BEWARE: AF lock and Matrix metering behavior!!!

Started Jan 2, 2008 | Discussions
jp Senior Member • Posts: 1,151
BEWARE: AF lock and Matrix metering behavior!!!

Just warning those who program one of the 3 buttons as AF lock only (cf-f4,cf-f5 or cf-f6) like I do

When you use matrix metering with AF-S mode, then pressing and holding the AF-lock button, does not just interrupt the autofocus like on a canon or sony like you would expect, it also "preconditions" the exposure according to the frame and active AF point at the moment the button is pressed (just like what happens when you half press the shutter in AF-S+matrix: it does nor really lock, but the measured result 'sticks' into the metering)

So if you want to make a shot with preset focus distance by using the AF lock function, then first compose the correct scene BEFORE pressing the AF lock button or you may obtain incorrectly exposured results.

I do not know whether many people use this technique but those who do use it and are used to the way other cameras work (like my 40D for example), remember this caveat. Those who don't should not worry about it

Another thing I just discoverd is the fact that the AF-lock button on the professional telephotolenses also exhibit this behaviour. Since I almost exclusively use this type of lens in AF-C mode, I never noticed this before. At least there is a consistency in this somewhat 'strange' behaviour.

cheers
JP

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People shoot people.

Tonik Contributing Member • Posts: 558
Re: BEWARE: AF lock and Matrix metering behavior!!!

I am not in front of my camera at the moment but there is a good fix for this in the menu's. Set the shutter half press to just focus lock and use the AE button to exposure lock. So you can focus lock on your target then move the frame and it will re-meter....I think that is what you are bringing up, if not please ignore my post

jp OP Senior Member • Posts: 1,151
You mean cf-c1 but that is not what I mean

Tonik wrote:

I am not in front of my camera at the moment but there is a good fix
for this in the menu's. Set the shutter half press to just focus
lock and use the AE button to exposure lock. So you can focus lock on
your target then move the frame and it will re-meter....I think that
is what you are bringing up, if not please ignore my post

I do not have the cf-c1 active like you mean. That one completely locks exposure, and it does this as wel in af-s as in af-c mode. The thing I am talking about is more like a prebias, not a complete AE-lock, and it only occurs in AF-S mode.
Thanks for your contribution anayway

Cheers
JP

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VadimOm Senior Member • Posts: 1,903
Re: BEWARE: AF lock and Matrix metering behavior!!!

Are you saying AF-lock ONLY assigned to AE-L/AF-L button pre-biases exposure, even though you're intentionally avoiding locking/biasing exposure by selecting AF-lock ONLY?

Jim Keye Senior Member • Posts: 1,889
the solution:

quit using matrix metering.

Seriously.

herr_bob Contributing Member • Posts: 607
Re: BEWARE: AF lock and Matrix metering behavior!!!

In matrix metering, the current AF point has a weight on the metering, i.e., if you keep the composition and change the AF point, the metering will also change. So when you use AF-lock, the metering give a reading according to the current composition and current AF point, then when you recompose, the metering will change, but still related with the chosen AF point(though it is not active now).

On the Canon 40D, if you use the evaluative metering, the reading is not associated with any AF point, i.e., if you keep the composition and change the AF point, the metering remains the same.

Could this explain your situation?

Elixir Contributing Member • Posts: 679
Re: the solution:

Jim Keye wrote:

quit using matrix metering.

I may do that myself. Getting excellent results from center-weighted. In fact, I believe all my friends in the 1980s bragged about that same center-weighted exposure system. I'm starting to see what they meant.

steves7k Contributing Member • Posts: 626
Re: BEWARE: AF lock and Matrix metering behavior!!!

hi

the reason behind matrix partial AElock when you focus lock and recompose is that matrix puts extra weight on the focus point and nikon thought that if you focused on something it must be inportant so they try to keep the exposure close but make some smaller adjustments for the new surroundings so as to not compleatly loose the exposure for the subject that you focused on in the first place ( did that make any sense )

this hole focus lock and recompose is an out dated technique from the days when there was only 1 focus point

you have 51 focus points to chose from so maybe you should try just moving the focus point for the comp you want
its quick and easy once you get used to it
thats one of the reasons they give you 51

Murray Bowles
Murray Bowles Senior Member • Posts: 2,051
If you must use matrix metering

and want to do focus-and-recompose, it seems like there are two possibilities:

for AF-S, AF-L/AE-L might not be too bad if the composition doesn't change much -- at least the metering is based on the correct AF target

for AF-C, don't use AF-lock at all and hope the camera can follow the subject as you recompose

Jim Keye Senior Member • Posts: 1,889
and if

you subject doesn't fall exactly where one of the 51 sensors is?

larsbc Forum Pro • Posts: 13,625
Re: BEWARE: AF lock and Matrix metering behavior!!!

steves7k wrote:
[snip]

this hole focus lock and recompose is an out dated technique from the
days when there was only 1 focus point

It's still a useful technique today, when you're shooting in poor lighting. The peripheral AF points have a much harder time locking focus than the center point.

larsbc

Murray Bowles
Murray Bowles Senior Member • Posts: 2,051
focus and recompose

just seems way quicker and more natural to me. Maybe the solution is just to shoot wider angle, keep the subject centered, and do the composition in PP.

jp OP Senior Member • Posts: 1,151
Yes, that is exactely what I am saying (nt)
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Murray Bowles
Murray Bowles Senior Member • Posts: 2,051
Re: Yes, that is exactely what I am saying (nt)

Do you think it is ONLY "remembering" the AF-point's position and miscalculating because of that, or do you think it is remembering more?

jp OP Senior Member • Posts: 1,151
Not my kind of solution

The superb matrix metereng was THE primary reason I switched to Nikon when I went digital 4 years ago, after being a very satisfied EOS user for 17 years.

I had one of the very first EOS620s and EOS1 in europe (went to the states to get them before they came out here).

I was always satisfied with the exposure meters of the canons since I used negative film.

But because digital is so picky of exposure, I decided to go Nikon when the D70 came out instead of buying the 300D, although the IQ of the latter looked better to me, especially at higher sensitivities.

I also rcently compared the matrix metering of my D300 with the evaluative metering of my 40D and in 90% of the difficult cases, MM faired clearly better.

So, you can see, your solution may be fine but it is not an option for me: I rely on MM and am very satisfied with the results. There are just a few caveats like this one and if you know them and take them into account, then it works fine.

From using my 40D a few months, I created the habit of pushing the AF-lock button at any convenient moment before making the composition, which is clearly a no-no with the D300. Once you know this, the problem is no more. Which is why I wanted to make this warning.

Jim Keye wrote:

quit using matrix metering.

Seriously.

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jp OP Senior Member • Posts: 1,151
Yes, there is more

If we just ignore AF lock buttons for a moment, and just analyze the way matrix metering works, it becomes clearer. (AF-S mode)

When you half press the shutter, the matrix meter calculates the exposure and remembers it but it does not do EA lock. When you recompose, the meter wil reconsider the new scene to modify the previously measured exposure to take into account new areas that deviate in brightness very strongly from the original scene. The originally measured result is slightly adapted to take into account the new composition. But the result can differ strongly from what this new scene would be metered from scratch. This strategy is good since it allows you to aim and recompose and stil have the camera do some thinking for you.

Unfortunately this strategy of 'storing the exposure' also occurs when you do not want it like in these cases:

  • autofocussing with the AF button and holding it

  • locking AF with the AF lock function button.

Wheteher these are intentional decisions from the designers, or simply the result of the way the firmware is programmed, we will probably never kwow.

Important thing is, is that we are aware of it and can take it into account when using our nikons with MM

cheers
JP

Murray Bowles wrote:

Do you think it is ONLY "remembering" the AF-point's position and
miscalculating because of that, or do you think it is remembering
more?

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jp OP Senior Member • Posts: 1,151
This behviour does not apply to AF-C (NT)
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jp OP Senior Member • Posts: 1,151
No that is incorrect

herr_bob wrote:

In matrix metering, the current AF point has a weight on the
metering, i.e., if you keep the composition and change the AF point,
the metering will also change. So when you use AF-lock, the metering
give a reading according to the current composition and current AF
point, then when you recompose, the metering will change, but still
related with the chosen AF point(though it is not active now).

That is indeed the way you would expect it to behave but it does not.

It persists the metering of the whole scene at the moment you press AF-lock and then uses this metered result as a strong bias for the eventual metering.

On the Canon 40D, if you use the evaluative metering, the reading is
not associated with any AF point, i.e., if you keep the composition
and change the AF point, the metering remains the same.

Incorrect: the canon evaluative metering DOES take the af point into account. It puts even a lot more bias on the zone under the AF point than the matrix meter (way too much to my taste by the way)

The difference however is, is that half pressing the shutter on a canon in single servo AF always locks the exposure, on the nikon it does remember but not lock the exposure, unless you set cf-c1.

But the AF lock button on the canon does not influence the exposure while on the D300, it remembers the exposure at the moment the af-lock is pressed and only modifies it very slightly as you compose the new scene.

Could this explain your situation?

So no it does not

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jp OP Senior Member • Posts: 1,151
This technique is sometimes necessary

steves7k wrote:

hi
the reason behind matrix partial AElock when you focus lock and
recompose is that matrix puts extra weight on the focus point and
nikon thought that if you focused on something it must be inportant
so they try to keep the exposure close but make some smaller
adjustments for the new surroundings so as to not compleatly loose
the exposure for the subject that you focused on in the first place (
did that make any sense )

When I lock focus with shutter half press, I do agree that it is a good implementation, with a separate AF lock button however, I find it debatable.

Anyway, I was not complaining that this is bad or a bug or anything, I was just issueing a warning that it does work this way and that the unaware operator will obtain undesired results.

this hole focus lock and recompose is an out dated technique from the
days when there was only 1 focus point
you have 51 focus points to chose from so maybe you should try just
moving the focus point for the comp you want
its quick and easy once you get used to it
thats one of the reasons they give you 51

I do not agree.

I too try to use the 51 autofocus points as much as I can but this is not always possible
1) the side points are less effective and not cross sensitive

2) sometimes you do not have no contrast to focus on and you have to use another object at the same distance

3) sometimes you want to prefocus to ensure that you will not miss the action when it occurs and press the af-lock button to prevent the camera from focussing when you press the shutter

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jkoplen Junior Member • Posts: 32
Re: This technique is sometimes necessary

jp,

Thanks so much for your input. If I had read it one day earlier, my camera would not be on its way to Nikon in USA for inspection.

My problems did occur with AF-L ONLY and matrix metering. I spoke with Nikon tech support twice. Apparently they were also unaware that thiis is "normal" for a D300.

Hopefully, your explanations will save others from needlessly sending their camera to Nikon.

Julian

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