How Many Megapixels Do You Need?

How Many Megapixels Do You Need?

How many pixels do you need in an image? Sounds a simple enough question and the inexorable megapixel race doesn't seem to have ended, in much the same way that the PC processor race marched on unabated for several decades.

Of course, megahertz envy ultimately resulted in people not caring a whit anymore. A 4GHz whizbang processor didn't make a blind bit of difference compared to a 2GHz popzoom processor when it came to finishing writing a Christmas thank you letter to Auntie Marge. That said, you try loading a 50MP image into a PC with an integrated graphics card from five years ago with 1Gb of main system RAM and, well, you'd better have plenty of spare time on your hands!

And so the principle is very similar to the photographic world and the ever-ramping up of the MP count for all those pixel peepers. So let's start with the premise of the actual use of the resultant image - why are you actually taking a photo? That then drives the second question - what are you going to do with it?

If you are shooting for a client, then they will have in mind a single, or more likely, a range of applications for which they want to make use of your images. Anything from Instagram, to photobook, to canvas, to full page ad, to billboard. Each of these is displayed/printed at a different size, on different media, with different print techniques.

So how many pixels do you need? The answer, which might seem counter-intuitive at first, is as many as you need to see. There are two closely related aspects to this:

1. Viewing Distance: as a rough rule of thumb, the viewing distance should be 1.5 to 2 times the diagonal length. This then gives an estimate of how far away you should be for the human eye to focus on a whole image. The bigger an image gets, the further away you need to be (doh!).

2. Pixels per Inch (ppi): for the viewing distance above, how many pixels do you need to fool the eye into believing you are showing smooth continuous tones? The short answer is at least 3438/viewing distance.

A 6x4 photo? You view it about 12" away: 3438/12=286.5. Hence the recommendation for 300ppi.

An A1 canvas? Maybe 4ft away: 3438/48=71.6. Hence the recommendation for 72ppi.

So where does the magical 3438 number come from? Well, this is based on the visual acuity of a "normal" human eye where 1 arc minute (0.000290888 radians) of angle is how much resolution the eye can see. With some High School trig (remember SOHCAHTOA?!) we calculate

1/ppi = 2 x Viewing Distance x tan(0.000290888/2)

1/ppi = Viewing Distance x tan(0.000290888)

ppi = 3438/Viewing Distance

As long as you keep your ppi above this value you should be fine at this viewing distance. If you have an image (such as a photo montage) where people are likely to look at parts of the image or where people may simply want to look at it more closely (possibly a landscape), then you'll need to up the ppi. It's also a timely reminder that, for an A1 canvas, you are only looking at a minimum 4MP requirement (although you might want it to be higher). Resolution, therefore, provides latitude in your photographic workflow - latitude to crop-in, rotate and in any other way you see fit, lose pixels during post-production.

Image courtesy of OpenClipart-Vectors

(Original inspired by having this problem and some great StackExchangers)

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20 Comments

Marius Pettersen's picture

Why do people always bring up print as the justification for needing, or not needing, a high number of pixels?
I do not print large, but I do however want a large amount of pixels for post-processing work. Just like video professionals can utilize 4K data even if the end product is only 1080p, or going from 6K to 4K with panning, re-framing or post-stabilization.

Anonymous's picture

I think it's a way of showing how the average person severely overestimates the amount of pixels they actually need, which is true.

Marius Pettersen's picture

I was born in the analog era, thank you.

Marius Pettersen's picture

More or less so. I only 'played' with analog compact cameras in my youth before purchasing DSLRs. I've also shot a fair amount of 120 films in the Mamiya M645, but not enough to say that I'm skilled with analog photography or have a deep understanding of the chemical processes.

Marius Pettersen's picture

I'm always glad when a tense comment thread flows over to a decent conversation. Too bad such a thing is rare on the Internet.

Because large prints are the only common avenue of display where the resolution can exceed the resolution of most cameras. It's hard to find a camera under 16mp these days, and even with heavy cropping that leaves you plenty of resolution for a 4K monitor/TV (8mp).

A 30x40 print at 300dpi is over 100mp, so the difference between a 24mp camera and a 50mp camera could be significant if the viewing distances are close.

Marius Pettersen's picture

Understandable, but my main grind is about the argument: "I/you don't print large, so I/you don't need many mega pixels."

Anonymous's picture

It's actually pretty rare for people to view prints at recommended viewing distances. The exceptions to this would be billboards simply because the average person cannot physically get closer to a billboard than the prescribed viewing distance. I generally find that be it prints or screens, people tend to be a lot closer than recommended because it adds to the immersion.

I have several 30x40 prints in my house and I can definitely tell you that when people come over and look at them, they almost never stand back and do so. They get right up close with some people even examining certain sections from inches away.

Given the fact that the vast majority of people don't print nor do they look at prints, however, I think a better metric would be to evaluate the question using the medium people primarily use to view their images: the screen. We're largely moving past standard HD at this point, so outside of mobile devices, you're probably going to be looking to be able to display your images on a 4K monitor or TV. Given this, you're probably looking at something in the 9-megapixel range as the bare minimum so that you can view your images full screen. That's assuming that you don't want extra "headroom" to crop down your images, of course.

Anonymous's picture

For the screen, it's really easy. What are the pixel dimensions of your screen? That's pretty much the minimum pixel dimensions that your final image should be. No need to think about viewing distances or anything like that. The hardware itself answers the question for you.

Time to have my cake and eat it too! :-) Thanks for this article. It puts some concrete numbers to ***minimum*** requirements that we all should be at least aware of. I'm glad to see them stated so specifically! At the other end of the spectrum, Marius Pettersen makes the excellent point that more pixel density has tremendous value. As is often the case, the best answer is dictated by use and is usually a happy medium. The photographer has to balance the "megapixel race" against need. It's great to have high megapixel count to produce quality photos from (possibly severely) cropped shots. But how often does this happen? A lot? A little? Is it worth purchasing a 60+ MP camera when you need this only on occasion, or all you're doing is instagram? Or is a 10 MP camera enough to make billboards or A1 wedding portraits? Maybe a 24 MP is perfect for your needs, maybe more, maybe less. Do I need to say cost vs. return on investment? I've given only one or two simple examples of hundreds of scenarios, but you get the idea. It's like diet and exercise. Everyone has their views, but a happy medium on all fronts probably produces great results. Perfect results will cost a lot more. Let the flame wars begin! ;-)

Michael Dougherty's picture

With a high MP full frame body, I can digitally crop and still maintain a high MP image versus a crop frame body. I've been shooting a crop frame D500 (21MP) with a 150-600. That's actually 225-900 with the crop frame body. However, sometimes 225 is just to far. Now I've started using the full frame D850 (46 MP) which is actually a 150-600 but if I digitally crop the image at the high end in post processing, I get an effective 900 while still maintaining an image with an MP over 23 which is slightly better than the crop frame body. (My math may be off a little but you get the point.)

How many megapixels? It reminds me of a famous bit of dialogue from Key Largo.

Johnny Rocco: There's only one Johnny Rocco.
James Temple: How do you account for it?
Frank McCloud: He knows what he wants. Don't you, Rocco?
Johnny Rocco: Sure.
James Temple: What's that?
Frank McCloud: Tell him, Rocco.
Johnny Rocco: Well, I want uh ...
Frank McCloud: He wants more, don't you, Rocco?
Johnny Rocco: Yeah. That's it. More. That's right! I want more!
James Temple: Will you ever get enough?
Frank McCloud: Will you, Rocco?
Johnny Rocco: Well, I never have. No, I guess I won't. You, do you know what you want?
Frank McCloud: Yes, I had hopes once, but I gave them up.
Johnny Rocco: Hopes for what?
Frank McCloud: A world in which there's no place for Johnny Rocco..

Go make some great pictures. Jim Race.

Simon Patterson's picture

Megapixels are like money.

Q. How much do you need?
A. More

😀

Manfred Mueller's picture

Now where does your 1.5 - 2x viewing distance come from?

My CAPA (Canadian Association for Photographic Art) judging manual tells me I should be no closer than 2x the diagonal of either the print (for print competitions) or the screen (projection or computer screen). The rationale here is that the judge should be far enough away to take in the entire image without having to move his or her eyes. That rule forces the judges to view the image in it's entirety without getting into pixel peeping.

When I first got into photography many decades ago, we were taught to hold prints at "arms length", but no closer that 1x the diagonal. This was a bit of a "common sense" rule that suggested that people would examine image close up but without actually touching their noses to the image.

I had recently watched a video from a current fine art photographer who was advocating 2x - 3x the diagonal as the appropriate viewing distance.

The bottom line is that there is no consensus on the correct viewing distance and we should expect pixel peepers to get their noses close up to the images. When it comes to the majority of the images we take, these will be viewed on electronic displays that range from relatively small smart phone screens to relatively large computer displays.

When we print, all bets are off as the limits are imposed on us by the printing process and the location where someone decides to hang our work. My largest work hangs on a boardroom wall and measures 2.5m x 9m / 9 ft x 33 ft and was printed at 150 dpi. I'm pretty sure the 1.5x - 2x viewing distance rule does not apply in this situation...

Kirk Darling's picture

Viewing Distance: Viewing distance can't be dictated to the audience, unless they're physically restricted. Viewing distance is always reading distance, because people will wander close to study a photograph that interests them.

That's not necessarily true of other visual arts, but it is true of photography, because the unfolding of additional detail on close examination is one of the traditional joys of the photographic art. I once saw a guy whip a loupe out on a landscape in a gallery.

Pixels per inch: Given that people are going to mozy to within reading distance of a photograph they like, pixels per inch is always ~300.

Also: Cropping. Being able to crop is beneficial.

In truth the vast bulk of my work does not require a camera with more than 16MP.
As a matter of fact, when we first started shooting digital, we were astonished by the quality of 6MP cameras. We regularly made prints up to 16x20 that exceeded the quality of film on most occasions. Portraits up to 40x60 were mistaken for MF until you got close enough to see pixelation and that was about 8" away.

However as some have noted I have no control over how my clients will use my images.
As an architectural photographer I have clients who will print some images up to 8 or 9 feet wide for display purposes. OTOH, most of my images are web bound and really need no more than 2000 pixels on the long edge for any sort of use.

So while I would love to use an Olympus with a 12-100 lens, I use a pair of 50MP Canons with 4 lenses to ensure I have the tools to make the images I think (or fear) my clients want.

I am so looking forward to getting rid of a 40 lb. camera case.

Kirk Darling's picture

Some technical issues such as moire are also resolved by more resolution. I might say that I need nothing more than 12mp for my wall portraits, except that I was dogged by hair moire captured on the sensor with half-length portraits taken at 12mp on my Camnon 5D. The 20mp of the Canon 5DII fully resolved hair in half-length portraits. Photographers shooting any surface that has replicating texture--textiles, architecture--will need some level of resolution to eliminate moire in what they commonly do.

Gil Aegerter's picture

I have large prints on my walls from 6mp, 12mp, 16mp, and 24mp DX and full-frame cameras. You would be hard-pressed to tell which is which. But 24mp full frame files definitely take up more storage space.

Kirk Darling's picture

Depends on the subject. Some subjects have linear details easy to interpolate well--such as the lines and curves of an automobile--and need less capture resolution. OTOH, there is no limit to the amount of resolution a landscape needs, and blades of grass in a distant meadow or leaves on distant trees can't be interpolated--they either got resolved or they don't. A portrait actually has an envelope of necessary sharpness--it's sharp if it resolves facial hair and pores, but it need not resolve eyebrow mites. So it depends on the subject.

Craig Marshall's picture

This is only relevant for print resolution. Capture resolution is different because fine details like hair would require smaller photosites to capture details. I find I will be ok with 150 PPI will give me minimum details needed, but would much prefer 300 to capture all the fine details. This works good for me even when a lens is rated with half as much perceptual megapixels.
Viewing distance is also up for debate. I want to view the art piece from a distance and close up, and when I do I want to see all those details within details. I print up to 72 inches and feel the megapixel count would have given me everything only once 1000 megapixel cameras are practical. Bring on the megapixels