How do I use spot metering?

Question

What's the right way to use spot metering? Is it better to use in manual mode than one of the priority modes?

There's a question about when to use spot metering, but none that serves as a tutorial to describe how to use it.

Please address the issue of exposure compensation. I am under the impression that spot metering is hard to use in aperture priority because you can't (to my knowledge) over/under expose an image in these modes since the camera will adjust other variables to get the proper exposure. (So I need to meter off something that is gray-ish since I can't meter off something black and compensate?)

This question is motivated by another one that I asked here. It became clear that I didn't really know how to use spot metering.

Answer 1

I wrote a tutorial about this very subject on my website. You can read it here.

To summarise, there are two advantages to using it in manual mode:

1. Once you've set your meter for the prevailing lighting conditions, you shouldn't need to worry about the exposure again (unless you need to change the aperture or shutter speed, or the lighting changes significantly)
2. Using manual mode allows you to move beyond the 2 stop range you get on most cameras with exposure compensation.

Learn the zones (see link); then find something in your image that you want to assign to a particular zone. Spot metering is the best way to isolate a small part of your image without any extraneous elements getting in the way of the metering. I now shoot almost exclusively with manual spot metering.

Of course there may be valid reasons why it's not appropriate to use manual mode - you may be shooting in rapidly varying lighting conditions, for example. But learning how to use spot metering in conjunction with manual mode will add another tool to your belt.

Answer 2

Overview

In general, the workflow goes like this:

• Choose metering area
• Adjust and meter
• Retain the metering
• Compose, focus and shoot

Since there are so many steps, spot metering tends to work better for planned shots, although it can perform quite fast when linked with AF point (details under "Adjusting and metering"). For rapid shooting in changing light, matrix metering would be faster to use, but less precise.

Now, let's see the details for each step.

Choosing metering area

Choose an area for which you have a vision about how it should be exposed. This is the key point of spot metering - executing your vision. At this step, it doesn't matter if the area has to be at the neutral 18% gray level, or something dark/light.

Adjusting and metering

Adjusting means that you set your exposure compensation according to which way from the 18% gray level you want the selected area be exposed. You want it lighter - use a positive compensation; you want it darker - use a negative one. Knowing the zone system will help you determine the amount of exposure you need - adjust exposure by 1 stop for each zone further away from the 5th. In the rare cases you want to go further than offered by exposure compensation range of your camera, you'll have to use manual mode.

Point the camera so that metering spot is lined up with that area. Usually the metering spot is at the center of your viewfinder.

Some cameras allow linking metering to selected AF point; that option is quite sensible for the common case where you focus and set exposure by same object (your subject). It will make your workflow faster as you can combine all three last steps into a single press of shutter - you can perform focusing at the same time and won't waste time recomposing.

Note that in full manual exposure mode, the order of sub-steps is reversed - you first meter, observe where your camera reports the exposure related to neutral level, then adjust any parameter of exposure triangle (or a combination of them) to get desired exposure level for the area.

Retaining the metering

Details of this step depend on your camera model, settings and exposure mode. This step can be skipped if you don't need to recompose for actual shot.

In full manual mode, you don't have to do anything in this step - just stop fiddling with controls and exposure stays as it was set, thank you very much.

In (semi-)automatic modes, however, you have to tell you camera to stay at current exposure level. This can be done activating exposure lock.

On many cameras, holding the shutter button half-pressed can be configured to lock exposure settings, but if it also triggers auto focusing, that would also result in focus locking. In many times that may be okay, but if you're going to significantly recompose and use shallow depth of field, it may cause focus being slightly off. For that reason, you may want to use manual focus or set your camera to use back-button focusing, if it supports that.

The most universal way would be to note exposure parameters, set the camera into manual mode and dial in the same exposure parameters. If there's a chance that the next step will take long time, switching to manual mode should be the preferred way - on most cameras, if not all, exposure lock has a timeout limit in the magnitude of 10 to 30 seconds.

Composing, focusing and shooting

Now that you have the exposure settings in place, you only need to worry about composing the picture, focusing and taking the shot. Details of these operations are already out of scope for this question, as there's nothing specific to spot metering to be done here.

Answer 3

Aperture and shutter priority don't determine the metering; they determine what the camera will adjust in order to get the correct exposure at the metering used. In other words, if it is possible to get the correct exposure using your selected aperture, then aperture priority mode and spot metering are perfectly compatible. And yes, exposure compensation is how you would adjust the "zone" or "placement" of the object you metered. For instance, if you meter a typical Caucasian skin, the meter will give you a value that will result in a photograph that will be about one stop under-exposed or so. In order to place the skin tone at the right value on the scale, you'd use exposure compensation to "overexpose" by one stop.

Use spot metering when you know where the object you're metering ought to fall on a tonal scale. Metering off of "something black" is probably not going to be a good idea as a general rule -- black velvet is almost completely non-reflective, and is not going to give the same reading as a black automobile, a black T-shirt etc. On the other hand, if you know where you want the darkest darks and the middle tones of your black car to sit on the tonal scale (and the car is what you are trying to photograph) then metering the car and "underexposing" by two or three stops will get the car right -- but perhaps at the expense of other things.

It's all about picking out what's most important and making sure that one thing is properly exposed in an automatic mode. In manual, you have the opportunity to meter several areas in the scene and come to your own decisions concerning where sacrifices need to be made when they need to be made (will you have to let the shadows block or the highlights clip, etc.).

Answer 4

For many scenes you cannot take a picture that will successfully capture details both in shadows and in highlights. Consider a man standing behind the sun. Either a man will be fine and the sky will be plain white or the sky will be great and the man will be plain black.

In order to correctly expose that picture you have to decide on the compromise between the light and the dark parts of the image and here's when Adams' Zone System comes in. It divides the visible world into a count of exposure zones, each one exposure step apart of the other. Using the spot metering tool you can measure an exact Shutter/Aperture to expose the measured object in Zone 5. Knowing that and the desired zone of the object you can make simple calculations to get a perfect exposure.

Let's say you measure the man against the sun. The face looks nice in zone 5, but you need to save some sky, so you shall expose him in zone 4, which is one step darker. So, if the spot metering gives you f11, you will have to subtract one exposure step and shot the man at f16.

This kind of metering is hard to get with matrix/centerweight metering, so this is why spot metering exists: for the sake of Adam's Zone System.

Answer 5

Here is what I've found is the fastest yet most effective way to use spot metering:

• Set your EV compensation to +X.X EV (Typically 2 or 3 EV) to push that metered gray point to the white point
• Spot meter on your white point, this is the section of the scene you want to be nearly blown out. Ignore specular reflections as they are typically many times brighter and don't contain useful detail. You should let those clip
• Set exposure lock
• Recompose for the shot

More information can be found here.

It's basically like the Zone System, but only caring about zone VIII, which is usually the easiest to tell. Even if it isn't a high contrast scene and has no white points, you can apply -EV in post and get the benefit of less noise, since there is more detail in the highlights.

This basically the ideal ETTR for digital systems.

Answer 6

You usually use spot metering along with exposure lock. You just point the center of the view finder at the object that you want correctly exposed (as the spot metering only measures a small spot in the center), lock the exposure, then compose the image and take it.

How exposure lock is done differs between cameras. On some you press the shutter halfways, others have a separate button.

There is no problem using exposure compensation in any automatic mode, as long as the camera actually have a setting for exposure compensation. The camera will automatically expose according to the offset that you have chosen, not according to the correct exposure.

(If the camera doesn't have any exposure compensation setting, your only chance to do that is to use manual mode, or measure the light on a surface that is darker or brighter to fool the automatic exposure.)