Digital Zoom vs. Cropping: Are They the Same Thing?

#python #imageprocessing #computervision

If you have ever zoomed in on a photo on your phone and noticed it gets blurry, or tried simulating "zoom" in code by slicing an image, you may have wondered: Is digital zoom just cropping? Let's unpack this.

So, What is Digital Zoom?

Zooming simply means enlarging a picture in a way such that the details in the image become clear and visible to see. In essence, zooming in lets you see the details more clearly. It does not involve any moving optics. Internally, it usually does two things:

Crops a region of interest (ROI) from the center (or target area) of the full resolution sensor image.
Upscales that cropped region to match the original resolution (eg., via interpolation: bilinear, bicubic, Lanczos, etc.)

What does this result in?

Same pixel count as the original image - but with less real information. Upscaling cannot reconstruct the lost details by cropping.

Check out this code snippet that demonstrates the zooming in concept.

from PIL import Image

def zoom_image(input_path, output_path, zoom_factor, center_x=0.5, center_y=0.5):
    img = Image.open(input_path)

    # Calculate the crop box (the area we want to zoom in)
    crop_width = img.width / zoom_factor
    crop_height = img.height / zoom_factor
    left = int(center_x * img.width - crop_width / 2)
    top = int(center_y * img.height - crop_height / 2)

    # Crop and resize back to original dimensions
    cropped = img.crop((left, top, left + crop_width, top + crop_height))
    zoomed_img = cropped.resize((img.width, img.height), Image.Resampling.LANCZOS)
    zoomed_img.save(output_path)

You may ask, What is Cropping then?

Cropping simply selects a region of the image without resizing. As a result, you get fewer pixels, but every pixel is real sensor data. In essence, cropping is cutting off the parts you don't want in the image. Cropping is lossless in terms of pixel fidelity, i.e., just fewer pixels.

Let's look at what cropping an image in concept with this code:

from PIL import Image

def crop_image(input_path, output_path, left, top, right, bottom):

    with Image.open(input_path) as img:
        # Define the crop box: (left, upper, right, lower)
        # Note: (0, 0) is the top-left corner
        cropped_img = img.crop((left, top, right, bottom))

        # Save or display the result
        cropped_img.save(output_path)

So, what are the key differences at a glance?

Feature	Digital Zoom	Cropping
Output size	Same as input (upscaled)	Smaller (native resolution)
Details	Reduced (because of interpolation)	Preserved (no interpolation)
Use case	Live preview, thumbnails	Post-processing, analysis
Reversible	No (pixel information lost in upscaling)	Yes (if we keep the original image)