This technique obviously has a long way to go before its useful with a quality outcome, but for a start I'm pretty happy given the in camera shot was actually black with dark warning on nearly every pixel.
To achieve this result I took two images through the black glass (16 stop neutral density Hitech Firecrest), one for the sky and background which was a ten minute exposure and the second which was 4m52s which was the duration of the show.
|Image 1: Ten minute exposure to capture background and sky, iso 400 f/10 24mm|
|Image 2: 4m52s exposure to capture the fireworks, iso 200, f/4|
I blended the two images together manually in photoshop using a layer mask and various sized brushes from large to get the bulk of the fireworks to 1 and 2 pixel brushes pixel peeping to get the light trails emanating from the main blasts. I could not use any kind of automated blending because of the bright noise pixels in the underexposed fireworks shot.
I then had a fight with noise and used a combination of the denoise filter in PS, NIK Define2 and finally the colour noise reduction in LR.
|Image 3: Blended denoised outcome shot.|
I'm fairly happy with the outcome as a first try - obviously much to learn, I think next time I might leave the exposure running so the same exposure captures the fireworks and the background. I'd need to get further away from people though as everyone popped on a torch or bright phone light the moment the fireworks ended.
In hindsight there are a few things I should have done differently on the both captures
- Should have used the lowest ISO the camera can do - ISO 50 to reduce noise.
- Should have used a brighter lens - I would still do the background at f/10 for the depth of field, but the fireworks could have been done at f/1.4 using my brightest lens which would have helped a lot in reduction of noise.
- I ended up cropping the image so did not need to go so wide - had I gone with 50mm instead of 24mm I would have had a much sharper brighter outcome as that lens is f/1.4 wide open; and
- Learn layer blending to improve the outcome - the fireworks - especially at the top plume are quite blurry in the final outcome but they were not like that in the original - this is simply my lack of skill (and probably patience) in my layer blending. The denoising robbed some of the detail too.
- I could try taking the firework image and making the non fireworks areas transparent then automatically blending for brightness - that might help. In fact I think I'll try that.
|Image 4: A different background and a different layer blending method (described below)|
Here is another version, I took the background as an image I did about 20 minutes earlier so the sky was brighter and instead of hand painting in the fireworks, I made most of the fireworks transparent by brushing away the content using smaller and smaller brushes as I pixel peeped around the fireworks. In this one I didn't have the denoise issues around the fireworks so they're nice and crisp however, they are not as dramatic because of the brighter sky. I think I prefer this outcome as the fireworks become more a part of the whole composition. I don't doubt I will keep playing with these ideas to make a technique for this.