How Much Resolution Do You Really Need?

The spec sheets on today’s flagship digital cameras are impressive to say the least. Nikon offers 36, Sony 42, and Canon’s 5DS a staggering 51 megapixels. And that’s just right now, and that’s not touching the likes of Hasselblad and Phase One 100MP offerings. Ever since digital cameras began to supplant film as the industry standard, resolution has been the headline feature of every camera along the way.

The trend of packing more and more pixels onto our image sensors doesn’t seem to be slowing down, as every year we marvel at the newest staggering megapixel number, only to see that number surpassed mere months later. However, in this relentless contest for king of the resolution mountain, it seems rare that we stop and ask the question: how much resolution do we actually NEED?

Naturally, the answer to this question will be a personal one and dependent on a number of factors, but there are a few constants that can help you as a photographer, to answer it for yourself. For example, we have standards in place to describe the abilities of the human eye, and unless your intended audience is a nest of bald eagles, these guidelines can suggest the point of diminishing returns, resolution-wise.

What is Camera Resolution?

First, it’s important to understand what we’re talking about when we describe the resolution of a photo. Resolution is essentially the ability of the human eye, camera sensor, printer, or screen to differentiate between two points. To calculate this ability, you need two key pieces of information: the distance between the two points, and the relative viewing distance from those points. Imagine that you’ve lit two candles, placed them 1 foot apart, and drove a mile away from them. At this distance, you would most likely be unable to tell if you were looking at one candle or two. In order to see that there are, in fact, two candles, you would need to move them further apart, or get closer to them.

This relationship between size and distance can be described as an angle, in degrees or arcminutes, where 1 degree = 60 arcminutes. For reference, the size of the full moon when viewed from earth is about 1/2 degree or 30 arcminutes. This is a necessary basis element to understand.

Resolution of The Human Eye

I hear you asking, nay, begging the question: what is the resolution of the human eye?

20/20 vision is described as being 1 arcminute. This equates to a factor of 1/3438. Using the candle example again, this means that someone with 20/20 vision would be able to begin to differentiate the candles, 1 foot apart, from 3,438 feet away, or 0.65 mile. Of course, most adults have worse than 20/20 vision, and some have better than 20/20, but as this is the standard for fighter pilots, let’s use it as a basis for our measure of what we “need” in a photo.

However, we don’t generally look at photos from over half a mile away, so this number still doesn’t get to the heart of the matter. Let’s convert this to a more applicable scale. To determine the resolution of the human eye at a distance of 12 inches, we would divide 12 by 3438, and we get about 0.0035 inch. 1/0.0035 = approx. 286 ppi (points/pixels per inch). Finally we’ve gotten back to a measurement that photographers are used to. So, if we slightly exceed this number, 286, up to a nice round 300 ppi, people with 20/20 vision will no longer be able to see individual pixels, and your photo will look smooth and sharp, from a distance of 12 inches.

Best Resolution for Print Size

This benchmark of 300 ppi still doesn’t quite bring us to the resolution we need, as ppi only describes pixel density, and is therefore only part of the equation. In order to determine the ideal resolution of an image at 300 ppi, you still need the finished size of the image. Or, to work backwards, using the resolution of your camera, you can determine the maximum print size at this magical pixel density.

If you’re shooting any of these high-megapixel cameras and printing your photos at smaller sizes than those listed above, you’re theoretically wasting resolution, as the camera is capturing more detail than the human eye can see. Of course, it’s often useful to be able to crop into these large files and retain detail and image quality, and it’s always better to have too much resolution than not enough. If you already know the final size at which a particular image will be printed, you can easily determine the minimum resolution you’ll need by multiplying the dimensions (in inches) by 300.

Viewing Distance Matters with Resolution

We’re almost there, but this still only tells part of the story. As we remember from everyone’s favorite candle allegory, we still need to factor in the distance from which your print will be viewed. The magical 300 ppi pixel density is calculated based on a viewing distance of 12 inches. If you hold your hand 12 inches from your face, you’ll see that this feels incredibly close, and holding it any closer begins to feel downright uncomfortable.

This brings up another aspect of human eyesight, that our eyes can only actually resolve fine details to their full ability in the central area of our field of vision, an area of our eyeballs known as the fovea, which contains the greatest concentration of cone cells. This area takes up only 1% of the retina, or roughly 2 degrees. Take a moment to look at the keyboard of your computer, and then try to read the words on the screen with your peripheral vision. You can’t do it, right? This means that even if we’re looking at a small printed picture, say 4” x 6”, 12 inches from our face, we can’t actually resolve all of the detail at the same time. So, trying to look at a 24” x 16” print from 12 inches away is illogical.

If, instead of the 1 foot viewing distance, we stand 3 feet away, suddenly we only need a 100 ppi pixel density to meet the standards of 20/20 vision, and the 36mp file from your D810 can be printed at about 73” x 49”. You may have begun to see where I’m going with this. As print sizes increase, you need more pixels, but you’ll also most likely be viewing these larger prints from a further distance, so you don’t need as much pixel density, so you actually don’t need as many pixels. Essentially, we end up back at the measure of resolution as an angle. If a photo takes up 10 degrees of our field of view, whether it’s a 4” x 6” at 1 foot or a 20” x 30” at 5 feet, our eyes will be able to resolve the same amount of detail.

Often, when speaking about high megapixel cameras, I hear the comment that you would only need ‘so much’ resolution if you were blowing your photos up to billboard size. In reality, billboards are viewed from so far away that the pixel density can be greatly reduced without anyone noticing. If the closest you’ll be able to get to a billboard is 30 feet, the image could be printed at 10 ppi, and your eyes wouldn’t be able to tell the difference. At 10 ppi, you could blow a 12mp iPhone image up to 38 ft. x 21 ft. without the image appearing pixelated. Or, to think about it in another way, the next time you see a billboard, from a reasonable distance, hold your phone in front of it, about a foot from your face, and you’ll see that your phone screen will probably hide the entire billboard from your view. That tells us that your eyes are able to resolve the same amount of detail in your phone screen as in that massive billboard.

Resolution for On-Screen Displays

If you don’t intend to print your work, and online publication is the final destination of your image, then your resolution needs are even less demanding. HD and UHD displays look incredibly smooth from recommended viewing distances, but the actual resolution in megapixels isn’t actually particularly high.

Without zooming, these are the resolution limits of these various displays. When viewing an image at full size with pixel dimensions exceeding those of your display, your graphics card will simply discard the superfluous information. Again, much of the resolution of your image will be wasted, and your 51 megapixel image will only serve to impress the pixel-peepers.

Resolution of Camera Lenses

But wait, there’s more. Your camera sensor will only be able to capture as much detail as your lens can resolve. The resolution capabilities of a lens are more difficult to quantify, as this step of the image-making process is still analogue. DxoMark has developed a measurement of this ability called “perceptual megapixels” which measures the effective resolution of a camera/lens combination. This P-Mpix score is listed as a single number, but we also know that most lenses have an optical sweet spot in the middle, and lose some sharpness towards the edges of the frame, so it’s still difficult to determine how those perceptual megapixels are distributed across an image. Still, this score is helpful when comparing lenses, and once we know the final print size and pixel dimensions we need, we can use these scores to determine whether a given camera and lens combination will deliver the detail we need.

A bad lens can easily turn a high megapixel camera into a low megapixel camera. For example, according to DxoMark, the mighty, 36mp Nikon D810 can resolve only 10mp when fitted with a $300 Nikkor 28-200 f/3.5-5.6G. So, while the full potential of your camera’s sensor may be overkill for your final prints, it’s difficult to deliver that level of detail to your camera’s sensor, and a low quality lens can quickly handcuff it’s performance.

In Conclusion

I could have saved myself a lot of work by titling this article “It’s better to have too much resolution than not enough” and leaving it at that. The truth is that the digital cameras being produced today are incredibly good, and the effective difference between 24 and 51 megapixels is usually, in practice, imperceivable, unless of course you crop or zoom in to where you can only see a small fraction of the full picture.

The bottom line is this: unless you’re making large prints from your photos, and displaying those prints somewhere that they will be carefully examined, such as an art gallery, much of the resolution of your camera will most likely be wasted most of the time. Does this mean that you should sell your DSLR kit and shoot everything with your 12mp iPhone camera? Of course not, because, first of all, it’s better to have too much resolution than not enough, and also, image quality is dependent on more than just the ability to render fine details. It’s also important to take into account a camera’s dynamic range, bit depth, high ISO capabilities, and other characteristics that may be more difficult to quantify than resolution.

Buying a camera is a personal choice, dependent on a variety of factors, preferences and needs. Resolution is often the highlight feature pushed by camera manufacturers, but should clearly not be the sole consideration of photographers. When comparing cameras, consider how your photos will be displayed, decide on the minimum number of megapixels you’ll need, and move on to other features and abilities.