Web Analytics RiceHigh's Pentax Blog: Two Important Custom Functions for Proper Metering

Thursday, January 22, 2009

Two Important Custom Functions for Proper Metering

There are two Custom functions (CFs) in all Pentax DSLRs which many new users (or even more experienced old ones) are asking about frequently. They are the CFs of "Link AF Point and AE" and "Lock AE with AF locked". I shall explain more below what these two functions are about and what proper setting should be selected for each of the CFs.

1. "Link AF Point and AE":-

I think to be more exact for better understanding, some words should be added for the description, just say if it is called "Link AF Points to the Evaluative Metering", I bet it would be better.

Now, let's look what exactly this CF means. Normally and by default, the 16-Segment Multi-Pattern Evaluative Metering does not take information from the AF system for determining the exposure value. However, if this CF is enabled. The Evaluative Metering will take into account which AF point(s) is/are in focus and which "corresponding" (see more below) metering segment(s) at these point(s) will be given more weight.

So, would enabling this CF help to improve metering accuracy and reliability? Putting aside the in-born deficiency of the Pentax DSLR metering system (which metering levels are much dependent on lens, metering mode, colour temperature and etc., even for the same objects), there are two reasons why this "feature" will not work well:-

(i) Object(s) that is/are in focus is/are not necessary the point(s) of interest, nor, each individual's reflectance it known;

(ii) The AF points of the SAFOX VIII system are not exactly aligned at the centrals of the metering segments. See the illustration below:-

Almost all Pentax DSLRs made since 2003 are having the same architecture as above, except the *ist DL/DL2 and K-m which some of the 11 AF points omitted. As you can see, only the dead centre metering segment has its AF points aligned exactly, whilst for all the remaining 9 surrounding metering segments, the "corresponding" AF points are not aligned, and some are even at the dead corner of the metering segment. And, there are still 6 remaining metering segments which do not have any AF point to cover.

Furthermore, the issue gets more complicated when the user selects an AF point manually or just let the camera decide for it. Just say if the user (as many users are) selects only the central single AF point most of the time, weight will only be given to the centre metering segment, but anyway it is *just* the basic assumption made by the Evaluative Metering, regardles of what AF point(s) the user/camera chooses. Afterall, the evalutive meter always assume the subject is in the centre of the frame, unless the CF "link AF point to AE" is enabled. If this is the case, and if the camera is let to select the AF point(s) (depending on the model, one or more AF points will be selected (e.g., the *ist D would select only one AF point whilst the *ist DS selects more than one point, as indicated), then those selected points will be given top priorit(ies) in the Evaluative Metering calculations.

After all the above explanations, I think the only case which the above Pentax CF should be enabled is only WHEN the Pentax DSLR is put into Auto AF point selection mode and the user does NOT recompose. This is the only scenario that the Evaluative Metering will do its job correctly as only under this scenario all the *preliminary* assumptions made by Pentax are valid. But do remember that the *assumption* of a subject is in-focus should be given more weight in metering does NOT usually hold - just imagine about taking landscapes, e.g., everything in the frame are at infinity and thus they should be all in-focus, so?

2. "AE-L with AF locked":-

As continued from the last part, the issue gets even more complicated when the user uses the AF lock and re-compose. If this CF is enabled, just say with a single AF point in Single-AF mode, the measured AE value will automatically be locked once the AF is completed. On the other hand, if this CF is disabled, once upon recomposition, the calculated Evaluative Metering value once again could be wrong, as the measured exposure value is Not Locked (but then this is by default), since the central metering segment of the evaluative metering is now pointing to another subject other than the interested object. If the user selects the Multiple Auto AF point mode and recompose, again, everything would be meaningless *just because* the AE is not locked and he/she recomposes, again! (but default it is!)

In fact, all the old days Minolta AFSLRs with multiple AF sensors (since the Dynax 7000i), some Pentax Z model AFSLRs, and every Canon and Nikon DSLRs will have AE locked by default or compulsorily when their cameras are put in Evaluative Metering mode. For many of them there is not even a CF to disable this for the Evaluative Metering. This leaves alone Pentax who just overlooked the problem and thus many Pentax DSLR users have been receiving more errors in the photos they took just because of this "simple" oversight.

Fortunately, the description of this CF is somehow clearer than the above one and at least it is more easily understood by most. As I have just said, I think this CF should be enabled by default, especially with Evaluative Metering, no matter which AF point or what AF point selection mode is selected. To de-link the AE lock from the AF system (after AF is to be locked) is just an *unique* oversight by Pentax indeed, which opens up this option is almost meaningless, for most users and will just create more troubles with even more exposure errors, which holds 100% true for the Evaluative Metering, at least.

If Centre-Weighted Average Metering is chosen and when the Single AF point at Central is used, enabling this CF could give more convenience for avoiding pushing the AE-L button all the time before recomposing. Well, this time it can be well assumed that the subject which is focused is just the point of interest, in this case! As for Spot Metering, since precise "circling" of an areat and careful selection of mid-grey objects is required, enabling this option may not be as practical, though.

Since Evaluative Metering is the default metering mode which is designed and aimed to be fool-proof. I highly urge Pentax to change their default CF setting, so as to be truly fool-proof, for the only reason, merely.

Least but not least, I must mention about the poor and unclear documentation which is also the culprit of creating the troubles, in addition to the inappropriate default setting. All the Pentax DSLR user's / operating manuals are simply poorly written IMHO, which the users could by no means know more about the actual practical meanings (not even to talk about more in-depth) technical knowledge) about many features and functions of their cameras - and of course, better appropriate modes and settings could not be selected by many novice (or even some more experienced) users, as a result. Afterall, all the confusions and inaccuracies show how unthoughtful Pentax really are. In contrast, my Canon 5D has far more thoughtful default settings which are more logically put as well as better documentation, which the difference is clear and has been shown. If anyone who is unconvinced, just go to the Pentax and Canon official websites to download some of the user manuals to compare!


  1. I love the way you turn an additional Pentax feature (the ability to link AE to AF in evaluative or not - nobody else gives you the CHOICE) into an "oversight" or a demonstration of how Pentax is "inthoughtful"(sic)...

    There are a few additional questions on this:

    Relatively to your comments on why link AE/AF won't work on a Pentax (but are super duper on a Canon): "(i) Object(s) that is/are in focus is/are not necessary the point(s) of interest, nor, each individual's reflectance it known;"
    ==> Why would you want to link AF to AE if the focus point isn't the subject of the photo?

    "just imagine about taking landscapes, e.g., everything in the frame are at infinity and thus they should be all in-focus, so?" ==> so no need to link AE to AF for landscapes... which is the default Pentax setting: what was the problem again?
    You should actually ask why on earth Canon implemented the AE/AF link as the only "option" as it clearly will give you wrong results general photography like landscapes...

    As to having AE lock activated by default when AF is OK... isn't it the reason why there is a AE-lock button? The fact that Pentax, again, gives you the CHOICE of coupling AF lock to AE lock or not is a bonus, not a defect. I can't count the number of times where I used spot metering on a difficult subject on my 5D and took a crappy picture just because I forgot to hit the AEL button before recomposing: I really whish Canon would allow me to select the option of locking AE and AF at the same time... but I can't!

    Well, as you can see, I politely disagree with your conclusions and I feel they denote a strange twist of mind where the brand offering the more options is being brought down because of the supposed additional complexity (OMG! I have to make an informed decision!! The camera isn't thinking for me, what am I going to do!!)

    As a conclusion: RTFM!! ;-) If I could understand it, I guess anyone could...

  2. You've missed the point totally.

    I am not against the feature, but it should be enabled by default for the **Evaluative Metering** (I think I have to mark by double stars!, as you still are unable to read that!) as what Canon, Nikon are doing with their DSLRs and Minolta and Pentax (particular models) did, for the EVALUATIVE METERING mode.

    Re-read my message above, I think I have stated it very clearly.

    My main message and recommendation in my this article is to tell people to enable it!

    In fact, I did always enable this "feature" with my K camera long ago. Just in case I didn't and don't like the metered area, I could and can still hit the AE-L button well before I do the AF. The main trick is to half press the shutter release button or the AF button (in case if set) before the AE-L memory expires. That's it.

    The only real meaning in disabling the CF is that real-time metering is desired. But for most cases, it is not so critical. If true real-time ambient light metering is required, only the old Pentax LX could do that.

    It is true that Canon 5D does not have this custom function but it has already given you by default, in the **Evaluative Metering** mode. For CWA and Spot Metering modes, the AF and AE are de-linked, so you have to hit the AE-L to do so if wished, as you've said. But isn't the Evaluative Metering the **Fool-Proof** mode as designed but not the CWA and Spot? (People who uses CWA and Spot might prefer for more freedom.)

  3. And, lol, I think you're still confused much with the two Custom Functions and what they are (and why they are two different things!)..

    Its very clearly that you have mixed up something for what you've shown in your response.

    Just think about the following Question: Can you "link AE to AF" in the CWA and Spot Metering modes? :-)

    Re-read my post, the answers are already all here and there!

    And, to asnwer your question. I think I have also mentioned in my post that the "Subject of Interest" is quite sure if the user uses only the Single Central AF point. Under this condition, the "lock AE when AF locked" CF can be useful, in all the 3 metering modes.

  4. So you're way of simplifying things is to have different default behaviors for different metering modes...? Seems more complex to me...

    I see it that way: either you use the AEL button to lock exposure... or you don't: operator's choice.

    It does require the operator to understand what he is doing and decide what works best for him: maybe not foolproof but then again the best advice to give to a fool is to eduquate him into being smart instead of trying to mitigate the consequences of his foolishness... ;-)

  5. "And, lol, I think you're still confused much with the two Custom Functions and what they are (and why they are two different things!).."

    Well thank you prof but I think I understand the two CF and what they do quite well despite my limited habilities.

    "Can you "link AE to AF" in the CWA and Spot Metering modes? :-)"

    No, my point was about the possibility of having a simultaneous AE/AF lock in CWA or spot when using center AF point so I think we agree on this point that Pentax's option is intersting to have in this case.

  6. So, we have come up to an agreement up to this point on all the factual technical points.

    But I still don't agree with the default setting for the second CF by Pentax here. And, I don't think *confused* Pentax users are "uneducated" (the manual says nothing useful, but only clone the words as displayed in the camera's menu!) or those users need to "mitigate the consequences of his foolness". My point is that Pentax *should* set a *fool-proof* default setting, even though there is an option. They just shouldn't to make the default behaviour of their DSLRs are totally different from all others but which is not so reliable and making sense for *most* situations and for *most* users.

  7. Anonymous23/1/09 04:43

    Maybe i am an uneducated Pentax user, but all you've done is confuse me more. It could the language, is english your native language RiceHigh? I think there are times you say "just" when you mean "only".

    Please double check when and where you use "AE" and "AF" and also the order of the CFs you describe. on the K20D for example, the order of these two options in the menu are reversed.

    I'm open to what you are trying to say, and trying to learn, but i just don't get it. Could you please describe the precise order of the steps each Custom Function makes and under which conditions? (under which AE & AF modes). Would be very much appreciated, thank you.

  8. Anonymous23/1/09 05:11

    these two CFs do the same thing (lock both exposure and AF on the active AF point) except one is for ALL 3 metering modes and the other is only for Evaluative Metering mode correct??

  9. Not correct.

    And, I just like much to use the word "just" as what Phil Askey and his crew do.

  10. Anonymous24/1/09 00:35

    what? that wasn't an attack RiceHigh.

    um, thanks for clearing absolutely nothing up at all and "just" adding confusion.

  11. Pardon? You asked me if your statement/supposition is correct and I clearly answered your question. It is NOT correct. So, what do you want?

  12. Anonymous24/1/09 01:43

    did you not see the first question in my first post? about describing the precise order of how those CFs work?

    and if my question in the second post is not correct, then please explain how its not. I asked that question for a reason. thank you.

  13. Why the CF No. (and their order) matters? What are you trying to suggest actually?? And, the CFs only have the option of "Disabled" or "Enabled", what "precise order of steps" are inside "each" CF???

    I think only you yourself could answer the very special questions created by your ownself! I appreciate very much your super creativity, anyway!

  14. Anonymous24/1/09 15:36

    "creativity". That's a nice choice of word. That's exactly what photography is about, and some here seen to forget that.

  15. However, when we are talking about Engineering and Science, everything should be absolute and "accuracy" should be the key word to follow. So, its totally different.

    Without a properly engineered machine as the photographer's tool, which should be accurate and reliable, any "creativity" is worthless.

  16. Anonymous25/1/09 01:14