Understanding Image and Print Size August 13, 2015 10:13

Cropping & Aspect Ratio

Will My Photo Need To Be Cropped?

Don't confuse cropping with resizing. Cropping is about composition such as getting rid of something, changing the focal point, or making a picture fit a certain size. Resizing an image has more to do with resampling the pixels within a photograph.

There is a good chance that if you want your digital images to fit a common print size, your photo will have to be cropped to fit your preferred output size.

Aspect Ratios and Factors

Cropping has everything to do with aspect ratios and factors. We’ve outlined the process below to provide a quick, easy to understand overview.

The aspect ratio of an image describes the proportional relationship between its width and its height. In still camera photography, the most common aspect ratios are 4:3 for "point and shoot" and 3:2 for Digital SLR's. That means that the height of the camera’s images is 3/4's, and 2/3's of their respective width.

Common Aspect Ratios

Camera Aspect Ratio Factor Digital Point and Shoot 4:3 (Width:Height) 1.3 Digital SLR 3:2 (Width:Height) 1.5

Although all cameras have a native aspect ratio and factor, aspect ratios will differ between camera manufacturers and even between different model's made by the same company. Some cameras have more than one aspect ratio available or cropping within the camera. Be sure to check your owners manual.

So How Does It Work?

A DSLR that has a 3:2 aspect ratio captures an image measuring 300 × 200, or 600 × 400 or 1350 × 900. So long as the relationship between the width and the height is always 3 to 2, the aspect ratio does not change even though the size of the image does. In our example 1350 px ÷ 3 ='s 450 px as does 900 px ÷ 2 ='s 450 px.

Another image is shot with a 4:3 aspect ratio. The horizontal and vertical size of this image in pixels is 400 × 300, or 800 × 600 or 1800 × 1350. Proof, 1800 px ÷ 4 ='s 450 px and 1350 ÷ 3 also ='s 450 px.

Print Sizes Are Aspect Ratios

Print sizes are aspect ratios. Simply switch the width around so that it is the first dimension, and then reduce it down to their lowest values. 

Print Size to Aspect Ratio Comparison

Typical Print Size Width and Height Flipped Aspect Ratio
4 × 6 6 × 4 3:2
5 × 7 7 × 5 7:5
8 × 10 10 × 8 5:4
20 × 30 30 × 20 3:2

 

If your camera has a 3:2 aspect ratio, you can print a 4" × 6" or a 20" × 30" image without fear of losing any of your image. However, cropping is inevitable if you want to print an 8" × 10", or a 5" × 7" because the aspect ratios are completely different. The proverbial "square peg into a round hole".

The following table summarizes the most common aspect ratio's and their respective proportional image sizes.

Aspect Ratio
4:3 3:2
Proportional Size
3 × 4" 2 × 3"
6 × 8" 4 × 6"
9 × 12" 6 × 9"
12 × 16" 8 × 12"
15 × 20" 10 × 15"
18 × 24" 12 × 18"
21 × 28" 14 × 21"
24 × 32" 16 × 24"
27 × 36" 18 × 27"
30 × 40" 20 × 30"

Calculating the Factor

You can figure out the factor by taking the image sizes listed (referenced in pixel dimensions, e.g., 3072 × 2304) in your manual and dividing the larger number by the smaller. In this case, that would be 3072 ÷ 2304 = a factor of 1.33, which tells us that the length is 1.33 times the height.

Calculating the Aspect Ratio

Aspect Ratio can be calculated by dividing each of the dimensions by their greatest common divisor or GCD, for example, 3072 ÷ 768 = 4 and 2304 ÷ 768 = 3 (where 768 is the GCD) for an aspect ratio of 4:3.

Alternatively

One way to determine if an image will require cropping to fit a particular print size takes your camera's pixel dimensions and divide each by 300∗ (which represents the most common resolution for producing a quality print). As an example let's say our image is 3072 ppi × 2304 ppi. Dividing each value by a resolution of 300 would give us a 10.24" × 7.68" print.

If you want this print to be 8" × 10", then...

  • Multiply 8 by the factor of the image, in this case 1.33, which gives you 10.64 (always multiply the shortest side of the desired print size).
  • Our image will size to 8" × 10.64" therefore .64 of an inch will have to be cropped off to fit to an 8" × 10" print.

As you can see from the examples above, there are very few choices in common print sizes that will not require some cropping given the pixel dimensions, aspect ratio and factor.

Note∗ To obtain the best quality possible, we print at 360 ppi.

Sampling and PPI (pixels per inch)

Pixels per inch (ppi) is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors.

PPI also describes the pixel resolution of an image within a specified space; for instance, a 100×100-pixel image that is a 1-inch square has a resolution of 100 pixels per inch (ppi). When referred to in this way, the measurement is meaningful when printing an image. Professional quality photographs usually require around 300 - 360 pixels per inch when printing.

To achieve the best quality prints, Photohop recommends using a target resolution of 360 ppi and strongly suggest a resolution of not less the 240 ppi, especially if you are printing fine art or high-quality photo image. Other prints such as posters, PhotoTex®, or digital paintings, which are viewed at a greater distance, can go as low as 180 ppi, but it is not recommended as noticeable softening of the image will occur below 240 ppi.

In some cases, resampling is possible. Resampling is the mathematical technique used to create a new version of the image with a different width and, or height in pixels. Increasing the size of an image is called upsampling; reducing its size is called downsampling.

When images are upsampled the number of pixels increases by referencing the original subject. New image detail cannot be created that was not already present. As a result, the more images are enlarged, the softer they become since the amount of information per pixel goes down.

Conversely, when images are downsampled, information is discarded to make the image smaller. If you downsample and then upsample an image, you will not get back the original detail. Downsampling a soft image can make it appear sharper even though it contains less information than the original.

If you want to print an image at 16"×20", and the native resolution of the image allows for an 8"×10" print at 240 DPI; then the image will have to be upsampled to a 16"×20." This action can only be done by adding pixels to the image. The more pixel you have to add the softer the image will become. In this example, we would have to resample the image by 200%. We don't recommend upsampling an image by more the 200%, as this results in a noticeable loss in image quality. However, if "really big" is what you want, be prepared for a softer image and loss of detail.

Some images that have had artistic effects applied are not affected as much by resampling and look good at almost any size.

At Photohop, we use special techniques and software that minimizes the effects of resampling an image, producing better results than most photo editor software.

Resolution & Print Size

Resolution

The resolution of an image is measured by the number of pixels per inch (referred to as ppi) that make up a digital image. The higher the number of pixels per inch, the better the precision or resolution of an image. For example, a resolution of 300 ppi means that the image is made up of 90,000 (300×300) pixels per square inch.

Camera resolution refers to how many pixels wide and high a camera captures an image and reflects the total number of pixels as megapixels, for example:

A camera that has a resolution of 3072 pixels wide by 2304 pixels high is a 7-megapixel camera. (3072×2304=7077888 pixels ÷1,000,000 = 7 megapixels)

Another way to interpret this is that a 7-megapixel camera captures images that are 3072 pixels wide and 2304 pixel high at the camera's highest quality setting.

Screen (display) resolution is the same principal as your camera, but the math is quite different.

Example:

A display with a diagonal measurement of 21.5", set to 1920 pixels high × 1080 pixels wide, yields 102.45 ppi. Using Pythagorean Theorem; take the diagonal resolution in pixels dp (square root of the sum of the pixel width to the power of 2, plus the pixel height to the power of 2) divided by the diagonal size in inches, equals the ppi for that particular monitor.

Print resolution When you print an image, the ppi of the image is translated to dpi (dots per inch). When you first open an image in an image editor or viewer, it typically opens at 72 ppi. This resolution is the lowest acceptable level for displaying your image on a monitor (screen) and is not acceptable for making a quality print. To achieve optimal results we suggest 360 ppi and recommend you do not print below 240 ppi. The lowest acceptable resolution for printing is 150 ppi, but you will certainly lose quality. The pixel dimensions of an image divided by the target print ppi will give you the native size that the image can be printed.

Example:

An image that is 3072 pixels wide × 2304 pixels high, with the height and the width each being divided by the targeted print resolution of 360 ppi results in a print measuring 8.5" × 6.4."

Print Size - How Big Can I Go?

The amount of detail that the camera can capture is determined by the native resolution, which is measured in pixels. The more pixels a camera has, the more detail it can capture.

To better understand print sizes, refer to the following table to give you an idea of the amount of resolution you need to make a photographic print. Keep in mind that resolution is only one of several factors required to produce a top-quality print.

Print & Camera Resolution

Least Better Best
180 ppi 240 ppi 360 ppi
2 megapixels (1600x1200) 8.9" x 6.7" 6.6" x 5" 4.4" x 3.3"
4 megapixels (2240x1680) 12.4" x 9.3" 9.3" x 7" 6.2" x 4.7"
5 megapixels (2560x1920) 14.2" x 10.7" 10.7" x 8" 7.1" x 5.3"
6 megapixels (3032x2008) 16.8" x 11.2" 12.6" x 8.4" 8.4" x 5.8"
7 megapixels (3072x2304) 17.1" x 12.8" 12.8" x 9.6" 8.5" x 6.4"
8 megapixels (3264x2448) 18.1" x 13.6" 13.6" x 10.2" 9.1" x 6.8"
10+ megapixels (3648x2736) 20.3" x 15.2" 15.2" x 11.4" 10.1" x 7.6"

** Note: the information in this table is based on the assumption that images are captured at a cameras highest resolution/quality.

As you can see from the dimensions above, if you are looking for a more common print size such as an 8 × 10, chances are your image will need to be cropped.

To determine how large you can print your image given the ppi range as stipulated above, refer to the Image Quality and Size Calculator.

Resampling an image does have an effect on the quality of the image. An image can only be resampled to a certain point before there is a noticeable loss of quality. The image sizes listed above do not require resampling.