Cropping digital images presents an effect on image quality that was not as prevalent in film image capture. When using film, photography using low ASA of 64 or less created a foundation of clarity and fidelity that facilitates cropping and a very extensive range of image output sizes. Further, when digitizing a film original, cropping was done in-process, so the resolution of the output file remained the same for full frame as it did for cropped frame.
Digital photography creates a fixed output of image pixels, full frame, as the master file. Cropping is done within that, subtracting pixels from the original file. The effect can be apparent when a large scale output is desired from a digital image that has been heavily cropped.
Digital File Cropping Effect on Image Size
The following illustrates this effect on finished image size and effective MP (without using enlargement to correct the output image size.)
|15% Crop (MP)||25% Crop (MP)||40% Crop (MP)|
|30||6,720 x 4,480||5,712 x 3,808 (22MP)||5,040 x 3,360 (17MP)||4,032 x 2,688 (11MP)|
|45||8,192 x 5,464||6,963 x 4,644 (32MP)||6,144 x 4,098 (25MP)||4,915 x 3,278 (16MP)|
|80||12,288 x 6,480||10,445 x 5,508 (58MP)||9,216 x 4,860 (45MP)||7,373 x 3,888 (29MP)|
|100||11,648 x 8,742||9,901 x 7,431 (73MP)||8,736 x 6,557 (58MP)||6,989 x 5,245 (37MP)|
|400||23,200 x 17,400||19,720 x 14,790 (292MP)||17,400 x |
13, 050 (227MP)
|13,920 x 10,440 (145MP)|
For this reason, where film photographer often left the subject very loose in the frame, then cropped it down during scanning to digital, digital photography often demands pre-cropping the image in a frame, holding tight on the subject, or capturing multiple images in different zoomed-in states, to provide the opportunity to produce the desired high quality end product needed for an assignment.
Available Crop to Fit a Large Target Image (8K Display)
But this is really not the full story. Let’s say that the assignment is to produce an image that will ultimately be displayed on a 8K Display full screen. How much room is their for cropping to fit that display in each of the captured files?
|8K Image||Available Crop|
|30||6,720 x 4,480||7,680 x 4,320 (33MP)||-14%H x 4%V|
|45||8,192 x 5,464||7,680 x 4,320 (33MP)||6% H x 21%V|
|80||12,288 x 6,480||7,680 x 4,320 (33MP)||38%H x 33%V|
|100||11,648 x 8,742||7,680 x 4,320 (33MP)||34%H x 51%V|
|400||23,200 x 17,400||7,680 x 4,320 (33MP)||67%H x 75%V|
This means that the 30MP image will need to be enlarged 14% to fit full frame, with only 4% available in the vertical dimension for cropping. On the other extreme, the 400MP image will need to be cropped or reduced significantly to fit the 8K image target, affording the photographer significant opportunities to refine the finished frame and content.
Medium Size Target File
On medium size projects, let’s say an image of 4200H x 3000W, or 14″ x 12″ large format magazine full page bleed, the following is a more realistic picture.
|14″ x 12″ Image @300dpi output||Available Crop|
|30||6,720 x 4,480||4,200 x 3,000 (13MP)||38%H x 33%V|
|45||8,192 x 5,464||4,200 x 3,000 (13MP)||49% H x 45%V|
|80||12,288 x 6,480||4,200 x 3,000 (13MP)||66%H x 54%V|
|100||11,648 x 8,742||4,200 x 3,000 (13MP)||64%H x 66%V|
|400||23,200 x 17,400||4,200 x 3,000 (13MP)||82%H x 83%V|
In this case, even the 30MP camera provide adequate room for generous cropping and framing adjustments.
In other words, the trick is to know early in an assignment, what the finished use of an image will be, and make adjustments in approach to produce the best match of image file size and requirements. Then, when collecting images, crop in-camera to a greater degree than one might have in the days of film, to increase the utility of the finished images. Or, in some instances, it may be required to capture an open framed image, coupled with zoomed in images, that can be combined later to provide the required page fill.
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