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Thread: Spot Metering Question

  1. #1
    Peter Ryan's Avatar
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    Spot Metering Question

    I use sinlge point focus and matrix metering.

    Recently with a group of photographers one was using spot metering but not fully understanding it. I know a little about using spot metering but could not answer the questions relating to how spot metering works when: -

    1. You us multi point focus - where does the spot meter take it's reading if you move the active focus point off centre?

    2. If you use single point focus and focus lock on a subject off centre I assume you also have to lock the exposure reading when you re-compose the shot.

    Your thoughts would be appreciated.

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    Re: Spot Metering Question

    With my Canon 5D Mk11 manual, spot metering is approx 3.5% of the centre of the viewfinder, so that if you single focus point off the centre, then it still uses the center 3.5%.
    From Digital Picture "AF point-linked spot metering is only available on 1-series bodies, and only when you manually select an AF point (you can't select more than one). In automatic AF point selection mode on a 1-series, even if you have spot metering set, the camera uses evaluative metering."

    The exposure is locked when the shutter button is 1/2 pressed, or you could use AE lock

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    Re: Spot Metering Question

    Peter, my Nikon works pretty much the same as Ken describes. Spot metering is center weighted with options to adjust the size of that"spot". Perhaps you are thinking the point of focus and exposure metering are more closely related? Not so. I can move focus to the sides while taking exposure from the center. How I facilitate that depends on whether I am recomposing or not and in single or continuous focus as is applicable to the subjects movement.

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    Re: Spot Metering Question

    "AF point-linked spot metering is only available on 1-series bodies, and only when you manually select an AF point (you can't select more than one). In automatic AF point selection mode on a 1-series, even if you have spot metering set, the camera uses evaluative metering."
    Jeez . . .

    Before I read this thread, I never knew that some cameras that can actually jumble up the auto-focusing and the exposure-metering functions.

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    Re: Spot Metering Question

    On your first question...it depends on the camera. With Canon, spot-metering is always based on the center of the frame. With Nikon, spot-metering follows the selected focus point. The Canon 1D series has a custom setting that allows spot metering to follow the focus point.

    Your second question is a little more tricky. Again, what you do depends on your camera, your settings, and the lighting conditions. But first, we need to be on the same page in regards to spot metering.

    More often than not, spot metering gives the wrong exposure. The value of spot metering is that it gives a known exposure, which can then be compensated. So you can meter evergreen trees and underexpose by -1 EV, or yellow aspen leaves and overexpose by +1. Either way results in the same exposure settings for the same lighting. This is the application of exposure knowledge. If you're not actively compensating your tones, then you shouldn't be spot metering.

    The default exposure-lock behavior is different between Nikon and Canon. Nikon bodies don't lock exposure on the half-press...Canon bodies do. Both allow this behavior to be changed through custom settings. Also, the behavior of the Exposure Lock button can be changed, and the options vary considerably between Nikon and Canon. With Nikon you can lock exposure and autofocus while the button is pressed, or only exposure, or only autofocus, or lock exposure even after the button is released (for the length of the meter timer.) With Canon the AE Lock button always locks exposure while pressed or for the length of the AE Lock timer, which is 4 seconds. The 1D series has a "Timer after release" that defaults to 2 seconds, but can be extended to maintain an AE lock for up to 60 minutes. The 6D and 1DX finally have an "AE Lock (hold)" function that holds an AE Lock until the * button is pressed again.

    Constant-light
    On Canon's with center spot metering only, you need to spot meter your reference (which may or may not be your subject,) lock exposure by pressing and holding the * button, and then recompose and half-press to focus on the subject at your off-center location (or just full press and take the picture.) With a Nikon you only need to meter separately if your exposure reference is not your subject (that's what I usually do...I spot meter the gray card in my pocket and lock exposure.) If your exposure is based on your subject, then you simply frame and shoot. And of course, in both cases the exposure should have been properly compensated for via the Exposure Compensation function.

    Changing-light
    Think small puffy clouds that are racing past the sun, stage lighting that's changing, etc. In these conditions, having a spot meter that follows the focus point is extremely helpful. With a Nikon, the exposure isn't locked on the half-press, and so you can move your focus point, half-press to focus, and then take the picture when you want...knowing that your exposure will be calculated at the time you press the shutter. Since you're metering the subject, the exposure will be consistent (and if you apply the correct Exposure Compensation, the exposure will be correct.) On Canon cameras (and Pentax and others) that only spot meter at the center, there's no way to have a consistent exposure because you can't reference your exposure from the off-center subject. So your exposure is always based on whatever the background happens to be at the moment. You can try to lock exposure, but under changing light conditions, the exposure is only correct at the moment you press it. After that, the exposure can be off. So with these cameras you're forced to keep the subject at the center, where you can control the exposure.

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    Peter Ryan's Avatar
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    Re: Spot Metering Question

    Thank you everyone for your insights and in particular Graystar for your extremely comprehensive summary. I should have said at the start that I use Nikon but did not realise the difference between Nikon and Canon in this regard. I do understand what you are say Graystar and if I am in such a position again I think I will need to download your words of wisdom to remember the Canon setup, as I do know the Nikon.

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    Re: Spot Metering Question

    Quote Originally Posted by Peter Ryan View Post
    ...did not realise the difference between Nikon and Canon in this regard.
    All these cameras have the same basic M, A, S, and P modes, and also share the idea of a few functions, such as exposure lock and AutoISO. But the differences in implementation of the extended functions is astounding. It makes writing any kind of book on using these advanced functions practically impossible. It's to the point where if you hand me a Canon I feel like I'm holding my old Canon F-1...I simply can't work the way I do with my Nikon.

    As noted, spot metering follows the selected focus point on a Nikon. The AutoISO function has a Minimum Shutter Speed setting. An AE Lock can be held for multiple exposures by extending the meter timeout. AE Lock and Exposure Compensation work in M mode. And when you have an AE Lock in place, nearly any adjustment to the camera can be made, including selecting a different exposure mode or updating menu options, without losing the lock. No other camera offers this level of functionality, and I've grown used to having it. With the previous generation of high-end bodies, Canon added the Minimum Shutter Speed option to AutoISO (but inexplicably limits the fastest speed to 1/250s) and with the latest generation added the "AE Lock (hold)" option for the AE Lock button. But these are still missing from the intermediate and entry level range of bodies. Pentax and Sony also have similar limitations to varying degrees. Pentax offers a bewildering array of ways to adjust aperture/shutter/ISO...but offers no easy method to hold an exposure lock for more than one shot.

    Even with flash use there are huge differences. A big one is that the Exposure Compensation control on a Nikon also affects the flash, where on a Canon it doesn't. This is a mixed blessing...when providing a key light, it's nice to control the flash exposure the same way you do ambient exposure. With Canon you have to adjust the Flash EC. But at other times you need to control the ambient exposure separately, and that's difficult on a Nikon. The Nikon D4 and D600 finally got a custom setting to disable the link between EC and flash exposure, allowing easy control of the ambient background while precisely controlling the flash exposure separately.

    So as the cameras have been evolving, the best functionality from each brand is slowing being adopted by other brands. But for the moment, I feel that Nikon tops all the other brands...especially when it comes to bodies in the lower end of the range. Even a Nikon D3200 has an AE Lock (hold) option and an AutoISO Minimum Shutter Speed range that goes as fast as 1/2000s.

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    Peter Ryan's Avatar
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    Re: Spot Metering Question

    Your knowledge on the different cameras is extensive Graystar, do you review these for a living. I am very apprecaitive of your reponse here.

    As for Nikon, I can understand that the camera exposure compensation would effect exposure including flash but can you offset this by dialing down the flash exposure compensation?

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    davidedric's Avatar
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    Re: Spot Metering Question

    Although it doesn't help the OP who's a Nikon shooter, I understand that with Canon spot metering the target spot is interpreted as a neutral grey.

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    Re: Spot Metering Question

    Quote Originally Posted by Peter Ryan View Post
    Your knowledge on the different cameras is extensive Graystar, do you review these for a living. I am very apprecaitive of your reponse here.
    You're welcome...and no, I don't review cameras. I wish that camera reviewers would get into these details, but they never do. I just know this stuff through forum posts like this here and elsewhere...someone asks a question and all this info comes out in the discussion.

    As for Nikon, I can understand that the camera exposure compensation would effect exposure including flash but can you offset this by dialing down the flash exposure compensation?
    That's exactly right! That's what you do...first you set your ambient exposure and then you "counter" the EC with the opposite FEC. And every time you change EC, you have to change FEC. I call it the FEC shuffle...it's annoying. And, of course, you're limited to +/- 3 EV.

    Nikon has a page describing the new "uncoupling" feature, found here...
    http://www.nikonusa.com/en/Learn-And...th-the-D4.html

    I love Nikon's assessment of the current situation...

    "Confusing! Nikon thought so too."

    After describing the new feature they say...

    "We think this should make the whole process that much easier to remember and result in some amazing images where the flash and background exposures can be quickly and easily set."

    And yet the D800 doesn't have this feature.

    I actually sent Nikon a message through their website asking for the uncoupling feature about 6 months before the D4 was announced (I also asked for the front wheel to be tilted upwards.) Though I'd like to take credit for it, I think my timing was a bit too slim. Sony cameras already had this custom setting (inherited from Konica/Minolta) so again, another example of a function being adopted across brands.

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    Re: Spot Metering Question

    Quote Originally Posted by davidedric View Post
    Although it doesn't help the OP who's a Nikon shooter, I understand that with Canon spot metering the target spot is interpreted as a neutral grey.
    Nikon and Canon are the same in this respect. If you spot meter a neutral surface, take a pic, and demosaic with no additional processing other than white balance, you get sRGB values of 100, 100, 100 from either camera. That's about 12.7% gray (it also varies +/- 10 depending on the lens and even the metering mode.)

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    Re: Spot Metering Question

    Spot metering really comes into it's own for scanning a scene to see what the light level range is. With some mental arithmetic it's then possible to set and exposure and know with some certainty the degrees of over and under exposure if any. Once that is established it's often possible to pick something in a scene and expose on that basis. Trouble is that there is a need to know what the camera really makes of the readings when a shot is actually taken. Having used this metering on film cameras I always get the feeling that there is something not quite right on digital versions of it. It could be used as sort of remote spot meter and I suppose it could still be on portraits and that sort of thing once some one is familiar with the results it gives on a face for instance.

    John
    -

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    William W's Avatar
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    Re: Spot Metering Question

    I use Canon DSLR's.

    I use Spot Metering often: probably more than 50% of the time when I am shooting using only the available light.

    In these shooting scenarios I would be in M Mode (Manual Camera Mode), so any discussion for me about 'Exposure Locking' or 'Exposure Compensation' is irrelevant.

    I usually would take two and sometimes up to four Spot Meter readings from the scene and then calculate the exposure I require.

    So in simple terms, I use the Spot Meter Functionality of my Camera’s TTL metering system, just as how I would use normally my Hand Held Spot Meter - which doesn’t get used very much anymore.

    WW

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    Re: Spot Metering Question

    Quote Originally Posted by William W View Post
    I usually would take two and sometimes up to four Spot Meter readings from the scene and then calculate the exposure I require.
    Sounds like three readings too many to me. Why do you need anything other than +2.7 (varies with camera) over the brightest hightlight that contains detail you want to keep?

    I mean you just spot meter the brightest detailed highlight, center the meter, add 2.7 EV, done...take your picture. Is there more? When would you ever need to underexpose the brightest highlight with detail that you want to keep? As far as I'm aware, that just drops shadow detail and increases noise.

    Now...if you're taking spot meter readings for GND selection, that's different. But otherwise...just trying to understand what it is you're calculating.

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    William W's Avatar
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    Re: Spot Metering Question

    Quote Originally Posted by Graystar View Post
    . . .Why do you need anything other than +2.7 (varies with camera) over the brightest hightlight that contains detail you want to keep?

    I mean you just spot meter the brightest detailed highlight, center the meter, add 2.7 EV, done...take your picture. ...just trying to understand what it is you're calculating.
    I’d normally take only two spot meter readings (as I mentioned).
    These would be from ‘known’ Subjects in the scene upon which I could base a precise exposure. For example: skin tones; foliage; concrete are key ones I tend to use.

    The brightest highlight in the scene I want to keep might be the detail in white dress; or the detail in a black coat; or the detail in a mauve hat.
    I find it easier and quicker working with the skin tones or foliage which are in the same lighting condition as that particular item, than to be calculating (or guessing) what relationship that particular ‘maximum highlight item’ has to PHOTOGRAPHIC GREY.

    I would initailly take TWO meter readings to confirm - this is especially important when working in part shaded areas as one might be comfortable with blowing SOME of the skin tones, particularly in a back lit shot.

    Occasionally, I want to keep the shadow detail, at the expense of more of the highlights being lost: that’s where I might take three or very occasionally four spot readings.

    Perhaps two on various highlights and two on the deeper shadows, thus allowing a quick thumbnail of the EV Range of the whole scene and enough detail to make an informed choice of exposure with a pretty good idea of what will and will not be recoverable in Post Production.

    ***

    I do not fully understand is this comment of yours:

    “Why do you need anything other than +2.7 (varies with camera) over the brightest hightlight that contains detail you want to keep? . . I mean you just spot meter the brightest detailed highlight, center the meter, add 2.7 EV"

    What DO understand about this statement is one can (should) expose to the right and recover highlight details in post production and the amount of that ‘over exposure’ will vary slightly between cameras (sensors).

    What I DO NOT understand about this statement is:

    We are specifically discussing Spot Metering and the comment seems to imply that NO consideration is being given to the COLOUR of the ‘highlight’ being Spot Metered – maybe that is understood by you, but it certainly is not clear to me in what has been written.

    WW

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    Re: Spot Metering Question

    Quote Originally Posted by William W View Post
    I do not fully understand is this comment of yours:

    “Why do you need anything other than +2.7 (varies with camera) over the brightest hightlight that contains detail you want to keep? . . I mean you just spot meter the brightest detailed highlight, center the meter, add 2.7 EV"
    The brightest highlights with details that you want to keep, as opposed to, say specular highlights or highlights that contain detail, but that you've determined you're going to allow to clip.


    What I DO NOT understand about this statement is:

    We are specifically discussing Spot Metering and the comment seems to imply that NO consideration is being given to the COLOUR of the ‘highlight’ being Spot Metered – maybe that is understood by you, but it certainly is not clear to me in what has been written.
    It's true, I was presuming whites as with clouds or clothing, and I didn't make that clear. Rich colors are difficult to meter, and you really have to have your exposure knowledge down pat...for example, bright red gets no adjustment. I made the following color samples and used it to get an idea of what 1 EV and 2 EV differences look like...
    brightnessscale.jpg


    Okay...I get the picture, so to speak (although I think you were stretching it there a bit with brightest highlight being in a black coat ) We just have different styles. My preferred exposure method is to meter my gray card, as it takes less than 10 seconds to set exposure. But that only works in constant-light conditions, and only if I'm standing in, or can get to, the same light that's falling on the subject. If I can't get to the light, then like you said, skin tone, foliage, blue sky...the traditional, naturally occurring references have to be employed. Finally, as I described above, if the light is changing then I have to determine the correct exposure using the subject as a reference and employ that as an Exposure Compensation to automatic exposure. With EC set, metering the subject always gives correct exposure even as light changes before my eyes.

    You mentioned backlighting and shade, and that's part of a broader subject I never see discussed inclusively...that of having two light sources to deal with. There are always forum discussions on backlit subjects, sunlit areas with shadows, indoor shots with blown windows, sunset shots, overcast days, etc...and they all fall under the subject of scenes lit by two light sources. Exposure can only be set correctly for one light source, and the human eye pulls some neat tricks to make the lighting appear more even than it is...confusing photographers even more. I think this is where we differ in styles. You'll take 4 readings, whereas I'll set exposure for the subject's light. But I'm usually street shooting, so that obviously has a large effect on my approach to exposure. I just don't have the time for a detailed evaluation of fleeting scenes. I have to use fast techniques that work. That's why I use exposure lock with a gray card and natural references, and if I have to I'll make EC decisions on subjects on the spot.

    And of course, as you know it's more difficult than using Matrix or Evaluative metering...but few things worth doing are easy!

  17. #17
    xpatUSA's Avatar
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    Re: Spot Metering Question

    Quote Originally Posted by Graystar View Post
    Nikon and Canon are the same in this respect. If you spot meter a neutral surface, take a pic, and demosaic with no additional processing other than white balance, you get sRGB values of 100, 100, 100 from either camera. That's about 12.7% gray (it also varies +/- 10 depending on the lens and even the metering mode.)
    If anyone should wonder why that isn't 18% gray, the well-respected Doug Kerr beats the subject to death here:

    http://dougkerr.net/pumpkin/articles...eflectance.pdf

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    Re: Spot Metering Question

    Phew! Does this make it easier? Let's suppose William is taking a photo of a sunset. On Manual setting he aims his camera at a part of the sky away from the sun. He checks the exposure numbers, speed and f/stop. Then rather than "locking in" that exposure by pushing a button, he sets his camera to those settings, speed and f/stop, and then leaves those settings there even though his camera is screaming at him to change the settings when he re-aims his camera back toward brighter setting sun. Therefore, too, his exposure value +/- setting, also remains at the darker sky setting when the camera points back toward the sun.

    Another point in Graystar's section, Changing Light in his original post: With the Nikon (maybe Canon, too), the focusing can be switched to be a function of the AE/AF Button. So, pushing that button sets the focus, keeping it held down allows you to click the shutter at any time and still have the focus set. No need to push the shutter release button half way.

    Example, lets supposed you are shooting a soccer game. You push the AE/AF button to focus on a player and suddenly another player runs between your subject player and the camera. The camera will not try to focus on that in-between player running by. You have already locked in the focus on the original subject player.

    BTW Very informative thread. I had always assumed that the spot-metering followed the focus point. Luckily I have a Nikon!

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    Re: Spot Metering Question

    Quote Originally Posted by rambler4466 View Post
    Does this make it easier?
    Turning dials while watching a little indicator moving vs. just pushing a button...I'm gonna have to vote "not easier" on this one.


    Example, lets supposed you are shooting a soccer game. You push the AE/AF button to focus on a player and suddenly another player runs between your subject player and the camera. The camera will not try to focus on that in-between player running by. You have already locked in the focus on the original subject player.
    I tried AF-ON...didn't like it. Couldn't see the logic in pushing two buttons to take a picture 999 times out of a thousand so that the one time I don't want to focus, I only need to push one button. As for your example...if I half-press to focus on a player, the camera won't try to focus on the player running by if I just lift my finger a little. And owners of Nikon pro bodies (and now the D7000 and D600) don't even have to do that...there's a custom setting that will delay the activation of focus when obstructions momentarily get in the way (pro Canon bodies have it too.)

    In any case, I don't have an AF-ON button on my D90, and AE Lock is too valuable to me. But it's a personal choice, and there are lots of people who prefer AF-ON.


    Luckily I have a Nikon!
    Now THERE'S something we can agree on!

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