Camera metering, what it's doing, and why it's dumb.

[arthurking83]

A brief how too on using your camera's metering feature.

All modern cameras have a meter built into them.
Most seem to do the same thing with their various modes, and the type of lighting you are using will determine how the meter works too.

This info is about non flash/strobe photography only.
I have limited to no knowledge of flash photography and the way the camera meter is used in that scenario.
(anyone with extensive knowledge on that topic is free to add any info they want too)

Also, for those that find any concepts in this thread hard to grasp, feel free to ask questions.

I'll try to keep it as simple and understandable as I can.

From the little that I know of other(non Nikon) brands, they all appear to operate in similar ways.

Firstly, camera makers seem to call their metering modes by various names.

Nikon use Spot, Centre Weighted and Matrix as they're three metering modes.
Other names for these can be Overall, Evaluative, Multi or Multi Pattern, Pattern, Partial, Zone, .. and probably a lot more.

Matrix metering is the same as Evaluative, or Multi/Multi Pattern, or Zone.
Some cameras have more than 3 different metering modes, for eg. Canon seems to have 4, with a mode called 'Partial'.
What this is and does is something I know nothing of.

What is important is that you know and understand what each mode does.

For this thread tho, I'm going to stick with the two modes that I use most which are Spot(99.9%) and Matrix(0.1%) of the time.
Centre Weighted I occasionally use if I use the speedlight, which is rare in itself, so I may use CW mode 0.00001% of my camera usage.

My second priority is to help those that don't fully understand their metering system, and to try to explain how a camera meter is not quite smart, or as I prefer to describe it .. quite dumb.

We'll start with spot metering mode, as this shows more than any other mode how dumb the metering system is.

two images that clearly demonstrate this point are:


109280 ................... 109278
................ #1 Spot metering with 0Ev exposure comp .................................. #2 Spot metering with 0Ev exposure comp.

Two seemingly different exposures yet using the same exposure control(metering more) and exposure level(compensation set to 0), yet the exposures are wildly different.
As is obvious in these images with the red squares .. spot metering was read off two different colours, or as the metering system see them .. tones.
What's obvious is that in #1, the metering is taken from the black area and with 0Ev compensation, and #2 is read off the white area, with the same 0Ev compensation.
if the camera's meter was 'smart' it would know what I know, and that is that one box is black, and the other is white and take this into account and adjust itself accordingly.

BUT! it's not smart, and now I have to get off my lazy pain inducing posterior and do some of my own adjustments to the camera to account for it's lack of a brain.
it should be fairly obvious what needs to be done, but for those that are unsure, the solution is very simple.
Exposure compensation!

When the camera's metering sees the scene, it really only sees everything more or less in black and white. That is a series of tones.
Apparently some, most or all Matrix metering settings take colour into account .. but from my experience(and Nikon is supposedly one of the better systems) it doesn't seem to understand deep blue skies very well.
matirx metering and landscape photography are a pair of concepts that work well together to drive me raving mad.
I think I may remember once where I used matrix metering .. and that my have been out of forgetfulness in returning back to spot mode.

So(trying to describe this part in brief) .. if I'm smart, and want to use (my preferred) spot metering mode, I have to set exposure comp depending on the area that I'm metering off.
if I'm photographing people, I use 0 compensation, if I'm doing landscapes, I'm looking for either shadows or highlight that are important not to destroy.
If I'm not 100% sure which in landscape shooting, I meter off green grass and use about -0/7Ev(gets a more saturated colour rendering), or if yellow dry grass, anywhere from +0.7 to -0.7 depending on time of day and overall light conditions. The point is I'm making my choice of exposure based on the meter reading knowing how it will render the colour I'm most interested in.

These aren't hard and fast rules, and each camera model will even determine the amount of exposure compensation I use. Higher ISO levels also play a factor in this too. the higher, the more I tend to exposure(set +ve compensation) .. but I still tend to prefer spot metering through all of this.

A bit of a gotcha to be aware of too.
Not all cameras metering systems work the same too. Nikon cameras will spot meter off the focus point you have chosen to focus with, so if you use the central one or if you use a peripheral focus square, they all still spot meter from that point. I've yet to come across a Nikon camera that doesn't work in this way.
But I've read that some cameras don't operate in this way(Canon :confused:) so if you're in spot mode, and want to use it, be sure to know how it works.

That is, if you're shooting portraits in a very dark(is. black) or bright(i.e. white) environment with a camera that doesn't spot meter from the chosen focus point(i.e. it spot meters only from the central point), and you are using a periphery point for focus of the person, then you would make the necessary adjustment to the meter to get the correct exposure, or use an alternate metering mode.

Also note if you use manual lenses the results can also vary wildly too ... but this isn't yet part of this writeup .. we'll save that for a later reply.

Now onto exposure compensation and how to smarten up the metering system.
below are the same two images as above, the difference is that exposure compensation has been set for one image to -2Ev (still in spot mode), and the other is set to +2Ev (in spot mode)

#3109279 #4109277

So in #3, as the red square is still set in the black area I set it to -2Ev, and conversely for the image with the red square set to the white area I set compensation to +2Ev.

As the meter only sees in terms of grey tones, metering with a 0Ev exposure setting means that the camera will try to render that area of the scene as a grey colour.
if the meter were truly smart, even in spot mode! .. it would still use the colour matrix system to oversee the entire scene, make a determination that there are different tones(darks lights and mid tones) and then set it's own version of smartness to the exposure setting.
As it is, this doesn't happen.
So by manually setting the exposure compensation what I'm in effect doing is resetting the meter to tell the camera that this is a darker or lighter tone that it thinks it is.
if I don't set any exposure compensation the camera thinks that I think it should be a grey mid tone.

The exact amount of compensation is not actually important, and even varies across different models from the same manufacturer. This is because of different sensor technologies, and the way the manufacturer decides is the best way for certain cameras to render the scene. These decisions could be to minimise noise/grain levels from certain cameras.
And also be aware that some lenses see differently and transmit different amounts of light through to the sensor.

But in the two images above, for #3 I set -2Ev compensation to tell the camera that this is a black area(ie. make it dark .. not grey as it tried to do in #1.
In opposition, for #4, I set compensation to +2Ev, which of course is the opposite direction in that I'm forcing the camera to render that white area white .. and again not the grey it did with 0 compensation.
(I hope this makes sense to those that are unsure).

There is another thread that talks about Exposure values or Ev levels. This is not the same thing.
These images are all shot under the exact same Ev. My dinky little study with those atrocious CFL lights that have terrible CRI values!
The only processing on these images has been to equalise WB to one point. WB variance has an affect exposure levels to a degree too.

The Ev of the scene is between 5 and 6(1/3s and f/4 @ ISO 100). This is inconsequential to the topic of the is post.
My use of exposure compensation in the above images is to simply tell the camera that the tones I want rendered are not what it thinks they are.
Like I said the camera is quite dumb and doesn't know this(well at least Nikon cameras are!). I don't know what other cameras do(ie. Partial mode on Canons) so your camera may be different.


But in saying that, for many situations the camera's meter isn't as dumb as it may initially appear(or I thought it was)! :p
This next image is just about spot on, and when compared to the other two images that I thought were rendered just about spot on for their respective spot selection point Matrix mode at 0Ev compensation turned out quite well.
109276

The way I understand this scenario is that the matrix meter chip/sensor saw the scene as both black and white and compensated for both tones.
Nikon's supposedly use a colour sensor for their matrix chip, but as I alluded too earlier, Nikon engineers may need a refresher course on what constitutes a deep blue sky scene!

So you would assume that this is the utopia that you were chasing. But I can tell you 100% for sure and without hesitation that this is far from the truth.
While it does an admiral job of trying to get a balanced exposure, in this instance(#5) this was only true because the dynamic range of the scene was easily within the range of the camera.
FWIW, the last three exposures are well within a range that could be described as consistent exposures(about 1/3Ev or less).

Where Matrix fails(for me) is in the fact that it can vary so much between similar scenes. It makes no difference if that scene was a portrait or a landscape scene, I find too much variability of the exposures. Hence my preference to use spot metering for the majority of any photo shooting I do.
This isn't to say that Matrix is no good .. it may work perfectly for you, as will Centre Weighted .. that isn't the point of the thread.
The real issue is that you are reliant on what the camera engineers think makes up a good overall exposure for the scene.
Go from a Nikon to a Canon camera using a similar lens, and the exposures will most likely be different if you rely on matrix metering.
From my memory of Canon matrix mode(years ago now .. it seemed to be more biased to protecting the highlight areas at the expense of shadow areas.
Nikon's also do this to a degree, but not as hard and fast as I remember the Canon to do.
(no offense or derision is meant towards Canon here .. this is just an observation I made from playing with a few Canon cameras years ago .. it may well be different now anyhow)

Had a chat with a member a few weeks ago and one of the topics that came up was about metering and exposures. The idea of this topic came to me way before that as well, which was actually over a year ago as a response to a thread I've completely forgotten about somewhere in the archives of AP.
So the idea of this post is to help those that don't fully understand their camera's metering system, and the way it relates to exposure control for a reasonable output.

Also note that this isn't the definitive how too on the topic of camera meter systems or exposure and exposure control.
It's meant as a (hopefully) simple way to describe as much as possible this topic, and hopefully to get those that don't fully understand it all to ask questions relating to their camera's metering.


The reason that this is important for many people is when they rely 100% on the cameras metering system to expose the scene.
And for those that think Manual mode is the only way you have true control over exposure, they may have to think again!
This fact is only true if you also don't rely on the camera's meter for the exposure!
That is, you shoot in Manual mode AND either use external lighting and or external exposure meters.
if you shoot Manual and adjust the exposure settings in the camera relative to the camera's meter .. you're not really using Manual mode. You're still relying on the cameras automatic meter for your shots!
Manual mode in this scenario only ensures that the cameras exposure settings are consistent from one exposure to the next .. for example if you are doing a multi shot panorama and other situations where consistent exposure is required.

Hopefully, this thread will help some members to understand some of the concepts that are important in photography. exposure isn't the only important concept, but once you have this technical concept under control, you can forget about it as something to concern yourself with and concentrate on the other aspects that create good photography.
Once you understand exposure and metering(and even how your camera exposes and meters) ... it becomes second nature to you and you're free to think about other stuff.

ps. questions welcome .. and no question is to silly to ask. The chances are that if YOU are thinking of it others are probably wondering the same point too. So if you have any questions just ask.

[Gonk0]

Hey AK your attachments aren't working, I get the message: Invalid Attachment specified. If you followed a valid link, please notify the administrator (http://www.ausphotography.net.au/forum/sendmessage.php)

[nimrodisease]

Thanks for the write-up... I don't have time to read it all now, but will certainly come back.

Unfortunately the attachment links don't seem to work though!?

[arthurking83]

Yeah! dunno what happened looking into it now.


... updated ...

I think I sorted it(hopefully)
For some odd reason, in the few seconds between previewing and hitting the submit button, all the attachments simply vanished.

[Gonk0]

The attachments are now working AK, and thanks for the explanation on exposure compensation and how it all works with the cameras meter.

I will now have to do some testing of my camera to see how it all works together, as I usually just use evaluate which is canon meter mode, but I have just started to use manual mode
when taking some long exposure photos and adjusting the exposure to suit the shot at the time, but still using evaluate as my meter mode, which I will have to broaden my mind and step out
of my comfort zone and try the others that are available to me.

I was a great write up and very informative for a beginner like myself, I thank you for taking the time to do a writeup like this for everyone.

[arthurking83]

I think I may have made some not too flattering comments about manual mode.
But the truth is I still like Manual mode and you have pointed out another reason to use it ... longer exposures.
Of course you can't technically do longer than 30sec timed exposures, but Manual mode does allow the use of bulb mode.

For manual and bulb mode operation it doesn't really matter which metering mode you use .. the meter at such long exposures is probably redundant anyhow.
Meters only work down to a certain level .. usually about 0 to -2Ev depending on camera model.

Depending on the lens used .... for eg. if you're using an f/2.8 lens a 30sec exposure means that light levels are at about -2Ev .. basically the borderline of a functional camera meter or not.
But the fact that the camera may be reading a shutter speed of 30sec doesn't necessarily mean it's an accurate reading.
(this assumes an ISO setting of 100 too tho).

What you would be doing in such conditions, is to set ISO to a much higher value .. ISO1600 is a good random value to use.
You would then get a meter reading based on ISO 1600 and reduce the shutter value by the correct amount of stops from ISO1600 down to ISO100(5 stops).
So if your camera said a shutter speed of about 1sec(at ISO 1600), you'd compensate shutter by 5 stops too.
So from 1sec increased by 5 stops means a shutter speed of 30sec if you set ISO back to 100.

The purpose of this exercise is to show that metering could still be made even if the shutter time for the exposure exceeds what the camera can do automatically.

So if you got a meter reading (a big IF) while you set your camera to ISO25600 and it read out a shutter value of 1sec and f/2.8, you can then extrapolate that into a bulb time if you wish.
Once you do the math with that one you'll understand.

I used to do this with the D70s regularly in those days.
Even for exposures that only needed something like 20 or 25sec, I'd get a flashing -- indicator in the viewfinder showing that the cameras meter rating was exceeded for the conditions(ie. too dark).
So I'd bump ISO to 1600, take a reading and compensate for the number of stops to get a bulb reading. Of course it was too dark to read my watch so I'd just stand there counting 1banana, 2banana, 3banana, 4 ..... etc, etc.

[Steve Axford]

I read your article with interest, Arthur. I think I probably use similar methods to you. My camera is usually set to spot metering, though I will change it to evaluative sometimes for landscapes. I will then use exposure compensation on Av to correct for the camera's light meter, and then sometimes switch to manual to get consistent exposures. Macros usually needs manual as the camera's meter can be way off and I may need anything from +1/3 to -3 to get the correct exposure. It will also change a lot as I move the camera a little bit, or even just refocus, so manual keeps the exposure consistent. With macros I use the rule "better too dark than too light". I also try to have the subject at about 70% brightness rather than close to 100%, even if it is the brightest object in the frame. It looks better that way. Just a note. If I am taking pure red or yellow subjects, I need to underexpose as the meter AND the histogram are wrong for developed sRGB images. They are ok for AdobeRGB, but not sRGB. Maybe that's a Canon issue, maybe it's an issue with colour gamut - not sure.

[bcys1961]

Thanks for the article Arthur. Very informative as usual.

So where does the AEL/AFL button on the back of my camera fit into all this and how should I use it?

[bitsnpieces]

It's meant to lock the exposure you've decided to use.
If I may intrude this wonderful post from arthurking83 a little and add input with regards to the spot meter, and also the use of AEL (feel free to correct me if anything else)

This is only an extreme example to show how the AEL works

Here is the original subject, but it's darker than it should. Stars are black nonetheless. You want to meter it to a neutral gray. The middle is just for reference so you can see how exposures change. Also, imagine the right hand side is closer to you than the left, so focus comes into play for this example.
109314

So we want this guy to be brighter to be correctly exposed.
109316

So what you see here is that the stars are still all black, which is correct because that's their colour. The tones are now at the correct exposure, meaning lighter in colour (top right is the correct tone). You'll notice the bottom right, it's not completely black, that's correct - it's a dark colour, but not completely black. The top left is also not completely white, just a bright colour. The designer wanted the stars to be that strong black that stood out regardless of the dark corner - again, extreme example so you know how AEL works.

Now, what I want to do is focus on the bottom left star, but I want the image to be bright, and have the bottom right corner correctly exposed. 1 is to manually brighten up the image whether by ISO, aperture, shutter speed, exposure compensation, whatever you want. 2, is to spot meter to the bottom right, so that becomes the grey, but meaning everything else will be brighter also because it thinks everything is dark gray like the spot you meter, when in reality, it isn't.

109317

So as you can see, the bottom right is now correctly exposed, but everything else, including stars, are brighter due to the necessary brightening it believe it needs. That's how spot meter works - takes one spot, thinks that's the whole image when it isn't.

Our goal now is to focus on the bottom left star. We like this new bright look, we want to focus there. What happens when we move the camera to focus there?
109318
The spot meter sees the new change and will meter to that, making it only a little brighter as it sees that that portion wasn't ask dark, didn't need as much brightening. But this isn't the exposure we want.

So this is where AEL comes in.

We put it back to the bottom right, get the exposure we want, lock it in with AEL, now, move to the bottom left, what happens?
109317

It still keeps the exposure you wanted, because that's what you've locked it to - it won't change.

We now focus on that bottom left star with the exposure we want:
109319

The other stars a blurry (I used MS Paint so yeah yeah)

This is handy outside as maybe you have a very bright sun in the background, the camera picks that up, your model is now darkened. Spot meter to the model, and the model is fine, but everything else is lightened, so the background is now too bright, so you spot meter to the grass instead, or pavement, which is relatively neutral. So the background sky is still bright, but isn't overblown. The model isn't perfectly exposed, but she isn't a silhouette. AEL - lock that exposure in, now move back to the model, focus on the model, pop up the flash and bang - now she's exposed correctly and the background isn't overblown in colour.

In short, AEL locks a exposure preference you have made and then allow you to focus on any part of the image without it remetering that portion. Hopefully it was understandable. If not, hopefully arthurking83 had more time to provide better samples and a better explanation. :P

I haven't noticed a AFL on the Sony cameras, either that's just another term for AEL or something else, I don't know. If it's just another term, then same as above. If it's Auto Focus Lock, I believe it's a similar concept but for focus instead.
You pick a point you want to focus on, lock it, then compose your image whilst that point is still in focus, and you can do all the fancy changes you want to your exposure, white balance, etc.
I think Sony has this, just isn't labeled and you have to push/hold a button near the lens - I have to check again.

P.S. I probably could have had more accurate colour examples using Photoshop and just using the exposure settings... Too lazy to open up Photoshop, yet willing to go through the hard yard on Paint... Oh life...

[bcys1961]

Thanks David.

[shaneando]

Very interesting topic. Not sure which Nikon cameras you are using Arthur but the lower end models, up to and including the D7100, definitely put a bias on the exposure in matrix metering based on what is under the chosen focus point.

I've seen up to a full stop of variation in the same scene by simply selecting a different focus point. Something to definitely be aware of.

Also there are apparently some subtleties with spot metering and which focus mode you use. Thom Hogan has some details, I'll try to dig them up.

Shane