How critical is it to expose raw images correctly?

Question

I have been thinking about the Zone System and its goal of maximising image quality given the limitations of film sensitivity and the necessity to choose specific developing parameters.

Clearly you can't extract information that was never recorded or was burnt out but given that raw images can be tweaked in post processing, how critical is it get things exactly right at the outset? Does for example +/- 1 stop exposure compensation have any value beyond the quality of a jpeg (if you shoot raw+jpeg)?

Perhaps what I am asking is how much latitude do I have with post processing raw images? (Nikon D90 and P7100). I have zero experience with raw so far (in case you hadn't guessed).

Answer

It's very important, but depending on the dynamic range of the scene you're shooting. If you have bright skies, but also deep shadow areas, then the dynamic range may exceed what your sensor can capture. You will lose detail in the shadow or in the highlights, or both, even shooting in RAW. Shooting RAW means you retain more of the original information, but it doesn't extend the dynamic range of the camera.

If the scene is lower contrast you may have leeway to under- or overexpose by a stop and still not blow highlights or lose shadow details.

You can get an approximation of this by looking at the histogram (although it is derived from a JPG version of your image, so isn't 100% reliable).

There is more information and a higher signal to noise ratio in the higher exposure values than there are in the shadows. So if you do have leeway to adjust exposure, you're generally better off to "expose to the right" (moving the histogram to the right hand side) and then you can adjust the exposure down in post processing if you want to. If you have most of the histogram to the left, and try to raise the exposure in post processing, you're more likely to end up with noise or banding.

More about expose to the right here and here

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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 are questions about when to use spot metering, but none that serve 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, and @rfusca suggested I ask a separate question about this.

Answer

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.

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What is exposure compensation?

Question

What does exposure compensation do?

If I take a photo with a given shutter speed, aperature, and ISO, and then take the same shot with +1EV or -1EV, what is actually happening?

Is this just a gain control on the sensor?

Can you achieve the same thing by changing ISO?

Answer

Exposure compensation changes the target exposure. Normally the camera is trying to work the settings to get about 18% grey (reflectance), but with exposure compensation of +1EV you are basically just saying to the camera "I want to expose this scene 1 stop lighter than the normal average."

Changing ISO vs Exposure Compensation

• Manual shooting

  • Changing the ISO would have the same effect on exposure that Exposure Compensation would have in AUTO mode. However, so would changing your shutter speed or aperture.

• Automatic modes

  • The camera will change either ISO, aperture, or shutter speed as needed to achieve the correct exposure, so adjusting your ISO will only change the shutter/aperture that the camera sets. Changing the exposure compensation will change the target exposure.

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Should I use “exposure compensation” setting while shooting?

Question

Should this be done using image processing software like Photoshop or with the camera's built–in function?

It seems to me a program like Photoshop could use much better algorithm than what is available in the camera; or does the camera do something else before taking the image?

Note—I'm asking about standard shooting mode, with HDR (High Dynamic Range), Exposure Bracketing, ADL (Active D-Lighting) modes turned off.

Isn't the logic here the same as the logic with digital zoom, which is useful only when shooting in a compressed format as the zoom is done before compressing. Or is there something else?

Answer

Exposure-Compensation affects how the camera takes the photo.

No matter what you do later, you cannot get what was outside of the captured dynamic range back.

So, the answer is YES and it is probably the most commonly used setting of all.

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How should I learn to understand Canon's metering?

Question

Spot metering is my preferred metering mode on the Canon 50D. This allows me to nail the exposure first time -- 99% of the time -- by using AE-lock on my subject and applying the desired amount of exposure compensation. However, I find this too slow for some situations, so would like to learn to use one of the "full frame" metering modes (i.e. Evaluative or CWA).

I have tried using both of these modes. On the whole, I found Evaluative to be unpredictable. I found myself having to second-guess when and by how much the camera had automatically added exposure compensation. On the other hand, I found CWA to be much more predictable, but slower as it was still necessary to AE-lock and recompose for off-centre subjects.

It seems that Canon designed Evaluative metering largely as a replacement for CWA, and that by not using this, I would somehow be missing out on much of the ability of my camera.

How would I be better investing my time?

a) Using Evaluative metering and learning to recognise how it responds in certain situations so that I can second-guess it.

b) Using the much simpler CWA metering and learning to read the scene to determine the required amount of exposure compensation.

If only Canon had an "active focus point weighted average" mode, it would be perfect!

Answer

Metering will always be trial and error, because the camera assumes everything you're shooting reflects 18% of the incoming light back at the camera. It has no way of knowing whether your subject is white or grey, or even what part of your scene is the intended subject!

The closest the camera can get to knowing the latter is by looking at the currently selected focus point, as you suggest. In fact some Canon bodies do offer spot metering linked to the active focus point. I suspect it's only on the more expensive models. Edit: yes it seems to be just the 1D/1Ds models. And it's only activated when you're using a subset of the 45 AF points, which is disappointed (you'd think it could use the closest spot metering point when you have all 45 activated).

Contrary to ElendilTheTall I use Evaluative mode almost exclusively (no I don't use the camera on auto ;). The reason for this is I don't want to be constantly recomposing to meter. Also whilst Evaluative might not get it right as much as spot metering, but when spot metering gets it wrong, it can get it wrong by a huge margin. Example, if you happen to have a deep shadow in the centre of the scene you'll massively overexpose.

At the end of the day metering is fixable in post as long as you're close, unlike for example, focus. So when shooting quickly I concentrate on nailing the focus, even if it means the metering is off. Evaluative allows me to do this. If I'm not shooting quickly I'll bracket exposure or use manual to get the exposure exactly how I want it.

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