Understanding metering, part two: what to use, when

In part one we examined why metering is important, and how the basics of how meters work. In today’s article, I’ll take a closer look at the different types of  metering, how they differ, and under what situations they should be deployed.

metering-viewfinder

A sample viewfinder – in this case, a rough representation of the Nikon D2H/ D2X finder.

With that background out of the way, let’s look at how the various metering options work, and what typical situations they might best be deployed under. Cameras typically have three options, or some variation upon that. Within these options, it’s also usually possible to fine tune various aspects of the meter’s operation. I’m going to leave out handheld meter operation since this is something that’s almost never encountered today. An important point to note is that all meters can be fooled by situations of uniform luminance, so don’t trust the readout blindly. Remember, meters function by averaging the entire evaluated area out to middle gray; this means if your evaluated area is meant to be black or white, you’re going to need to add or subtract some exposure compensation. For predominantly light/ white scenes, you need to add; for dark scenes, subtract. This holds true for every one of the different metering methods detailed below.

Average
The simplest form of metering evaluates the frame as a whole, and tries to expose it to middle gray – under the assumption that there will be shadows and highlights, but these will average out. Seldom used today because you will almost always require exposure compensation (making it unsuitable for the point and shoot crowd which constitutes most of the global camera market), but has the one enormous advantage of behaving predictably under every situation.

Spot
The simplest form of meter is the spot meter. This evaluates luminosity at the desired point only, ignoring everything else in the frame. There are two important things to be aware of with a spot meter: the location and size of the spot. The metering spot’s location is either in the center of the frame, or tied to the selected or active autofocus point; the logic there is that you would typically want to ensure your subject is both in focus and properly exposed. Variations on the spot meter include types that are biased for highlights or shadows – i.e. you meter a shadow or highlight and it doesn’t turn out over or underexposed. Don’t forget to add appropriate exposure compensation.

The size of the spot is also very important – don’t be fooled into thinking that it’s a tiny, precise eyedropper the same size as your autofocus area box – it isn’t! Most consumer cameras have a spot size that’s about 2.5% of the frame area, which is actually quite large – imagine your frame divided into six vertically and horizontally, i.e. a grid of 36 boxes; a 2.5% spot meter is the size of one of these boxes. Professional cameras might have a 1% spot meter; imagine a 10×10 grid of 100 boxes, and this is pretty much what you’ve got. In our sample viewfinder above, the cyan box is a 1% spot meter, tied to the active (red) AF point. Keep this in mind as you’re moving it around. If your spot meter is tied to the center of the frame, then you’ll need to assign another button – perhaps the shutter half press – to lock exposure once you’ve metered for your subject (unless it is of course dead center, which is highly unlikely).

The obvious question would be why spot meters aren’t smaller – firstly, you don’t actually want them to be that acute, otherwise moving the camera by a fraction of a degree might yield a vastly different (and incorrect) exposure – they’d be too sensitive to use. Secondly, some averaging is still a good thing – you can move the camera around a bit until the spot falls onto the right mix of light/ dark to give the desired exposure. With practice, this can be much quicker than using exposure compensation.

Use the spot meter in situations where your subject is in very different light to the rest of the frame – either much brighter or much darker – in order to ensure that the focus of your shot is properly exposed. It’s great for high key or low key images – put your subject in the shadows or highlights respectively, and spot meter there – or even general situations under which the luminance of your composition doesn’t vary that much. I don’t generally use it for street photography or fast moving situations, because it requires precision and/ or a little meter-and-recompose dance that can cost you valuable time.

One tip: the way I use the spot meter is always either covering my subject, if the subject is darker than the rest of the frame; or, on the highlights plus a bit of dark area if your subject is lighter than the frame. This effectively tricks the meter into adding a bit of exposure compensation to average out the bright/ dark areas – you need to do this to prevent your highlights from falling into middle gray and consequently completely losing your shadow information. It also adds a bit of speed in operation since you don’t have to muck around with exposure compensation.

Spot meters only came about when the metering cells in cameras could be made small enough to evaluate only a portion of the frame; they’re common now because our metering sensors are made up of hundreds, if not thousands, of discrete individual elements.

Centerweight
In our sample viewfinder, the circle around the center AF point represents the centerweight meter area border. That sounds like a bit of complex mouthful, but in reality it’s not. A centerweighted meter divides the frame into two areas – the circle in the middle, and the border. The circle in the middle is presumably roughly where most subjects are going to be framed, which in turn you would like to expose properly etc – it is metered separately from the border area. The two metering values are combined in a predetermined ratio – usually 70-30 in favor of the central portion, sometimes 60-40 – to determine the final exposure value.

Centerweighted meters are the predecessor to matrix metering – they try to average things out over the entire scene, and make a sensible assumption or two about what you would like to expose for. Modern cameras allow you to change the size of the center area – the D800E, for instance, allows a spot anything between 8mm and 20mm in diameter. The default center area is usually etched onto the focusing screen for reference. Note that centerweighted metering was the successor to evaluative metering, and shares its advantage of predictability: if you put your subject in the circle, chances are the exposure will be right; the advantage it has over evaluative metering is the ability to bias the exposure towards your subject.

In situations where spot metering would not be suitable – action, for instance – I actually prefer using centerweighted metering to matrix in unfamilar cameras; at least I have some idea of how the meter will respond. There’s nothing more frustrating than missing a shot to over or underexposure because matrix metering has gotten things very, very wrong.

Matrix
Matrix metering is either a miracle or a curse, depending on where you stand. For those who don’t want to take control of their cameras, matrix metering provides a higher ‘hit rate’ than evaluative or centerweight; the problem is, you have absolutely no idea when it’s going to get it wrong, and how much by. This can be rectified with experience with a certain system; as you encounter more situations, you get a better idea of when the camera is going to miss. It’s for this reason that the only time I use matrix metering in a situation where delivery is critical is when I’m shooting cameras I’m familiar with – the Nikons, and the OM-D. Everything else is either spot or centerweight.

That doesn’t of course explain how it works. The frame is divided up into a number of areas – up to 100,000 of them in the Canon 1Dx – and a reading taken of each area, for both luminance and color. The camera then either compares this to a database of similar situations (i.e. photographs converted into 100,000 or however-many pixel maps, along with exposure values) and then determines the exposure. If the camera can’t find a matching situation, then it makes an intelligent guess about what the exposure should be based on a combination of overall scene luminance, color, and the current AF point. With this many variables, it’s actually surprising that the meters get it right such a high percentage of the time – perhaps there are only so many possible luminance maps?

In any case, matrix metering tends to be more reliable in situations that don’t have extreme contrasts, or bright point sources in the frame, or very small subjects. Under quickly-changing circumstances, it’s the method of choice – it might get things wrong, but most of the time it will save you from having to move around the spot or use exposure compensation. For most users, matrix metering is sufficient, and you can always add or subtract exposure compensation and take another shot. It’s also worth noting that matrix meters that use the imaging sensor are much more accurate and reliable than those that have separate metering sensors simply because the tonal response characteristics of both match, making overexposure almost impossible. Presumably, these should also run some form of ‘expose to the right’ algorithm for digital cameras, but then again perhaps not as it would only be useful for RAW shooters.

I think considering some examples would be useful at this point. Let’s take a few of the images from my recent Introduction to Wildlife workshop:

metering-1-master

This image could be taken care of by either spot or centerweight; I have no idea if matrix would have been accurate or not. For centerweight, you would need to ensure the central spot is over the subject area, like so:

metering-1-cw

This implies a lock-exposure-and-recompose is necessary – or, perhaps not seeing as I intended to crop the final image to a more square aspect ratio anyway. You might wonder whether the 70-30 distribution – specifically the metered portion falling on the black water – would throw things out; in this case, actually it helped. The center portion would have metered the white bird to middle gray, i.e. too dark; the outer portion metered the black water to middle gray, i.e. too light. They averaged out.

metering-1-spot

We could also have used the spot meter, in a few different ways. For location A, no compensation would be required so long as we took a bit of the dark portion and a bit of the highlight portion – i.e. enough to average out to middle gray. Location B would have required some positive exposure compensation as it is a highlight, in zone VII-VIII or so. Location C falls in zone V anyway, which is middle gray – so no exposure compensation would haven been required. In this case, I would have picked location C if using an AF lens (I wasn’t) as it’s of both the right luminance value and subject distance – alternatively, the head would have been a good choice, too.

metering-2-master

Here’s our second example. This is a much trickier situation because of the thin rim of backlight around the bird; you don’t want to overexpose that else you’ll lose all tonal detail in the feathers.

metering-2-cw

You can see here that centerweighted metering wouldn’t work; the highlight areas – in this case, the subject from the meter’s point of view – is just too small. It would expose for the dark area and result in blown highlights. Spot metering, on the other hand, is ideal:

metering-2-spot

Location A is obviously nonsensical because although it might be the same luminance value as most of the bird, that isn’t the part we’re exposing for; using location A would result in huge overexposure. Location B is fine, and the highlight area is small enough that it wouldn’t require any exposure compensation since some of the dark background is also included – this is actually what I used – C and D are also workable options, though C might require a little negative compensation.

How about a few more examples?

_5014972 copy
Clearly, spot metering on the eye is the only way to go – all other options would have resulted in overexposure and both detail loss and an imbalance in the composition caused by the eye of the viewer not going to the intended area.

_5014910 copy
Actually, any metering option would work fine here – the scene is divided into relatively large portions of different luminance. If you used spot on the feathers, you’d have to add a bit of exposure compensation to keep things white; if you used center, you’d have to lock exposure and then recompose.

_5014793 copy
Our frame is fairly consistent in luminance, so once again, any metering method would work. However, all would require a bit of positive exposure compensation as the overall tone of the subject is light, and should be kept high-key.

_5015451bw copy
Small white subject against a dark background, intense contrasts – spot meter.

_5014828 copy
Here’s one situation where matrix metering would actually work better than the other options: you have relatively even luminance across the frame, a strong colored background (making centerweight possibly inaccurate) and a fast moving subject (making spot metering impractical).

Of course, knowing which metering method to use in a given situation is quite useless unless you have things set up so that it’s easy to switch between them; otherwise, pick one and get used to the way it operates. If you can lock exposure separately from focus, then you don’t really need to use exposure compensation most of the time – the spot meter is all you need. If you can’t be bothered to do the finger dance, well, that’s why matrix was invented. Needless to say: as ever, practice is the key to mastery. MT

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Understanding metering, part one: introduction

metering-2-master
An image from my recent Introduction to Wildlife workshop, and a very tricky metering situation – more importantly, do you know why, and what to do in a situation like this to achieve the desired exposure outcome?

One of the more important – yet almost always overlooked – aspects of camera operation is metering. Simply put, the meter determines what your final exposure is, and how bright or dark your image looks relative to the scene. Unless you are shooting manual – and even then – the camera’s exposure is determined by the meter. Add the fact that the eyes of a viewer tend to go to the brightest and/ or highest contrast portions of an image first (i.e. this should be your subject) – and it’s clear to see why it’s absolutely critical to understand both how metering works as a fundamental concept and any camera-specific peccadilloes that might exist. The last thing you want is to find that your camera drastically underexposed a once-in-a-lifetime shot of some critically important event because you didn’t know (or forgot) that the meter was extremely affected by point light sources*.

*This can actually happen. The meter in the Leica M8/9 is extremely sensitive to direct point light sources, and can often yield nonsensical readings of say 1/1000s ISO 160 for a shooting aperture of f4 at night – that’s because it’s picking up a street lamp. One can only hope the new M is less affected by this – the only solution to the problem I’ve been able to find is just go 100% manual at night.

How meters work
Depending on which exposure mode your camera is in, the meter will try to find a combination of settings that creates an image that averages out to middle gray in luminance, i.e. the histogram average is around level 127 or thereabouts. There are three exposure parameters the camera can use to control the amount of light reaching the image processor – note that the sensor is also now involved in the process – shutter speed, aperture and digital gain, i.e. ISO. If you fix any one of these variables manually – say by shooting aperture priority at a set ISO – then the camera varies the remaining parameters according to a set of rules in order to achieve the ‘correct’ exposure. If the correct exposure is out of adjustment range – e.g. the required shutter speed for a given aperture is too high – then you’re going to land up with an over or underexposed image.

In program mode, the camera controls both aperture and shutter values depending on its preset program; the photographer can usually shift the program to a different combination of values which still yield the same net amount of light hitting the sensor. In shutter priority, the user fixes the shutter value manually, so the camera alters the aperture. In aperture priority, it’s the other way around. In manual mode, the user fixes both values – the only thing the meter can do is display how far off the manually chosen exposure is from the correct exposure, or alter the ISO or flash. If auto-ISO is activated, then the camera will always default to the lowest possible ISO within the specified range in order to keep the shutter speed at or above a certain value – either user selected or 1/ focal length in second. (Note that for some cameras, using manual shutter and aperture values will cause the camera to shift the ISO rather than display the variance from correct exposure.)

Simple enough, right? So why are there so many different metering methods? My OM-D, for instance, has no less than five: matrix, centerweight, spot, spot low and spot high. The differences are down to the area of the frame the meter evaluates when deciding what the correct exposure should be. Note that in all situations, it will still try to expose the considered portion of the frame to middle gray – except this area might change. One uses the different metering methods in different subject situations. We’ll get into that in more detail later; first, there are a few more things that need explaining by way of background.

The meter itself is a photovoltaic cell, or combination of cells, whose output voltage over a certain area varies depending on how much light lands on it. The more light, the higher the voltage, which is translated into a brighter exposure. A particular chemistry’s electrical response is a fixed property of the material, and therefore consistent across different situations and cameras. Note that some meters require power to give a readout – this is because a base voltage must be applied across a semiconductor for it to respond to light, or to amplify the signal to a point where it gives an output that can be displayed – CCD meters are like this, for instance – other types of semiconductor photovoltaics do not require power because they already produce current on their own the minute light hits them. (Solar cells, for instance, fall into the latter category.)

Note that not all cameras have built-in meters; very early film cameras generally did not, and required the use of a separate handheld meter, or a particularly sensitive eyeball. My Nikon F2 Titan, for instance, comes standard with the unmetered/ plain DE-1 prism/ finder. Early Leicas are the same. A whole variety of hotshoe-based clip on meters are available, as well as handheld types. Modern digital cameras either use a separate metering CCD, usually located in the viewfinder (for an SLR) or use the imaging sensor (for any live-view based cameras) – this is obviously the most accurate possible method of metering given that the metering sensor also perfectly represents the response of the imaging sensor. (This was not always the case with film and separate meters; it was therefore highly important to know the characteristics of your particular chosen film.)

Incident vs reflective
All cameras’ built-in meters are of the reflective type. This is to say that they measure the amount of light reflected from the subject and hitting the camera; the advantage is that you don’t have to stand in the same light as your subject in order to obtain a reading – potentially problematic if your subject is say, a landscape that’s several kilometers away – but at the same time, they suffer from the disadvantage of not being able to obtain an accurate reading for very reflective subjects. Incident meters are always handheld (but handheld meters can be either incident or reflective) and are placed in the same light as the subject in order to obtain an accurate exposure reading. The photosensitive portion of the meter is covered by a matte white dome in order to ‘average out’ the light measured by the meter.

Exposure compensation
The use of exposure compensation is simply translated into an offset of the zero point of the meter. For instance, if you dial in +1 EV exposure compensation, then the meter will add this to the calculated exposure value before displaying the final settings.

Flash metering
Flash meters a slightly more complicated. There are two ways to determine how much flash power is required to achieve the correct exposure. The first is by using an incident light meter next to the subject, firing the flash, and setting the camera with the valued displayed on the meter. This is the most precise method, but again, is often impractical if you do not have time to repeat a shot. The second, more common method, uses a very short duration and low-power preflash of known output in conjunction with the reflective meter to determine how much additional power is required to make up the gap between the trial exposure and a correct exposure, with the given camera settings. The adjustment to flash power is made almost instantaneously and a second, correct power flash is fired along with the exposure. This entire process is so fast that there is almost zero added lag. The disadvantage again is that partially transparent or reflective objects may not be correctly exposed as the metering type is reflective-only.

Histograms and expose to the right
The exposure histogram represents the evolution of the light meter into the digital age. It not only shows you what the average exposure should be over the entire frame, but how that exposure is distributed. For instance, it is important to know whether you have one uniformly gray area across the entire frame, or say two halves of the frame divided into 100% black and 100% white areas. A simple exposure meter that evaluates the entire frame would give identical readings for both situations. However, in the second situation, you would probably expose for the highlight areas to prevent loss of tonal detail. This would actually result in a final exposure that is slightly darker then what the whole-frame evaluative meter would suggest. Learning to read a histogram, is therefore a very useful tool for digital photography. Histograms and digital actually come with two others very useful tools. The first is the ability to display areas of the image that are overexposed – usually in the form of a flashing highlights warning; the second, is the ability to redraw the histogram based on the specific area of the image displayed. Note that availability of both of these functions depends very much on the camera you’re using. Some cameras are able to display histograms and overexposure warnings for individual color channels, as well as overall luminance.

Metering is actually much more critical in the digital age, simply because of the tonal response characteristics of the imaging medium. With film, there was a degree of nonlinearity and reciprocity era which translated into a little bit of latitude in photosensitivity; for negative film, this may vary by as much as 1 to 2 stops: the same exposure with different batches of film, even if the same emulsion type, may not necessarily result in the same final luminance. Add variation in the developing chemistry to that mix, and you can see why having high precision wasn’t all that critical. (Slide film is a different story; it’s very sensitive to over or underexposure.) However, digital photography is nothing if not repeatably consistent. Two identical cameras with identical exposure settings will yield an identical image under any fixed given situation. Changing the exposure by as little as a sixth of a stop will be consistently visible.

There’s also one additional characteristic of the digital medium that we need to take into consideration. This is to do with signal amplification and noise, and also the origins of the ‘expose to the right’ motto. Exposed to the right refers to ensuring that the histogram graph touches the right-hand (highlight) side of the scale, but does not exceed it. The reason for this is to capture as much total information as possible, with as little noise as possible. Underexposure in a digital image may be corrected for by increasing brightness. This is achieved by amplifying the signal; doing so also amplifies any uncertainty in the signal, which translates into increased amounts of digital noise – obviously not a desirable characteristic in an image. The advantage of exposed to the right is that we maximize the amount of signal and minimize the amount of noise. The brightest tonal values in a digital image also contain the most information simply because of the way digital sensors respond to light. This translates into maximizing latitude for post processing, higher color accuracy, and less noise – in short, making the most of your image quality potential.

We therefore want to expose the image as brightly as possible, and then adjust the tonal map later in post processing – or do we? The reality is that in most situations this holds true. However, due to the nature of the total response of some sensors, there may be situations under which we do actually want to underexposed overexpose slightly in order to create a particular look due to the nonlinearity of tonal response. Of course, if you are a JPEG shooter and do not post process at all, you should expose at your intended final output level.

Note that this is much more of an issue for digital cameras than film ones, as the tonal response of film is non-linear – however, underexposure in a digital camera will usually result in undesirable noise when the luminance value is brought up to the desired level because it can only be done by amplifying a small signal. This in turn amplifies the uncertainty in the signal, i.e. noise.

White balance
One additional complication brought upon the digital photographer has to do with white balance and color temperature. Different colors have different luminosity values even under identical illumination; this is to do with the wavelengths that are reflected or transmitted to the imaging device, and their associated energy (luminance) levels. From a perceptual point of view, we see this as different brightness**. White balance is an important setting that comes into play here: it acts as the zero-offset point for color, effectively adding or subtracting different amounts of exposure compensation from the various channels to compensate for the ambient light. (This is how whites can still be rendered as white under extremely warm incandescent light if the correct white balance is used.)

**Nikon’s color matrix metering system has long compensated for this by using a metering CCD that had a color filter array over the top, both to aid scene recognition as well as increase exposure accuracy when presented with strongly colored subjects – for instance, yellow objects always render brighter than reds or purples of a given reflectance even if they’re illuminated under identical light – the color matrix meter compensates for this by increasing or decreasing the exposure if a scene is predominantly of one color or another.

We have several considerations here. The first and most obvious is of color accuracy – even so, this can be compensated for with the eyedropper tool in Photoshop providing we can find something white in the frame to set as a baseline. The less obvious problem is to do with individual channels. If the white balance is incorrect and a channel is overexposed, there is no way to recover this information afterwards. It is therefore important to set a white balance that is in the right ballpark – it doesn’t have to be perfect – to avoid this. Similarly, extreme underexposure of a channel will result in a lot of noise when compensated for afterwards. Generally, the auto white balance function in most cameras will get you in the right ballpark, but you will need to make adjustments afterwards.

The auto white balance function actually works in a similar way to an exposure meter – except instead of trying to average the scene out to middle gray in luminance, it tries to average out the scene to a perfectly neutral color.

The confluence of exposure metering and autofocus
As if the whole metering thing wasn’t complicated enough, DSLR manufacturers have started to use the metering CCDs to aid autofocus – after all, it’s an additional source of information that can be used to help track subjects especially when the mirror is down, and the main imaging sensor is not available. The flow of information is two way and affects both autofocus and metering. The autofocus system uses the spatial and color information from the metering sensor to track subjects by color and location across the frame, especially if they move out of coverage of the autofocus sensor array – the metering sensor always covers the entire frame. The exposure meter uses the autofocus information to determine which area in the frame is being focused on, and is presumably the subject, which the photographer presumably wants to have correctly exposed – in matrix metering mode, exposure is thus biased towards whatever subject is underneath the active autofocus point, or points.

I’m sure you can see there are a lot of presumptions involved. This of course means that the camera doesn’t always get either exposure or focus right when left to its own devices; the metering sensor may lack the resolution to distinguish between the desired subject and another similar-looking one, resulting in focusing errors; or the meter may be too heavily biased towards the area under the active focus area and thus yield erroneous exposures. A situation in which this might happen is say if your subject is much larger than the active focus area, and of a different luminance value. Anything small and reflective almost always causes problems, too.

The bottom line is that it pays to take control of both your meter and focusing system: without this, you can never be fully certain of what your camera is doing; I seldom use auto-anything especially with DSLRs since they do not meter off the imaging sensor (unless in live view, of course).

To be continued in part two! MT

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Visit our Teaching Store to up your photographic game – including Photoshop Workflow DVDs and customized Email School of Photography; or go mobile with the Photography Compendium for iPad. You can also get your gear from B&H and Amazon. Prices are the same as normal, however a small portion of your purchase value is referred back to me. Thanks!

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