Link AF Point and AE - DOH!!

[ramble]I'm trying to rest up a lot to get over what ever it is that's bugging my health at the moment, and decide to have a casual look over my Pentax gear. While playing around and switching between the 18-55 and 17-70 and metering against a table lamp I came across this startling realisation...[/ramble]

This is a real "Doh!" moment here and one that's definitely worth posting

When I set up my K10D & K20D I thought that the "Link AF Point and AE" would be a good idea - so I set it to option "2 = on". Well it turns out that either I misunderstood the manual, or it wasn't clear enough Occasionally I've had some rather odd exposures, but put it down to haste... now I'm coming round to a different view!

As many will know, I always focus using the "AF" button on the back, preferring to leave the shutter release button for just that (it makes things so much quicker). I also use centre AF point (selected on the rear).

When the "AF and AE" point are linked in the custom menu, this is what happens:
* When you let go of the "AF" button the metering is "Matrix" (or whatever was set on the dial)
* When you hold down the "AF" button the metering switches to "Spot" metering.
* Consequently, if you press the shutter while holding down the "AF" button you can get a vastly different metering!

Doh!!

Edit: Even more bizarrely, the spot metering is only chosen when the focus is achieved! If you switch to manual focus, then holding "AF" with an out-of-focus centre will use matrix, while holding "AF" with an in-focus centre will switch to spot metering! (I'm off to get that little buglet to Pentax )

In fact, even when you use a selected focus point with "AF and AE linked" it seems to take the reading from only the metering segment closest to the focus point. So, even if you had matrix metering selected then the metering will be spot, or an average area.

Now, I suppose the manual said that... however, I still expected the matrix metering to work as matrix (perhaps weighted towards the selected AF point). But from my little check here that's not what happens!

Also worthy of note is that "AE-L" effectively captures the meter readings of the 16 segments, and pressing the "AF" button will still switch over to use the spot/selected segment. Releasing will revert back to the "stored" matrix metering value. (Hope that makes sense! What it means is that you can't use this feature to temporarily switch to spot metering and hold that value by pressing both the AF and AE-L buttons.)


I've not yet checked whether this affects the flash metering...

Hope that little gem helps you out!!!
Matt

Nice one Matt! I use the AF button rather than the shutter too, but I have my "Link AF & AE" switched off (default). Interesting to know what happens if I were to enable it.

I have mine set the same way, but I use the shutter button for both AF&AE

The way I understand it is that while the camera is set to matrix metering, if link AF & AE is enabled the camera will in effect work in a similar way to centre weighted, ie; normally the centre portion of the image will expose brighter, but with AF & AE linked this will be linked to the focus point.

A bit like having C/W metering but concentrated on any of the focus points in the frame, think of it as "selective" C/W metering so if the focus point is say to the left of the frame then that is where the camera will meter.

I believe the camera must be set to Matrix for this to work.

so should the ae-l with af locked be on or off,:

Up to you - depends on what you want and your style of shooting.

Moral of the story, always use manual if you want to know what is going on.
I get too easily confused with auto stuff and have the camera set to use matrix metering (mostly) with center autofocus point set with the shutter button, and the camera to meter in manual, it seems to work for me but I could be just lucky. On occasions where the lighting is a bit tricky I will change to spot metering and choose the area that I want to expose for, but I find that most of the time matrix works pretty well.
It's nice to have options though and nice to while away the time just playing with what is a very sexy piece of kit.
Right I'm off for a cold shower now.
Harry