Cropping photos for print

In a previous article I looked at how to resize images for web display; now it’s time to consider how to  size photos appropriately for printing, specifically how to ensure that you can print onto a given size of paper without stretching or squashing your image, or having to accept uneven borders.

It seems a simple question, but I have to confess that it was one that had me stumped for quite a while when I started to print digital images on a home inkjet. I tried all manner of programs but they didn’t seem to work. Later, when I started using Lightroom, I purchased a very good book which devotes around 30 pages to the print module. Even this weighty tome failed to mention the important point, which is:

The ratio of your image width to image height must match the ratio of the paper  width to height.

It seems so obvious now that I’m almost embarrased to admit that it took me a while to understand. But in my defence, I’d like to point out that camera designers and print manufacturers have done little to harmonise the dimensions of their sensors to match common print sizes – even though sometimes the same companies produce both products.

“Full frame” digital cameras have sensors measuring 24mm * 36mm (the same size as 35mm film). The number of pixels will vary from one model to another, but the ratio of the long side to the short side will always be 1.5:1. This can also be expressed as a 3:2 ratio. For example, the Canon EOS5D Mark II has a sensor with 3744 * 5616 pixels, approximately 21 megapixels. The ratio is 5616/3744 = 1.5

“APS-C” digital SLRs – the vast majority of digital SLRs on the market today –  also have this 1:1.5 ratio. The physical size of the sensor is smaller than the full-frame cameras, but the ratio of the width to height is the same. For example, the Canon EOS450D has 2848 * 4272 pixels, about 12.2 megapixels. The ratio is 4272/2847 = 1.5 again.

Digital compacts usually have  a ratio of 1.33

So now we know the most common aspect ratios for digital sensors:

Full frame and APS-C = 1.15 or 3:2
Compacts = 1.33 or 4:3

We can compare this to the sizes of paper which are commonly available for printing digital images. If you’re printing at home your likely choices are 6″ * 4″, 5″ * 7″, A4, and A3, in increasing order of size. If you’re sending your images to a commercial printer then a wider choice is available.

Taking the smallest size, 6″ * 4″ paper has an aspect ratio of 1:1.5 or 3:2. So a print from a full frame or APS-C camera will print onto the paper without any cropping. An image from a digital compact cannot be printed without cropping or uneven margins.

5″ * 7″ paper has an aspect ratio of 1.4 – so it’s not an ideal shape for printing from any digital camera.

A4 paper (the dimensions are 210mm * 297mm) has an aspect ratio of 1.41. Not ideal again.A3 paper is twice the area of A4 but the aspect ratio is exactly the same as A4.

The following table expands this analysis to a wider range of sizes, with the blue rows showing the paper sizes which are a good match for uncropped images from full-frame or APS-C DSLRs and the yellow rows showing the good matches for uncropped images from compact cameras. I’ve taken the print sizes from those offered by two popular commercial printers.

Target
print size in inches

Print size ratio

Short Edge

Long Edge

long:short

short:long

4

6

1.5

2:3

5

7

1.4

5:7

8

10

1.25

4:5

8

12

1.5

2:3

8.3

11.7

(A4)

1.41

8.3:11.7

11

14

1.27

11:14

11.7

16.5

(A3)

1.41

8.3:11.7

12

16

1.33

3:4

16

20

1.25

4:5

20

30

1.5

2:3

24

36

1.5

3:4

30

40

1.33

3:4

36

48

1.33

3:4

40

60

1.5

2:3

Whilst it’s handy to know what paper sizes are a good match for uncropped images, it would be highly limiting to stick to those sizes, and in any case you will often want to crop your images to suit the subject matter.

Therefore it is very helpful to know how to crop your images to match any paper size.

I will show how to do this using two software options, Adobe Lightroom and GIMP.

Cropping for print using adobe lightroom

I’m assuming here that you’re already reasonably familiar with Lightroom so I’m not covering every single detail here.

It’s a good idea to create a “virtual copy” of your image before you start cropping. Although Lightroom will allow you to undo any changes, it’s handy to be able to view two (or more) versions of the image side by side so you can decide which you prefer.

The “Photo” menu includes the “Create Virtual Copy” command:

 Here we see the original image alongside the virtual copy, in Grid View:

Once you have your chosen photo selected in any Lightroom module, you can press “R” which is a shortcut to the crop option in the Develop module.

Above, you can see the options for the Crop tool. Notice the row that includes “Aspect:”, “Original”, and the padlock icon. When the padlock is closed, the crop is constrained to either the same aspect ratio as the original image, or a selected alternative ratio. When the padlock is clicked to make it open, any crop can be selected. For our purposes, we need to leave it closed.

Cick on the arrow next to “Original” to see the available choices:

There are a number of options provided with Lightroom, which includes some paper dimensions such as 4*6 and 5*7. Then underneath the row “Enter Custom ..” are custom aspect ratios which I have entered. In this example, we are going to crop the photo to fit A4 paper, which has dimensions of 210mm * 297mm.

Click “Enter Custom” and this dialog box appears:

Here I have entered the numbers 21.00 * 29.7 to represent the A4 aspect ratio. In later versions of Lightroom, you can enter up to 4 whole numbers in each box, so you wouldn’t need to change the millimetres into centimetres as I’ve done here, but the result is the same.

Once you press enter you will be presented with a crop box of the requested dimensions which you move about as you wish:

And after pressing Enter to confirm the crop:

The next time you do this, your custom crop will appear in the list of available aspect ratios:

You can create as many custom aspect ratios as you wish, to suit different paper sizes.

The next step in printing would be to go to the Print Module; this is well covered in other material available on-line or in books so I don’t intend to cover it here.

Cropping for print using Gimp

Assuming you have opened your chosen image in GIMP, then the shortcut to open the Crop tool is SHIFT+C. Pressing “C” by itself is the shortcut for the Clone tool, so don’t forget to hold down SHIFT while pressing C to get the Crop tool.

Above you can see that I have opened an image in GIMP and started the Crop tool. The Crop tool has a number of options and the important ones for our purpose are shown below.

If the checkbox to the left of “Fixed” is not already selected, select it now. The numbers shown below that are the current aspect ratio, expressed using the actual numbers of pixels in this image. You can see that it’s close to a square (1:1 ratio) and we need to change so that it can be printed to an A4 page (the same ratio applies to A3).

We know from the table above that the ratio we need is 1.4:1, so enter that value in tthe box:

Well, actually I entered the ratio the other way round which suits a “portrait” style crop where the longest edge is the vertical edge.  If you find that you’ve entered the aspect ratio the wrong way round, you can change it with the two small icons to the right which select portrait or landscape format.

The next step is to drag the crop boundary to where you want it to be. How you crop is up to you but here’s what it looked like in my example:

Note that because you have chosen a fixed aspect ratio, the possible crops are constrained. Here is the image after pressing the Enter key:

I think you can see that this particular image is not really ideal for printing to A4 paper because the required crop is too narrow to include all of the circular centre of the flower. I could have chosen another image to demonstrate, but this example shows that you don’t always have a totally free hand in choosing a paper size.

So how big can I print my image ?

Everything covered in this post so far is just about getting the image in the right proportions to print – we still need to consider whether we have enough pixels to get an acceptable print in a particular size.

That depends upon the resolution at which you print, expressed in DPI – dots per inch – although it makes more sense to think of Pixels Per Inch.

A commonly quoted resolution to aim for is 300 dpi. If the longest edge of your image is 3600 pixels, then you can print on a paper with a long edge of up to 12 inches. (3600 / 300 = 12).

In years gone by, people used to use a resolution of 240 dpi. This was considered a good resolution to use a few years ago. The reason that many people now advise the higher resolution of 300, is mainly because the increasing size of sensors means that they can, rather than any intrinsic shift in the value of the human eye to evaluate a printed image. My suggestion would be to use 300 dpi if you can, but if that won’t allow you to print at the size you want, then use 240 dpi. By the way, 300 and 240 are not “magic numbers” and you can print at 267, 283, or any other number.

The type of paper (or other surface) you are printing on also effects the useable resolution. For example, prints on canvas are often made at a lower resolution.

The table below shows how many pixels you need to print at a certain size at either 300 dpi or 240 dpi.

Required image size

Target print size in
inches

300dpi

240dpi

Short Edge

Long Edge

Short Edge

Long Edge

Short Edge

Long Edge

4

6

1200

1800

960

1440

5

7

1500

2100

1200

1680

8

10

2400

3000

1920

2400

8

12

2400

3600

1920

2880

8.3

11.7

(A4)

2490

3510

1992

2808

11

14

3300

4200

2640

3360

11.7

16.5

(A3)

3510

4950

2808

3960

12

16

3600

4800

2880

3840

16

20

4800

6000

3840

4800

20

30

6000

9000

4800

7200

24

36

7200

10800

5760

8640

30

40

9000

12000

7200

9600

36

48

10800

14400

8640

11520

40

60

12000

18000

9600

14400

Here are a few more examples taken from the table above.
Suppose you have a 12megapixel digital compact with sensor dimensions of 3000 * 4000 pixels.

According to the table, you could print up to A3, using a resolution of 240dpi, because this size would need 2808 * 3840 pixels which is slightly less than the resolution of your camera.

I say “according to the table” because I can’t predict whether the resulting print would meet your expectations for image quality. The requirements for hanging in an exhibition may be more exacting than for a print stuck to your fridge. Also, the resolution alone doesn’t tell us whether the camera is a high-end expensive device or a sub-£100 item.  Really, you just have to try it with your camera and see how it looks; but I think you can use the table to rule out extremes and note that you’re unlikely to get an acceptable 40″ * 60″ print from an 8 megapixel camera.

If you’re using GIMP and would rather not have to refer back to this table, then GIMP can give you the same information.

The “Print Size” option on the “Image” menu shows this dialog box:

The image used was a 34 megapixel file created by scanning medium format film at 2400dpi. Changing the units for width and height from pixels millimetres to inches, and entering 16 for width and 20 for height shows this display:

You can see that this image could be printed at  16 * 20 inches (a standard print size) at a resolution of over 300dpi.

And finally

It is becoming increasingly common for cameras to offer alternative aspect rations as well as their “native” aspect ratio. For example a camera with a 3:2 aspect ratio may have menu options that allow you to change to 4:3, 16:9, or 1:1 – with some reduction in total resolution as the cameras won’t use the pixels on the sensor which fall outside of the optional aspect ratios.

This feature may well be useful to you if you prefer an alternative aspect ratio and you want to be able to view the effect on screen rather than cropping on the computer.

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6 comments

  1. That was a very interesting and informative read. While the crop is non destructive in Lightroom, ACR and the same can be achieved in Photoshop using the crop tool in a non destructive way, it has always bugged me that if I frame an image in a particular way why should I have to lose part of it to match a specific paper size … I would much prefer to keep to the aspect ratio of the camera, or as near to it as possible and interpolate from there, printing to a larger paper size if needs be and using the borders as natural matting … for those with not enough fingers or are a bit dumb when it comes to maths (like me) this might be a useful tool http://www.gt-photography.com/matworks.html .
    How big can we print, as rule of thumb to the dimensions set out above … Perfect Resize by onOne makes claims to be able to resize a 12 megapixel file produced by an “APS-C” digital SLR to six feet without any major loss of detail or softening … interpolation from within PS can be achieved successfully by up to 25% of the image size by increasing the size by 1% increments … it may sound a long drawn out way to achieve something but it need only be done once if you create an action for it.
    Having said that how difficult would it be to give us photographic paper sizes that we could actually use without having to worry if were going to lose part of the image

    • Thanks Ed. You’re quite correct that accepting wide borders is one way round the problem. I sometimes do this with square images (eg from a twin lens reflex) as even the more flexible sizes offered by darkroom papers don’t accommodate square images. So I print a 7*7 image on 8 * 10 paper then trim off 2″ blank space at the bottom.

      I wonder if those onOne people could create world peace as well ? They seem to be able to do anything !

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