Resizing photos for web display

This is a basic technique post about resizing your images for web display, for example on flickr. If you’re already in the habit of reducing the size of your photos before uploading them to flickr, then you might want to skip this one.

So why would you want to resize your photos for the web – why not update the biggest file you have and let people see the image in all its’ glory by selecting the flickr “view all sizes” option ?

Here’s a few reasons:

Perceived image quality

For many cameras, and I’m going to stick my neck out here and say most of the cameras commonly in use, the photos they produce don’t actually look too good when you look at them full size, ie the actual pixel view. If you’re rich enough to own a full-frame digital SLR, or wise enough to use medium-format film, you probably don’t need to worry. But for most digital compacts, for 35mm film, and for the lower-end ranges of APS-C format DSLRs, it does seem to be an issue.

When a viewer chooses to view, say, a 12 megapixel file in “original size” then they will see a file around 4000 * 3000 pixels. Since the computer screen is much smaller than this, the viewer has to scroll around to see the detail – and what they might well see is mushy-looking grass, tree branches with purple fringing, and noise in the shadows. Maybe some dust marks as well.

That doesn’t mean that those cameras don’t produce images which are perfectly acceptable when viewed onscreen at a suitable resolution, or printed to a suitable size; it just means they weren’t designed to produce files which stand up to “pixel-peeping”, by which I mean close-up examination full size.

Nor does it mean you can’t produce a reasonable size print to an acceptable quality. You could print that 12 megapixel file at 240 dpi and produce an A3 print that looks good at “normal viewing distance” – which means the viewer will be 3 foot or more from the print. If you do see someone pushing their face right up against your exhibited print, they’re probably another photographer …

 

UPLOAD TIME

It’s always going to be quicker to upload a smaller file than it is to upload a bigger one. Incidentally if you don’t have a flickr “pro” account, flickr will limit the size of your displayed image to around 1000 pixels – but you will still have spent longer uploading the larger file.

POTENTIAL FOR IMAGE THEFT

The subject of image theft from the web could occupy many thousands of words, but I’ll try to keep it brief. Some people worry more about it than others. I tend not to worry too much because I reckon the people who steal images were never likely to buy them anyway, so I’m not losing any income.

That said, if someone does steal your image then they can do more with a large image than they can with a small one – so limiting the size of the image has some part to play in discouraging image theft.

A recomended approach

The approach that I take is that full-size images stay with me unless there is a very good reason to let them out of my possession. When I upload an image to the web, I output a version that is sized for that purpose. When I send an image to my own printer, or send it out to a commercial printer, I output a version sized for that purpose, matched to the paper size in use.

So how big should an image be for web display ? Unfortunately that’s not an easy question to answer, because screen sizes vary. I’m writing this on a computer which has a monitor with a resolution of 1600 * 900. Elsewhere in the house there’s a laptop with 1200 * 800 and a notebook computer with 1024 * 600. You never know what size monitor people are going to use to view your image, or even the ratio of width to height.

A compromise solution for flickr is to limit the size of your image to 1000 pixels wide, for images which are horizontal in format. Only very old monitors will be too small to display 1000 pixels in the horizontal axis.

When you resize, you would normally specify only one dimension and allow the other dimension to be resized according to the width to height ratio of the original image. This is because you don’t want the image to be unnaturally squashed or stretched.

What about vertical or square images, where the height is equal to or greater than the width? That is more of a problem because, whilst most monitors are more than 1000 pixels wide, they aren’t more than 1000 pixels high.  So viewers are likely to have to use the vertical scroll bar to view the full height of the image.

Perhaps I’m being a but fussy here, because restraining your image to 1000 pixels still resolves the image quality issue, and most of the time flickr users aren’t looking at the “original size” view. But if you want most viewers to be able to see the original size view without scrolling, you’ll need to keep the height below 1000 pixels.

For that reason, try limiting the height of vertical format images to 800 pixels (or square images to be 800 * 800). Unfortunately that means that the width of your vertical images will be a lot less than the available space.

Incidentally, there are some monitors which can be rotated, which might be useful if you want to get the best view when you are editing a vertical image. I might consider that next time I buy a new monitor.

How to resize your images

The method you use to resize your image will depend upon what software you have available. My image-editing “toolkit” is to use Adobe Lightroom (an old version 2) for the majority of tasks and GIMP for anything more complex. As I learn more about Lightroom I find I use GIMP less and less. GIMP is free to download and use from www.gimp.org. I don’t have Photoshop and don’t fancy paying around £500 to get it so can’t help directly if you’re a Photoshop user, although I imagine the method will be fairly similar to GIMP.

In the following sections I’ll illustrate how to resize an image using Lightroom, GIMP, and also Microsoft Office Picture Manager, which you might have on your PC already if you’re running Windows.

Resizing with Microsoft Office Picture Manager

If you can’t see Microsoft Office Picture Manager, then open the Start Menu and type “Picture Manager” in the Search box – it should appear. Make a copy of the image you want to resize and load the copy into Picture Manager.

Choose “edit pictures” from the toolbar, then you’ll see the screen above. I’ve pencilled the option “resize” so choose that next.

The simplest method is to use the option “Predefined width and height” which offers some standard sizes. The first option is “Document – Large (1024 * 768). Further down the right hand side, Picture Manager shows you the original size and the new size. Note that the new size in the image is not, in fact, 1024 * 768 but 839 * 768. This is because the width: height ratio of the original image is not suitable for resizing to 1024 * 768 so the software has chosen a size which avoids squashing or stretching the image.

If you’re happy with the suggested size, press OK. If you want to choose a different size, you can use the option labelled “custom width and height”. However, note that if you enter the first value, width, the second value does not automatically change to match the image ratio – so you’ll have to work out for yourself what the height should be.

Once you’ve made the sizing changes you want, do “file, save”. If you forgot to make a copy before you started the programme, do “file, save as” to give the changed file a different name. It’s handy to include the image size in the file name, eg “1000 px wide red dahlia.jpg”.

Resizing with GIMP

Make a copy of the image you want to resize then load your image into GIMP:

Then choose “Image, Scale Image”:

Now you will see this dialog box which shows the current image size:

In this particular case, the height is greater than the width, so I want to set the height to 1000 pixels and let GIMP calculate the appropriate width to match the image ratio.

Unlike Microsoft Office Picture Manager, GIMP will automatically calculate the second value for you. If you actually wanted to change the height without changing the width, or vice versa, then click on the anchor icon  to the right of “height” and this will break the link between height and weight. More likely, if you find that GIMP doesn’t calculate the second value for you, you’ve accidentally pressed the anchor, and need to press it again to restore the link between the two values.

Don’t be tempted to change the X Resolution and Y Resolution boxes – the value of 72 pixels per inch is the default for screen display. Values such as 300 or 240 dpi are used for print output, which I’ll cover on another article.

Neither do you need to change the “Quality” box, which is only needed when you are increasing the size of your image.

When you’re happy with the size you’ve set, press “Scale” for the changes to take effect, then do “File, Save” to save the file. If you forgot to make a copy of the file before starting GIMP, do “File, Save As”.

resizing with Adobe Lightroom

If you use Adobe Lightroom, then the simplest method of resizing is with Lightroom, as it will fit in well with your workflow. There is no need to explicitly make separate copies, as Lightroom will handle this for you.

Here I have a picture showing in the Library module and have chosen “File, Export”:

I am using Jeffrey Friedl‘s Flickr export add in in Lightroom 2. If you have a later version of Lightroom, the flickr export function is built-in to Lightroom, but the principles are the same. He has some other Lightroom add-ons that may be of interest to you.

The “export destination” is set to “Temporary Folder (will be discarded upon completion)” which is self-explanatory. With Lightroom, you can make several different exports with different file sizes, without having to keep the file copies.

If you scroll down you will see the “Image Sizing” dialog box below:

Note that Lightroom uses the words “long edge” and “short edge” rather than “height” and “width”. This means that you could choose to always export your images to flickr with the long edge set to say, 1000 or 800 pixels, without having to worry about whether they are horizontal or vertical. Personally I set horizontal images to 1000 pixels on the long edge and vertical ones to 800 pixels on the long edge.

If you choose the “Width & Height” option, you can enter both dimensions but it is up to you to choose dimensions which suit the image ratio and avoid squashing or stretching.

The “Dimensions” option is similar to “Width & Height” but allows you to set the dimensions in inches or centimetres instead of pixels. That use relates to printing rather than web display – I’ll cover printing in another article.

The “Resolution” option also relates to printing and will have no effect when uploading to the web.

You might want to choose the “Don’t Enlarge” button as this avoids the possibility that you accidentally set dimensions which are larger than the original image. You might sometimes want to do this deliberately but it’s best to avoid doing it accidentally.

Once you’ve specified the size options, you just click “Export” and Lightroom will then create a temporary file of the size you’ve specified, upload it to flickr, then delete the temporary file.

However before pressing Export you might want to save these settings as a “preset” which you can choose again.

Give the preset a useful name such as:

Then click “Create”. The next time you want to do an export to Flickr with a 1000 pixel image you just select “File, Export with Preset”, and choose your preset from the list:

The next thing you will see is your photograph on flickr. Using Lightroom as your sizing and upload option also allows you to do things like adding your images to sets and groups, which can be saved in the preset.

In a future article I’ll look at sizing your image for print, including adjusting the image dimensions to match the size of the paper – something which had me stumped for a while when I started printing my images.