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Introduction

abstract . overview
new . since last release
install . getting started
colorspaces . linear
gamut . and color
list . of all actions and scripts
migration . from FFDD5

Examples

contest . user photos
development . details
stitching . workflow
darken . LDR development

Pre-Processing

shooting . bracketing technique
batch . blending

Development

knee . highlight control
dodge . custom exposure
gnd . graduated exposure
contrast . adjustments
zone . check development
gray . b&w conversion

Post-Processing

denoise . noise reduction
sharp . bring in focus
double . image size
grain . add film like grain
gloss-diff . removal

Miscellaneous

blog . notes and future
performance . photoshop tips
cache . photoshop cache levels
license . agreement

Miscellaneous / Blog / 20060922

On the Topic of Long Exposures For Night Photography

First the visual results. The setup is a simple still life, Canon 5D camera on tripod, no mirror lockup, 29mm f/22 ISO100 with 10 minute exposures timed from the shutter release (after the light goes out). Tried with and without the Canon 5D's in camera long exposure noise reduction. Took shot with lens cap on (dark), and also subtracted dark frame from noisy frame. Each shot labeled with the time it was taken, and each crop from the same location on the frame. Images processed with levels (to make it easier to see) and USM, but no noise reduction.

Key Results

1. The long exposure noise tends to be the same across frames taken with the same shutter time within a three hour time period. The noise on the 9:35 frame, 10:52 dark (lens cap on) frame, and the 12:54 frame all have nearly the same noise.
   
   
2. The long exposure noise on the Canon 5D is mainly in the reds. Look at the 10 minute exposure taken with the lens cap on. This fact might be useful in the creation of a long exposure noise filter.
   
   
3. Subtracting out the lens cap on exposure looks better than the Canon 5D's built-in long exposure noise reduction. Note this only works if you process the frame in a linear colorspace. The Canon 5D's built in long exposure noise reduction (see the custom function menu) tends to blacken the noisy red spots. While simply subtracting the frame taken with the lens cap on (same 10 minute exposure) tends to remove a majority of the noise without adding new artifacts in the darks.

Comments

So the idea of shooting the same long exposure at night multiple times and averaging the results will not work, because the noise will be mostly the same in each exposure. Seems like the best solution is to take the first shot as normal, then immediately take the next shot with the same apature and shutter time but with the lens cap on. Then subtract out the dark frame in post processing. This same process should work for long exposure bracketing. Assuming you have enough backup batteries!