Photography Light Meter

The photography light meter comes in two generalized forms. The Reflective meter, whose design is intended to read reflective light from the photographic scene. And the Incident light meter, which is designed to read the amount of light that is illuminating the scene. The latter of these two may also include some ambient light being reflected around the scene.

You can get excellent results with specialized meters designed for solving problems, and when you know your way around them, they are great time savers. For those people that just want to stick with the in camera light metering system, let me direct you to the pages Understanding Exposure and Metering Modes. These pages on in-camera light meters will show you how your internal metering system reacts to light, and will help you get familiar with learning to interpret those light meter readings and making adjustments via Exposure Compensation .

Hand held photography light meters provide exposure constancy, and speed in challenging lighting situations. Modeling, Portraiture, Motion Picture and Video, complexed Studio lighting setup's are common environments for hand held light meter work. For Landscape photography where film is shot using the Zone System, the spot meter is still as valuable as it ever has been.

Photography Light Meter Calibration

All light meters are calibrated to read the light presented to them and then return an exposure that will be averaged to a middle tone value. If we ignore color for a moment and just consider a gray scale of B&W tones from brilliant white to absolute black, then this middle tone value would be a middle gray value half way between these two extremes. For most light meters, whether a hand held meter or an internal camera meter, both are calibrated to just a bit darker than the middle tone value, and are often set to some unique preference value by the manufacture of the product.

In your image editing software program you will have a color picker that displays what constitutes your color's profile. The brightest white you could have would be at 100% brightness, and the darkest Black would be a "0" value. There are 255 different brightness levels in all. For all color neutral tones the RGB values are all equal in values to each other. So the brightest white would have the RGB values of R-255/G-255/B-255, and black would be R-0/G-0/B-0. Since middle gray is the half way point between white and black ( 50% brightness ), it's brightness value would be half way between 255 and 0, which is R-128/G-128/B-128.

It is not too uncommon to get a different calibration settings between different makes and models of hand photography light meters and cameras, but for the most part they are usually just a little darker than the middle tone value, which is perhaps less than a 1/3 of a stop down (or darker) from the middle gray value. A common Photography and Printing Industry reference for the middle tone value is printed on an 8x10 piece of cardboard, and gold standard use by many photographers is the Kodak 18% gray card. Kodak filed for bankruptcy in it's digital imaging section, but there may be a few places that still sell these.

The little volcano shaped area at the bottom of the diagram is showing Middle Gray value of 50‰ brightness. Often times light meters will be calibrated to just a bit darker than this middle gray value where this brightness values will show up being around 118-122 in the color picker of your image editing application.

I have heard of people having as much as a full stop difference between their various camera and light meter equipment, but I would say this is pretty rare, and it has never been my experience to have such a strong difference in exposure readings.

Kodak Gray Card
Using a Reflective photography light meter reading
taken from a Gray Card at your subject's position,
will yield the same exposure result as an Incident
light meter reading should. Having said that it is
possible to tilt the Gray card a little improperly
which results in picking up a little light glare,
this can sometimes effect the reading while you are
searching for the middle tone value. This is one
reason the Incident Light meter is preferred as it
is immune to such a problem. The incident light meter
has a little plastic white dome that sits over the
light sensor of the meter. Because the dome is a
sphere, glare is not an issue effecting the reading.

If you have a hand held photography light meter that needs calibration, you can usually have that work done locally in your town, or at a quality factory authorized camera repair shop. You can usually adjust the these meters yourself if you feel you are careful enough with the setting up of lights to evenly light a gray card as a target so that you can take pictures and compare your different metering equipment. The trick is knowing which one of your light meters is giving correct reading between the two. It is not that hard really to solve this riddle, you use your color picker in your image editing software. If you reference the RGB values I gave you listed above on this page you can then solve question. In any case, on most dSLR cameras, you can't adjust your light meter on your own, that generally needs to be set by an authorize repair shop. But you can if you would like, tune your handheld light meter to your camera.

For light meter calibration adjustment, check with your hand held light meter manual and it should tell you where an adjustment on the light meter can be made. All new light meters are already factory calibrated. The adjustment process might be as easy as making an adjustment in the menu settings or turning a small adjustment screw with a small screw driver.

If your camera light meter readings and handheld light meter readings are within 1/3 of a stop, then I would call it good, you don't need to bother with any adjustment

If your two readings are greater than that, and you don't want to risk making adjustments on your own to the hand held meter sensitivity setting, you can still use your light meter by over compensating the difference by off setting the ISO value on the hand held meter, to match the meter readings on your camera. This is the blessing of reciprocity, where all adjustments between Sutter Speed, Aperture and ISO are all in equal units of light adjustment with each other. This is the same basic idea as the Exposure Compensation function on your camera.

On a cloudless, clear, blue sky day, you can check a spot meter by pointing it north and about 35-60° above the horizon, this exposure should an equivalent exposure to the exposure for the Sunny 16 Rule. This should work for your camera also if you are using a narrow enough angle of view on your telephoto setting.

Photography Light Meter Calibration

The Incident Photography light meter

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The incident photography light meter is really a reflective light meter that has a plastic dome that slides over the light sensor so that the meter does not read the reflective values of your scene. What it does read is the the light that is inside the dome, and this means all the light that reaches the dome in a 180 degree sphere. The light meter is calibrated to read the light that is transmitted though the dome's plastic, if the dome is moved out of the way off to one side, then light meter's calibration changes to read the reflected light of the scene just as any other reflective light meter would.

Most of these light meters are multi functional, being capable of reading flash strobe light in incident or reflective light reading modes, making this a vary valuable tool.

Some of these photography light meters may even have an accessory lens that can fit over the front of the light sensor allowing for narrowing the angle of view of light in reflective reading meter mode. They may call this Spot metering, but your angle of view might be from about 15° down to about 8°, which might be better thought of as partial metering mode by definition.

If you are working with your camera on a tripod, a hand held meter allows you to move freely around your work area without having to reframe your camera view every time you need to answer a new exposure question about your scene, this is true for incident meter as well as the spot meter.

Incident Photography light meter
Incident light meter readings are designed to read the light that illuminates the scene, so to get accurate results you often have to read the light from the subject location.

Incident Photography light meter readings are a fast accurate way to read the light that illuminates your scene without the light meter being influenced by the subjects individual reflectivity and surface qualities. If you want dark objects or light objects to appear in your photos the way their natural reflectivity appears in the scene, then this is an accurate reading for that purpose, and the quickest way to get that result.

The incident reading is taken with the dome over the light sensor while standing in the same light that your subject is being illuminated by. This means you will stand in front of your subject with your light meter while pointing the dome back at the camera so that the light that illuminates the dome is the same light your subject receives. Reading the light from your primary subject's position, relative to the lighting, is vary important when working around man made light sources. Light intensity diminishes at the rate of the inverse square law. Meaning that every time the distance that light travel is doubled while leaving your light source, the light loss is squared. Using an incident meter often means you don't have to calculate this every time you need to move lights around, your incident exposure reading is only concerned with the light that is reaching the meter, so the readings are accurate.

If you are using multiple hot lights or multiple flash heads, and you want to know how bright one light is compared to another light in your set up, you can point the light dome towards the individual lights one at a time, and determine the difference in light intensity between them. This can help you figure out lighting ratios. If one light is too hot, you back it off some, or add a scrim to it in order to reduce it's power output. A scrim is a metal screen not unlike a the old metal screens that would be found in a screen door.

If you are shooting movie or video scenes, you can duplicate the lighting between different scene and sets, and the get constant results of the light on your actors face, because you are always reading the light where the actor will be standing in each scene, that is if you keep good records. Plus it is not often that the actors are around when you are setting up lights for the next shot or portrait. With an incident meter, they don't have to be around, you just have to know where you want them to stand or sit. Your exposure reading will be correct for recording that subject at it's natural reflectance in the scene.

Another advantage of the incident reading is the dome it's self. It has a 3D curved shape (sphere), which will pick up highlights and shadows, and those varying values will be a part of your reading result. Having those highlights and shadows calculated into the exposure saves you from blowing out the highlights in your shot.

Here is two pictures of our cooperative exposure subject, Ping. I took these pictures on an overcast day, and I later combined them down the middle. In each shot, I read an incident exposure from my subject position and point the dome back towards the camera. Note that in one shot my back ground is all black and in the other shot my background is all white. A reflective light meter reading for each of these would give radically different exposure results for this same subject. The incident meter reading allows my subject to be expose constantly without regard to different scene reflectance. You will notice a little bit of contrast change between the two pictures, but that is to be expected because of the difference in surrounding reflectance, but I have not lost details in my white or black in either exposure of my subject.

Incident Photography light meter readings

The exposure readings used for both of these images were incident meter readings which yield similar results despite the dramatic change in background reflectance and surroundings.

In the below photograph is an incident reading of a snow scene where sun light is coming from the side. The same light is falling on me at my camera position as is falling on my scene, so in this case I can use the incident meter right were I am standing. I orient the face of the dome to be pointing at the same plane of my camera's image sensor, so that I am getting the same amount of sun and shadow as my scene is from this angle. I reduce my meter exposure by -1/3 just to ensure I have all the highlight detail I want for the picture. The snow texture in the highlights and shadows is quite noticeable, it is a perfect exposure and I did not have to calculate the middle exposure between the highlights and shadows. A reflective light meter reading for this general scene would yield an not so pleasing dark picture of about -1 1/2 stops under exposed.

GirdWood Alaska - Randy Smith Photography © 2011.
Incident Photography Light Meter Reading

Photographers who commonly shoot portraits might purchase an incident meter that accommodates a larger dome size then the Gossen light meter that I am using here in these examples. The larger the dome size that is being used is considered by some to have a greater statistical accuracy advantage at calculating the shaping of light to shadow across the dome.

Incident Photography Light Meter

Photography Spot Meter

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Spot meters are only reflective light meters, and with most spot meters these days, you will be able to read flash strobe light as well as hot lights. They may have an angle of view from about 1° up to about 3.5°. My camera has a spot reading function of 3.5 degrees and many other camera makes and models do also. The video play back of the LCD screen of the digital camera and the Histogram Graph give you vary detailed information, that can allow you to adjust your exposure to protect those valuable highlights of the image. This might be enough to convince some people they don't need to purchase a spot meter.

The whole point of a photography spot meters is to solve where to best place exposure to suit the dynamic range of the scene for the medium you are capturing the image on. The one degree spot meter is like a surgeons scalpel that allows a person to take a reading with a narrow angle of view, and the narrower you can read light accurately, the more accurate your over all calculations can be. However, you must be fairly knowledgeable and critical about how you want reading of those lit areas to appear in your final calculation of the exposure.

If you notice the width of the exposure readings in the photograph below, that is about the area of a one degree reading. The 3.5 degree angle would be useless in this scene for getting specific detail about scene brightness, so your camera spot meter would not be much help in determining exposure in this scene unless it also had have a vary narrow reading area for sensitivity.

Now to be fair and clear, I did shoot this scene using my camera's internal light meter and not a Spot Photography light meter. I took into consideration where I wanted my highlights in this scene and then examined my histogram, so for digital still photography this process works fine. For shooting large format film, motion picture film or video sets, studio lighting and portraiture, you might fine the speed and accuracy of using a spot meter quite useful.

Spot Photography Light Meter Readings

For an exposure calculation, you might lock into the light meter an exposure you took from the highlight of a cloud and then another exposure of a shadow under the log in the forest, the light meter can then calculate an exposure average between these two settings. This is a way of choosing an exposure for making the best use of your dynamic range within the scene.

Since we use several different mediums these days, your spot meter might allow you to set the limits of your dynamic range for your camera sensor, or video sensor, or film, or slide copier. It is not uncommon for all of these devices to have different dynamic ranges associated with different mediums (film or electronics). When your meter readings indicate that the limits of the dynamic range have been exceeded, the light meter will indicate a warning. You then would know you will have to compromise by composing the shot differently, or by adding supplemental lighting for the shadows.

If you are shooting film, especially B&W negative, then you can choose to learn and shoot the Zone System which is a form of matching film type with to unique exposures, development times, and paper types, to maximize the dynamic range for the final print. It is a whole different art form to itself.

The spot meter allows you capture all light meter readings you need from the camera shooting location, unlike that of the requirements of the incident light meter in a studio setting, where you need to be moving around reading light from the subject locations.

Working with a spot light meter is a more methodical calculating process to ensure exact exposure placement. It is not uncommon for lighting and set directors to use spot meters to check the lighting reflections of a scene and get an idea of how intense they will be. In the past, Polaroid film was also used for the purpose of checking the lighting and scene continuity of the set. Today digital camera pictures, video play back on monitors, histograms, and laptops are all used for checking these issues.

Since most Spot meters and Incident light meters read flash strobe light intensity, the light meters place a convenient test fire button on your light meter from which to trigger the strobes so that you can the lighting intensity around your scene or set.

Photography Light Meter - Spot Meter

Photography Light Meter - Summation

The Incident photography light meter is ideal for quick assessment of complex light situations of contrasty scenes, studio or set lighting, portrait exposures indoors or outdoors. This is a very powerful and flexible light metering tool for any type of photography.

I would say the only reason everyone is not using one regularly is because people are using their camera light meter which can save weight and space in the camera bag and to limit the need to transfer exposure readings from the meter to the camera body. They opt instead, to shoot with just the in camera light meter and attempt to out guess the reflective meter readings and adjust with Exposure Compensation. I am not making a statement of bad of good, it's a different process. The Incident light meter is an added expense but for serious work it is a valuable tool that is consistent for exposure readings, and in studio situations it is quicker than even using the in camera meter.

The Spot photography light meter is a carefully calculating exposure assessment tool. It will slow the pace down quite a bit for taking pictures, but this tool is invaluable to the analytical photographer. It is often use by photographers that shoot large format film. Often times exposure calculations include concerns of development times, and the type of print paper one intends to print on. It can take a lot of time to set up for a photograph, so you need to be able to ensure you have the exposure right before you leave the scene. I would assume that in this day and age a photographer might also shoot a digital image just to proof their calculation before committing the image to film. The large format film photographer still would have to compensate for bellows adjustments.