Aspect ratio and frame sizes

Two frames can be the same size and the wrong shape. That sentence explains most of the disappointment in home framing, and once you understand aspect ratio the problem essentially stops happening.

What an aspect ratio is

An aspect ratio is width divided by height, reduced to the smallest sensible numbers. It describes shape only — it says nothing about how big something is.

A 6x4 inch print, a 30x20 inch poster and a 6000x4000 pixel file are all 3:2. They are wildly different in size and identical in shape, which means the same image fills any of them perfectly.

Two images with different ratios can never both fill the same frame. One of them has to be cropped, bordered, or distorted. There is no fourth option, and "distorted" is never the answer.

Where the common ratios come from

3:2 is the shape of a 35mm film frame — 36mm by 24mm. Digital sensors in interchangeable-lens cameras copied it, so essentially every DSLR and mirrorless camera in the world produces 3:2 files.

4:3 came from early television and from the 4/3-inch sensor format, and it is what most phone cameras produce by default.

4:5 is the shape of large-format sheet film — 4x5 inches, and 8x10 inches. American portrait studios worked in that format for a century, which is why 8x10 and 16x20 are the dominant US print sizes despite matching no camera in current production.

√2:1 is the A-series, defined so that halving a sheet preserves the ratio. It is used almost everywhere except North America.

16:9 is a video and display shape. It exists in print essentially only as a deliberate crop.

Frame sizes are the sediment of these separate histories, which is precisely why they do not form a coherent system.

The ratio families

Two things stand out. The 3:2 family is the largest and the most useful for photography. And 11x14 — one of the most stocked frames in existence — belongs to no family at all.

What cropping actually costs

Filling a frame whose ratio differs from the image means removing a strip from the long edge. The amount is not always small.

Seventeen percent of the width of a portrait is roughly an arm. Thirty percent of a 16:9 landscape is most of the reason it was shot in 16:9. These are not rounding errors, and a print lab set to "fill" will apply them without telling you.

The three honest solutions

Print to a matching size. The cleanest answer. Pick the frame from your file's own ratio family and nothing is lost. This is what the aspect ratio calculator is for — enter your dimensions and it lists every standard frame within 2% of your shape.

Crop deliberately. If you must crop, do it yourself, looking at the image, rather than letting an algorithm centre-crop it. You will make a better decision about which arm to lose than the lab will.

Mat the difference. Print at "fit" so the whole image lands with borders, then cover the borders with a mat cut to the image's real ratio. The mat opening follows the artwork; the frame does not have to. This is the technique that lets a 16:9 panorama live inside a perfectly ordinary 16x20 frame, and it is why professional framing rarely worries about ratio mismatch at all.

Resolution is a separate question

Aspect ratio tells you whether the image fits. Resolution tells you whether it will look good doing so.

For photographic quality, aim for 300 pixels per inch: a 16x20 print wants 4800 x 6000 pixels, which is 28.8 megapixels. Very few cameras deliver that at the long end of a print run.

In practice, viewing distance rescues you. A large print is looked at from further away, and 150 DPI is entirely acceptable for anything 20 inches or more on the long edge viewed from a normal distance across a room. That halves the pixel requirement, and a 24-megapixel camera comfortably produces a 24x36 poster on that basis.

The rule that fails is upscaling. Software that invents pixels can make a file the right size; it cannot add detail that was never captured. A 1200 x 800 image printed at 24x36 looks exactly as soft as it is, regardless of what the file's dimensions claim.

Working it out for a specific print

1. Get the file's pixel dimensions, or the print's inch dimensions.
2. Divide the larger by the smaller — that is your decimal ratio.
3. Find the family it belongs to in the table above.
4. Choose any frame from that family that suits the wall.
5. If nothing matches, mat it rather than crop it.

If you would rather not do the arithmetic, the calculator takes your two numbers and returns the simplified ratio, the metric conversion, the pixels needed at 300 DPI, and the list of matching frames in one go.

Cropping deliberately

If the ratios do not match and you have decided to crop, do it yourself rather than letting a print service centre-crop for you.

Open the file, set the crop tool to the frame's ratio, and move the crop box around before committing. Two things to check: that nothing important sits within about a quarter inch of the new edge, since the frame's rabbet will overlap that much again; and that the composition still works, because removing 17% of a frame's width often removes the balance the photograph depended on.

For portraits specifically, watch the space above the head. A 3:2 portrait cropped to 4:5 usually loses width, and the instinct to keep faces centred can push the crop into hands and elbows at the edges.

When the mismatch is the point

Occasionally a ratio mismatch is worth keeping rather than solving. A panoramic image floated inside a conventional frame, with a wide mat above and below, reads as deliberate and often better than either cropping or a custom panoramic frame.

The technique is the same as any float mount: the mat opening follows the artwork's real ratio; the frame does not have to. See float and floater frames for how to mount it.