This button has two main functions that can be incorporated in various working methods involving exposure locking. It also has an extra function for the flash meter capability (see AE-L section under Flash). The AE-Lbutton can:
a)lock an EV setting in manual and automatic modes.
b)be used as a brightness range checking facility in standard terminology or Zone System terminology.
a)When the button is pressed (fig 1), the light metering facility is locked to the EV setting at that moment. An L (= locked) symbol appears between the shutter speed and the aperture indication (fig 2) on the grip LCD and viewfinder LCD to confirm the status. Press the AE-Lbutton again to unlock (toggle function).
In the locked setting, the aperture and shutter speed become in- terlocked. In this way, a new aperture/shutter combination that still represents the same EV, can be rapidly chosen. For example, if you set the shutter at 1/125s and the aperture at f/8 and lock them together, you can access new EV-equivalent combinations of, for example, 1/30s a f/16 or 1/500s at f/4 just by moving the front control wheel.
In practice this means you could, for example, in auto mode, posi- tion the metering area (spot setting) over an area in the subject that you determine to be equivalent to a mid-grey and lock it with the AE-Lbutton. You can then recompose the picture with the metering zone positioned over an area much brighter or darker while still retaining the original exposure setting and choose a new combination of aperture and shutter speed settings.
b)The AE-Lbutton also allows the spot metering function to make tonal comparison readings and brightness range checks. When the AE-Lbutton is pressed, the metered area is saved as a mid-grey. When the spot area is then placed over another part of the scene, the new area is then compared to the saved area and the difference can be read off the scale seen in the viewfinder. For example, in a landscape situation you could meter the foreground, lock the reading with the AE-Lbutton (thereby locking that area to be reproduced as the equivalent to a mid-grey 18%), point the camera at some rocks to see by how much darker they are compared to the foreground by the EV difference read off the scale (illus 3).
If you have chosen Spot together with Zone display (see Custom options) as well as one of the automatic modes A, S, P or Pv, point the spot marking at an area that you decide should be a Zone 5 and click the AE-Lbutton (illus 4). The meter will now display different parts of the subject as zone values (illus 5) in the viewfinder LCD, instead of EV deviations, as you move the spot marking over the subject. (Included are Lo and Hi (illus 6)to signify areas beyond the range of the film).
Alternatively you can choose to re-classify an area as another zone and then check the rest of the subject to see how other areas fall on the zone scale. Do this by following the above procedure and then turning the rear control wheel until you see the new desired zone value in the viewfinder LCD. You will also see the new exposure that will now produce that new zone. For example, you might have measured a rock at zone 5 but wish to make it darker. By moving the rear control wheel you could re-classify it as zone 4. You will then be able to see, for example, whether white clouds are now falling within the exposure range by their new Zone classification.
