For any off-camera photo image printing, editing, and general touch-up, it’s critical that one start by using the original* full-sized photo image file that is, the first possible un-corrupted digital file available from the camera.
An image contained in a JPEG file will display beautifully on any PC, Mac, Tablet, or smart phone screen and, will be automatically resized and/or cropped to fit and adapt to the device’s screen specs and/or the criteria of an application-program currently in control. This is completely transparent; For example: A JPEG file from a new DSLR camera may contain a 6000×4000 canvas**. Despite a different shape and resolution, the image in this JPEG file displays perfectly on a Galaxy smart-phone’s 5” widescreen screen and also on my PC’s old VGA CRT monitor. …Amazing!
Before adjusting the file size, shape of image, or size of image, one must first ask: Is the image need resizing or does the file itself need to be resized?
File resizing: Sometimes the original JPEG file size is way too big, that is, is comprised of too many digital bytes and bits; typically too big for texting and/or, takes up too much disk space. Unfortunately, you cannot, repeat, cannot reduce the size of the JPEG file without impacting (corrupting) the image file it contains. There are three ways to downsize a too-big JPEG file. The first is to increase the JPEG files bit compression. Do not lower the quality/compression level more than 50% of the original Image Quality option setting. Any reduction reduces the quality-purity level of the image but keeps the images original size/shape/resolution. Using Photoshop or other editing software, always use “save-as” with a new file-name. The other ways to reduce file size is the impact the actual image size and/or shape contained within the file.
Resizing Image: Reduction of the image file in terms of pixel resolution is another way to reduce the file size. This is called “down-sampling.” Some photo editing software tools also allow one to increase size (resolution) too. Obviously any resolution increase will the also increase the new file size. Any resolution (ppi) changes do not impact image’s original shape/aspect ratio. Resizing by changing resolution (ppi) is in effect changing the canvas** size of the image. This is pixel manipulation and downsizing by photo-editing software tools is quite efficient, upsizing is not. In Photoshop, use the “image-size” function and specify a smaller (or larger) and “resample”. Again, it’s best to “save-as” with a new file name so you can salvage the “original” file.
For example: An original image inside a big JPEG file may have been 6000×4000 pixels. This would be capable of printing with excellent detail onto a big 20”x 13” canvas (photo-paper) at 300ppi. Cutting the canvas in half by reducing the resolution would result in an image file containing the same image albeit on a smaller canvas at 3000×2000 pixels. The lower resolution image would be limited to print onto a smaller 10”x 7”canvas in order to retain similar quality and detail.
Cropping for Printing: Cropping*** an original image inside a big JPEG file may have been 6000×4000 pixels and trimming is almost always required when preparing a photo file for printing. The shape of the camera’s sensor (as contained in the original photo-file) is more than likely unable to fit onto a standard photo-paper size.
For example: An image inside a smart-phone’s original JPEG file may have been 2500×2000 pixels. It must be cropped to a 2500×1667 image to fit onto a 6” x4” photo-paper. The resolution remains unchanged. The size of the JPEG file will be reduced.
Cropping for Composition: The original image may have a subject that is better viewed if centered or visa-versa. Or, there may be an annoying item on the sides in the image. Cropping (out) sections and/or enlarging a particular area are often desirable with or without cropping for a certain print-paper aspect ration. When done to the extreme and the original is at a high resolution, cropping is a way to substitute for a telephoto lens. I do it often. An original image inside a big JPEG file may have been 6000×4000 pixels. Cropping and trimming impacts the canvas, not the resolution. Image quality is not impacted albeit, cropping effectively is like magnifying the image.
Trimming: When only minor crops are performed and without any drastic shape changes, it’s called “trimming.”
* Original Full-Size Image File: All digital cameras create a digitized image in its full size and shape that matches the camera physical sensor. That image is digitized and saved within a photo-file that is formatted in an industry-standard way, most often a JPEG type. The first straight out-of-the-camera photo-file is considered the “original full-sized” photo. In reality, the camera’s on-board computer-processing unit likely modifies the original captured image information before it presents the JPEG. Some of these modifications can be drastic, changing the size, and shape, as well as the image itself. Smart cameras even have post-processing and effects that a user can automatically apply and, this is really replaced the actual original. So, a tip to the wise: Check your camera’s settings. Also note that professional cameras usually provide “RAW” files containing unformatted bit-by-bit photon data that matches exactly what was recorded in the physical sensor. This is the optimum “original” and can be viewed, edited, and converted into a JPEG file by using the camera manufacturer’s supplied custom software tools.
** I use “canvas” to put “crop” terminology in perspective. A photographer places an image on a canvas just as an artist paints a creative image onto a real canvas. The camera’s original canvas “size” is depicted/measured in “pixels” that is, picture-elements rather than in inches/centimeters. The canvas’ “aspect ratio” is the relationship to its horizontal & vertical dimensions. A camera’s canvas size is its starting dimensions conforming to its photocell arrangement in the camera’s sensor and contained in its original output photo-file.
*** “Cropping” means a cutout of the canvas. This is akin to taking a scissors to the canvas and cutting out the part that one wishes to preserve.