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How many megapixels do you need?
In buying a digital camera, the first question one will run into is "how many megapixels" do I need? As this is the most visible advertised feature in digital cameras these days, it's important to understand what you truly need.
How large do you want to print your photos?
When it comes down to it, deciding on the resolution for your digital camera is as simple as determining what you would like to do with the printing. If all you want to do is capture basic snapshots that are intended to print as a 4" x 6" (4x6) photo, then it is possible to get away with even a 1.0 - 2.2 Megapixel (MP) camera, depending on the final output quality desired.
However, most of us like to be prepared for the fact that someday we may capture a photo that deserves "blowing up" / enlarging or even cropping. This is where high-resolution digital cameras come to shine.
| Print Size | Minimum Resolution (200 DPI) |
Best Resolution (300 DPI) |
|---|---|---|
| 4 x 6 inch | 1.0 MP | 2.2 MP + |
| 5 x 7 inch | 1.4 MP | 3.2 MP + |
| 8 x 10 inch | 3.2 MP | 7.2 MP + |
| 11 x 14 inch | 6.2 MP | 13.9 MP + |
| 13 x 17 inch | 8.8 MP | 20.0 MP + |
| 20 x 30 inch | 24.0 MP (*) | 54.0 MP + |
Important Notes:
- The above table indicates the achievable print sizes straight from the camera. Given the techniques shown on other pages (using Photoshop or IrfanView), one can achieve print sizes greater than what is shown above.
- The data in the above table is easy to calculate. One takes the print size and multiplies the
two dimensions (in inches) to get the area in square inches. Multiplying the area by 0.04
for minimum resolution and 0.09 for best resolution will give the megapixel
counts stated above.
Where do those 0.04 and 0.09 come from? It is generally accepted that the minimum resolution in a print that is suitable for close viewing is about 200 DPI (dots per inch). So, for a one-inch square, we would need 200 dots x 200 dots = 40,000 dots = 0.04 megapixels. Similarly, the resolution at which the human eye would have a hard time "seeing the dots" is at or about 300 DPI, which corresponds to 300 x 300 = 90,000 dots or 0.09 megapixels.
- NOTE *: With very large prints (eg. 20x30 inch prints), one can get away with much lower effective resolution (than say 200 DPI) because the viewer will often see the print from a distance. Therefore, the rule of thumb suggesting a 200 DPI minimum is not as realistic. It is possible to get reasonable prints at this size from 10 megapixel cameras, for example. One can also take advantage of some excellent image editing software packages that are able to interpolate (up-res or up-scale) "natural" images to much greater resolutions very effectively (e.g. Genuine Fractals).
Why do some Canon cameras set the resolution to 180 DPI?
For the longest time, I tried to figure out why the Canon 10d generated photos that were tagged with a resolution of 180 DPI. Why not 300? After giving it some more thought, it likely that they wanted the 6.0 megapixel camera to automatically imply a print size of 11 x 14 inches for advertising purposes (but notice that this requires a lower resolution than the 300 DPI that is standard for printing).
Reader's Comments:
Please leave your comments or suggestions below!In Photoshop, open your original photo, set the image>image size "resolution" to 300 and make sure you have "resample" deselected.Now when you save it and have a look at file>file info, it will show 300 DPI. Nothing is actually changing (unless you saved it as a JPEG which will compress the image each time you save it, losing quality), but it should keep the magazine company happy because they don't know what they're talking about. What they want is a high resolution (X by Y = pixel dimensions eg 2256*1496 px) image of high quality (point and shoot cameras like to have higher MP specs than DSLRs as a selling point but they produce lower quality images).
THANKS, Laura
Thanks again.
Thank you!
I have a question about photoshop and cropping/printing. I am trying to fiqure out cropping and resoloution. I have a canon slr xti camera and fiqured out how to properly crop so that when I print, my photos come back the way I thought that they would. I am confused about resolution. In photoshop when I change my size to crop I was told to make sure that the resolution is no smaller than 300. Can I just leave the resolution blank?? If not, will a resolution of 300 be okay for all sizes of photos?? Do I just change the resolution on the display page in photoshop or do I have to go into image at the top and then go into the resize section?? I have alot of quesions...please help!!
I have a SONY Mavica 3.5" 10X digital camera. Can you please tell me what my settings should be so the picture can be attached to an email and arrive the correct size and not be blurry? Thank you so much!! I'm desperate.
I read your article and it is very informative. However, I have a question to ask regarding the dpi. I brought Panasonic Lumix FX50 and the maximun dpi is 72 dpi. Compare with another camera like the canon ixus series, the maximun dpi is 180 dpi. when i use my personal computer to compare both photos taken with the same resolution from pansonic and canon, I find that the photo taken from the canon is much more sharper. And when I zoom in the photos, canon still produce sharper image than panasonic. Does that mean higher dpi is better ? If yes, then I guess camera that offer superfine image quality is better than camera that offer only fine image quality. Thank you for reply
Thanks again!
J
This is one of the ways in which the digital format differs from film -- in film, people have grown accustomed to some degree of film grain. Digital noise and JPEG artefacts are not nearly as accetpable. Low compression artefacts are disturbing because they are easily identified by their repetitive patterns and hard-edged nature (not organic).
Thanks for your answer ahead of time! I need all the help I can get. I've left my personal email with you.
Thx
JB
You don't need to, and in fact cannot, change the DPI resolution as recorded by your camera. To be completely honest, it's a meaningless number. Your camera records a fixed number of pixels (3.2 million) in large mode. It is your choice (in the photo editing software) what physical dimensions you want for the final print. Obviously, the larger the print, the lower the DPI that you are effectively using.
When enlarging, it is best to choose a minimum DPI that is acceptable for your final output (most people aim for 300 DPI, but 240 or even 200 may be acceptable). Since your camera records 2048 x 1536 pixels, this means ~ 5" x 7" @ 300DPI, 6.5" x 8.5" @ 240DPI or 8" x 10" @ 200DPI. You can define this resolution (or conversely, output dimensions) in Photoshop (via the Image Size dialog using the Resample option). This will override any DPI setting that your camera may have written into the original file. The whole concept of DPI settings from the camera has certainly confused a lot of people!
Just as an aside, the superfine mode simply describes the level of compression that you are using (not resolution), which is another contributor to overall image quality.
i wont to supply a client a CD of imager that they can print out ONLY them self’s at 4x6. so they can not enlarge them and have to come back to me for the enlarges
thanks
Thank you!
What I believe they may be refering to is the resolution selected when scanning photos that were taken with film. Scanning a typical 4x6" print (processed from film) at 300dpi will deliver a pixel resolution of about 1800 x 1200 pixels, or just under 2.2 megapixels.
For most judging purposes, 2 megapixels should be reasonable enough to make an assessment of the merit and quality of a photo. Therefore, I feel that submissions direct from your Canon Ixus 55 should be more than fine for their competition rules. Good luck!
I've been trying to get people to think in terms of the number of *independent* pixels in the image. More pixels doesn't necessarily mean "better picture" if, for example, the lens resolution isn't up to the task.
This is especially true with point-and-shoot cameras, which have gone from 2 Mp (or less) up to 5-6 Mp in recent years without any increase in sensor size -- which they have to keep small to keep the lens and the rest of the camera relatively small, the battery life up, etc.
At about 5 Mp the individual pixels are about 2.4 microns in each dimension, which requires a lens to be able to resolve over 400 lines (or 200 line pairs) per millimeter for each pixel to be seeing something different in the original scene being shot. Manufacturers generally don't quote the figures for what their lenses can do, but I doubt anyone could afford a 400 lines/mm lens.
The moral is that you're not getting the number of pixels you think you are unless the designers have made a very good match between the optics and the sensor. It's always less, when looked at from an information flow perspective through the entire system. So generally the only way to get more real Mp's is to get a bigger sensor.
* MB
P.S. - "Why do some Canon camears set the resolution to 180 DPI?" I think you're basically right. In practice, even 125 DPI can be OK for casual use. People have become so accustomed to the low-res nature of TV that just about anything goes as far as the non-photographer is concerned so long as the individual pixels don't leap out at you.Great comment! Yes, absolutely. No discussion of megapixels should be without mention of its relationship with the sensor size and the lens. Many of us were shocked when the first 8 megapixel digital Point & Shoot camera came out, the Sony F828. It didn't take long before it's terrible noise characteristics became news -- a function of the pixel element size (and poor signal-to-noise). You mention the other side to this, the lens' resolving power -- this is almost never mentioned because there is no obvious metric that can be used as a selling point for the average consumer. Most people will assume that the lens doesn't have any important characteristics other than its zoom range. Very few will ever go and look at the MTF charts on Photodo, but this is the missing element. Thanks for your insight.
do you know about a similar web site in french?
thank you.
Hi -- In all honesty I am not sure, but if you like, you can get Google to translate any parts of this website (or any others) if you like. Simply go to:
Google Translate
Then enter my web page address into the box titled: Translate a web page, select English to French and press Translate.