How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

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Size, resolution, and formats… What do pixels have to do with it?

Do you buy your camera based on its number of megapixels? Are you having problems sharing your photos online? Does your print look low quality even if it looks great on the screen? There seems to be a lot of confusion between pixels and bytes (image size and file size), quality and quantity, size, and resolution.

So let’s review some basics to make your life easier, your workflow more efficient, and your images the correct size for the intended usage.

How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

This image is sized to 750×500 pixels at 72 dpi, saved as a compressed JPG which is 174kb. Let’s look at what all that means.

Is resolution the same as size?

One of the biggest misunderstandings comes from the concept of resolution. If this is your case, believe me you’re not alone.

The problem is that resolution can refer to many things, two of them relate to the problem at hand. Further on I’ll explain these two resolution concepts, however, they have one thing in common that I need to clarify first. They both have to do with pixels.

You’ve probably heard a lot about pixels, at least when you bought your camera. This is one of the most available and “valued” specs on the market so I’ll start there.

What is a pixel?

A digital photo is not one non-dividable thing. If you zoom in far enough you’ll see that your image is like a mosaic formed by small tiles, which in photography are called pixels.

Pixel grid - How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

The amount of these pixels and the way they are distributed are the two factors that you need to consider to understand resolution.

Pixel count

The first kind of resolution refers to the pixel count which is the number of pixels that form your photo. In order to calculate this resolution you just use the same formula you would use for the area of any rectangle; multiply the length by the height. For example, if you have a photo that has 4,500 pixels on the horizontal side, and 3,000 on the vertical size it gives you a total of 13,500,000. Because this number is very unpractical to use, you can just divide it by a million to convert it into megapixels. So 13,500,000 / 1,000000 = 13.5 Megapixels.

Pixel density

The other kind of resolution is about how you distribute the total amount of pixels that you have, which is commonly referred as pixel density.

Now, the resolution is expressed in dpi (or ppi), which is the acronym for dots (or pixels) per inch. So, if you see 72 dpi it means that the image will have 72 pixels per inch; if you see 300 dpi means 300 pixels per inch, and so on.

The final size of your image depends on the resolution that you choose. If an image is 4500 x 3000 pixels it means that it will print at 15 x 10 inches if you set the resolution to 300 dpi, but it will be 62.5 x 41.6 inches at 72 dpi. While the size of your print does change, you are not resizing your photo (image file), you are just reorganizing the existing pixels.

Imagine a rubber band, you can stretch it or shrink it but you’re not changing the composition of the band, you’re not adding or cutting any of the rubber.

Pixel Density 72dpi - How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

Pixel Density 300dpi - How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

In summary, no resolution is not the same as size, but they are related.

So quantity equals quality?

Because of the aforementioned correlation between size and resolution, a lot of people think that megapixels equal quality. And in a sense it does because the more pixels you have to spread out, the higher the pixel density will be.

However, on top of the quantity you should also consider the depth of the pixels, this is what determines the amount of tonal values that your image will have. In other words it is the number of colors per pixel. For example, a 2-bit depth can store only black, white and two shades of grey, but the more common value is 8-bit. The values grows exponentially so for example with an 8-bit photo (2 to the power of 8 = 256) you’ll have 256 tones of green, 256 tones of blue, and 256 tones of red, which means about 16 million colors.

This is already more that the eye can distinguish which means that 16-bit or 32-bit will look relatively similar to us. Of course, this means that your image will be heavier even of the size is the same, because there is more information contained in each pixel. This is also why quality and quantity are not necessarily the same.

Therefore quantity helps, but also the size and depth of each pixel determine the quality. This is why you should look all the specs of the camera and its sensor and not just the amount of Megapixels. After all, there’s a limit to the size you can print or view your image, more than that it will only result in extra file size (megabytes) and no impact in the image size (megapixels) or the quality.

How to choose and control image size and file size?

First of all, you need to choose the outlet for your photo, there is a maximum density that you need. If you are going to post your image online you can do great with only 72 dpi, but that is too little for printing a photo. If you are going to print it you need between 300 and 350 dpi.

Of course, we are talking about generalizations because each monitor and each printer will have slightly different resolutions as well. For example, if you want to print your photo to 8×10 inches you need your image to have 300dpi x 8″ = 2400 pixels by 300dpi x 10″ = 3000 pixels (so 2400×3000 to print an 8×10 at 300dpi). Anything bigger than that will only be taking up space on your hard drive.

How to resize in Photoshop

Open the menu for the image size and in the popup window, you need to tick the Resample Image box. If you don’t activate “resample” you will only be redistributing the pixels like I explained at the beginning of the article.

You can also choose to tick the Constrain Proportion if you want the measure to adjust according to the changes you make. So the width adjusts when you change the height and vice versa.

How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

8×10 inches at 300 ppi, this is the size needed for printing an 8×10. Notice the pixel size is 3000 x 2400.

750×500 pixels at 72 ppi. This is web resolution and is the exact size of all the images in this article. The size in inches is irrelevant when posting online – only the pixel size matters.

On the top of the window, you’ll also see how the file size changes. This is an uncompressed version of your image, it’s the direct relationship I explained in the first part of the article: fewer pixels means less information.

How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

Now, if you still want to change the file size without resizing anymore, you have to do it when you save the image. Before saving your photo you can choose the format you want:

Formats - How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

If you don’t want to loose any information you need to save an uncompressed format. The most common, and therefore easier to share is TIFF.

Tiff - How to Understand Pixels, Resolution, and Resize Your in Photoshop Correctly

If you don’t mind losing a little information as long as you have a lighter file, then go for a JPEG and choose how small you want it. Obviously the smaller you set it, the more information you will lose. Fortunately, it has a preview button so you can see the impact of your compression.

JPG high quality.

JPG low quality. Notice how it’s pixelated and breaking down? If you crunch it too much or go too low quality you risk degrading the image too far.

Conclusion

So there you have it. So quality, quantity, size and resolution explained and they all have to do with pixels, as they are the basic units that constitute your image. Now that you know you can make the best choices to print, share and save your photos.

Read more from our Post Production category

Ana Mireles

is a photographer and artistic researcher. She has been awarded and exhibited in Mexico, Italy, and the Netherlands. Through theory and practice, she explores the cultural aspect of photography, how it helps us relate to each other, the world, and ourselves. She has also a passion for teaching, communication, and social media. You can find more about her and her work at her website or acquire some of her works here.

  • Thank you! This is a great foundation for knowing how to optimize your photos for web use.

  • You lost me at the point you told me a 16 bit image is heavier than an 8 bit image.

  • @carlcrumley:disqus Meaning you’re confused by that, or don’t agree with what is written?

  • KC

    It’s a complicated topic on so many levels, and Ana has a done a great job here.I’m going to touch on Photoshop last. It would just confuse things. What’s relevant are prints and displays/monitors. You can make up your minds from there.

    Most print houses want to see a file that’s at least 300dpi. The reason is that’s the resolution nearest the print heads ppi. Confusing? Yes. But you don’t want their computer or printer “up sampling”. The file size is not relevant. The best answer there is to ask them what dpi. If you’re printing in-house 300dpi is a sweet spot – from Photoshop. (It doesn’t matter from LIghtroom.)

    I don’t agree with 72dpi for online. That setting needs to go away – fast. That’s for old CRT displays/monitors. Most people are going to view your images on a reasonably current computer display/monitor, tablet, or phone. The resolution on those devices is well over 200dpi now, closing in on 400dpi. Once again, 300dpi is a sweet spot.

    Depending on where you’re uploading to, they may have their own recommendations, they may resize/resample, or they may display the image exactly as you uploaded it.

    Touching on bit depth in very broad strokes, 16bit is fine. JPEG’s are typically 8bit, Raw is near 16bit. More bits equal more info for editing and displaying. Many display/monitors are now capable of showing more than an 8bit depth.

    But, that brings up color space, and a whole other level of confusion and debate. JPEG is sRGB, Raw is RGB (see bit depth above). But JPEG can be RGB and 16bit. It’s just a “tag”. Some display devices and browsers can see and interpret the RGB tag.

    Does it matter for printing? Not much. It’s all going to be converted to CMYK.

    OK, onto Photoshop. Historically, it’s a program for creating and compositing images. It’s a pixel level editor. It has no clue what an image is. It reads the “tags” and file extensions. Camera files don’t show 72dpi in their data. It’s not in Lightroom or Bridge, either. Photo editing was added on, more as a method for adjusted scanned images, not cameras. ACR came later.

  • Ana Mireles

    I’m glad you found it useful, thanks for your comment! 🙂

  • Ana Mireles

    Sorry to hear that, let me know if I can clarify something or share with us your opinion!

  • Ana Mireles

    I’m glad you liked the post and even more that you continued to elaborate on the subject, thanks a lot!

  • Leigh Garner

    I rarely take the time to respond online, but this post made me feel the need. While I admire the effort to explain what is a complicated topic all it really does is complicate and confuse matters further regarding DPI/PPI.

    Suggesting that changing the DPI somehow reorganizes the existing pixels like stretching an elastic band, is simply wrong. Pixel dimensions are the only thing that control the size of an image on disk. The DPI may be used as a modifier of Pixel dimensions, but suggesting DPI changes pixels like an elastic band is just adding to the confusion in my opinion.

    Displaying an image online (on Flickr for example) at 6 DPI, 72 DPI or 300 DP with the same Pixel dimensions will make no difference whatsoever (please try it). DPI is just a number used to scale images for some output methods (ie. Printers or resampling in PS). Only the actual Pixel dimensions make a difference to the size of an image on disk.

    The best and full explanation that I urge you all to read is here: https://www.scantips.com/no72dpi.html

  • KC

    It’s a great topic. People get so lost in the “numbers” it’s easy to get caught up in “I need more camera” syndrome. It’s all about final image. I doubt anyone has heard “it would be a great image if it was X dpi”.

  • Mister Williams

    Actually the author is correct in using the elastic band analogy.

    The lower the DPI that you present an image, the more ‘stretched out’ it will look visually.

    She wasn’t talking about doing anything to the pixels, merely presenting them visually at a higher or lower number of pixels for every inch of space.

    Software for presenting images on screen or on print is designed to ensure that there are no empty spaces or gaps between pixels, because that would look a bit rubbish! So if you lower the DPI to something ridiculous like two dots per inch, or pixels per inch, the software that is presenting the image on screen or sending to a printer will fill in the gaps between the pixels using ‘interpolation’ algorithms.

    That will give you the visual effect of stretching out the image, therefore making the elastic band analogy correct. The image would increase in physical size (on screen or print) but would look blurry or fuzzy due to the stretching out effect.

    The use of the word ‘re-organise’ needs to be taken into context, and I think the meaning is clear even to a beginner.

    The DPI parameter that we set when taking or processing an image is used only for informational purposes by the software that is processing it. So the processing software can ignore the pixel density settings if it wants to.

    Therefore…..What websites like Flickr do is that they ignore all the pixel density parameters that come with the image, regardless of what they are set to.

    Flickr only looks at the pixel dimensions of the image, or to put it another way, the number of pixels that the image consists of.

    Flickr ajusts the image using its own parameters and algorithms (applying its own pixel density parameters) so that the image looks its best at the pixel dimensions that Flickr is displaying the image at.

    One last thing, the DPI setting is what controls the image size, not the pixel dimensions. You did get that bit wrong. Like I said before, the DPI is informational only, and set by the user….or to put it another way, an instruction from the user – ultimately to the screen or printer, that will have the effect of printing or displaying an image of the same pixel dimensions, but at a different physical size.

  • Leigh Garner

    I think it’s probably a matter of semantics as I agree with many of your points (apart from the bit you told me that I was wrong!)

    Image size is, as we’re talking digital, logically the actual pixel dimensions on disk.

    The DPI is a parameter used by certain software to interpret the pixel dimensions to display or print the image at a certain size. As you have said (which I like), the DPI is for ‘informational purposes’.

    Changing the DPI does not change the image size. It changes the image output size in certain applications that choose to use it.

    It can be used as a factor to change the image size if in Photoshop (or similar) you choose to edit the document size and resample the image.

    These are facts.

    Here’s a video from the excellent Julieanne Kost which shows resampling in action.

    https://helpx.adobe.com/photoshop/atv/cs6-tutorials/understanding-resize-vs-resample.html

  • I just mean I don’t understand the use of a term referring to weight in this context. How is 16 bit “heavier” than 8 bit?

  • Confused. I don’t understand the use of a term referring to weight in this context. How is 16 bit “heavier” than 8 bit?

  • Ana Mireles

    Ah ok, sorry for the confusion. What I meant is that it will be a bigger file size, so it would take more space in your hard drive because it will contain more information even if it looks the same.

  • It means it is a MUCH larger file with a lot more data contained within it. 16 bit is 2 to the power of 16 or 65,000+ tones per color (RGB) and 8 bit is 2 to the power of 8 or 256 tones per color (RGB). Does that help?

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