[mikesbike]

I think what the OP is asking is when back button AF is set does the button still perform AE-L.

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.

[DW58]

The back AF setup is not for metering.

Then why do exposure values change when using the back button to focus on a bright subject then a dark subject? Without ever touching the shutter release.

[Kobie]

Okay, there seems to be some confusion regarding the use of the term "metering".
Given that the OP was asking about how they would meter by using BBF (Back Button Focus) I'm guessing as well that they're talking about AE-L.
There's 2 separate things being discussed here which is making things confusing.

1) The camera will meter the same using BBF or Shutter release button for focus. That metering doesn't change just because you aren't using the shutter button to focus.
2) If you have a model that only allows the AE-L button to be used for BBF then no you can't lock the exposure since you've told the camera to use that button for focusing instead.
3) If you do have a model that has a separate rear button for focus and a separate AE-L button, then yes you can lock the exposure and still use BBF.
4) If you can program a different button for AE-L to keep it from being the same button used for BBF then that would be a solution (I don't believe this is possible though).

So in a matter of speaking, everyone is right, the camera will meter. It just may not allow you to lock the metering.
Only Exposure lock shows * (AE-L).
Normal automatic metering won't show anything.

[pschlute]

Then why do exposure values change when using the back button to focus on a bright subject then a dark subject? Without ever touching the shutter release.

Because the meter will operate and change exposure parameters (depending what mode you are in) whenever the camera is "active". That means it is turned on and has not fallen "asleep". Waking the camera from "sleep" is done by either half pressing the shutter button, or pressing the BF button, if enabled for focus.

[DW58]

The use of back button focus does not effect the metering system at all.

The back AF setup is not for metering.

Forgive my ignorance but these are the quotes I don't understand. When set up for back button focus my K-70's metering IS affected when pressing the back button. In Av mode the shutter speed will change depending on the brightness of the subject I back button focus on. The OP asked how to meter with his K-70 when set for back button focus. I replied "press the back button." Works for me.

[pschlute]

Forgive my ignorance but these are the quotes I don't understand. When set up for back button focus my K-70's metering IS affected when pressing the back button. In Av mode the shutter speed will change depending on the brightness of the subject I back button focus on. The OP asked how to meter with his K-70 when set for back button focus. I replied "press the back button." Works for me.

See my reply just above your post. Pressing the backbutton focus button does not activate the meter per se, but wakes the camera. Same as using a shutter button half press, or simply turning the on/off switch on. Whenever the camera is awake, the meter is on.

[DW58]

See my reply just above your post. Pressing the backbutton focus button does not activate the meter per se, but wakes the camera. Same as using a shutter button half press, or simply turning the on/off switch on. Whenever the camera is awake, the meter is on.

Bottom line is you can achieve an accurate meter reading by pressing back button to focus. As per OP's original question.

---

Last edited by DW58; 04-12-2019 at 03:40 PM.

[PDL]

Forgive my ignorance but these are the quotes I don't understand. When set up for back button focus my K-70's metering IS affected when pressing the back button. In Av mode the shutter speed will change depending on the brightness of the subject I back button focus on. The OP asked how to meter with his K-70 when set for back button focus. I replied "press the back button." Works for me.

No, you are using the wrong words, The metering is not affected by pushing the back button, it is turned on. By being turned on, it will set the shutter speed as the brightness changes.

This is by design and intent of back button focus. When you activate the camera by pressing either the shutter button, the LV button or the back button focus button. The meter in the camera is turned on by pressing these buttons. Is it absolutely necessary to press only the shutter button to start the exposure sequence? --- No - absolutely not.

---------- Post added 04-13-19 at 12:02 AM ----------

Please re read the following.

When the camera "goes to sleep" it will come back to the present (rather than the trite term come back to life) by pressing either the Menu button, info button, AF button etc. When you press a either the LV button/switch, AF button or shutter button, the meter starts and the camera is getting ready to take a picture.

Now, as for the K-70 being able to differentiate between back button focus and exposure lock (manifested by displaying a * in the viewfinder) I can not say as all of my cameras have separate buttons to control AF and AEL.

[DW58]

No, you are using the wrong words, The metering is not affected by pushing the back button, it is turned on. By being turned on, it will set the shutter speed as the brightness changes.

This is by design and intent of back button focus. When you activate the camera by pressing either the shutter button, the LV button or the back button focus button. The meter in the camera is turned on by pressing these buttons. Is it absolutely necessary to press only the shutter button to start the exposure sequence? --- No - absolutely not.

---------- Post added 04-13-19 at 12:02 AM ----------

Please re read the following.



Now, as for the K-70 being able to differentiate between back button focus and exposure lock (manifested by displaying a * in the viewfinder) I can not say as all of my cameras have separate buttons to control AF and AEL.

Yawn.

I understood the question to be simpler, though not well phrased. I understood the OP to mean how does one get an exposure meter reading when the half-push on the shutter button is no longer being used for autofocus. The answer is that the half-push on the shutter button makes the exposure meter show what it's thinking; that function has nothing to do with which button's controlling AF.

[rod_grant]

You have got me thinking!!!!
This may not add to the discussion re K70, but for anyone with a KP it might be helpful.
I have a KP and the AF and AE-L button are one and the same.
I always use BBF and assumed (without checking the book!) that when I focus and then recompose, my exposure would be set for the object on which I focused.
This has proved to be true, but as I found tonight, only when AF is set to AFS.
When set to AFC, the meter reads the area central to my recomposing.

edit
This is of course predicated upon menu settngs C1 - 5 - ON and C5 - 6 - ON

[mikesbike]

Then why do exposure values change when using the back button to focus on a bright subject then a dark subject? Without ever touching the shutter release.

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.

[monochrome]

You have got me thinking!!!!
This may not add to the discussion re K70, but for anyone with a KP it might be helpful.
I have a KP and the AF and AE-L button are one and the same.
I always use BBF and assumed (without checking the book!) that when I focus and then recompose, my exposure would be set for the object on which I focused.
This has proved to be true, but as I found tonight, only when AF is set to AFS.
When set to AFC, the meter reads the area central to my recomposing.

edit
This is of course predicated upon menu settngs C1 - 5 - ON and C5 - 6 - ON

My KP came with the FX2 button on the upper left rear preset to change to AEL when the rear AF / AEL button is set to AF only. I love having the camera set up this way. So spot meter on your preferred Zone, hit FX2 (with left thumb (* lights in VF), compose, AF and shoot. Meter does not change Av, Sv or ISO.

[Edit] Come to think of it, I’ve never tested this with AF.C

[RichardJ]

I have a K70. The metering becomes active when any button is pressed on the camera and it will actively then meter the scene, adjusting shutter/aperture/ISO as appropriate depending upon what mode you are in. It does this for 10 seconds. If no further buttons are pressed within those 10 seconds, e.g. to take a photo or to auto focus, then the metering system disengages. It wakes up again for another 10 seconds when a button is next pressed, and so on. The length of time the meter is active can be controlled through menu C1-3 "Meter Operating Time". You have options for 10s, 3s, 30s, or continuous. The default is 10s.

This metering behaviour happens regardless of whether back button focus is enabled or not.

When back button focus is enabled, on the K70, you can no longer use the AE Lock feature to lock exposure. This is because the only button that you can set to do AE-L, is the same button you've just changed to do back button focus instead. Effectively, back button focus means you can no longer lock the exposure, the camera will always auto meter instead. That auto metering will start as soon as you press any button on the camera.