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With Exposure Compensation, we only correct the exposure when the camera has done a poor job, and this is recommended whether you shoot RAW or JPEG..
With Exposure Optimisation however, we go a step further and use the histogram to set the exposure for the best quality RAW file - not for a pretty preview / review image. This approach helps to deal with high contrast scenes, combat noise and obtain the highest quality post processed images.
These techniques are recommended for those who shoot RAW. Don't do this if you shoot JPEG!
A histogram is a graphical representation of the tones in your image. Shadows are on the left and the highlights on the right, with mid-tones in-between.
This tutorial explains how it's possible to optimise exposure using the histogram - in some cases, paying no attention what so ever to the exposure meter!
My method of exposure optimisation mostly adopts the principles of Michael Reichmann's 2003 tutorial, "Exposing (to the) Right" (ETTR), and his 2011 follow-up, Optimizing Exposure (the latter more asks why cameras still don't have a setting to do this automatically).
Basically, you set exposure so the histogram is as far to the right as possible, but returns to the bottom without clipping (being cut-off or spilling over).
So if your camera's exposure setting shows a histogram like the one above, simply adjust the exposure so the histogram is pushed to the right (shown right). Of course, not all histograms look like these; the optimisation process, however, is mostly the same.
You get a better RAW file doing this because (without being too technical) there's far more "space" for the data and less noise on the right than there is in the shadows on the left.
It can be compared to lowering ISO below base, although conversely, we can benefit from the technique at slightly higher ISOs where necessary.
In landscape photography, the angle of view is usually wide (zoomed back), and there are often both bright sky and deep shadows in the composition. Such contrasty conditions can cause tones to spill over at each end of the histogram (shown left [above-left on mobile]).
Optimisation is just the same here; simply adjust the exposure so the histogram is as far to the right as possible but returns to the bottom without spilling over (shown right [above-right on mobile]).
In this case, you've saved the highlights from being blown-out. Shadows will perhaps spill over on the left side of the histogram, but this is fine; the RAW file will actually contain lots more shadow data than what's shown here.
Caveat: If a small horizontal line persists at the very bottom of the graph at the right, don't try to pull that back in; it represents tiny specular highlights and you'd ruin the image's overall exposure in doing so. Just let it clip.
If you use a DSLR, you will first have to go with the camera's metered exposure (or estimate any compensation) and look at the histograms on review (after taking the shot).
Check the colour histograms (shown right) to see if any colour channels are clipped / blown (spill over at the right) or if there is room to push them further to the right without clipping.
If highlights are clipped on any channel, reduce the exposure and re-shoot. Likewise, if there is room to push the histogram to the right before clipping, increase exposure and re-shoot.
Live view cameras, such as CSCs, SLTs, super-zoom / bridge and advanced compacts, usually have the option to display a real-time mono histogram of the frame's exposure before you've taken the photo; using this can help to capture an optimally exposed RAW file on the first shot.
Even if you have a live view histogram, it's still best to check the colour histogram (shown right) on review, because one channel (usually red) can have blown / clipped despite it not appearing so in the mono histogram. This is especially the case when photographing brightly coloured objects.
If any colour is blown, you have to re-shoot at a reduced exposure.
For this reason, I recommend selecting a default review display setting with colour histograms (if your camera has them) and preview image.
In aperture priority, shutter priority or programmed auto, your camera is always ready to use at the metered setting, whatever the light, so one of these may be your preferred choice.
In in these modes, you have to use the EV button to apply compensation for an optimised exposure; this must be set back to zero afterwards.
With a DSLR, I would probably use this method.
In fully manual mode, you set your own combination of aperture and shutter speed and use the EV meter (see below) to see how this compares to what your camera's metering thinks is middle grey.
I wouldn't use this method with a DSLR (using the default optical view), because without any sort of preview of the exposure, you either have to guess a degree of compensation or run with the metered reading - the latter, one of the other modes can do instantly!
The only advantage is not having to set EV back to zero after taking a compensated shot.
If your camera is designed primarily around electronic viewing (CSCs, SLTs, super-zoom / bridge and advanced compacts), by using the live histogram, you can pre-optimise the exposure of every shot, quickly and easily. Yes, it's still best to check the colour histograms on review, but you have a great chance of an optimised exposure - first time!
I use this method with live view cameras. I find it much quicker than compensating with aperture priority. With aperture set and fixed, I simply frame-up the composition and scroll the command wheel (adjust shutter speed) until the live histogram is optimised - simple!
Of course, this isn't for sports and action photography, but we're talking about shooting the optimal RAW file here, and particularly in conditions where dynamic range can be a challenge for camera and photographer.
The histogram represents the JPEG generated by the camera processing. But we're interested in the capturing the optimal RAW file, which doesn't involve the camera's JPEG processing at all, so it's best to make the histogram represent the RAW file more accurately.
To do this, go to the menu and set contrast, saturation and sharpness to their minimum values (often a negative, -2, -3 etc).
At the default settings, the histogram will show clipping before the RAW data has clipped. Setting them to the minimum resolves (some of) the inaccuracy. Also, turn off features like "Active D Lighting" (Nikon), "Dynamic Range Optimisation" (Sony) and "Auto Light Optimiser" (Canon); these features are good for shooting JPEG, but, again, adversely affect the histogram for RAW exposure optimisation.
The more accurate the histogram, the better optimised the RAW file will be.
Practice and get to know your camera's histogram.
More often, a slight amount of clipping in one or more channels proves not to be the case when you come to post process the file - there's often a bit of hidden head-room. So in very contrasty conditions, allowing a little restrained highlight clipping can pay dividends with the shadow data. Also, light sources and, as previously mentioned, specular highlights, are best left to clip.
Contrarily, don't push too far to the right if there's a lot of important highlight detail in your image (high key), because if one channel gets clipped and recovered in post processing, the colour rendering may be affected.
"But should I still expose to the right if I'd have to use a higher ISO?"
Yes, up to a point. And that point depends on what camera you're using:
However, make sure you're using the widest aperture and slowest shutter speed that circumstances/ requirements will allow before increasing ISO.
That's it. Exposure Optimisation - optimised RAW files for better photos.
If you use a live view camera, it's quick and easy to get the optimal exposure using the live histogram (with no need to look at the exposure meter) and if you shoot RAW I would consider this for your shooting routine.
With a DSLR, I would shoot at the metered setting and re-shoot with compensation if the review histograms show I should.
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