spot metering - how to use it?

[theregoesjb]

I have a canon P&S: 'sx 130 is'.
Under the brightness settings there is "evaluative" , "center weighted" , and "spot" which says "set brightness based on center frame area". Is this the same as spot metering?

I recently had an issue taking a late day picture where it came out looking pretty bad from the poor lighting conditions, sky looked good but landscape was underexposed. but it was recommended maybe spot metering in multiple areas and taking a few shots and then merging them in editing software. It was stated this is difficult because it requires not moving the camera.

So my question is how do people do this? do more advanced cameras allow you to spot meter different areas of the frame (without moving it)?

 

[Big Mike]

There are two (somewhat separate) issues here.

Firstly, yes that is your spot metering option. Although, it's likely not a tiny spot, but just an area in the centre of the frame. The difference between that 'metering mode' and the other modes, is that 'spot' will ignore the areas around the outside of the frame, and only use the centre area to determine the exposure. This would be a good choice when your subject is a lot more important that the background.
The evaluative mode takes into account the whole scene, as does centre weighted...but center weighted gives more importance to the centre area.

So in your case, you could have use the spot mode and metered on the landscape, and it would have been better exposed...but then the sky would have been overexposed...which brings us to the second issue....dynamic range.

Cameras are only capable of capturing a certain range of brightness. We call this the dynamic range. Many scenes that we encounter in the world, will have a greater range of brightness than what our cameras can capture. So as a photographer, it's your task to recognize this and make a choice. Choose what parts of the scene you want to be exposed properly and be content with the limitations of your camera. Let that limitation drive you to be more creative with your composition and choice of what to photograph.

You mentioned taking multiple shots...and yes, that is a technique that you can use. It's commonly called HDR (high dynamic range) the idea being that you take several photos with different exposure values. (so that you get good exposure of both the sky and the landscape etc.). You can then use software to merge the images and hopefully end up with a good image that has a greater dynamic range than you camera could capture with a single photo. And yes, it works much better if the camera is in the same position, so using a tripod would be a good idea.

 

[tirediron]

Yes, this is exactly what spot-metering is meant for (one of the things at any rate). The technique described is known as a high-dynamic range merge, commonly called an "HDR". Some cameras will allow you to select which focusing point the spot-metering point is slaved to, however, if yours doesn't, simply meter your scene, note the readings, compose your scene, and shooting, adjusting your settings according to your notes.

 

[theregoesjb]

great explanations, thanks a lot

as far as noting the settings for the HDR merge, its just aperture, shutter speed, and ISO correct? my camera will auto focus... do i need to account for this in any way?

 

[tirediron]

You don't need to account for focus, and you will probably achieve best results by setting your ISO to a single value (the lower the better) rather than leaving it in Auto. Ideally, set your camera to either aperture or shutter priority (I prefer aperture) so that only one exposure component changes during the process.

If you leave your ISO in auto, your run the risk of one image using a much higher ISO than the others and therefore having significantly more digital 'noise' in the image.

 

[RichardsTPF]

Ideally, set your camera to either aperture or shutter priority (I prefer aperture) so that only one exposure component changes during the process.

I would prefer shutter priority for multiple shots for HDR, thus the DOF won't change. Am I correct?

 

[tirediron]

Ideally, set your camera to either aperture or shutter priority (I prefer aperture) so that only one exposure component changes during the process.

I would prefer shutter priority for multiple shots for HDR, thus the DOF won't change. Am I correct?

Nooooooo... with shutter priority, you set the shutter-speed, the camera will choose an aperture to give the correct exposure, so, there is a chance of changing DoF. The reality is that with landscape shots, you're probably going to be focused at or near infinity, so an aperture change of a couple of stops won't effect things, but... why take the chance?

 

[RichardsTPF]

You are right. I was thinking about use same aperture with varies shutter speed. But I got confused with the term at that moment. My bad.

 

[unpopular]

Oh good greif people.

I do not know about your particular camera, but spot metering is just that, a meter taken from a single point. Typically in-camera this point is shown by a square or circle in the middle of the viewfinder or, perhaps in your case, LCD viewer. It will meter within that spot.

As with all reflectance metering, spot metering will tell you the amount of light reflected from that which it is metered "off" of, but will not tell you how much exposure is needed for any given subject. This is where metering gets kind of tricky.

When your camera was built and the meter calibrated it was calibrated to a certain standardized circumstance, when the meter reads null (usually denoted by a '0' in the camera) then it matches those same circumstances. This does not, however mean that the exposure will be correct, Only if the amount of light reflected off the subject is equivalent in value to that of when the meter was calibrated will the exposure be correct.

This is why some photographers use a grey card. A grey card is supposed to be the standard to which all meters are calibrated. Some people will say otherwise, and I don't know how legitimate this argument really is - but the idea is that this grey card standard is what the meter means by "zero".

So, if you spot meter something darker than this grey card, and use auto exposure modes or dial in "zero" in manual exposure, the image will come out brighter than you might expect. If you do the same with something lighter, then the image will come out darker.

More specifically, if you meter something that reflects one stop more light than the middle grey which the meter is expecting, you are actually under exposing my one stop since the camera is expecting less light. In fact, the luminosity of that point which you metered will be approximately 50% if you were to measure it on the computer or with film using a densitometer (that is, in a perfect world, in reality it might differ somewhat), regardless how bright or dark that spot actually was. Everything else will become equally brighter or darker until it goes beyond what the sensor (or film) can capture.

Likewise if you spot metered off of something darker than middle grey it would be over exposed. Again, because the meter is expecting something lighter at that given measurement.

"Expectation" is kind of a funny word, also. Meters are just dumb devices that measure light as compared to some standard.

Array metering, which you might be more familiar with his a bunch of spot meters in one, and this allows the camera to make an informed decision about the scene in various ways. Ultimately, it's looking for that "middle grey" that it was calibrated to.

Now, this might make spot metering sound useless, but in fact it isn't. Spot metering permits the operator to make informed decisions about the scene. I shoot exclusively in spot meter, manual exosure for this reason. Spot metering will tell me if the hilights or shadows are out of the camera's practical dynamic range of about 5 stops, and will allow me to make decisions about what areas I can permit at the expense of lacking detail. For example, if the subject requires lots of hilight detail, but there isn't too much in the shadows I care about, I can place the hilights in a lower "zone" than if exposed "properly". Likewise, I can predict how much shadow detail I can include while avoiding over exposure in the hilights. Spot metering permits detailed information to the photographer that allows greater control over exposure.

However, if your camera does not permit manual exposure, or at least exposure compensation, then spot metering would be useless without a grey card or some other reference.

ETA:

I reread the OP, and realized that I did not really address the last question: how do you actually use spot metering.

To use spot metering you either point the spot at a grey standard, such as a grey card. Green grass or the bluest part of the sky often works as well. Using manual exposure, dial readings off these kinds of subjects, those that reflect equal amounts of light as they absorb, to "zero" will give you a "proper exposure".

Likewise, you can meter off something that is approximately a known amount lighter or darker than something that absorbs half the light it reflects. To do this, you'd adjust exposure to correspond. So if you estimate that a sidewalk is one stop lighter than green grass, you could meter off the sidewalk and increase by one stop's equivelent exposure, i.e, if the sidewalk measures f:16, 1/120, the "proper" exposure would be f:16, 1/60, or f:11, 1/120, or +1EV.

The same would go with something darker. If the asphalt is about two stops darker than green grass, and measures f:5.6, 1/30, then the proper exposure would be f:5.6, 1/120; f:11, 1/30 or -3EV.

Notice that I place "proper" in quotes. That is because you can place anything anywhere depending on how much detail you'll require elsewhere. I don't go about thinking of "how much lighter is this than middle grey", but rather I think of it in terms of Zones. If I want the shadows to have lots of detail, I make sure that I have at least two stops exposure under what the meter is telling me when I meter off the shadows. This places the shadows in Zone III, whereas what the meter reads is always Zone V, or middle grey.

That, though is starting to get into some more advanced topics and maybe something you want to file away until you understand metering on a more intuitive level.