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How Much Resolution is Enough?
I keep seeing comparisons on the web of digital vs. film. How many megapixels will it take to equal what we get from film? I’d like to drop 35mm from the equation and ask instead, “How many megapixels will I need before I stop caring about increased resolution?” As my daughter would say, “Are we there yet, Daddy?”

To answer this question for myself, I decided to take a clean, well exposed image from a sharp lens using a camera mounted on a sturdy tripod and make 4” x 6” prints at various resolutions using an Epson R2400 printer in order to determine just how much detail is enough. What is the limit of detail that either my eye can detect or my printer can reproduce?

I used a single unsharpened image scaled to 4” x 6” at 420, 360, 300, 240, 180, and (just for kicks) 120 pixels per inch. Each image was then independently sharpened at 100% view using a calibrated and profiled monitor and a high quality edge-sharpening technique. Prints were made on Ilford’s high gloss photo paper (to minimize dot gain) at the highest quality settings using a custom printer profile.

At handheld viewing distances, the 120ppi print was easily detectable as “soft.” Frankly, it looked a lot like the prints I used to get from a one hour photo lab using moderate quality consumer zooms. Not bad, exactly, but enough to detract from the overall quality of the image.

The 180ppi print was also fairly easy to detect. Overall contrast was lower than the higher resolution prints, and the image didn’t seem to “pop” as much as the rest. It didn’t look at all bad, but when comparing it to the rest of the images it was noticeably less sharp.

After eliminating these two, things got pretty tough. Here comes the big surprise—the 420ppi print was the least sharp of the remaining images. I suspect this is because of the sharpening technique used. Since the amount of sharpening applied was appropriate for a 100% view of the picture, effectively the 420ppi image got the least amount of edge sharpening and so looked just a touch soft compared to the remaining images. I expect that with slightly more aggressive sharpening, the 420ppi print would have been indistinguishable from the rest.

Frankly, the differences between the remaining images—240ppi, 300ppi, and 360ppi—were too subtle to name a clear winner. I could find no element of detail that was visible in one that wasn’t also visible in the other two. To my eye, they were indistinguishable without magnification. All three images appeared sharp and full of fine detail at virtually any viewing distance.

So how large a print can I make from a 6 megapixel digital SLR at a resolution that will stand up to the most critical eye? Would I see any difference in my prints if I were using an 12 or 14 megapixel camera? Could I see that additional detail in my prints? Would I get sharper images on paper? Remember, it’s all about the final product!

Well, a 6 megapixel camera such as the Nikon D70s produces an image that is approximately 2000 by 3000 pixels. Dividing by 240, this would mean that I could make an 8” x 12” print that could be viewed from ten inches and still appear critically sharp. Certainly, the print would be as sharp as the Epson R2400 is capable of producing.

What about larger prints? The rule of thumb on viewing distance is that it should be equal to the diagonal of the print size. This seems to give the human eye the best compromise of being able to evaluate the entire image without moving the eye while still seeing as much detail as possible. If in fact the viewing distance were to increase linearly with print size, there is no reason I couldn’t make prints as large as I wanted from a 6 megapixel camera and still have them appear critically sharp. Viewing distance is the key, of course. If the viewer is going to walk right up to a 20" x 30" print and examine small areas of detail, then the equation breaks down.

So would I ever want more than 6 megapixels? Sure. I’d like to have room for cropping. I’d also like to be able to make 20” x 30” prints that could be examined closely and still appear sharp. In the real world, though, it is clear that camera resolution is rarely the limiting factor now in an image’s sharpness. Accurate focus, good handholding technique (or the lack thereof), a fast enough shutter speed, and enough depth of field will have a much more profound effect on perceived sharpness in the final print than actual camera resolution.

Are we there yet, Daddy? Not quite, but it won't be much longer.

- Jared Willson


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