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Forum: Troubleshooting and Beginner Help
04-13-2019, 10:58 AM
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When you activate focus, you are also "turning on" all of the camera's functions. Once activated, you can use the focus button or half press over and over on a subject as the subject might possibly move forward or backward so you would need to refocus, and if the lighting has not changed in the frame, exposure values might remain the same- or might change.
Two separate systems are involved here. The focus button per se does not govern exposure. However, if you are to zoom quite a bit as described above, your camera's Program mechanism will likely kick in to compensate for a difference in the lens focal length as it changes while you zoom. You see, this programming by the camera is usually a good thing for hand-held shooting, because as you zoom into more telephoto range, this will make hand-held shots more difficult due to increased effects of your own slight body motion, which can cause lack of clarity in your image. The camera's compensation to offset this will be a higher shutter speed to better freeze motion. If the lighting in the frame is in the same balance as before, the exposure will still be the same, but the higher shutter speed will then be compensated by a wider aperture setting (smaller number).
As to going between lighter and darker subjects, again this has nothing to do with your focus button per se, other than when you activate focus you also activate the camera's other systems. When your camera is set on any of the auto exposure modes on your mode dial, and you go between dark and light subjects, your camera's program is doing what it is supposed to do- react to the camera's meter reading of the lighting. But there will be times when you would not want that to happen. Then employing the Manual Mode will be the thing to do.
I'll give you a real-situation example. Shooting an indoor roller hockey event, where the venue at one end of the rink has a back wall and a side wall with very large windows, but none at the other end. So, during daytime, as the skaters skate from the non-window end over to the window end, their background, and the lighting in the frame, changes dramatically. If left in any auto-exposure mode, the camera would react to the meter's reading of all that light from the windows, setting exposure values accordingly, but this would result in the skaters rendered much darker- as shadows or silhouettes. The cars parked outside would be well-exposed. So the thing to do is switch to Manual Mode and spot metering, take meter readings of a mid-tone area under the building's lighting at the non-window end to set your exposure parameters. Of course, with fast action, a high shutter speed will be needed, along with an attendantly wide aperture, and also necessitating a boost in ISO to get a meter-centered exposure setting.
Once this exposure setup is satisfactory at the non-window end, the exposure values are left alone and will not change in Manual Mode, even though the meter is warning of under or over exposure as you follow the skaters, and as the lighting changes. But as you activate your AF button, you will then see that your exposure values will still remain unchanged. You can and do just ignore the meter at this point. You have eliminated the camera's ability to react to the meter and change exposure. This has no effect on the AF button's function.
There are numerous other scenarios of this sort where this kind of approach will be needed. It also demonstrates the AF function's being a separate function to exposure function. In the realm of using auto exposure modes, they are simply coincidental.
Another way to understand this is to switch to MF and focus your lens manually. Your camera will still provide a focus confirmation in the VF. But, if you activate the camera's metering system by the shutter half press, which can also be used for AF when the camera is set to AF, your exposure will change according to the lighting, even if you have not achieved good focus with your manual focusing.
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Forum: Troubleshooting and Beginner Help
04-11-2019, 06:15 PM
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As I indicated, unfortunately the two uses of this button are alternate uses. The only way to have both functions in the rear is on a larger flagship model like the K5 and K-3 series bodies, which have an independent button for each function in the rear.
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Forum: Troubleshooting and Beginner Help
04-11-2019, 05:31 PM
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The back AF setup is not for metering. It is for focusing- Auto Focus to be specific. But the button is alternately used as an AE-L button instead of a back AF button. In that case, it also does not meter, but it will lock the exposure meter that has been set already by metering. This is used under various conditions where a metered value is needed to be preserved, while lighting situations may be changing. When the button is set for that use, then you get your AF via the front shutter release button by using the half/press to obtain AF and to hold focus.
Metering takes place in a manner according to where you have set your mode dial. If you want your camera set for fully automatic exposure metering, where both the shutter speed and the aperture will be set by the camera according to the camera's meter reading of the lighting, then the mode dial can be set either on the green AUTO mode, or on the "P" mode. The "P" mode is better because the green AUTO mode does not allow access to many adjustments and to many control features, including the exclusive Pentax Hyper Program operation, where you can override the camera's setting of either aperture or shutter speed. You can also have the camera automatically adjust the ISO according to the light meter reading as well in this mode.
There are other automatic exposure metering modes on the mode dial to choose from, which are not fully automatic because you play a role, such as selecting a preference of aperture or shutter speed, while the camera automatically compensates by setting the other. These are the Tv or Av modes. Consult your handbook for the other modes. There are good reasons for being able to take control of these aspects of photography instead of always just having your camera on auto-pilot.
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