This post is part of a series of posts about image quality. This post is going to be looking at Stitching Images together to potentially raise the quality of an image. You can follow the Image quality series here.
The idea behind super resolution came from astrophotography and the principle of stacking images. Stacking is when you place one image above another forming a stack. When an image is taken the sensor records the scene exactly the same (if the light is consistent) except for digital artefacts like noise, which are randomly created and so different in each image. This is why stacking an image has the ability to reduce noise. With every two images that are stacked you are in effect reducing the amount of noise by half in relation to its corresponding ISO. Simply, four ISO 3200 images stacked have a similar level of noise to that of a ISO 800 image. The drawback is that you need to have a scene with no movement, because movement will be seen when the stack is created. In astrophotography this process of stacking images enabled photographers to create the look of a longer exposed image, showing star trails in the sky, at the same time as reducing noise created from a longer exposure.
Photographing for Stacking
All of the images need to be taken from the same point with no movement in between, this means that you need a tripod. On a DSLR it would also be a good idea to lock your mirror up so as not to have any reverberation from the mirror moving. Your subject also needs to be still. If you are trying to recreate a faux long exposure you will need an intervalometer that will take a picture at the same exposure at regular intervals.
Stacking in Photoshop
In Photoshop go; File – Scripts – Stack and browse and choose your images. Alternatively you can open all the images and then select the station and choose the option Open Images.
Once you have your files stacked a tiny bit of math is needed to make sure the mean of all the layers is showing through. The equation is 100/n = Opacity %, meaning 100/ the layer number = the opacity for that layer. If you have four layers your opacity settings would be – Background 100% Layer 1 50% Layer 2 33% and Layer 3 25% Layer 4 20% – you have to treat the background layer as layer 1 in the formula.
For this post I took a photo of my cactus.
I took the image on my smart phone, with no tripod I set the phone to burst mode (taking 20 photos in a quick burst) and then chose the first four to place into a stack. When stacking the images, because I didn’t use a tripod, I chose the setting align images.
The difference in the level of noise is quite impressive and wondered what 20 layers would look like.
With the 20 layer version the difference is slightly noticeable as it has slightly smoother tones but I feel that it isn’t as good as it could be, because when you get past 10 images you are dealing with decimals which can’t be set as an opacity setting. The reduction in noise has improved the image quality but still cannot be printed above its basic size.
This is where super resolution can come in.
What super resolution does is enlarge the image and then using the different exposures with different pixels in each place smooth’s out any artefacts creating a larger image than can be made with a single exposure from the same sensor. This process is good when needing to enhance fine detail that or developing more accurate pixels for an image. As with stacking it does have the benefit of reducing noise as well.
For super resolution your process will be similar but with two differences. First you don’t need a tripod but you do need to take shots at a faster shutter speed than your focal length to keep the images sharp (eg 50mm lens your shutter speed can be no slower than 1/60 sec). This is because just like when I created images for stitching you need a slight overlap of the images, though unlike stitching you need a 99% overlap. The best way to do this is handheld as between each shot you will have moved slightly. This slight movement between frames will not be a problem as Photoshop will be able to realign the images.
Super Resolution In Photoshop
Once your images are shot, you will need to open them in Adobe Camera Raw. Here make no adjustments, at the bottom click on the output options and set the resolution to the highest + setting (this setting is upsampling the image to its largest workable size by using a best match system to fill in the gaps of missing pixels as the image is enlarged). Load the files in a stack and use the same formula as when stacking images.
Due the new image file sizes, your final stacked file size will be considerably larger. The comparison between the original image and this will be astounding. My new super resolution image has a resolution of 240PPI and a print size of 19”x24”, making it a 26 megapixel image. Compared to the original with a resolution of 240ppi can only print at 10”x13”. Using super resolution has doubled the size of the image.
In comparing the two processes Super resolution is the better method for improving the resolution and reducing the noise in the image. Although this process doesn’t fix any issues with the lens, exposure setting or composition. It also needs a bit of planning and forethought when it comes to capturing the image. But in a pinch knowing how to do this could mean a low quality image can be raised up.
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