Subject: Re: Exposure to the right and tone placement
why is it not a good idea to *systematically* overexpose (as long as we
don't clip the highlight details which are important in our final prints/images)?
Of course, I'm not Bruce, but I did start this thread and at times my views have differed somewhat with the experts (including Bruce) and I present the following as somewhat of a consensus with which I hope Bruce will agree.
With regard to "overexposure" much depends on its definition. I would regard optimum exposure as overlaying the dynamic range of the scene with the dynamic range of the sensor in such a way as to maximize the capture of the scene levels. When the scene is less than full scale, you have some lattitude in highlight placement but it is best to place the highlight at 4095; the mid tones would be above the midpoint (2048). With Kodachrome this would be overexposure since the mid tones would be too bright, but it is proper exposure with digital since those tones can be re mapped. Since your metering might be less than perfect, it might be advisable to place the highlight slightly lower so as to leave yourself a little headroom.
If the dynamic range of the scene (or at least that portion of the scene you wish to capture) and the camera are the same, maximum levels will be obtained when the highlight is placed at 4095. If you overexpose and blow the highlights, highlight levels are lost and you have not gained anything useful at the shadow end. However, in this case you might not want to leave any headroom at the top but rather use highlight recovery in ACR if your exposure was slightly over.
If the dynamic range of the scene is greater than that of the sensor, you obviously have to make a choice. Placement of the scene highlights at less than 4095 will result in loss of data levels in the shadow areas. With the highlights placed above 4095, highlights will be clipped and lost, but additional shadow levels will be gained.
Since the camera meter is calibrated for the equivalent of 12% gray reflectance, it might appear logical to take a reading from a mid-gray area in the scene and base the exposure on that reading. However, this will not take the dynamic range of the scene into account and may not place the highlights at 4095.
If you are using zone exposure concepts, you want to place zone I at 4095 (expose for the highlights). The other zones will fall in place. Theoretically you could take a mid-gray reading and place the exposure so that the highlights are at 4095, but selecting the exact gray to meter would be difficult end error prone. Of course, the Zone System was not developed for digital, but many of the concepts are still valid. Adams exposed for the shadows (with negative film) and developed for the highlights. With digital, we expose for the highlights--the response of the sensor can not be altered, but the developemnt can be handled digitally.
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