Method and apparatus for capturing high quality long exposure images with a digital camera
View Patent ↗A digital camera captures high quality long exposure images by capturing and summing several images of the same scene. The effective ISO of the camera is reduced by scaling the summed image, thus reducing image noise and improving long exposure quality.
1. A method for capturing a long exposure with a digital camera by dividing the long exposure into a plurality of shorter exposures, wherein the digital camera includes an image sensor that (a) is assigned an inherent ISO speed rating that determines at least in part an overall photographic exposure upon the image sensor, and (b) generates an image signal for each shorter exposure and a dark current comprising both a fixed pattern offset component and a random noise component, said method comprising the steps of:
increasing the overall photographic exposure upon the image sensor;
capturing the shorter exposures as a plurality of image frames;
generating an image representation of the long exposure from the plurality of image frames;
generating a representation of the dark current in the plurality of image frames;
processing the image representation and the representation of the dark current in order to remove the fixed pattern offset component of the dark current from the image representation, thereby generating a processed image representation; and
scaling the processed image representation proportional to the increased overall photographic exposure whereby the scaling additionally reduces the random noise component in the image representation.
2. The method as claimed in claim 1 wherein the step of increasing the overall photographic exposure upon the image sensor comprises reducing the ISO speed rating for the shorter exposures below the inherent ISO speed rating such that the image signal for each shorter exposure accumulates to a greater extent than it would, had the inherent ISO speed rating been used; and wherein the step of scaling the image representation comprises scaling the image representation proportional to the reduced ISO speed rating whereby the scaling additionally reduces the random noise component in the image representation.
3. The method as claimed in claim 1 wherein the step of generating an image representation of the long exposure from the plurality of image frames comprises the step of summing the image frames.
4. The method as claimed in claim 1 wherein the step of generating a representation of the dark current in the plurality of image frames comprises the steps of capturing a plurality of dark frames and summing the dark frames.
5. The method as claimed in claim 4 wherein the step of capturing a plurality of dark frames comprises capturing at least one dark frame before the image frames are captured and capturing at least one additional dark frame after the image frames are captured.
6. The method as claimed in claim 4 farther comprising the step of weighting the dark frames proportional to temperature change.
7. The method as claimed in claim 1 wherein the step of processing the image representation and the representation of the dark current comprises subtracting the representation of the dark current from the image representation.
8. The method as claimed in claim 2 wherein the step of reducing the ISO speed rating for the shorter exposures increases the amount of light incident upon the image sensor during the shorter exposures.
9. The method as claimed in claim 2 further comprising the step of displaying a plurality of different longer exposure times and at least one reduced ISO speed rating that is appropriate for each longer exposure time.
10. The method as claimed in claim 1 wherein the overall photographic exposure is increased by at least one stop.