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p.8 #1 · p.8 #1 · Easy ETTR technique using the Zebra pattern | |
dclark wrote:
I agree that ETTR should be practiced to get the best possible SNR even if it is not possible to get to base ISO, but if base ISO cannot be achieved due to the limitations on light and other shooting requirements (min shutter speed and f/#) then the strategy to get the best SNR is changed. To get the best possible SNR we want as many photoelectrons as possible. That means as much light on the sensor as possible without exceeding the photoelectron capacity of any pixels (i.e. any pixels for which we can't accept blown highlights). ETTR is the method of achieving that. If we can get enough light on the sensor to get to saturating pixels it is necessary to be at base ISO to keep from saturating the ADC before the pixels are saturated. If the light level is low, minimum shutter speed is high, and DOF requires a large f/#, it may not be possible to get enough light on the sensor to get to the maximum capacity of the pixels. The usual ETTR/bracketing strategy is not possible, since it is not possible to increase the number of photoelectrons in the shadows.
What is the ETTR/bracketing strategy when base ISO cannot be achieved? The only remaining parameter that can be varied is the ISO. How should ISO be set? The best practice is the usual ETTR method, increase ISO without saturating the ADC. With an ideal "ISO-less" camera this does not yield any real improvement. Since the ideal ISO-less camera has not been produced, there is some improvement, although with the best cameras it is very small. I believe it is not wise to push too hard since the gain is small and the risk of blown pixels is unrecoverable. Is it useful to bracket by increasing the ISO to get better SNR in shadows? Again the gain is very small for the best cameras but there is some improvement. Usually I don't bother.
There is an exception. It is useful to study the input referenced noise curve for your camera (http://www.photonstophotos.net/index.htm). That tells you a bit about how close your camera is to ISO-less, but more importantly it tells you if the camera has more than one analog gain (actually a change in capacitance at the sense node). For example, the A7R2 and A7R3 change gain at ISO 640. So with those cameras it is helpful to get to 640. For example, if your base frame is at ISO 400, getting a second frame at ISO 640 will get a frame with improved SNR in the shadows. Capturing a third frame at higher ISO is probably a waste of time.
Dave
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Thank you, this answers many of my questions. So there is marginal improvement, but not much. I'm guessing less than 0.3 stops? Is it worth it to get closer to the clipping point still in order to avoid color shifts? Or should I just stick to ISO 640?
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