Reciprocity Failure

[Ian_D_Griffiths]

Hi All,

I know this may come over as something as a 'nerdy' question but I plan on doing some long exposures on digital.

I recall from my college days that with film we allowed longer exposures than the meter indicated to allow for reciprocity failure.

For those who perhaps have not heard of this before, this is why many long exposures on film come out under exposed if you just used meter readings. It is caused by the decline in sensitivity of film emulsion the longer it is exposed to light.

Here is a link that will explain it far better than I can: Reciprocity Failure Correction

So, does the chip in a digital camera suffer with the same decline in sensitivity during long exposures?

[ChrisPerry]

I don't think so.
It used to be with film adjusting the exposure would effect the saturation or contrast of the image, and that's not so with digital - as far as I know.

My problem is WB on long or night exposures. What's correct? I can make the sky any color of the rainbow, but I want a baseline of some kind. Would that be daylight WB?

[Ken_Tanaka]

No, reciprocity failure is a phenomenon specific to chemical photography. Noise is perhaps the main issue in long-exposure digital work. My Canon bodies have a specific facility - "black frame subtraction"? - that greatly helps to alleviate such problems for long exposures.

My advice is to just shoot some test frames to learn your camera's particular behavior. Always shoot such exposures in RAW format to give yourself greater leeway. I also recommend shooting at the lowest ISO practical (I rarely travel above 1000). Honestly, I ignore meters when shooting digital at night. Inspection of a few test frames always gets me in the ball park.

[jlipkin]

I have very little experience with long exposures, but I do recall that Seth Resnik talked about this in a presentation I attended about three years ago. He showed two images, one shot at 14s, and another at 16s. The 14s image was fine, but the 16s image had terrible grain/noise. His point was that unlike analog processes where things like reciprocity failure or exposure clipping build gradually, digital has sudden cut offs.

I'd agree that checking the image after you shoot (and shoot RAW) it is a good idea. Be aware, though, that the in-camera preview does not show the RAW image, only a conversion into JPEG based on an arbitrary tonal response curve. In other words, you have more leeway adjusting your RAW images than the preview might suggest. You have a little more leeway in the highlights - what shows up as clipped may be recoverable within reason. Not quite a direct answer, but hope it helps.

[Dave New]

Folks that do astronomy work routinely take multiple exposures, say, of 30 seconds each, and then 'stack' them in layers in Photoshop or some other similar combining tool.

This helps avoid the tendency of the sensor to accumlate large noise levels over many-minutes-long exposures, and also by stacking the images, it becomes easy to detect and eliminate random noise.

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