A light meter is, as the name reveals, a tool that measures light. Virtually all cameras have built-in light meters, but there are also handheld light meters. So what is the difference between the one you already have in your camera compared to a handheld light meter?
The light meter in the camera has the limitation that it can only measure reflective light, i.e. the light reflected from the subject you are aiming the camera at. A handheld light meter can measure both reflective light and incident light. Incident light is the light that actually hits your subject, i.e. the light coming from the light source (not the light reflected from the subject). Some handheld meters also have the function of measuring reflective light with spot measurement, which means that you can measure the reflective light with great precision on the subject.
Another limitation of the camera's light meter is that it cannot be synced to studio flashes and measure the flash light. This is probably the area of use that most people associate with handheld light meters.
A light meter assumes that the light it measures is reflected from a gray surface with 18% reflection. If we convert it to sRGB, it's 118,118,118 as the color in Photoshop. So what does that mean in practice?
In most cases, the exposure is correct because in most photographs, the subject gives an average reflection of around 18%. But of course there are exceptions. If we e.g. exposing the sky on a sunny day after 18% gray, the whole scene becomes heavily underexposed because it is significantly lighter than 18% gray. Same thing if we expose a black object as 18% gray. The object will be displayed as gray and the surrounding, brighter environment will be overexposed.
In the example below, I have on the first exposure put spot measurement on the egg. Now the light meter tells me which settings I should use for the egg to be exposed as 18% gray, which we can see gives a fairly strongly underexposed image.
In the next picture I measure the incident light, i.e. I hold the light meter in front of the egg and point the measurement sensor at the light source and then set the camera according to the measured value. Now we get a more accurate exposure because the egg should be lighter in the scene than 18% gray.
With spot measurement aimed at the egg, the egg is exposed as 18% gray. When measuring against the light source, the light is measured as 18% grey.
Do you really need a handheld light meter now that everything is digital and you can see the image directly on the camera or even on a monitor when shooting tethered to computer? There are many pros and cons here. Many people think that handheld light meters are superfluous (even for flashlights) as you can take test pictures and look at histograms directly on the camera.
Personally, I think it is still as important with handheld light meters today, with the digital medium, as it was in the analog era. When shooting with flash light, it is obvious to me as it is both more efficient and more precise with a loose light meter than looking at the camera's LCD or the computer's monitor.
If you also have access to control the amount of light with 1/10 precision, as we have with Elinchrom's flashes, it is nice to be able to measure it to give all the shades of the image perfect exposure.
The light meter is an invaluable tool when lighting with flashes. Not only to get the correct exposure but also to measure the contrast between several flashes, e.g. when I want the headlight to be 1 step stronger than the fill light. Instead of guessing and taking test exposures, I can go to the subject to measure the light that falls on the subject and get the correct exposure to start from. This also enables you to reproduce lighting and have a more consistent workflow.
Another function I appreciate on the light meter I have chosen is that you see how many% of the measured light is flash light in relation to existing light. This is very useful when you want to balance flashlight and existing light.
Polarizing filters and ND filters change the exposure and there is nothing the camera compensates for. With a handheld light meter, I can easily key in which filters I use and the meter automatically compensates for the filter factor that the filter provides.
My light meter, a model from Sekonic, supports Elinchrom's radio system, which means that when measuring I can trigger my flashes and adjust the strength wirelessly directly from my light meter! So if you have Elinchrom, it's definitely worth looking at Sekonic light meters adapted for Elinchrom Skyport remote control.
ere, many people probably think that a handheld light meter is not needed at all as the camera's built-in light meter works excellently and has different programs for different situations. This is true quite often, but there are situations where the camera's built-in meter does not work with the same precision as a properly calibrated handheld meter. Here are some scenarios as I always use a handheld light meter instead of the camera light metering system.
When you are investing in a good handheld meter, I highly recommend that you calibrate it to your camera. The light meter shows right directly out of the box, but since ISO today is not exactly comparable between different sensors / cameras, it is a good idea to calibrate the meter so that it matches your camera and you will get the best results. Another advantage is that you can get information about the tone range on your camera's sensor, i.e. you can see on the meter when your camera will bleach out the low key / high key parts of the picture.
Hope you got some more information on what situations a handheld light meter can be useful!!
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