Print Media Decoded: Weights, Sizes, Grain Direction, and the OBA Problem Nobody Talks About

If you've ever tried to buy paper for photographic printing and found yourself staring at specifications like "80 lb text," "215 GSM," "grain long," and "OBA-free natural white" with absolutely no idea what any of it means or why it matters — you're not alone. Print media specifications are built on a foundation of 19th-century measurement systems, manufacturing terminology that was never designed for consumers, and marketing language that actively obscures what you're actually buying. This is the guide that fixes that.

Paper Weight: Two Systems, One Mess

The single most confusing aspect of print media is weight, and the confusion exists entirely because the United States refuses to use the same measurement system as the rest of the world. Most countries use GSM — grams per square meter — which is exactly what it sounds like: the weight in grams of a one-square-meter sheet of paper. Higher GSM means heavier, thicker paper. 80 GSM is standard copy paper. 200-300 GSM is fine art photography paper. 350+ GSM is heavy card stock. Simple, consistent, universal.

The American system is a disaster by comparison. In the US, paper weight is measured in "basis weight" — the weight in pounds of 500 sheets (a ream) of paper at its "basic size." The problem is that the basic size is different for every category of paper. Bond paper uses a basic size of 17" x 22". Text/book paper uses 25" x 38". Cover stock uses 20" x 26". Index uses 25.5" x 30.5". This means that 80 lb text paper and 80 lb cover paper are completely different thicknesses — the cover stock is nearly twice as thick as the text stock, despite having the same number on the label. It's as if someone designed a weight system specifically to confuse people, and then the entire industry decided to keep using it for two centuries.

An 80-pound text sheet and an 80-pound cover sheet have the same number on the label but completely different weights and thicknesses. The US basis weight system is a 200-year-old accident that the industry never bothered to fix.

For photographers, the practical conversion you need to know: 1 lb of text paper equals approximately 1.48 GSM (so 80 lb text ≈ 118 GSM). 1 lb of cover paper equals approximately 2.708 GSM (so 80 lb cover ≈ 216 GSM). But honestly — just look for the GSM number. Every reputable paper manufacturer lists it, and it's the only measurement that lets you compare papers across categories and brands without doing math. For fine art photographic printing, you're typically working in the 200-350 GSM range. Below 200 GSM feels too flimsy for a print you'd want to frame. Above 350 GSM is heavy card stock territory, which is great for certain applications but can cause feed issues in some inkjet printers.

Points, Caliper, and Thickness

Weight doesn't tell you everything about how a paper feels or performs. Two papers with identical GSM can have different thicknesses because of differences in density, fiber type, and surface coating. That's where caliper comes in — the actual measured thickness of the paper, expressed in thousandths of an inch (mils) or in "points" where 1 point = 1 mil = 0.001 inches. A 12-point card stock is 0.012 inches thick. The distinction matters because a dense, calendered paper can be relatively thin for its weight, while a fluffy cotton rag paper can be surprisingly thick despite a moderate GSM. When you pick up a sheet of Hahnemühle Photo Rag and it feels substantial and luxurious despite being "only" 308 GSM, that's the caliper at work — the cotton fiber gives it bulk and body that a resin-coated glossy at the same GSM wouldn't have.

Grain Direction: The Hidden Variable

Every sheet of paper has a grain — a dominant direction in which the cellulose fibers are aligned, created during the papermaking process as the pulp moves along the machine's forming wire. Fibers align parallel to the direction of machine travel, like logs floating down a river. When the continuous web is cut into sheets, those sheets will have either "grain long" (fibers parallel to the longer dimension) or "grain short" (fibers parallel to the shorter dimension).

Why does this matter for photographers? Three reasons. First, paper folds more easily and cleanly along the grain — fold against it and you get cracking, rough edges, and broken fibers. If you're creating cards, portfolios, or any folded print product, grain direction determines whether your folds look professional or amateur. Second, paper is stronger against the grain, which means a sheet fed into a printer against its grain will resist curling and deformation better during transport through the print mechanism. Third — and this is the one that catches photographers off guard — paper expands and contracts across the grain when humidity changes. In the grain direction, dimensional change is minimal. Across the grain, a sheet can expand enough to cause registration problems in multi-pass printing or visible cockling in mounted prints.

For inkjet printing, grain direction is less critical than it is for offset lithography, but it still matters for sheet feed reliability with heavier stocks and for the long-term behavior of mounted prints. Most fine art paper manufacturers specify grain direction on the packaging — look for "grain long" or "grain short," or check which dimension is underlined in the size specification (the underlined number indicates the grain direction). When in doubt, the fold test works: gently curve the sheet in both directions without creasing. The direction that bends more easily with less resistance is along the grain.

Paper Sizes: ISO vs. the US

The metric world uses the ISO 216 system — the elegantly logical A-series where each size is exactly half the next larger size, and all maintain the same aspect ratio of 1:√2 (approximately 1:1.414). A0 is one square meter. A1 is half of that. A2 is half of A1. The math is beautiful, and you can scale any document between sizes without changing proportions. The most common size, A4 (210mm x 297mm), is the metric world's equivalent of US Letter.

The US uses Letter (8.5" x 11"), Legal (8.5" x 14"), Tabloid (11" x 17"), and a collection of custom sizes that have no mathematical relationship to each other. They exist because they've always existed, which is the same reason the US system uses basis weight. For photographers, the important sizes tend to be defined by print output: 4x6, 5x7, 8x10, 11x14, 13x19, 16x20, 17x22, 24x36. These are ratio-driven for photographic display and don't align neatly with either the ISO or US document standards. Roll-fed wide-format printers typically use 13", 17", 24", 36", 44", and 60" roll widths, and you cut to whatever length the image requires.

Optical Brightening Agents: The White Lie

This is the section where your understanding of photographic paper is about to shift permanently. Optical Brightening Agents — OBAs, also called Fluorescent Whitening Agents — are chemical compounds added to paper coatings that absorb invisible ultraviolet light (in the 340-370nm range) and re-emit it as visible blue light (420-470nm). The effect is that the paper appears significantly whiter and brighter than its natural base color. Without OBAs, most paper substrates have a slightly warm, yellowish tone from the cellulose base material. With OBAs, the paper glows with a clean, cool white that catches the eye on the shelf.

Here's the problem: OBAs are performing a magic trick. They're converting energy from a part of the spectrum you can't see into a part you can, which means the paper is literally emitting more visible light than it's receiving in the visible range. Your spectrophotometer — the instrument used to build ICC color profiles — can be fooled by this. Reflectance readings in the 400-480nm range can exceed 100% when fluorescence combines with natural reflectance, creating measurement errors that propagate through your entire color management chain. This is why prints made on OBA-heavy papers can show color shifts that shouldn't exist according to the profile — the profile was built under one UV/visible light ratio, and you're viewing the print under a different one.

A paper with optical brighteners is literally emitting more visible light than it receives. Your spectrophotometer sees this as excess blue, your ICC profile tries to compensate, and your print comes out with a yellow cast that exists nowhere in your file or your screen.

The Metamerism Connection

OBAs are one of the primary drivers of illuminant metamerism in photographic prints — the frustrating phenomenon where a print looks perfectly neutral under one light source but shifts to a noticeable color cast under another. This happens because different light sources contain different amounts of UV energy. A print on OBA-heavy paper viewed under UV-rich fluorescent lighting will appear bright, cool, and neutral. Move that same print to a warm incandescent bulb — which emits almost no UV — and the OBAs stop fluorescing. The paper's natural warm tone emerges, and suddenly every shadow area in your carefully profiled print has a yellow or green tint that wasn't there before.

For gallery and exhibition work, this is a critical issue. Many galleries use UV-filtered lighting or LED sources with specific spectral profiles. Under these conditions, OBAs don't activate at all, and the paper reverts to its natural base color. A print that looked spectacular under the studio lighting where it was profiled can look markedly different on a gallery wall. This is why archival fine art papers — the ones serious printmakers and galleries prefer — are typically OBA-free or have only trace amounts. Papers like Canson Infinity Platine Fibre Rag, Breathing Color Elegance Velvet, and Crane Museo Silver Rag are designed to look consistent under any light source because they don't depend on UV fluorescence for their white point.

OBAs and Longevity

Beyond color management headaches, OBAs present a permanence problem. The fluorescent compounds degrade over time — accelerated by UV exposure but occurring even in controlled storage conditions. As the OBAs fade, the paper's apparent whiteness decreases and shifts toward its natural warm base tone. Your print doesn't fade (assuming you used pigment inks on archival paper), but the paper beneath it changes, which effectively changes the print's color balance. The whites get warmer. The overall brightness decreases. A print that was profiled and evaluated with active OBAs will not look the same in five or ten years when those OBAs have partially degraded.

The Wilhelm Imaging Research Institute — the industry's primary authority on print permanence — explicitly recommends avoiding OBAs when long-term image permanence is desired. Their accelerated aging tests show that OBA degradation can produce visible yellowing within timescales much shorter than the ink-fade endpoints that typically define a print's "display life." In practice, the paper yellows before the ink fades, which means the weakest link in your print's longevity might not be the image at all — it's the substrate.

How to Check for OBAs

The simplest test: shine a blacklight (UV-A lamp) on the paper. Papers with OBAs will glow a brilliant, unmistakable blue-white. Papers without OBAs will appear dull and natural under the blacklight — no fluorescence, no glow. Small battery-operated blacklights are available for a few dollars and are a worthwhile investment for any photographer serious about print quality. Beyond the blacklight test, most premium fine art paper manufacturers now explicitly state whether their papers contain OBAs. Look for terms like "OBA-free," "no optical brighteners," or "natural white" as indicators. Papers labeled "bright white," "ultra white," or "super white" almost always contain OBAs.

Putting It All Together

Understanding print media isn't about memorizing conversion charts — it's about knowing what the numbers actually mean so you can make informed choices. Use GSM for weight comparison and ignore US basis weight whenever possible. Know your paper's caliper if you're working with heavy stocks near your printer's feed limits. Check grain direction for folded products and large-format mounting. And make a conscious decision about OBAs based on the intended use of your print: temporary display and commercial work where impact matters more than longevity can benefit from OBA-enhanced whites, while gallery work, archival prints, and anything intended to last should be printed on OBA-free substrates with natural white points.

The print medium is the final creative decision in photography — the one that determines how your image physically exists in the world. Understanding what you're printing on is as important as understanding the camera that captured the image in the first place.