Luminous Landscape Forum

Raw & Post Processing, Printing => Digital Image Processing => Topic started by: richarddd on April 02, 2012, 05:50:23 am

Title: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 05:50:23 am
I'd like to take multiple short exposures and combine them in photoshop to simulate a long exposure, but can't remember the technique.

For example, instead of a 10 second exposure of a waterfall or other moving water, you would take ten 1.0 second exposures and combine them. It would be better to take the shots on a tripod, but I suppose you could use photoshop to align handheld images.

This would be useful if it was bright and you didn't have a sufficiently dark neutral density filter to allow a long enough exposure for a smooth blur.

I believe the technique is some variant of median image stacking (useful for eliminating things that move between shots, such as annoying tourists) or using blending modes or opacity adjustments on a number of layers, but I'd rather not reinvent the wheel if there's a standard technique.

EDIT: each shot would be correctly exposed.  The reason for not doing a single 10 second exposure would be that there is so much light you can't stop down enough to get correct exposure.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: hjulenissen on April 02, 2012, 05:55:33 am
I am not familiar with photoshop, but do you need a name for this technique? I would open the N photos, and take the average of them.

I believe this works better for diffuse/chaotic blurring than it does for e.g. star trails, as you will have discontinuities.

-h
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Tony Jay on April 02, 2012, 06:09:23 am
I believe this works better for diffuse/chaotic blurring than it does for e.g. star trails, as you will have discontinuities.-h[ /quote]

Because stars move slowly if you do not have too big a time interval between images there should not be any discontinuities.
Once the images are stacked another advantage is a zero noise result - the noise is averaged away.

I must admit I have never thought of using the technique to blur something like a waterfall - must try it.

Martin Evening's book on PS does describe stacking and would be a good resource for the PS processing.
I think that the in camera methodology depends on the subject and the lighting - however I would be happy to be contradicted on this since, as stated, I have never tried the technique in this context.

Regards

Tony Jay
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Bart_van_der_Wolf on April 02, 2012, 06:55:46 am
I'd like to take multiple short exposures and combine them in photoshop to simulate a long exposure, but can't remember the technique.

For example, instead of a 10 second exposure of a waterfall or other moving water, you would take ten 1.0 second exposures and combine them. It would be better to take the shots on a tripod, but I suppose you could use photoshop to align handheld images.

Hi Richard,

Since you are effectively under exposing each partial exposure, you need to Add the signal levels to get close to the intended brightness with Image|Apply Image... or Image|Calculations... . Unfortunately, this will also add noise (no free lunch here either). You can of course use mild noise reduction before adding for a better result.

Averaging is only useful if each individual exposure is basically correct, or if very weak signals need to be pulled out of the shadows.

Cheers,
Bart
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Christoph C. Feldhaim on April 02, 2012, 07:03:45 am
Just stack the images and apply 100/position percent of opacity to the layer.
Lowest Layer: 100/1 = 100%
Second lowest layer = 100/2 = 50%
Third lowest layer = 100/3 = 33 %
and so on ...
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Tony Jay on April 02, 2012, 07:17:57 am
Pretty simple rule of thumb.

I will certainly give this a go.

Regards

Tony Jay
Title: Re: Combine multiple shots to simulate long exposure?
Post by: john beardsworth on April 02, 2012, 07:22:45 am
Can you do it in camera? Some Nikon cameras have multi exposure and image overlay options.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: hjulenissen on April 02, 2012, 07:30:16 am
Averaging is only useful if each individual exposure is basically correct
My bad.

-h
Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 08:08:55 am
Since you are effectively under exposing each partial exposure
I fear I was unclear.  Each shot would be correctly exposed. 

Imagine you want a 10 second exposure to get the right amount of smoothing blur, but there's so much light on the scene you can't stop down enough.  At 1 second, you can stop down enough to get correct exposure, so you shoot 10 correctly exposed images.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 08:17:45 am
Quote from: Tony Jay
Martin Evening's book on PS does describe stacking and would be a good resource for the PS processing.
The Martin Evening & Jeff Schewe CS5 Ultimate Workshop book describes using median rendering on an image stack.  They use the technique to eliminate people from a scene, as I mention in the OP.

Will this work to simulate a long exposure, smoothing moving water?  I had been thinking it wouldn't work properly, but may well be mistaken.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 08:21:14 am
Just stack the images and apply 100/position percent of opacity to the layer.
Lowest Layer: 100/1 = 100%
Second lowest layer = 100/2 = 50%
Third lowest layer = 100/3 = 33 %
and so on ...
Will that work to smooth out moving water?

Now that you mention it, I vaguely remember seeing that technique somewhere before.  Is smoothing its main use or is it usually used in another context?
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Bart_van_der_Wolf on April 02, 2012, 08:45:16 am
I fear I was unclear.  Each shot would be correctly exposed.

In that case, use the averaging techniques mentioned by others. You will run the risk of other scene elements (e.g. branches/leaves in the wind) creating ghosting artefacts, unless you use Median averaging of an image stack (which would work less well with moving water that needs to be blurred).

Cheers,
Bart
Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 08:54:51 am
I just tried putting my camera on a tripod, then shooting an object, moving the object a bit, shooting again, move some more, etc.

Image stacking median just about made the object disappear while leaving the background visible.

Christoph's 100/1, 100/2, etc. and image stacking mean produced what looked like identical results, in which a ghostly version of the object appeared multiple times (once for each capture) against a constant background.

Each image was on a separate layer.  To do image stacking, highlight all layers, then Layer->Smart Objects->Convert to Smart Object.  For mean or median, Layer->Smart Objects->Stack Mode->[Mean or Median]
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Christoph C. Feldhaim on April 02, 2012, 09:01:43 am
The method I described basically is an averaging by using the mean as a measure of central tendency.
Each layer in the end image should have the same weight like this.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: hjulenissen on April 02, 2012, 09:11:14 am
I wonder if there is a lazy approach to this. What if I want waterfall blurring like a 10s shot, but only have patience to take 2 images, and exposure conditions suggests a 1/100s exposure?

For each pixel/region:
"amount of local blur = alpha * abs(image1 - image2)"

-h
Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 09:26:39 am
I suppose my next step is to shoot actual moving water, then try mean and median and see which works better.

Bart, if there's ghosting from leaves, you could pick the shot with the best leaves and mask to combine those leaves with the averaged water. 

Christoph, that makes sense, you are in essence taking the mean and so is image stacking mean.

For a very lazy approach, there's http://www.photoshopessentials.com/photo-effects/waterfalls/ (use photoshop's motion blur filter on the water).  This would not be my choice of techniques.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: RFPhotography on April 02, 2012, 11:27:08 am
Stack mode Mean or Median may do what you want.

You can also try using layers with the Lighten Blend Mode.

Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 01:08:50 pm
Lighten blend mode, with my test in which I shoot an object, move it, etc., results in seeing  the object ghosted in many places, but significantly darker than using image stack mean.   I'll have to add it to my list of things to try in a real test.

See above for discussions of image stacking mean and median
Title: Re: Combine multiple shots to simulate long exposure?
Post by: RFPhotography on April 02, 2012, 02:01:48 pm
I saw the above.  Your example was flawed, unfortunately, because you weren't working with the right conditions.  I actually did this with images of a static scene and moving water.  Samples below.  One of the two original images, Lighten Blend Mode, Mean and Median stack modes.

But yes, you do risk ghosting using whatever method if there are other parts of the scene that are moving. 

Title: Re: Combine multiple shots to simulate long exposure?
Post by: richarddd on April 02, 2012, 03:01:40 pm
I actually did this with images of a static scene and moving water.  Samples below.  One of the two original images, Lighten Blend Mode, Mean and Median stack modes.
The four pictures look very similar to each other.  I'd expect the combined pictures to look much smoother than the original images, as the scenario I'm trying to deal with is that it's too bright to take a single exposure that's long enough to sufficiently blur the image.  By combining a number of pictures, each with a relatively short exposure (so the water is not smooth in any one picture) you end up with a smooth image.

I may well be confused about exactly what you are doing
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Guillermo Luijk on April 02, 2012, 03:58:15 pm
If you really want to achieve the same as a long exposure you need to:

1. Make the delay times between shots negligible compared to the duration of each shot (if there are bright moving objects such as lights, some gaps can still be visible in the resulting image such as those seen on mutiexposure star trails).
2. Average the images with the following weights (upper to lower layer):
For 2 shots: 50%, 100%
For 3 shots: 33%, 50%, 100%
For 4 shots: 25%, 33%, 50%, 100%
and so forth
3. Use a linear (gamma=1) setting in Photoshop. No one has mentioned this, but as long as you use any gamma profile the result will not be a reliable simulation of a single exposure (remember a sensor is a linear photon counter).

If you accomplish with those 3 rules you'll achieve the same visual effect as a single longer exposure, but it will have a much higher noise quality since averaging will improve SNR.

Another drawback is that sharpness can be reduced because of minimal misalignment if you cannot lock the mirror up during all shooting time.

Regards
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Tony Jay on April 02, 2012, 04:29:16 pm
Thank you Guillermo.

Although I did not start this thread I have found it most instructive.
In the next few days I will test your suggestions.

Regards

Tony Jay
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Christoph C. Feldhaim on April 02, 2012, 04:32:32 pm
3. Use a linear (gamma=1) setting in Photoshop. No one has mentioned this, but as long as you use any gamma profile the result will not be a reliable simulation of a single exposure (remember a sensor is a linear photon counter).

WOW - I just had overlooked that.
Should have known additive operations are not allowed at non 1.0 gamma..../me slaps forehead
Title: Re: Combine multiple shots to simulate long exposure?
Post by: RFPhotography on April 02, 2012, 04:37:45 pm
The four pictures look very similar to each other.  I'd expect the combined pictures to look much smoother than the original images, as the scenario I'm trying to deal with is that it's too bright to take a single exposure that's long enough to sufficiently blur the image.  By combining a number of pictures, each with a relatively short exposure (so the water is not smooth in any one picture) you end up with a smooth image.

I may well be confused about exactly what you are doing

They do look similar at such small size.  But if you do the same things on larger images, you'll see the differences, which are in the water areas.


3. Use a linear (gamma=1) setting in Photoshop. No one has mentioned this, but as long as you use any gamma profile the result will not be a reliable simulation of a single exposure (remember a sensor is a linear photon counter).



This is achieved how GL?
Title: Re: Combine multiple shots to simulate long exposure?
Post by: wolfnowl on April 03, 2012, 01:41:22 am
This might be of use to you: http://www.wolfnowl.com/2010/12/photographing-moving-water/

Mike.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: Guillermo Luijk on April 03, 2012, 03:18:27 am
WOW - I just had overlooked that.
Should have known additive operations are not allowed at non 1.0 gamma..../me slaps forehead

Additive operations are 100% allowed at non 1.0 gamma, but they will not exactly mimic what you get in a single exposure (I did a comparision one day but cannot find it now, will repeat it again tonight just for curious). I was just being strict here though since the results between adding images using 1.0 gamma profiles and any other arbitrary gamma will not differ much; in fact you may even prefer those obtained using gamma profiles. In addition to that the 1.0 gamma images must have been neutrally developed (not contrast, bright or saturation adjustments yet), then added, then processed.

This is achieved how GL?

Just need to convert images to linear versions of your profile (sRGB, Adobe RGB,...). You can build a linear version of any profile straight in PS. I explained how in the DCRAW Tutorial (http://www.guillermoluijk.com/tutorial/dcraw/index_en.htm), in the 'COLOUR PROFILE' section near the end. But as I said, this is just to be strict, I don't think the mess is worth.

Regards
Title: Re: Combine multiple shots to simulate long exposure?
Post by: RFPhotography on April 03, 2012, 07:41:01 am


Just need to convert images to linear versions of your profile (sRGB, Adobe RGB,...). You can build a linear version of any profile straight in PS. I explained how in the DCRAW Tutorial (http://www.guillermoluijk.com/tutorial/dcraw/index_en.htm), in the 'COLOUR PROFILE' section near the end. But as I said, this is just to be strict, I don't think the mess is worth.

Regards


Converting the images to the LinearRIMM profile available from Color.org would also work, no?  That assumes that the RAW image is tagged with the ProPhoto space to begin with.
Title: Re: Combine multiple shots to simulate long exposure?
Post by: milt on April 03, 2012, 11:27:26 am
Unless you are absolutely fixated on Photoshop, there is an easier way to do this.   PhotoAcute will smooth out the water, do ghost removal on the static parts, improve noise over the static parts, and as a free bonus, quadruple the number of pixels.  I have found it best to feed PhotoAcute lots of images (I see improvements up to around 15-20).  I have never tried hand-held shots.

--Milt--