Can you explain what exactly exposure target level does. I am using Lightroom and I save recover quite a bit in some shots,this is directly relevant with the target but if its possible an explanation would be awesome.
Can you describe further what it is you are saving? I assume it's highlights.
AutoETTR is first and foremost a white point selection, the white point being the brightest pixels that retain detail (not clipped).
If you have AETTR set to no clipping (0% highlight ignore, clipping mode set to no clipping, midtone and shadow limits off), there will be no clipped pixels. Here, the exposure level target defines where to set that white point. An exposure level target of -0.5EV, tries to place the brightest pixels in the image as far to the right of the histogram as possible without pushing any pixels to clipping.
This might appear as if you have pushed to much detail to white. It has, but you need to be aware that none of that detail has actually been clipped to white. There is a defined difference.
Have a read of
this page. I have linked directly to the zone system on that page.
You can see that there are 3 zones above the highlights that still retain detail. These 3 zones are the areas where the camera is performing it's best. This is where AETTR is very useful, as it's designed to make it easy to push as much of the detail into these 3 zones as possible.
When you take one of these images into lightroom, you might see an extreme amount of very bright pixels, this is a good thing. Remember earlier that AETTR was set to ensure that no pixels have actually been clipped to white. To
fix such an image in LR, you simply have to lower the exposure. Take the exposure slider in LR and move it to the left.
If you were to set the exposure target level to -2EV, you are telling AETTR to to make the brightest pixels in the image -2EV below clipped white. The image might look like the correct exposure when taken directly into LR, but you have wasted 2 full stops of the sensors best performance.
Now, if we decide we want to allow some clipping (specular highlights, or just a simple exposure decision), the exposure target now becomes the point where pixels at this set clipping point are mapped. So if we decide to allow 3% of pixels to be clipped and we set the exposure target to -2EV, those 3% of pixels will be allowed to be above the -2EV point and all other pixels will be below this point.
Now, if we decide to allow no pixels to be clipped to white (as per the first example above), but we have the midtone SNR limit set to 6EV, we are now saying, I don't want any pixels clipped to white, but if the midtone point of the exposure has a Signal to Noise Ratio lower then the point set in the midtone SNR limit settings, let's allow some pixels to be clipped to white to ensure that midtone level has the request Signal to Noise Ratio.
http://www.magiclantern.fm/forum/index.php?topic=5200.msg70150#msg70150
