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Posted in : Featured / Photography Principles / Shooting Film on March 11th, 2012.
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In our last article, we discussed how to meter using your in-camera metering systems, the differences between the different modes and when some are more advantageous than others.  We also illustrated some of its advantages and disadvantages, and where it may possibly be able to get confused.

One of the things we really go into, that will almost certainly confuse your camera’s meter, is flash.  This is essentially because your camera’s meter is only looking at the ambient light in the scene through the viewfinder.  As the flash isn’t going off constantly while the camera’s looking at the scene, it doesn’t know that it needs to account for it.

This is one of the circumstances under which a handheld incident light meter can prove extremely useful, although it is not the only time when it can prove to be an advantage.

So, how does an incident meter differ from a reflective meter?

Where a reflective light meter’s sensor looks at the amount of light reflecting off the surface of an object through the viewfinder, in order to determine an average exposure, an incident light meter actually looks at the light being emitted from a source and the sensor measures how much light is actually hitting it.

So, are these just for flash then?

Most light meters offer 2 or more different modes for measuring light – some have several advanced modes for metering all kinds of different types of lighting setups & combinations, but pretty much all of them have these two in common – one for measuring flash, and one for measuring ambient light.

The ambient light meter is as it sounds, it looks at the ambient light in the environment and feeds back a reading.  Typically you can measure this in one of two ways, either aperture priority or shutter priority.

In aperture priority mode, you tell the meter what ISO you’re using, what aperture you want to shoot at, it measures the light and tells you what shutter speed to use.  This is handy for situations where your depth of field is important, and you want to focus in on specific parts of your scene while letting the rest drift off out of focus.

In shutter priority mode, you tell the meter what ISO you’re using and what shutter speed you want to set and it tells you what aperture to use.  This is useful when you’re trying to either freeze or capture motion.  If you want to shoot at 1/4000th of a second to stop a runner in their tracks, or pan with the runner on a slightly longer exposure to give some motion blur to the background, shutter priority is where you want to be.

One thing to remember is that 18% grey means nothing to incident light meters.  That’s solely to do with reflective light meters and how your camera is attempting to see the light.  You’re only ever really going to get accurate metering on a camera’s reflective meter if your subject is actually holding an accurate 18% grey card (let’s ignore histograms for now).  Incident light meters just look at the amount of light hitting the sensor, and give you an appropriate reading based on the information you tell it (ISO and either aperture or shutter speed).

So, how would I use an incident meter differently than the reflective meter in the camera for ambient light?

As incident meters are looking at the amount of light they get hit by, that means that they need to be positioned where your subject is.  So, if you’re photographing somebody in the park, for example, you’d hold the light meter up to where their face will be.

There is debate about whether you should point the light meter at the light source itself, or back at your camera.  Personally, I find that there is no single answer and it’s a situational judgement call depending on the shot and the result you’re trying to achieve.  Typically, I will point the light meter towards the key light (whether that be the sun, a light on the wall in an interior, or at the main flash unit).

You press a button, and the meter feeds back your ISO, aperture and shutter speed.  Once you set your camera to manual mode and enter these settings, you should (in theory) get perfect exposures every time.  I say in theory, as it will depend on the lighting conditions under which you’re shooting, and how you’re using your meter.

As you’re in manual mode, if somebody turns one of the lights off in a room, or a cloud comes up and covers the sun, you’ll probably start to get underexposed images.  Likewise, if you meter when the lights are low or the cloud’s already in front of the sun, if the cloud goes away or more lights get turned on, now you’ll start to overexpose.

The light meter will give you back the settings you should be using under the lighting conditions you metered.  If those lighting conditions change, you should meter again.

If I have to keep going over to my subject to meter, isn’t that a lot more hassle than just using the camera’s reflective meter?

Personally I don’t think so, but it depends on what you’re shooting.  If you’re wandering around an event capturing moments as they happen, under varying lighting conditions, your camera’s built in meter is probably going to give you the best results – but it’s still a “best guess” (albeit a very educated one with modern DSLRs and final generation or two of 35mm SLRs).

If you are shooting under more controlled conditions, and are able to pose your subject, or have control over the light that you’re shooting under, a light meter is typically going to give you more accurate results consistently.

As to whether it’s more or less hassle or not, if you want accuracy and consistency using your camera’s reflective light meter, you’re going to need to have an 18% grey card with you.  Every time you need to meter, you’re going to have to get that card, hand it to your subject, have them hold it while you point your camera at it, fiddle with some settings, get the card back from your subject, put it away, and then shoot.  If you’re using an incident meter, assuming you don’t change your ISO and aperture (or shutter speed if you’re in shutter priority mode), all you need to do is walk up to your subject, press a button, walk back to where you’re shooting from, and enter the settings the meter has given you (or have an assistant meter and yell out what aperture or shutter speed you need to set).

Most of the time, I’m shooting under controlled conditions, it’s not typically “spur of the moment” events, and the timing is not critical.  Accuracy, however, is.

I get it now, so what about with flash?

Flash works a little bit differently to metering for ambient, as with flash your exposure is determined almost entirely by your aperture (and ISO), and not your shutter speed.

Click to ExpandWhy shutter speed doesn't matter with flash (click to expand)
Let’s say my camera’s set to ISO200, f/8 with a shutter speed of 1/60th of a second.  If I take a shot without using flash, my image is completely black.

Now I add a flash to the mix, I meter it and adjust my flash power’s settings to make it read f/8.  I take a shot and now my subject is beautifully exposed.  If I change my shutter speed from 1/60th of a second to 1/120th of a second, nothing happens, I get the same identical shot.  I bump it up to 1/250th of a second, and it’s still exactly the same.

This is because the duration of the flash (the amount of time the flash head is outputting light) is extremely fast, usually faster than 1/1000th of a second, so no matter how long the shutter stays open, it sees all of the light being emitted from the flash during that time (and without the flash, the scene is black, so ambient light is not contributing and changing that).

If, however, I adjust my aperture from f/8 to f/5.6 (or go from ISO200 to ISO400), my camera will shoot everything 1 stop overexposed.  If I stop down to f/11 (or drop my ISO to 100), now everything will be a stop underexposed.

Note : This information is only valid at or below your camera’s sync speed – the maximum shutter speed at which your camera can work with a standard flash (see your manual, not all of them go to 1/250).

Also Note : This information is only valid when your ISO, aperture & shutter speed combination is fast enough to crush all ambient light.  If you’re mixing flash & ambient lighting, shutter speed will still affect the amount of ambient light hitting your sensor. (Thanks for reminding me Jamie!)

Typically, when your scene or subject is lit solely by flash, you’ll be in shutter priority mode.  You’ll give your meter a shutter speed (which is largely irrelevant, as noted above), press a button, fire your flash, and when the meter sees your flash go off, it tells you what aperture you need to set on your camera in order to achieve a good exposure.

Cool, but I want to shoot at f/4, and my meter’s telling me to use f/8.  What can I do?

Well, you have a few options here.  The first one is the obvious one, set your camera to f/8 and deal with the extra depth of field.  Sometimes you may be forced into this situation if you’re unable to use one of the other solutions suggested below.

As we know from the Inverse Square law, when we double the distance between the flash and the subject, we quarter the amount of power from that light hitting the subject.  Or, we lose two stops of light.  Doubling the distance of the flash from the subject and metering again should bring that f/8 right down to f/4 where we want to be.  But, as with changing your aperture above, this affects the quality of the light and how hard or soft the shadows are.

Another option is to lower the power on the flash.  If we’re currently at full power, dropping the flash down to 1/4 power also loses us two stops of light without changing the quality of the light and without affecting the shadows.  If your flash meters f/8 at 1/8th power, you’d need to drop down to 1/32nd power to lose two stops.  If your flash is already at its lowest possible power setting, there’s ways around that too.

If you can’t (or don’t want to) move your flash further from your subject, you don’t want to change your depth of field, and you’re already at the lowest flash power setting, you can drop your ISO.  If you’re shooting at ISO200, going to ISO50 (if your camera is capable of it) will lose you those two stops.

If your camera’s ISO can’t quite go low enough, you could also use Neutral Density.  You can either put ND gels over your flash head or you can use ND filters over your lens.  Either way will drop the amount of light emitted from the flash and seen by the camera by however many stops of ND you apply.

If you put 2 stops of ND gels over your flash, your meter should now tell you f/4 and everything’s good.  If, however, you put two stops of ND over the end of your lens, your flash meter will still tell you f/8 (as the flash itself isn’t being affected), so you need to remember that you have 2 stops of ND over your lens and compensate accordingly by opening up your aperture to f/4.

You could also combine some of the above solutions.  If your light meters f/8 @ ISO200 and your ISO only goes down to 100, you could lose one stop by dropping your ISO to 100, and then add just 1 stop of ND over the lens (or the flash).  Or you could be happy dealing with slightly more depth of field and set your aperture to f/5.6.  If you’re happy to settle for a tiny bit of extra sharpness, you could just move your flash about 40% further away instead of doubling the distance.  You’ve already lost one stop by going from ISO200 to ISO100, so now you only need to lose one more instead of two.

One thing you quickly learn about photography, is that everything is a compromise.  It’s a trade off between this or that, and when you take from somewhere you have to give back somewhere else in order to compensate.  Once you start to figure that out, where those compromises and trade-offs can be made, and find your preferred way of doing things, that’s when you start to develop your own style which makes your images stand out from everybody else’s.

Awesome! So, when else might an incident meter be useful?

One of the advantages of digital photography is that you can instantly see your results on the back of the camera.  You take a shot, you bring up the image review, you can check the histogram, and immediately correct any issues that may arise in your exposure.

The same cannot be said when shooting film.  I have several film cameras, some including rather fancy meters similar to those which we see in DSLRs, like my Nikon N90s (the camera which captured the image at the top of this article – metered with my Sekonic L718 light meter), and some which don’t have any kind of metering at all, like the 60 year old Agfa Isolette and the 80 year old Voigtlander Brilliant.

Even though my N90s has a built in meter that’s pretty reliable, it can still get confused – the same way DSLR meters do.  Using a light meter gets around those issues and helps me to get an accurate result every time, and I know that when the time comes to develop the film I’ll be getting exactly what I expected.  With cameras that don’t have a light meter at all, they’re pretty much essential.

So, with film, light meters are extremely useful, even if the camera has a built in meter, to help ensure accurate exposures, pretty much every time – when used correctly.

With digital, they’re not as essential as they are with film, as the histogram is a pretty wonderful tool (although still not necessarily completely accurate or perfect).  That said, they are a great time saver, even with digital, to help you get the shot that much faster when you know how to use one.

Speed and efficiency of workflow on a shoot can make the difference between a nice happy and relaxed portrait session, and your subject looking bored and uncomfortable halfway through the set and in a large number of the images.

Comments

  1. [...] If we look at the simplest device used to balance our colour temperature, a neutral grey card of solid density throughout, on a combined histogram display we will see a single spike shooting up somewhere along the histogram.  Theoretically, straight up the middle is good with perfect exposure on an 18% grey card (your in-camera processing settings may shift this up or down depending on your brightness and contrast, even though you are shooting a correct exposure according to your in-camera meter or a handheld light meter). [...]