful light metering and Auto Exposure Lock (AE-L). When taking photos of a very bright subject like a white house with a DSLR, the author suggests taking the light meter reading off a mid-tone, such as grass. But the problem is, my camera will also focus on the nearby grass when I press the AE-L button; then the more distant white house will be out of focus. How do I solve this?
—Lonnie T.

In order to ensure precise focus on the nearby subject and exposure based on the more distant mid-tone area, I used manual focus and metered the grass that was in the same light (direct sun) as the white statue. (Hearst Castle, San Simeon, CA) © 2013 Peter K. Burian

In order to ensure precise focus on the nearby subject and exposure based on the more distant mid-tone area, I used manual focus and metered the grass that was in the same light (direct sun) as the white statue. (Hearst Castle, San Simeon, CA) © 2013 Peter K. Burian

The AE-Lock technique is certainly very useful, Lonnie, but, as you indicate, it’s essential to be able to focus at one distance and to take the light meter reading from an area at another distance. Note too that you must take the light meter reading from a mid-tone in the same light as the primary subject. (i.e. If the white house is in shade, do not meter a grassy area that is lit by the sun.) And yes, it is important to separate the Autofocus Lock and the Auto Exposure Lock functions. Clearly, your camera is set to lock both the focus and the light metering when you use the AE-L button; it’s probably also set to do both when you maintain slight pressure on the shutter-release button.

Here are two methods that allow you to focus on a subject at a certain distance while metering an area at an entirely different distance.

1. Set the camera for manual focus by flipping the AF/MF switch on the lens to MF.  This will make the entire process very simple. Point the lens at the white house (in your example) and rotate the narrow ring on the lens until the house looks perfectly sharp. Since the camera/lens is set for manual focus, there is no need to maintain slight pressure on the shutter-release button while recomposing. The camera will never try to change the point of focus.

Point the lens at the mid-tone area that’s in the same light as the primary subject (grass, in your example). Depress the AE-L button (marked as AE-L or with a * symbol) to hold and lock the exposure. Keep it depressed while recomposing for the desired composition. (During this time, neither the exposure nor the focus will change.) Take the photo. You’ll get exactly the desired results: sharp focus on the primary subject (distant house) and accurate exposure based on the light meter reading of the mid-tone (nearby grassy) area.

2. If you insist on using autofocus, however, you’ll need to use a different tactic: modify the camera so the AE-L button locks only exposure and not focus. (By default, most cameras’ AE-L buttons lock both aspects.) Find the custom function menu item that defines the role of the AE-L button. Select the option that causes the AE-L button to provide only AE Lock and not AF Lock as well.

Then point the lens at the nearby mid-tone and press the AE Lock button to take the meter reading. Keep the AE-L button depressed while recomposing. Next, point the lens at the intended subject and touch the shutter-release button to activate autofocus. While re-framing to make the final composition, also maintain slight pressure on the shutter-release button to keep focus locked on the primary subject. Using this technique, you’ll again get the desired result in terms of both focus and exposure.

Nearly all DSLRs and other cameras for photo enthusiasts provide a menu item to determine the function of the AE Lock button. (The name of the function will differ from camera to camera, however.) One of the options allow for setting the button's function to provide only AE Lock and not AF Lock.  © 2014 Peter K. Burian

Nearly all DSLRs and other cameras for photo enthusiasts provide a menu item to determine the function of the AE Lock button. (The name of the function will differ from camera to camera, however.) One of the options allow for setting the button’s function to provide only AE Lock and not AF Lock. © 2014 Peter K. Burian

 

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