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.
Why Create a Stitched Image
One of the main areas we can judge the quality of an image on is its resolution in relation to its print size. The resolution of a digital image can be changed but it will have an effect on its print size. An image at 72PPi (Pixel Per Inch) may say that it is a 10”x8” image but the print quality of 72PPI is not optimal for a good quality image. 240PPI is considered the minimum resolution for a good quality image when printed. This means that the 10”x8” image at 72 PPI when changed to 240PPi becomes 2.4”x1.92”.
Stitching is a way that we can raise a potential image’s quality by placing multiple images together to make up one image. This means that the image is not one frame but many, creating a bigger image in pixels and therefore a bigger printed image.
Before You Start taking Pictures
You start creating a stitched image before you even take the picture. Let say you come across a scene and you only have your phone or a small point and shoot with you. You want an image of the scene and know that it needs to be at least 20”x16”, as this size print would fit nicely in the gap on your wall, and your camera can’t produce those size images. The solution is to divide the scene into sections, take pictures of these individual sections and in Photoshop or another image editor, stitch the images together to create the scene you wanted to capture. This process is similar to that for creating stitched panorama images.
The scene I wanted to capture is of a track next to a railway line leading to a road as the sun is going down. I took this on my phone and the image quality is not brilliant. The original image has a resolution of 72ppi and is an 8 megapixel image. Because I wanted to have a larger printable image of this scene, I took several images with the aim of stitching them together.
When Shooting
I usually take a reference image first and compose my shot in a single frame. I then look at my reference image and map out how many images I will need to take. Sometimes this is only 4 images with about a 1/3 overlap in each shot. I overlap by 1/3 so as they are easier to stitch together. There can be times when I take up to 12 images ( four images in three rows). The images you have to stitch together the larger the file size and time it takes to stitch together.
Best practise is to use a tripod so the images are all taken for the same point. I also usually set the focus and exposure to manual to also make sure there is no difference in exposure and focus. With exposure readings I take an average reading across the whole scene to make sure it is evenly exposed. Any minor changes in exposure can be edited in post.
If the image is a landscape I take the individual frames as portraits and a portrait image is taken landscape frames.
When using a mobile, to take the pictures, focusing and having the correct exposure will be a major issue. To try and keep the focusing set to one point, I choose one point and touch focus with the camera interface. Exposure is something I have do by eye and use exposure compensation if needed to keep a similar exposure across the whole scene.
Stitching
Stitching cannot be done in Lightroom but you can edit a stitched image there. To stitch an image you need to use Photoshop or an equivalent program. Since I have Photoshop I will be writing about how to do it here.
1) File – Automate – Photomerge. If you use Adobe Bridge you can select the images and then automate through bridge.
2) In the dialogue box select your files with browse, select auto on the left side (I have found this creates the best images) and tick all the boxes at the bottom. This will allow Photoshop to do the work of blending and correct lens distortions.
Once you click OK, how long the stitching process will take depends on the size and number of images.
3) I straighten and correct shifts using Filter-Lens Correction. To do this you either need to flatten the layers or turn them into a smart object ( I like to turn them into a smart object so if I need to I can correct any blending issues in the future.
4) Edit
The resolution of the image has not changed but the number of pixels in the image has. With stitching it has created a 21megapixel image. Of course this does not compete with the DSLRs out there since the resolution is still 72ppi. The impact from stitching is that I can print a bigger than before. The original realistic print size was 8”x10” with the stitched image we can now print around 10”x16” inches.
Not a Magic Bullet Solution
Stitching does not improve the camera that an image is taken with. If your camera has a noisy sensor and your lens not sharp then your image may still not be of the best quality. What stitching does do is allow you to create an image that can be printed bigger and has more pixels than a single shot. Stitching can also be used, when your lens is not wide enough to capture an image and you cannot move back anymore. Allowing you to capture an image you may otherwise couldn’t.
Remember Your Poor Computer
The image below is made of 12 HDR 16bit images. the stiching process took about 30 mins and created a 6gb file (with the layers not yet flattened, once all flattened the image file size was about 1gb) at about 32megapixels in size with a print size of 26” by 42”. This image could be a huge power drain on your computer and if your machine is not powerful enough and it can slow it right down. I saved the original stitched file (with flattened layers) and then created a duplicate file at a smaller size and more realistic for my needs. Since I edited the image in Lightroom if I ever need a much larger version, I can import this one into Lightroom and sync the adjustments.
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