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Exposure to the right (ETTR) is a technique of setting the exposure of a scene such that few or no pixels are are over exposed (blown out) thus maximizing the information captured without loosing information to over-exposure. This may produce a darker image but this can be corrected later in post processing. The exposure curve is thus as far to the right as possible without it being clipped.
There are two methods (three really) of using ETTR:
The first method involves taking 2 images. A shot is taken then ETTR looks at it and it modifies the exposure for the next shot. There are two ways to use this method: Always on and Auto snap. The Always on setting sets the ETTR for the next shot on every shot you take for the next shot. The Auto snap setting takes a shot, calculates the ETTR, and then takes the second shot immediately.
The second method uses the live view momentarily to run through a series of exposures until the exposure is correct without taking a photo. You then take the photo.
ETTR sets the shutter speed and ISO. It does not change the aperture setting thus not altering the depth of field of the scene.
In the Expo Tab (Exposure Menu of ML) you will find Auto ETTR:
Pressing the Q button brings you to the settings menu of the ETTR function:
Trigger mode: Always ON, Auto Snap, Press SET, HalfS DblClick. The mode that AutoETTR uses to perform it's function.
For Timelapse set it to Always on.
Slowest shutter: 1/60 The slowest shutter that will allow for the current scene. . For a night time lapse set it to 30 seconds.
Highlight Ignore: 0.2% Percentage of (highlight) pixels to ignore when calculating ETTR. This is used if you want to allow overexposure in the highlights, for example room lights. Typical setting is 0.2% and means ignore almost nothing. If you want to ignore more highlights try 1, 3, 5 or even 10%. If the timelapse has the sun or moon in it, you may want to raise h.i. to 5% or more. Otherwise the scene will be too dark.
Midtone SNR limit: 6 Is defined in EV levels of noise in the Midtones form 1 to 8 EV. The higher the SNR the less noise. Typical settings from 4 to 8 EV. For a Timelapse we recommend 4, 5 or 6.
Shadow SNR limit: 2 Is defined in EV levels of noise form 1 to 4 EV. The higher the SNR the less noise. Typical settings from 2 to 4 EV. For a Timelapse we recommend 2 or 3.
Exposure Target: A general exposure target for the scene, cannot be set to zero but it could be set lower to expose darker. This is not really used very much, usually left at -0.5
Link to Canon shutter: This links the Slowest shutter to the shutter value you set via small scroll wheel. This is very useful for quickly changing the Slowest shutter without having to go into the ML menu. Just set the shutter speed and on the next photo Slowest shutter = new shutter. You'll only be able to set the shutter speed if the camera is in M mode.
Link to DUAL ISO: ON or OFF. In ON position the camera will analyze the scene via Trigger mode (4) and decide if DUAL ISO is required or not. Requires the DUAL ISO module to be enabled.
Note: If Link to DUAL ISO is set to OFF, the module will not calculate the required DUAL ISO and the image will only be set to meet the ETTR requirements.
Show debug info: ON OFF. Will show some of the important calculation results of this module.
If you want more visual feedback as to what's going on, enable histograms and raw zebras:
In the Overlay Tab you can scroll down to Histogram and press Q to configure:
RAW EV indicator: OFF Here you can press the set button and select ETTR hint. This will display in the RAW histogram a hint of how many more EVs you can expose to meet the ETTR parameters set in previous menu.
Also in the Overlay Tab you can scroll to Zebras, press Q, and make sure that RAW zebras are ON. The RAW zebras will show on the image itself to mark the areas where a color channel may be overblown.
ETTR in RAW images - HOW TO USE IT (step-by-step):
If you do not have ML installed in your camera go here: RAW video & ML – Beginners Guide and FAQ – READ FIRST http://www.magiclantern.fm/forum/index.php?topic=5520.0
1. Go to Canon Menu a set Image Review: hold (This gives you a change to review the RAW histogram until you press half shutter to stop the preview, 2 sec is too quick to analyze the histogram but you can set it as desired NO LESS THAN 2 SEC!
2. Put the camera in manual M mode or Av mode (it makes no difference since you must the aperture yourself), and set ISO to any number except in Auto. It wont work in auto!
3. Press trash button to enter ML menu.
4. Go to the Expo Tab and enable Auto ETTR by pressing set.
5. Press Q if you want to change ETTR details. Default settings are good but you may want to adjust Slowest shutter depending on your needs.
6. (optional) enable Global Draw
6.1 Press Q and move to Overlay Tab. Make sure Global Draw in: On, all modes.
7. (optional) enable Zebras
7.1 Scroll to Zebras, press Q, scroll to the bottom and select Use RAW zebras ON.
7.2 Press Q
8. (optional) enable the Histogram
8.1 Scroll to Histogram and press Q
8.2. (optional) Scroll to RAW EV indicator and select ETTR hint by pressing set.
8.3. Press Q
12. Press trash button to exit ML menu.
13. Frame you subject and set the aperture and shoot a test photo.
14. Now look at the display and you should see the image. If the current M settings are not optimal the camera will calculate new settings to be used in the next photo.
The new important features that you are looking in the display are:
RAW histogram on the bottom right is a small graph.
a. Inside this histogram you may see near the top-left of the graph, in white lettering, the letter E followed by a number which represents how far you can go, in EV numbers, to reach ETTR, i.e E1.2 b. Inside the RAW histogram near the bottom you may see colored circles with numbers inside which represent the overblown channels (R, G, B) and the number of EV of over-exposure. To see this aim to an area with higher more light and take a pic. c. If there are overexposed areas you will see RAW Zebras in the color which is overblown.
13. Take another picture without changing the scene and you will see how the camera settings are adjusted to meet the ETTR requirements. If the camera beeps it means that the photo is out of specs and you should shoot again. In the tests I ran I found that in extreme situations the adjustment was completed in three photos. For most situations the adjustment is done in one shot.
With the Always on and Auto snap setting which takes images, it may require at least two images (sometimes three or four) to come up with the proper exposure. Alternatively you can set the Trigger mode to Press Set or HalfS DblClick in which case the camera will go to live view, analyze the scene, and return to photo mode with the correct settings.
When the algorithm cannot find an adequate exposure it will beep twice and give you a message in the LCD screen. ETTR: expo limits reached, indicating the ISO and speed limits used.
When the algorithm finds an adequate exposure it will beep once and give you a message in the LCD screen. ETTR settled at ISO xxx and the shutter speed. Note: the image review time in the LCD is set via Canon menu in Image Review. You may want to put 8 seconds so you can see the results in the LCD before it turns off.
It is best to enable RAW Histograms via Global draw. This will put a little histogram on the LCD during image review. This histogram can show an ETTR hint (how many EVs are required to reach ETTR), or the Dyanmic Range of the image. It will also show the clipped channels via colored numbered circles.
If you also enabled RAW zebras via Global draw, you will also see, if any, areas of the image are over-exposed. Black will be completely overexposed, then you can see red, green, and blue channels clipped areas shown with their respective colors. White will be dark areas completely under-exposed.