The subject of image resolution is one that often arises with our clients and can be quite confusing if you’re not a photo geek. Due to the complexities of the topic and especially the difficulty people have in communicating or understanding what they mean by ‘high resolution’, I’ve discovered a tendency for people to talk about resolution in terms of file size.
In this blog post, I will attempt to simplify these two terms in order to explain their relationship and why it is important to differentiate between them.
Please note, all images have been prepared for illustrative purposes only. They are not intended to technically reflect what they are representing, but rather to illustrate the concepts involved.
Resolution
A digital photograph is made up of pixels in the same way that a mosaic is made up of tiles. If you have more tiles, or more pixels, you’ll have higher resolution.
Let’s take a look.
1. This is a zoomed in view of a photo which doesn’t have many pixels. Let’s call it low resolution. (Like many things in life, ‘low’ is relative). You can see the individual pixels which make up the photo. If you were to count them, you’d find 140 pixels across by 93 from top to bottom (making 13,020, which sounds like a lot, but actually isn’t very many for a photo). The result is a very low quality photo.
2. This photo has many more pixels (540 pixels along its length). We’ll call it medium resolution in relation to the previous image. From the circled inset in the top left corner, you’ll see that it has some detail, but not a lot.
3. Meanwhile, this photo has many more pixels still. It is relatively high resolution. (7000 pixels wide making over 40 million pixels in total!)
So, more pixels = higher resolution, which means more detail and better quality. And that leads us to take a look at…
File size
If you increase the resolution of an image (add more pixels) the file size will naturally increase. This is logical.
However, there are a number of other factors that also influence the file size of an image, which may not seem so logical, such as:
1. The type of file. There are many different file types for saving images. The most common is a jpeg, but you may have also heard of gifs, tiffs and pngs to name a few. They all have their pros and cons, but the point is that the same image (with the same number of pixels / resolution) can be saved in different ways and have a completely different file size each time depending on which file type you choose.
2. The amount of ‘compression’ in the file – compression is a clever thing that some files do to reduce their size by abbreviating the information and reducing unnecessary data. Just like shorthand writing or braille can communicate the same sentence (or similar) using less ‘data’. Gifs do this by reducing the number of colours in the image from 16.7 million to just 256 – that’s a big saving in data and who really needs to see the difference in an image between deep deep blue and deep deep deep blue?
3. The amount of detail in the image. A plain white box on a plain background is easier to describe than a complex cityscape. Smooth areas in an image take up less file space than detailed areas. Think of it this way, it’s easy to describe a bunch of pixels when they’re all virtually the same – if two pixels sat next to each other in an image are identical, you only need to describe one accurately – the other can just be described as ‘the same’. The more pixels which are the same or similar, the less data required to describe them and the less file size. (This is also the reason that photos taken on a higher ISO, if you know what that is, end up being quite big in file size – it’s because of the increased grain or noise in the photo that is complex for the file to describe – but that’s a bit geeky and not necessary to understand here).
Let’s look at all three of these concepts now.
An example of how file types can mislead in terms of resolution:
This is representative of a jpeg, with a file size of approximately 150MB.
This is representative of a tiff at 300MB – double the file size of the previous jpeg.
Which has more detail? It’s the opposite of what you’d expect. The tiff is twice as big a file but contains just 1/16th of the resolution (81,000 compared to 1.3 million pixels). This is because tiffs are not good at compressing data – in order to create a file which is a comparable file size to the jpeg, the resolution needed to be reduced significantly – the tiff only has 350 pixels along it’s longest side compared to 1400 for the jpeg.
So, as you can see, in this case, the smaller file (the jpeg) has more detail and is therefore better quality than the big file (the tiff).
An example of how compression can reduce file size without compromising resolution or quality:
This is an example of the resolution or detail contained in a zoomed in portion of the image, and is representative of what would be seen if it were saved as a tiff. The overall file size of the entire image would be approximately 120MB.
This is an example of the resolution or detail contained in a zoomed in portion of the image, and is representative of what would be seen if it were saved instead as a jpeg with minimum compression (the setting we normally use). The overall file size of the entire image would be approximately 5.8MB.
You’ll see that there is no discernible difference in quality – indeed the resolution, or number of pixels is identical and the jpeg is preserving all 16.7 million colours.
Yet the jpeg is less than 1/20th of the file size! This is an example of why jpegs are the standard for image delivery. Why would you choose a file which is 20 times bigger for absolutely no visible difference in quality?
An example of how the amount of detail in an image can affect the file size:
In this example a section has been taken from the sky and saved separately as a jpeg with minimum compression. The resulting file size is approximately 250MB.
This section is taken from a detailed part of the image but contains exactly the same resolution (both examples contain a section of 2845×1897 pixels). The overall file size in this case is 1.4MB.
Basically, the only thing that is different between the two photos here, is the content – the blue sky vs the carving. The resolution is the same, the file type is the same, the compression is the same, it’s even taken from the very same frame, but the file size is nearly 6 times bigger in the case of the detailed section.
This explains why for example, products shot on a plain white background, tend to generate much smaller file sizes than expected.
Conclusion
File size is not the same as resolution. Whilst file size can sometimes provide an indication of resolution, this is misleading because resolution can be very high whilst file size is relatively low and vice versa, depending on other factors.
At Fisher Studios, we deliver photos to our clients with the maximum number of pixels possible (the highest resolution) and save them as jpegs with minimum compression, which offer a tremendous reduction in file size with virtually no reduction in quality.
If you need them delivered in any other way, we can of course do that too. But otherwise, you can trust in us to provide you with the very best resolution and quality possible, every time.