Image Size, Resolution, Cropping and Resizing

After commenting on a cropped image of a bee in last week's assignment pictures I thought it would be useful to talk about image resizing and cropping.  I will use my own picture of a bee to illustrate this topic, the original was taken in an iPhone 13, in a square format and was saved as an HEIC file. 

HEIC, High Efficiency Image File, is Apple's proprietary means of reducing file size without losing quality.  HEIC files are smaller and higher quality than jpg files, the industry standard format.

The original image, in camera, was 3,024 x 3,025 pixels (picture element), how does this relate to image size?

Image Size

The size of image we can output from this photograph is determined by "how thinly" we wish to spread the pixels (i.e. the resolution of our final image).  This is dependant upon how we wish to view the photograph.

1. For printing to a book or small photo a  resolution of 300 Pixels Per Inch (PPI) is generally accepted.  My original image will therefore print at good quality at 10 inch x 10 inch.
2. For onscreen viewing you are limited to the maximum resolution of the screen so need fewer pixels per inch. The accepted norm is 72 PPI though some screens now demand higher.  My image will therefore display at 42 inches square - or at full screen on most desktop monitors with no loss of quality.
3. Other options exist, iy all depends upon the desired output size and viewing distance.  The larger the printed image, the larger the viewing distance will be and therefore we can get away with fewer oixels.

What happens if I want to crop in on the original to show only the bee.

Cropping the Image

Cropping an image is a legitimate way of improving composition but it does lose pixels and so must always be done with a view to how the final version is to be displayed.  Incidentally users of mobile devices who use digital zoom (squeezing in by your fingers) when taking the photo are cropping the image at this point.  They are reducing the area of the sensor that contributes to the final result; the image appears larger because it is being displayed over a larger area.

The cropped image - now 1877 x 1877 pixels

If I crop in to the bee my new image size is 1877 pixels square, total 3,523,129 pixels or 38% of the original.  If we apply our standard output resolutions to this image we find:

1. The largest print I can produce is 6 x 6 inches
2. The maximum screen display is 26 inches x 26 inches

My photo can display on screen at full size on a desktop monitor but is not going to be something I can hang on the wall at home!  But it is OK for sending by email for viewing on a phone without the recipient using a whole month's data allowance to receive the photo.

But I want a high quality 12 x 12 inch version to hang on the wall.

Resizing

If I want a large version of the cropped image to hang on the wall I need to find some more pixels from somewhere, my software has to fill in the gaps.  I think I need to resort to paid software for this, PhotoShop (but I I have not tested the many free photo resizing apps that exist as apps or on the web to see if there is a suitable free package. I suggest trying IrfanView, a free downloadable image viewer and editor.  I used it a long time ago

I need 3600 pixels in each direction but the cropped image only has 1877 - I need to double the size of the image.

Going back to my original (un-cropped) image in Photoshop I need to double its original size  Photoshop will resamples the image and inserts additional pixels to fill the gap. Then I can crop in on the bee to produce an image that is 3600 x 3600 pixels.  I have not uploaded the result here as it will not look any different when viewed on the screen.

Summary

You need to reduce the number of pixels in your photos if you are sharing them by email.  Most mail clients will do this for you as they do not want huge files "clogging up" their systems.  As a consequence emailed photographs are rarely suitable for quality printing at anything larger than small snapshot size.

For quality printing you need to preserve the number of pixels in your image.

Cropping "throws" many of your pixels away and so you may need to consider resizing your image in e.g. PhotoShop, before cropping.

For onscreen viewing you have more leeway as fewer pixels are needed to fill the screen.

Sharpening

All digital images need sharpening.  Sharpening should be applied as the last step in your digital workflow and is best set according to the size of the final image and how it is going to be output.  Therefore if you intend to crop and resize your image you should first remove any sharpening that has been applied by your camera or editing software.  This will need to be the subject of a future posting.