Double Exposure Time


Today’s Question: In your answer about in-camera noise reduction you explained that a second “black frame” is captured. Doesn’t that mean you are essentially doubling the time it takes to capture a photo? Wouldn’t a 30-second exposure then take a full minute to actually capture? And isn’t that a good reason not to use in-camera noise reduction?

Tim’s Quick Answer: Yes, in-camera noise reduction does double the total time required to capture a photograph, but in my mind it is well worth the extra time in terms of the improvement in quality (relative to noise levels) in the final capture.

More Detail: As noted in the previous edition of the Ask Tim Grey eNewsletter, in-camera noise reduction generally operates by capturing a “black frame”, which is essentially a duplication of the “real” capture with the shutter closed to prevent light from reaching the image sensor.

This black frame provides information about how the image sensor is performing in terms of noise in the same overall conditions as the photo you are capturing. That information is then used by the camera to subtract the noise from the capture.

To be sure, there are situations where it is more important to be able to work as quickly as possible, or to be able to capture more photos within a given timeframe. But when it comes to overall image quality I consider in-camera noise reduction to be tremendously helpful for long exposures.

You will generally find that in-camera noise reduction is not employed until you reach exposure times of around thirty seconds or more. As a result, the impact of the second black frame capture is somewhat significant in terms of total time. It is also significant, however, in terms of the reduction of noise in the capture.

So, recognizing that using in-camera noise reduction essentially doubles the amount of time required for each long exposure capture, I do recommend making use of this feature whenever time allows.