How to Resize Images Correctly using photoshop
Resizing your images with photoshop is easy right? You would be suprised how many people scale images using the wrong techniques.
How you wish to scale your images will determine what method you use to change their dimensions if you have a strict size that you require a document to be, then the Crop Tool is the only way forward. It might sound a little obvious, but you would be surprised at how many ‘Photoshop Professionals’ we’ve seen who create a new document, copy and paste the image in and then proceed to use the Transform function to scale down the image to fit within the doc. When the dimensions don’t fit the image. they then go on to totally distort the proportions forcing it to fit. This is clearly very amateurish, time-consuming and shows a lack of understanding of the power of our beloved digital imaging package. It’s a lot like owning a Ferrari and only using two of the six gears.
Quick crops
So with that pet peeve out of the way, we’ll continue. You can also use the Crop Tool to quickly change the size of a document if you only require either the width or height – simply enter one of the required values and crop away. This approach can be a little long winded for one image, but if you have a lot you need to get through then this will save you bundles of time.
The cropping process can be automated if you aren’t that fussed about the crop. Open the Actions palette, hit Record and use the Image Size function to reduce the height and width, then the Canvas Size to crop the image. This will maintain the proportions, but you’ll lose control over what portion of the image is lost, so it can be a bit of a gamble. Still, it’s better than the ‘Photoshop Professional’ method.
When using the Image Size function, you’re most likely to put in the required amount and then hit Enter. But there’s a little drop-down menu that’s often overlooked which contains three main options: Bicubic, Bilinear and Nearest Neighbor.
Usually you won’t have to change these and leaving it on Bicubic will do the job, but mastering them will improve the quality of your images whether you’re sampling up or down. When an image is re sampled, an interpolation method is used to assign color values to any new pixels it creates, based on the color values of existing pixels in the image. The more sophisticated the method, the more quality and detail from the original image is preserved.
The image you’re working on will determine what option needs to be employed. Bicubic is used when a smooth tonal range is required, which is why it’s set as the default – it’s perfect for most photographs. When used for blowing up images, a smooth blurring effect will appear around the edges. Bilinear is much the same as Bicubic but reduces the quality more. Again, when blowing up, blurring will occur but a stepping effect will be produced. Nearest Neighbor is best used for images with solid colors such as illustration; as the sampling process preserves hard edges, this method can result in jagged effects. This becomes apparent when distorting or scaling an image or performing multiple manipulations on a selection. The last method is the only one that doesn’t produce an antialiased effect on the edges.
In CS there are two additional options: Bicubic Smoother and Bilinear Smoother – these are pretty straightforward and self-explanatory.
Crop The Image
The Crop Tool can be used in a variety of ways. It can constrain proportions while changing the size of an image, reduce the height or width while taking full control over the resolution, and rotate and crop at the same time.
Image Resolution:
The rule of thumb with image resolution is quite simple: the more info, the better the quality of the final image. Camera phones take pictures at low resolution, so upscaling is unavoidable. Digital cameras also take at a low resolution, but the document size is bigger so quality is maintained, because when the dimensions are reduced the resolution is increased.
Testing
A fantastic way to see these methods at work is to create a new document at 100x 100px and 72dpi, and draw (without using the Vector Shape Tool) a black square in the center of the document leaving a white border. Now open the Image Size function and increase the size to 2500 x 2500px and 72dpi, then hit Enter. Zoom in on the edges to see what effect this has. Do this for each of the re sampling methods to see how they work.
When using the Transform Tool to scale an image, the Bicubic method is employed to re sample the image. As mentioned, this is pretty standard, but if you know another re sampling method that produces better results, first scale the image to the desired dimensions using the Transform Tool. Make a note of its new dimensions, then transform the image in its own document using the Image Size function and correct sampling method.
Resampling
As a rule, blowing up or re sampling images up is a bad idea, because there’s never enough information in the images to maintain the quality. But in this age of camera phones and the like, the occasional re sample is unavoidable, because the quality of a printed photograph at 72dpi will appear blocky, whereas a photograph sampled up to 300dpi will look better. A good way of re sampling is to do it in steps, IE from 72dpi to 200dpi then on to 300dpi. This method gives Photoshop the opportunity to fill in smaller gaps and therefore more of the quality is maintained.
Digital cameras also work at 72dpi but they overcome the resolution problem by having a higher document size, that is also referred to as resolution, such as 2500 x 1875px and 72dpi. As the document is larger, there’s more information contained within it, so when it comes to printing, the document is reduced while the resolution is increased. For example, printing an image with these values at 6 x 4 inches will increase the resolution to over 430dpi.
