How Does Photoshop Handle Resolution & Image Size

EmeraldHike

Well-Known Member
The entire topic of resolution and image size can get technical. Because of that, not many people are interested in it. It's sort of like talking about color and print settings. Even my eye glaze over when it comes to that stuff. It's important to learn about these things though because you'll probably need to know about them someday. My advice is to read a few blog or forum posts on topics such as these every so often, just to stay in shape. No one's asking anyone to become masters at the technical side of Photoshop, but a working knowledge can go a long way.

By this point, I'm sure you've heard of the term pixel. If you haven't, think about pixels in images as you'd think about molecules in a body. They're what makes the image, or body, up. Every image you work on in Photoshop is comprised of pixels. Every photograph you capture with your camera and every web page you look at is comprised of pixels. Pixels are everywhere and by counting them, you can tell how large or small your working file is or can be. Before I continue though, let me show you an up close example of what a pixel is.

Take a look at this image. It's of an ice cream cone. The image itself looks just like a cone, but if you enlarge it like I did, it looks completely different. I enlarged the image as far as it could go in Photoshop so I could see each and every pixel. Then, I took a screenshot and cut out a sample of what I saw. The sample is round.

ice-cream-cone-pixels.jpg

That round area with the distinct pixels in it are of a very small area of one of the chips covering the ice cream. So if you think about it, now that we know pixels make up an image, we can actually count all the pixels in an image to identify its size. More on that in a moment.

The pixels in an image tell us not only the size of the image, but what kind of detail it's got. Images with more contrast between pixels are generally sharper and those with less detail are less sharp, or blurry.

When you view images inside of Photoshop, the size you see on the screen may not be the image's actual size. Some monitors/displays are very high resolution, while others are low resolution. The way Photoshop works with pixels and displays is like this; it takes the pixels in an image and maps them to pixels in a display. So again, Photoshop maps image pixels to display pixels when the image is viewed at 100% size. So knowing that some displays can fit many more pixels in an inch of screen than others can, some images may appear much smaller than others do, when viewed at 100% inside of the Photoshop workspace. So don't be alarmed if your image looks larger or smaller on one device than on another. It's just the way Photoshop works.

Have you ever heard of a megapixel? I'm sure you have. One megapixel is 1,000 pixels in one direction multiplied by 1,000 pixels in another direction. So if you have an image that measures 1,000 pixels vertically and 1,000 pixels horizontally, you have an image that's one megapixel. If you've got an image that's 2,000 pixels horizontally and the same vertically, you have an image that's four megapixels. Every megapixel is 1,000,000 pixels. Obviously, the more megapixels your image is, the larger it will be, both in physical size as well as file size.

Let's take a moment to define what resolution means when it comes to computer monitors, televisions, and inside of Photoshop. When it comes to monitors and TVs, resolution mostly refers to dimensions, such as 1920x1080. Inside of Photoshop though, resolution refers to how many pixels are fit inside of an inch, such as 300ppi or 72ppi. As you may have guessed, ppi stands for pixels per inch. So if you're working inside of Photoshop and you're working on a file that's going to be used for the web, you'll likely set the file to 72ppi or dpi (dots per inch). If you're working on a file that will be printed, you'll likely set the file to 300dpi or higher. On your monitor, the file will appear to be the same size, but in reality, the pixel density will be much higher in the file that'll ultimately be printed.

Okay, here's where it begins to get slightly confusing. Stay with me here because if you do, you'll understand what I'm about to tell you very easily and clearly. Here goes. The question is, if you change resolution inside of Photoshop, will the size of a file change? Let's say you're working on a file that's five inches by five inches. You've got the file set to 300dpi, so the overall dimensions are 1,500 pixels by 1,500 pixels. If you were to change the inch dimension, but keep the pixels at 1,500x1,500, the file size wouldn't change. If you were to change the 300dpi to, say, 150dpi, but kept the pixels at 1,500x1,500, the file size wouldn't change. It's only when you change the pixels per inch that the file size would change. So if you increased the image to ten inches by ten inches and reduced the pixels per inch to match, the file size would stay the same. But if you kept the ppi at 300 and also increased the inch dimensions, yes, the file size would change. It would increase, because you changed the pixel dimensions, or resolution.

The topics of printing, preparing documents for print, and print resolutions are a huge ball of wax. I've been in this industry for years and there are still some areas I have difficulty remembering and understanding. Luckily, I'm not getting into most of this area today, but I will dabble in the periphery though. I'll discuss various resolutions you might want to use for various purposes.

In general, there are two different types of resolutions; low and high. Anything below 150ppi to 200ppi would be considered low and anything above 200ppi to 300ppi would be considered high. Low resolution images are common on the internet. The clearest of images can be viewed at 72dpi online. When it comes to print, I've always used at least 300dpi. There are some considerations that need to be taken into account when dealing with resolution though. I'll discuss them next.

If viewing images on a regular computer monitor or mobile screen at their intended size, there's no reason to have a resolution that's higher than 72dpi. If you were to go higher than that, you wouldn't be gaining much more clarity, but you would be increasing the size of the file and creating a longer download period. When it comes to print, if you want to take advantage of the technology and clarity your printer can offer, you should use a higher resolution; something in the range of 300dpi. Also, when viewing images on a device, you'll need to determine which resolution is appropriate based on the distance someone might be viewing it. If you're viewing an electronic device from an arm's length distance, you can use a lower resolution. If you're looking at something that's been printed fairly closely, you'll want to use a higher resolution. If you're looking at a billboard across the street, you can use a medium resolution. The closer people will view that billboard though, the higher the resolution you'd want to use.

Remember, even if the medium people will be viewing your images on can handle very high resolutions, this doesn't mean that you need to meet those resolutions. If you offer something lower, your image's pixels will be grouped with others around it to create the highest possible output.

There's a lot to know about pixels, resolutions, print, and web and the depth of knowledge can go on forever. I hope what I shared above has offered you a glimpse into the beginnings of this world though. If you've got any questions or concerns about any of this, please share down below.
 
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