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formula for camera megapixels to print size?

Megan Stone , Sep 26, 2009; 01:21 a.m.

i am aware that many people dont abide by such rules and one can push print size at times.
but i have been told that there is a formula to calculate what an appropriate print size is according to a camera's megapixels.
can someone share please...? :)

much thanks

Responses


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Matt Laur , Sep 26, 2009; 01:34 a.m.

Being conservative (erring on the side of a higher quality print), assume 300 pixels per inch.

So, a camera sensor that has, say, 3000 pixels across would produce a print that is 10 inches across in that same dimension.

Rather than thinking in terms of megapixels, think in terms of pixel dimensions. The file is (natively) some number of pixels wide, and some number of pixels tall. Just divide those numbers by the pixels-per-inch of the printing device/service, and you'll knw how many inches you can go with before having to upsize the image (or have the printing process do it for you).

Matthew Newton , Sep 26, 2009; 06:47 a.m.

A 5.3 mega pixel camera is approx. 1889x2835 pixels. This enables about a 6x9 print going with slightly more then 300 pixels per inch for the final print size (overly conservative). For every quadrupling of the pixel count you can double the print size. So to go up to 12x18 with the same very conservative DPI of the final print you'd need a 21.2 megapixel camera. To get a 24x36 you'd nee 82.8 megapixel. All it is is the square of the increase in the dimensions (IE if you went up 50% in size (1.5) then you'd need 2.25 times as many pixels).
Of course this is also for the same viewing area of the print. I don't think I'd be viewing a 24x36 print at the same distance I would a 6x9...so in reality for the 24x36 I'd probably only need 4-6 times as many pixels, not 16 times as many (4 times 4).
On a little looser, but still plenty acceptable DPI count (call it 240) that 5.3 megapixel print could also be printed at about 8x12 and still look just fine unless you were eyeballing it really close.

Howard M , Sep 26, 2009; 08:38 a.m.

there isn't just the x/y = 300 calculation that Matt refers to but also that there no one is going to be putting their nose up against a huge print. the viewer is going to be standing back and the larger the print, the further back in general. Then there is the issue of human vision acuity.

At the end of the day, the 300ppi is a nice target but in most cases, you can be down to 1/2 that number, in the mid-100's and not notice it.

Luis G , Sep 26, 2009; 08:50 a.m.

You may find this helpful:

http://www.bythom.com/printsizes.htm

Scott Ferris , Sep 26, 2009; 09:01 a.m.

Megan,

Like so many of these things it is very subjective. But 300 dpi has now come to be accepted as very very conservative. Different printers and cameras and the multitude of combinations all mean people end up with their own preferences but 240 is more than good enough even for most high quality applications. Also printers have a large effect on it all, they all convert a comparatively small number (pixels) into a much higher number (the huge number of print head jet blasts).

Anyway after that butt covering waffle, I, and many others ( third paragraph, http://wyofoto.com/EOS_IQ_shootout_2008/EOS_shootout_2008.html ), have never had anybody tell the difference between a 240 dpi and a 300dpi print.

So my 4.1 mp camera at 2480x1646 pixels will print out natively, perfectly, at 10"x7" realistically without resizing just sending to the printer to "fill" it does an 11x8.5, letter size.

My 15 mp camera at 4416x3312 pixels will print out natively at 18"x14".

My 21 mp camera at 5624x3736 pixels will print out natively at 24"x 16".

If you crunch these numbers you will see that there is close to a constant, if you take your print in square inches (10"x7"=70 square inches) and divide by the mp (4.2) you get 17 ish. So between 17 and 18 square inches per mp. If you want a known sized print then just divide the prints square inches by 17 and that will give you the mp you need.

An example cos that all seemed long winded :-)

Print size wanted 12"x18"

12"x18" = 216 square inches divide 216 by 17 = 12.7 so a 13mp camera

Hope this makes sense.

Ben Goren , Sep 26, 2009; 10:46 a.m.

Scott,

I’m nearly entirely in agreement with you as far as resolution goes; there’s just one obscure exception I’d like to point out. And that’s that, for art reproduction, the difference between 240 and 300 ppi is discernible, and so is the difference between 300 ppi and 600 ppi. The reason is that, as photographers, we can use post-processing techniques to enhance acutance, but art reproduction requires that you have the actual fine details visible rather than a facsimile thereof.

Whether or not you need 240 or 300 or 600 or whatever to reproduce a particular work depends (as usual) on the work, the demands of the artist, and all the rest of the usual caveats. Fortunately, paintings generally don’t move all that fast, so you have the (tedious) option of stitching a panorama by shifting the work around under the camera.

Or, of course, you could spend stupid amounts of money on a scanning back, etc.

Cheers,

b&

Mike Slivka , Sep 26, 2009; 11:30 a.m.

Though I don't use SmugMug to print, I found this to be a helpful guideline: http://www.smugmug.com/help/print-quality
Keep in mind that those are calculated using fairly high-quality images in the first place - if you're using old, junky overly compressed files, the number of pixels isn't going to matter that much.

Kelly Flanigan , Sep 26, 2009; 12:12 p.m.

In printing for the public my take is that folks who worry about megapixels versus print size create the VERY worst inputs. Their mindset is involved in what does not often matter; thus part of their brainpower is off worrying about nonsense; instead of a using a powerfull image.

Unless viewing distance is known and purpose; there is no answer to the weekly "how large can I enlarge" questions.
It will be asked until end of time.
It is like asking how long a pice of string or piece of rope needs to be.

A VGA image is good enough for a 12 by 48 foot billboard; ie overkill too.

For a super detailed map here we sometimes require an image that is at the print level at 360 to 400 ppi too.

The human eye only resolves a certain arc angle.
In a pro print there is an actual client; an actual purpose for the image.
Maybe it is a hockey dasher board where the viewing distance is 85 feet awayas a typical best case closest distance. Folks may want 300 ppi at 1 foot to be sharp; that is like 30 ppi at 10 feet; 3.5 ppi at 85 feet. Thus a 3 foot high dasher board only requires 36 inches times 3.5 ppi or 126 pixels. If it is 10 feet long then one has 120 inches times 3.5 or 420 pixels. A cell cam or Barbie cam bubble packs image is overkill for a 3x10 foot hoclkey dasher board. BUT a more newbie or amatuer dasher board client will "get the 300 ppi/dpi" requirement etched into their brains; and thus totally focus on this balls to the wall super detailed image; and DEMAND we learn something. One can got to the rinks and spot the newbies images; dull; lifeless with zero soul; BUT THEY LOOK SHARP IF ONE IS 1 FOOT AWAY!. Part of the cost of digital printing for the public is handholding; hearing lectures as new folks jump on the digital bandwagon. Its abit funny since I first printed some hockey dasher boards/vinyls 20 years ago with a pen plotter driven by a 286.

Consider what matters; ie viewing distance.

An image that is say printed at 300 pixels per inch viewed at 1 foot is considered as sharp to many.

Thus this equates to:

An image that is say printed at 100 pixels per inch viewed at 3 feet

An image that is say printed at 30 pixels per inch viewed at 10 feet

An image that is say printed at 10 pixels per inch viewed at 30 feet

An image that is say printed at 3 pixels per inch viewed at 100 feet

An image that is say printed at 1 pixels per inch viewed at 300 feet

Thus a billboard that on an interstate highway that is best case 300 feet away only requires a 1 pixel per inche image. If it is say a giant 12 by 48 foot billboard; that image needs to be only 144 by 576 pixels; one 13th a megapixel. ie barbie cam; ie way less than a cellphones image of 4 years ago. If one gets a flat tire right by the billboard and one has pilots eyes the image still is sharp; since it is 300 feet away.
Amatuers will always argue that one could in theory go over the barbwire fence; dodge some mad bulls; climb up the billboards tower and see the flaws of a 1 pixel per inch image. Becaise amateurs cannot *box in* or define *viewing distance* or a print/poster/etc; overkill is often the preached dogma. One might have a billboard that is 300 feet away best case; that on has to trespass, risk ones life to see its flaws and folks will still preach 300 dpi/ppi; off by a factor of 300 to one.

A photo or poster of billboards requirement for details is like as scale model in a movie; it needs to be good enough for the job; but not overly detailed to flush money down the drain with added costs and labor.
Many folks prescribe overkill; since it is hard to box in the viewing distance; it involves actual thought.
Thus it is their house they are selling; in like matter 300 dpi/ppi printing is like cutting each blade of grass with sissor and a micrometer; a prospective house buyer will inspect the yard at 1 foot for flaws.

Understanding how close one needs to be as to requirements is fundamental; whether one is making coffee; lunch or an image for a client.
Preaching 300 dpi/ppi is like measuring coffee with a Mettler balance; it does not hurt; it just wastes time and money.
Consider what matters; ie viewing distance.

Megan Stone , Sep 26, 2009; 12:31 p.m.

thank you everyone - that was helpful.
i print at 360dpi on my Epson 3800 - as i've been told thats the "magic" number... im now reading 300 and in some cases even less.

thoughts?


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