A Viewfinder Darkly

Photography tips and tutorials

Essential photography editing using the GIMP

May 22 2015

by Philip Northeast

The GIMP has a number of essential tools for creating a copy  your photograph into the right size, shape and resolution for its final use. This could be on the web page,  email, on social media , or even printed on paper for display in a photo frame.

Rotate

It is not always possible to hold a camera perfectly level while taking a photograph. Most of the time small errors can be ignored. Where there are obvious reference points for viewers, and the horizons or verticals are wrong, then this can distract from the main point of the photograph. Classic examples are pictures of the sea, where everybody knows  the horizon  must appear perfectly level.   

The horizon needs to be level in this type of photo

The horizon needs to be level in this type of photo

There are two  tools for rotating images in the GIMP.  In the image menu the transform item has options  for rotating the image in large fixed steps, such as 90 Degrees.

For fine adjustments go to the tools menu, in the transform tools option there is another rotate tool.  In the pop up dialog box there is a choice of either entering a precise rotation in degrees, or a freehand approach using a slider bar. As an aid there is a grid superimposed over the image so you can align one of the grid lines up with a convenient horizontal or vertical reference in the image.

The chequered background revealed when the photo was rotated

The chequered background revealed when the photo was rotated

The disadvantage of these fine rotational adjustments  to straighten or level a photograph is you lose some of the image. The GIMP shows areas uncovered by rotating the image, as a grey chequerboard.  Then the photograph needs cropping to produce a rectangular image without the chequerboard background.  This means that some of the photograph around the edges is lost, so it is worth taking some time lining up the original photograph.

The rotate tool dialog box

The rotate tool dialog box

Cropping

There are two ways of controlling the crop tool. Using the mouse free style to create the crop area on the image, or  use the control panel to enter precise coordinates for the start position and size of the cropped area.  The rulers around the edge of the window are a guide to determine the co-ordinates. Entering the numbers is a more precise way of positioning the area to be cropped, but a combination of both methods can be used to fine tune the crop area.

The crop tool in the tool box panel

The crop tool in the tool box panel

There is also a provision in the tool control panel to specify the aspect ratio of the crop area. Aspect ratios are important as many uses have different requirements and they may not match the aspect ratio of your camera’s sensor.

The aspect ratio is the relationship between width and height of the image. A common aspect ratio is a rectangle one and half times wider than it is high. These proportions were inherited from 35mm film. This is often referred to as a 6×4 aspect ratio because it matches up nicely with standard 6” by 4”  photo paper.

For a  slightly larger print, say using standard 10” by 8” photo paper has  a different aspect ratio, and most normal  images will not fit. You may get white strips on a couple of edges, or  miss some of the image you thought were going to get in the print.

Cropping to a fixed aspect ratio to suit print size

Cropping to a fixed aspect ratio to suit print size

Producing a copy in the GIMP and cropping it to fit the shape of the photo paper makes you in control of what is in the printed photo.  Of course printing is only one option.  For Instagram photos you need a square,  that is a 1×1 aspect ratio. Enter in the dialog box 1:1 in the GIMP and it will keep that shape as you adjust the size of the crop area.

For web page illustrations the aspect ratio must match the shape in the web design so you can use the GIMP to crop your photograph to suit.

Often there are compositional imperatives for cropping the image. The combination of shooting distance, photographer’s position, and available focal length, often means less than perfect composition in the initial photograph. The rapid advance in digital sensor technology is producing cameras with high resolution so the final photograph’s quality is not noticeably reduced by moderate cropping.

Why use GIMP

These are basic operations and there are countless simpler editing apps photographers could use to perform these tasks. This is only a small part of the GIMP’s capabilities and often are the start of  more complex editing processes  we will explore in other tutorials.

 

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