A Complete Introduction to Stacking Images

Transcription

1 A PDF guide by Version 1.0-3/27/05

2 Table of Contents TABLE OF CONTENTS...2 INTRODUCTION TO STACKING...3 WHAT IS STACKING AND WHY USE IT?...3 TAKING THE PICTURES...3 PREPARING THE PICTURES...3 NOISE REDUCTION...5 DIGITAL NOISE...5 STACKING TO REDUCE NOISE...5 EXAMPLE NR STACK...6 ISO PUSHING...7 EXPOSURE BASICS...7 PUSHING...7 EXPOSURE TIME...9 METHOD #1: LIGHTEN...9 METHOD #2: AVERAGE...10 METHOD #3: SCREEN...10 DYNAMIC RANGE...12 WHAT IS DYNAMIC RANGE?...12 EXTENDING DYNAMIC RANGE...12 APPENDIX...14 QUESTIONS, COMMENTS OR SUGGESTIONS?...14 THANKS...14 USEFUL STACKING LINKS...14 ver 1.0 3/27/05 page 2 of 14

3 Introduction to Stacking What is Stacking and Why Use It? So, you have heard people mention stacking images to produce some wonderful results, and you want to know just what it entails. Well, that or you may have opened this document by mistake, in which case I will suggest you pay closer attention to where you click. Either way, this tutorial will explain the basics of combining multiple exposures into one. This combination of images is what the term stacking refers to. Stacking is done using any imaging program that supports layers. Photoshop, Paint Shop Pro, and The GIMP (in descending order of financial burden) are three of the more common programs that can be used to stack images. Paint.exe just won t cut it here folks. There are several useful ways to stack images. The ones discussed here include noise reduction, ISO pushing, exposure lengthening, and dynamic range extension. These methods allow you do to produce images that are not possible at this time with a consumer digicam. Before we join the ranks of stacking masters, I d like to go over some basics. These are things that you need to keep in mind when you are dealing with images that you plan to stack later on. Taking the Pictures Stacking images starts before you even release the shutter. Stacking effectively is impossible if your images are not properly aligned. As a result, handholding while shooting pictures that you intend to stack makes it considerably more difficult to actually stack them. This means that you generally must shoot on a tripod. And even with the camera on a tripod, shooting images for stacking requires very gentle movements. Because of this, a remote can be a very useful thing. Using one will allow you to literally not touch the camera while you shoot the images to be stacked. When you are taking the pictures you also must pay attention to certain metering concerns according to what method you intend to use. It s also best not to change any camera settings other than those that you need to for the stack. This will be covered later in this document. It should also be mentioned that with the exception of an ISO push, these techniques aren t very viable for photographs of any moving object, since they require multiple, identical images. Preparing the pictures Once you ve taken them, you ve got to get your batch of perfectly aligned images ready to do whatever it is you want to do. First, get them all into your favorite imaging program. Once you ve got them opened up, you need to copy every image you have into one single, multilayer image. For example, if you start out with 7 images, you should have one 7-layer image before you proceed. Now, if you shot on a tripod with a remote and didn t touch the camera at all, you are likely ready to roll. However, if you did not use a tripod (or you did use a tripod, but were not able to use a remote), then you are going to have to be sure that all of your images are perfectly aligned before you start working your magic. To do this, use the difference blend mode. This mode subtracts the selected layer from the one underneath it. If the layers are identical, they will subtract away from each other and you ll end up looking at a black screen. If what you are seeing is not black, you will have to move each layer independently Start from the bottom and work up: turn off ALL layers but the background and layer 1. Turn layer 1 to difference, check alignment, and then re-set to normal blend mode. Then move to layer 2. turn it on, change to difference, check alignment, change back to normal. layer 3.. etc ver 1.0 3/27/05 page 3 of 14

4 until they line up perfectly. If you are having trouble lining up an image, and can t seem to get the black image that I described above, keep in mind that if your images are off rotationally or differ in focal length, you will have a very hard time lining them up. These problems require special attention which I will not detail here. Next time you ll need to be more careful with your movements. Now, you ve got a stack of images that are ready to make magic. ver 1.0 3/27/05 page 4 of 14

5 Noise Reduction Digital Noise Digital noise is very much like grain, a term from the ancient days when cameras were loaded with cellulose like material that was coated with an emulsion layer. This film, as it was called, produced a subtle, random pattern when exposed to light. Noise on the other hand is a by-product of heat (and other sources that are insignificant for our purposes) inside of a digital camera. The higher the amplification, the more noise is apparent, just as in an audio recording. Amplification in this case is commonly referred to as the equivalent ISO of a camera. As you increase the ISO on a digital camera, you are increasing the gain, and hence the noise. Stacking to Reduce Noise Arguably, the most common (and one of the oldest) reason for stacking is to reduce the noise content of an image. If your goal is to stack for reducing noise, then you are already nearly finished once you have all of your images aligned. Simply go to your image stack, and set each layer s opacity equal to 1/(n+1) where n is the number of the layer. The background stays at 100, layer 1 goes to 1/(1+1) = 1/2 = 50%, layer 2 goes to 1/(2+1) = 1/3 = 33%, etc. For example, using 10 images, the sequence is as shown in the table at right. As a note, technically the opacity for each layer is 1/n. however, this does not account for the fact that the background is the first layer, and layer 1 being the second layer, and so on. For simplicity s sake, I am using 1/(n+1) here where n is the number that your imaging software tells you. Now your image is substantially reduced in noise content. How much so, you ask? Well, that answer varies. In theory (mathematically), 2 images will reduce noise by half, 4 images by half again, 8 by half again, and so on. However, DSLRs typically have 2-3 stops less noise than a prosumer at any given ISO. So if you want ISO100 on your prosumer to compete with ISO100 on a 20D, you re going to need at least 7 shots. Yikes. Name (PS) Number Formula Opacity Layer /10 10% Layer 8 9 1/9 11% Layer 7 8 1/8 12% Layer 6 7 1/7 14% Layer 5 6 1/6 17% Layer 4 5 1/5 20% Layer 3 4 1/4 25% Layer 2 3 1/3 33% Layer 1 2 1/2 50% Background 1 1/1 100% according to a few tests that I have quickly conducted, this isn t really the case when speaking of ISO stops on a camera (realize that reducing noise by ½ and reducing noise by 1 stop of ISO isn t the same - your CCD isn t necessarily a linear device). Three images will reduce noise by 1 stop. For example, if you have 3 ISO800 images it will look like an ISO400 image when stacked correctly. The rough breakdown is as follows: 3 images for one stop, 7 images for two stops, 16 images for three stops, and 32 images for four stops. As you can see, it requires a few images and some work to reduce noise drastically. Remember, these numbers may not be perfect (and I am not purporting them to be), but they should be reasonably close to the outcome you should expect. I should mention that this method works only if the noise pattern is truly random. And generally speaking, it is. But dark current noise is not random, though it is very, very subtle and hence unimportant here. On the other hand, interference that the camera might pick up from any electromagnetic field is not random, and in some instances is not subtle. Stacking will do nothing to prevent or mask this type of noise. Luckily, dark current is subtle and interference is rare. Now, go shoot 7 base-iso shots and stack them. Boy does that look smooth. ver 1.0 3/27/05 page 5 of 14

6 Example NR Stack Here is a set of images that show how useful stacking can be to reduce noise. It shows quantitatively what can be expected of stacking to remove noise, and exactly how many shots you will need to reduce noise to a certain extent. This image set also shows a direct comparison to an approximately similar ISO setting. For an accurate comparison, feel free to zoom as far as you need to this PDF is very high resolution. Pay special attention to the detail that comes out of the underexposed areas once the stack is used. This is where noise, and thus improvement, is most obvious. These shots were taken with a Sony DSC-F828, on a tripod, with a remote. Note that sharpening was on (minus) for these shots. No alignment was needed. A Complete Introduction to Stacking Images ver 1.0 3/27/05 page 6 of 14

7 ISO Pushing Exposure basics Before we go over the types of stacking that change the exposure of an image, we should probably cover the basics of exposure. Think of the level of exposure as the water level in a bucket that is being filled by a hose that you control. The water level is decided by three things. The length of the time you turn the hose on, the size of the hose, and the size of the bucket. First, the size of the hose is analogous to the size of the opening in your lens, or your aperture. The wider you open it the more water (or light) will flow through it. Second, the on time of the hose is like the shutter speed you use. The longer you leave the hose on (or the shutter open), the more water (or light) will pass through it. Finally, the size of the bucket is just like the ISO speed you are using; the higher the ISO, the smaller the bucket. As your ISO goes up, the camera needs less light to fill up, just like the bucket does water. Now that we understand what dictates the exposure of an image, we can look at how to mimic these items. Pushing Push processing only uses the screen and multiply blend modes. Screening an image with itself (or another identically exposed image) will lighten the image by 2 stops. To lighten by 1 stop, change the opacity to 50%. To pull ISO into the opposite direction (here is your warning, this doesn t work well past 1 stop), use multiply. The same applies as for screening, 100% opacity denotes two stops, 50% denotes 1 stop. Now I said before that shooting these shots can be tricky, even more so than dynamic range stacking. Here s the reason. If you want to push a shot one stop (or pull it one stop), then you have to shoot it underexposed (or overexposed) by 1EV in camera. Now, you can be inexact and not see Pushing is most useful when your scene is beyond the capabilities of your camera. For example: when you need a fast shutter speed to stop motion but there is not enough ambient light to sustain that shutter speed, or when you need a slow shutter speed to blur motion and there is too much light and you have no ND filters. any disadvantages, but then the whole idea becomes much more random, and you will not know how far to push in post processing. For pushing to be most useful, it is usually done with only one shot, screening it on top of itself via a duplicate layer. This of course is because if you take two shots, you may as well have just taken a longer exposure in the first place. ver 1.0 3/27/05 page 7 of 14

8 While stacking to push ISO using a single shot, keep in mind that the noise will increase accordingly in the stacked ISO1600 or 3200 shot. And believe me when I say that a digicam ain t pretty at ISO3200. Conversely, the more unique frames you have, the less noise your final image will contain. But again, to take more than one frame, your subject must be stationary. If your subject is stationary, then why not just take one, longer exposure? Also worth noting is that if you do take more than one shot, you can only mathematically add two shots that are the same ISO. If you have an ISO800 and an ISO1600 frame, you must push the ISO800 frame one stop to ISO1600 before you can add it to the other frame. Of course, this only applies if you want the end result to work out correctly, mathematically speaking. The example I have shown above (apologies for the quality and subject both leave lots to be desired) is a frame shot at ISO800 in aperture priority mode. At ISO800 and f/2, the highest shutter that my camera could muster was 1/80s. That just wasn t enough to stop the action. By shooting at -2EV (two stops underexposed), or ISO3200, I was able to bump the shutter speed two stops to 1/320s, more than enough to stop a running dog. Of course, the full sized ISO3200 image looked fairly bad. But a grainy shot is better than one that s smeared across the frame, wouldn t you agree? With enough noise reduction and some clever processing, you can get a viable 4x6 out of an ISO3200 shot on a small sensor camera. To accomplish these shots, put the camera in aperture priority, use the highest ISO you can, lock the f/# wide open, and set the EV to -1 or -2 stops. This will give you the fastest shutter speed possible. Conversely, to pull a shot you can stop the lens the whole way down, use as low an ISO as possible, and then overexpose by 1 or 2 stops. As I mentioned previously however, trying to pull-process shots from most digital cameras doesn t work well due to the way highlights are treated. Where underexposed shadows contain detail that can be salvaged, blown highlights do not, and will consequently wind up being a solid gray patch. If you need a longer shutter speed, I recommend you look at method 2 under stacking to lengthen exposure.. ver 1.0 3/27/05 page 8 of 14

9 Exposure Time Method #1: Lighten The first way to simulate a lengthened exposure is to use the lighten blend mode. To implement this, simply change the blending mode of every layer except the background to lighten. This selects the brightest pixel of all the layers at any given pixel to be used in the final composite. The reason this works is that you are mimicking your camera s light gathering Lighten is best to use for a shot that tracks a subject which is very bright against a background that is fairly dark. Screening will brighten that background and averaging will darken the bright object, where lighten will do neither. Star trails are great example of a use for this type of stacking. This technique is the only one here that is not akin to anything achievable on film. behavior. Your camera will only see the brightest event to happen on any given pixel. If you leave your shutter open a long time and something bright moves across the scene, it is recorded on every pixel that it passed across, since the darkness that precedes and succeeds it is overwritten by that bright data. When you add a bunch of layers together in this manner, you start with images of that object being recorded on all the different pixels individually. By adding them you end up with the object recorded on all the pixels it passed across, combined into one single frame. This type of stacking will not blow highlights, since it only chooses the brightest information of the whole stack. Thus, the final image can contain no pixel that is any brighter than the brightest pixel of the input images. If no individual image contains a blown highlight, neither will the final product. Therefore, when metering these shots, meter the scene as if you were taking a single frame. As an added benefit, the lighten method also reduces noise drastically. Noise has a light and dark component. With the blend mode set to lighten, the darker part of the noise always gets ignored in favor of the slightly lighter pixel. After a few images, the lightest noise pixels are all that remain, and are much closer to each other in value. The noise is therefore reduced drastically, and the deviation from pixel to pixel is much less. Take a look at the images at above right. The first image was a single frame from a batch of about 90 shots. These 90 shots were taken over 45 minutes, at 30 seconds (15 second dark frame plus 15 second exposure per shot). The result of the screened stack is reduced noise, as well as the impression that the stars were recorded as they moved across the sky. Note that the exposure is changed minimally, but not completely as it would be using the screen blend mode, detailed in method 3 of this section. ver 1.0 3/27/05 page 9 of 14

10 Method #2: Average The next way to do this stack is exactly the same as if you were reducing noise. This method, like the previous one, will not blow any This type of stack is best to use if you desire a long shutter but do not have any ND filters handy. highlights. However, its outcome is much more akin to actually being a long exposure. This is because it uses - you guessed it - the average exposure on any pixel over the total time. Instead of taking the brightest pixel alone like the lighten mode does, the effect is as if the image had occurred over that length of time. A Complete Introduction to Stacking Images To meter for these kinds of images, simply meter the scene as if your intent was one single shot. The averaging will stretch time without changing the exposure. Very useful. Just as before, the average blend mode has the added benefit of decreasing noise. This shouldn t be surprising, as it is being used exactly as it was earlier. The images on the right show just what the effect of using average is on a set of exposures. The first is a single, 1/10s exposure, and the second is layered from a set of 10 shots. The second then is equivalent to a single, 1 second exposure. Again, feel free to zoom in and examine the images at a higher magnification. Method #3: Screen The third way to mimic a longer exposure is very similar to stacking for an ISO push (previous section). The reason this technique is so similar is that your exposure depends on three things: f/stop, sensitivity, and shutter speed (if you haven t yet read the exposure basics section within the ISO Push chapter, now would be a great time). By stacking images, you are effectively lengthening the shutter speed (the total exposure time is the sum of the individual exposure time of the stacked images). The end result looks the same whether your sensitivity or shutter speed was modified, so you can fake increased sensitivity by This type of stacking is the most using a longer total useful in a situation where your exposure (multiple camera cannot reach the required exposure (for example, over 30 short exposures). seconds on most digicams). Relating back to the exposure explanation used earlier, it doesn t matter whether the hose is left open longer or if a smaller bucket is used. Either way, the bucket will fill up faster. In o rder to use this effect, start with the second layer (layer 1) and change the blend mode to screen and the opacity to 50%. Work your way through all layers, doing the same to each one. Screen will tell your program to add the pixels to one another. Opacity of 50% denotes one full stop of exposure, or ver 1.0 3/27/05 page 10 of 14

11 twice the shutter time. This means that opacity of 100% would actually denote 2 stops, or four times the shutter time. Opacity of 100% is generally not used here since you would be implying that you shot 2 seconds of action over 4 seconds of exposure. This extra stop has to come from somewhere, since your camera only recorded one stop more action, and not two. Therefore, think of it as a combination of lengthened exposure (by one stop) and an ISO push (one stop), both in the same step. A caveat, metering these long shots can be challenging. For the star trail shot shown here, I had to meter the shot as if I were exposing for all 45 minutes. This presents a rather serious problem with digital cameras. Where film will essentially expose to very low light, a CCD has a threshold under which it will not sense any light. What this could cause is that your individual frames might be below the threshold of the CCD. If you have 100 exposures, none of which exceeded that threshold, your end result will be nothing but black. To prevent this, you will need to meter at least slightly above the CCD threshold for each frame. However, be warned that if your images are correctly exposed per-image, stacking using screen will blow the exposure completely. Then again, if your images aren t exposed well enough, you will have very little shadow detail or worse, a completely black frame. In addition to the metering problem, using the screen blend mode has the added disadvantage of making Dark Current noise very obvious. See the example stack. The example shot used here was a set of 60 exposures taken over the course of one hour. They were stacked using the screen mode. Notice how the exposure is completely changed. Also notice upon zooming in how obvious Dark Current noise (seen here as vertical scan-like lines) becomes. This can be fixed with most noise reduction programs, but is nonetheless a consideration. ver 1.0 3/27/05 page 11 of 14

12 Dynamic Range What is Dynamic Range? Putting it as simple as possible, it is the range of brightness which your camera can faithfully record in one image. Unlike our eyes, which have something on the order of 17 stops of dynamic range (DR), cameras only have about 5 usable stops of DR. This presents a problem when we try to capture a scene with very bright and very dark portions in the same frame. Things like sunsets with foreground detail, night cityscapes, or the example I am using here (see below right) can be very hard to capture while maintaining detail in the shadow and highlight areas. Dodging and Burning has been around for many years, and Photoshop maintains these techniques. However they come at the expense of noise and lost detail. If you expose correctly for each part of an image, you will be able to avoid both of these problems. Extending Dynamic Range First and foremost, you must shoot for this on site, and pay attention to what you are doing. If you want to stack images later for dynamic range, you need the appropriate exposures. This takes at least 2 exposures, and can consist of up to as many as you wish. Each must be exposed correctly for some part of your final image. The shot below consisted of an exposure each for midtones, highlights, and shadows. You can also choose to simply bracket the frames, using a spacing of your choice. I will not going to go into the technicalities of bracketing and the like here, since the title at the top of the page says A Complete Introduction to Stacking Images and not A Complete Introduction to Bracketing. Once you have your images in a stack and ready to go, you are faced with a perilous choice, one to which there is no truly accepted correct answer. How are you going to use the bright and dark portions of each image? There are two ways I will describe here, which I feel are the best. The first one is to use masks and paint away different layers, allowing the ones you want to show through. This can be tedious but yields great results in patient hands. For some shots, this process is sufficient. Using a gradient mask correctly will allow you to mimic a ND-grad filter. Again, I will not detail the process of masking here. Since I am about as patient as a 12 year old in line for a rollercoaster, all that hand painting and masking would take more time than I can sit still in one session. So I use the following method, in a short-nsweet form (and an easy one to follow while you work along): ver 1.0 3/27/05 page 12 of 14

13 1. move the correctly exposed image to the top 2. create as many blank layers as you have +/-EV 3. select a -EV layer a. select highlights; copy b. select one of the blank layers; paste c. repeat for all -EV layers 4. select a +EV layer a. select highlights, INVERT selection; copy b. select the other blank layer; paste c. repeat for all +EV layers 5. delete (or make invisible) the +/- layers Depending on your imaging program, the way in which you will select the highlights and shadows of your image may vary. Photoshop users can use Alt + Ctrl + ~ to select highlights, then simply invert the selection. Another way is to use color range to choose the shadow or highlight area of your choice, giving you the flexibility of the fuzziness control. This will leave you with a set of layers that each represents a separate area of correct exposure within the image. You should now have one image that is transparent in the highlight areas and one that is transparent in the shadow areas. Now, discard (or hide if you prefer to retain them) the +EV and -EV layers. Make visible the 0EV layer. You now have increased the dynamic range. This method also gives you the ability to change the highlight and shadow areas by using the opacity sliders on the two layers you created using apply image. Change their opacity and rearrange them until you are satisfied with the result. This can also be done with only two images, even if neither is correctly exposed for any region of the image (not sure why one would do that though...). When you first get your images stacked for this, you may feel that it looks bland compared to your original. This will be entirely possible, since the contrast will be greatly reduced. A little work and that feeling will subside. In addition, having multiple layers will allow you to pick and choose the things you want to edit and change. For example, you can edit just the highlights, or likewise just the shadows. Note in the included sample images the difference in the first and second frames. The first is one image, while the second contains 3 layers, one each for shadows, midtones, and highlights. Notice the blown highlights in the first, and how there is detail retained in those areas in the second. Also examine the shadow areas, for example all of the harsh shadows in the first shot. In the second shot, these shadows are subdued and allow detail to remain visible. ver 1.0 3/27/05 page 13 of 14

14 Appendix Questions, Comments or Suggestions? You can contact the author three ways: on AIM under the screen name monky9000, on the Sony Talk Forum at dpreview.com under the name monky, or via at Thanks Gaurav Khanna helping me proofread and offering suggestions for improvement Useful Stacking Links A repository of very detailed information and reviews on cameras. You will also find a forum comprising the largest online photographic community. There are some very knowledgeable people that call the dpreview forums home. This is a program which you guessed it stacks images automatically. This is a very useful piece of shareware that costs a measly 10 dollars. If you shoot using a remote and do not move at all, this is the only program you need. It does screen, average, and lighten blend modes, making it usable for ISO pushing, exposure lengthening and noise reduction. I have found the screen mode in this program is equivalent to 1 stop per image. ver 1.0 3/27/05 page 14 of 14