Combine multiple shots to simulate long exposure?

[richarddd]

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.

[hjulenissen]

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

[Tony Jay]

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

[Bart_van_der_Wolf]

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

[Christoph C. Feldhaim]

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 ...

[Tony Jay]

Pretty simple rule of thumb.

I will certainly give this a go.

Regards

Tony Jay

[john beardsworth]

Can you do it in camera? Some Nikon cameras have multi exposure and image overlay options.

[hjulenissen]

Averaging is only useful if each individual exposure is basically correct

My bad.

-h

[richarddd]

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.

[richarddd]

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.

[richarddd]

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?

[Bart_van_der_Wolf]

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

[richarddd]

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]

[Christoph C. Feldhaim]

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.

[hjulenissen]

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

[richarddd]

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.

[RFPhotography]

Stack mode Mean or Median may do what you want.

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

[richarddd]

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

[RFPhotography]

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. 

[richarddd]

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