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How Much Resolution Do You Really Need?

By Bing Putney on October 11th 2016

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?

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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 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.

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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.

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.

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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.

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.

Camera Megapixels Actual Pixel Dimensions Print size at 300 ppi
iPhone 7 12 4608 x 2592 15.36” x 8.64”
Fujifilm X-T2 24 6000 x 4000 20” x 13.3”
Nikon D810 36 7360 x 4912 24.5” x 16.37”
Sony a7RII 42 7952 x 5304 25.5” x 17.68”
Canon 5DS 51 8688 x 5792 28.96” x 19.3”

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

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.

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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.

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.

Monitor Type Actual Pixel Dimensions Megapixels
720p HD 1080 x 720 0.7mp
1080p HD 1920 x 1080 2mp
4k UHD 3840 × 2160 8.2mp
27″ iMac 5k 5120 x 2880 14.7mp

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 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.

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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.

[Rewind: Advanced Lightroom Processing: Pushing High Resolution RAW Files to the Extreme]

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.

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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.

About

Bing is a professional portrait and on-set still photographer who lives in Los Angeles, and frequently travels the world to explore new and interesting cultures and pastries.

Website: bingputneyphotography.com
Instagram: @bingputney

14 Comments

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  1. Jacobus DeWet

    Great read. For me, and I do print on a regular basis and sell prints, up to A0 size, the D4 produce great results

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  2. Rohan Mishra

    Can’t thank you enough for coming up with this article. Have clarified many of my queries. Thank you!

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  3. Mary Johanssen

    Disagree totally. High resolution is a waste of resources. I’ve printed 8 megapixels to 16×24 inches. It looked great. Only a real pedant looking for trouble would press their nose up against such a print and use a loupe to find pixels.

    99% of camera owners think they’re going to sell their snaps but they’re deluding themselves. Nobody much pays for what can be lifted off Flickr free.

    The reality is most photography is personal. In fact, I hardly ever pick up my camera these days. I use my cellphone. Nobody cares about images being the ultimate quality when the average cellphone produces darned good images. For most people, high megapixels is just pointless and gratuitous overkill.

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  4. Timothy Linn

    Very well written article, Bing. Clear, concise, and appropriately nuanced.

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  5. Bob Panick

    Probably the best article on the subject I’ve seen yet. Nikon has a list of lenses for the D8xx series that can resolve to the sensor, or at least close. I saw a LensRentals blog where they were testing the Canon 5Ds and if I understood it correctly, the best lens available was only in the low thirty MP resolution.

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  6. Paul Empson

    an interesting article…. I have upgraded to the D810 & D750 because I print, usually 24×12, wedding albums and wanted the peace of mind that the more pixels gave me.. if I decided to print a double page…. that said my 28×14 demo album was printed full of 12mp D700 images & looks great as was my biggest ever print 2mx2m (canvas)

    I’ve decided I’ve enough ISO performance & pixels in my current cameras and really can’t see an upgrade or camera change any time soon. I’m interested as to what the next innovation will be to separate us from our money.

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  7. Dave Norwood

    I’ve read many articles about resolution and what’s needed for good viewing but this one makes it easy to understand in a real world environment. Thank you.

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  8. Korey Napier

    Thank you for this article. This absolutely hits the nail on the head. People often forget that a 2MP image (1080p) has been and is currently being viewed on 40, 50, 60, 70, etc. inch tv’s for some time now. Viewing distance factors SO much into this. This is why I had no problem “downgrading” from a 24MP Nikon to a 16MP Fuji. I’ve been caught up in the “I need more MP’s!” mentality before, but have since realized that it is quite silly to some extent.

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  9. Kim Bentsen

    But there is more.
    Bayer interpolation causes a loss of resolution. I will always reduce the image 50% to make 4 RGGB pixels into one good pixel. My final output would eventually be targeted a 8K 100″ TV screen (not realistic yet). That means 7680×4320 pixels, and with a 50% reduction this means, that I need/want a 133MP camera :-). I do not target paper output or posters/billboards.

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    • Bing Putney

      Well, in a standard bayer pattern sensor, each effective “pixel” is actually 4 photosites, RGGB. So, while reducing an image by 50% may help effective resolution by making up for any lack of lens sharpness, it won’t make your individual pixels any “better.” As for the 8k screen, the viewing distance still comes into play. How close are you planning to stand to a screen that large?

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    • Kim Bentsen

      Good question. 2 meters (6.5 ft) viewing distance seems reasonable. Actually I need more than 133MP, because sensors are not in 16:9 format, so the image will need to be cropped. We will be at 200MP within 6-8 years, and then we have reached the goal.

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  10. Kim Farrelly

    I need 200Mpx, so I get really good at mapping and stitching in post.

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  11. David Heintz

    Excellent article, which presents some of the detail behind the facts most “pros” already know. I have pretty much understood this for years, but it does not explain this phenomenon:

    I take the same exact photo with my Nikon Df and D810, or at least as close as I can get it, and I process both files the same way in Photoshop. Use the same lens, of course.

    Then, I put them side by side, work the magnification (zoom) to be as close as possible, and, every time, the D810 image looks sharper, or “more resolute.” Actually, the Df should look best under these conditions (good light, no ISO issues) as the pixels in the sensor are larger. I can switch these around, etc. but my eyes pick them out each time. (iMac 5K 27′)

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    • Bing Putney

      Well, as soon as you zoom into those files, you begin to get into the realm of pixel peeping, and the D810 at 36mp will absolutely have more detail than the 16mp of your Df. The real test would be to print the images from both cameras at a reasonably large size, stand a reasonable distance away from them, and see if you can make out a difference.

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