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mads78 Mar 3, 2006 1:15 PM

Please help!

I have a 5 MP camera, my resolution size is set on 2560X1920, however, I am told I can not make prints above 12X18, even though in the chart below from the lab, it says I AM able to print above 12X18. I am preparing to get a new camera. I do not know what mega pixel size to get to make 16X20 or if I'm lucky 20X30 on occasion. What mega pixel size do I need for this, and why cant I print accordingly to THEIR chart?

Print Size Minimum image resolution required
4 x 6 640x426 pixels
5 x 7 1050x750 pixels
8 x 10 1280x1024 pixels
11 x 14 Enlargements 600x1143 pixels
12 x 18 Enlargements 1600x1066 pixels
16 x 20 Posters 1920x1536 pixels (not available)
20 x 30 Posters 2272x1500 pixels (not available)

JimC Mar 3, 2006 1:38 PM

You'll see all sorts of contradictions on what you need to print at a given size. You'll find a chart similar to the one you posted here:

But, keep in mind that the larger the print, the further away you'll typically be viewing it from.

The idea is to prevent pixelation (where you start to see the individual pixels in an image, because you don't have enough pixel density as your spread them out over a larger print area), while still maintaining enough "real detail" from the desired viewing distances.

But, many modern print services will automatically "rez up" your images to a larger size using interpolation.

This works by adding more pixels, based on the values of adjacent pixels, using fairly sophisticated algorithms.

This won't increase the detail captured by the sensor, but it does help to prevent pixelation to allow for larger print sizes.

You can also do this yourself. I typically use the Lanczos algorithm under the Image>Resize/Resample menu in the free Irfanview for this purpose, and try to aim for at least 240 pixels/inch of detail for most sizes.

For example, if printing an 8x10" print at 240 pixels per inch, you'd want an image size of 2400 x 1920 after cropping for the correct ratio of width x height.

8" x 240 pixels per inch = 2400

10" x 240 pixels per inch = 1920

Check out Mike Chaney's QImage Pro, too.Mike is using some pretty sophisticated techniques to interpolate images, optmizing the interpolation to match a specific printer when you print. See his "quality challenge" menu choice for more information on this part.

But, in practice, I've seen good looking 8x10" prints from 2 Megapixel models, which works out to around 150 pixels per inch, without even bothering to interpolate them to a larger size first.

At larger print sizes, you can get by with even less (because viewing distance will be greater).

The type of subject matter also comes into play. If you've got lots of fine detail (foilage, etc), then it may not enlarge as well as something like a portrait.

See THIS POST from Lin Evans explaining why.

I know someone that had a 20x30" print made from an image size of less than 3 Megapixels and was pleased with the results. But, that particular image did not contain a lot of fine detail (it was a mountain range with the sun going down behind it, but it did have a bird flying in it, too). This person cropped the image from a 5 Megapixel camera and had the crop printed at 20x30". :-)

In this case, the printer did the necessary interpolation, advisingnot to prepare the image by interpolating it first (the printer claimed that they could do a better job with their software).

Here is the printer used:

Here is what they have to say about it:



Of course, this printermay be the "exception to the rule". With many printers, you'll probably want tointerpolate the images first using a number of available tools (and some of them are free, like the resize/resample feature in Irfanview

bernabeu Mar 3, 2006 1:45 PM

a 'photographic' quality 12X18 @ 300 ppi would require 19meg file

most 'printers' have upwards interpolation software

this is an extremely complex question

(your chart contains many 'typo' errors)

TRY : for printing from digital files

in general you need a MINIMUM of 240 ppi to make an acceptable print, but, this depends on several factors incl. viewing distance and degree of detail in image

if you are looking to consistently make 16x20 prints you will need a 12-14meg camera

BUT, i routinely get excellent 11x14 and sometimes an acceptable 16x20 from my KM 7D (7.1 meg)


eric s Mar 3, 2006 1:47 PM

The size of a print you can make from a specific image is a difficult one, and I'll try to explain. There are many issues involved.

- The biggest question is how many pixels do you want to produce 1 inch of print. Common answers are between 200 and 300 pixels per inch. This is, of course, picture dependent (I get into this more below.) So based on that you should be able to get reasonable prints between 12.8 x 9.6" and 8.5 x 6.4". Too few pixels per inch and the printer will interpolate the data up to a higher numer of pixels per inch... which reduces quality.

- Subject matter. Some subjects don't need a lot of resolution to portray them. For example, most humans know what a human face looks like. Therefor, you don't want the absolute most detail in your print. In fact, most people don't want every pore in their skin showing up in the print. So portrate photographers often blur or "smooth" the skin slightly to reduce sharpness. Other things, like landscapes or wildlife images, often have to be very sharp. Printing them with low DPI (or enlarging them before printing) often produces unsatisfactory results. But that connects to my next point

- Sharpness. If you don't have enough resolution and want to enlarge the image so you can print it large, you must have a VERY sharp image. If you don't it will enlarge the bad parts and they will look worse.

- Viewing distance. Some large prints look very good at 8-10 feet away. But if you look at them closer up you'll say "look at how bad that looks! It has sharpening halos, bad detail... its a bad picture." Prints are made to be viewed at specific distance. The further away you are, the lower the dpi and the less sharp the print can be.

Now to those numbers. They look very low to me. Extremely low. 128 dpi for an 8x10 is much smaller than I would find acceptable. I don't know why they claim they'll even print those because I would expect them to not look good.


JimC Mar 3, 2006 2:02 PM

bernabeu wrote:

I'm no expert. Lots of folks here probably have more experience than I do. But, I do try to learn from others and pass on that info, as all of the forum members here do to help out each other, like I see you do often.

Also, I suspect that you've forgotten more about photography than I'll ever learn. :-)

For this question, there is no hard and fast rule. There are too many dependencies (for example: viewing distance, subject matter and user expectations for image quality).

As I mentioned before, Lin Evans has a very good post on why some subjects enlarge better than others HERE

One way to tell if you may be happy with the results is to crop a portion of an image, so that it's at the same pixel density at a smaller print size, as you'd have printing the entire image at a larger size.

Then, see how it looks from the desired viewing distance you'll have with a larger print.

I've got 8x10" portraits from a 2 Megapixel Nikon 950 that look great, even though there was no preparation at all (which works out to around 150 pixels per inch of detail).

But, for landscapes or images with lots of fine detail, a higher pixel density is desired.

Also, image quality is subjective, so there's no one right answer.

JimC Mar 3, 2006 2:26 PM

eric s wrote:

- Sharpness. If you don't have enough resolution and want to enlarge the image so you can print it large, you must have a VERY sharp image. If you don't it will enlarge the bad parts and they will look worse.
One comment on this...

I think Eric means "real" versus perceived sharpness, too. For example, if you have a camera model that sharpens images via image processing, it's doing things like increasing contrast at edges to give the illusion of more sharpness at smaller viewing and print sizes (and most models will do this).

This can lead to things like halos around edges where the image sharpening was applied, and can stick out like a sore thumb at larger print sizes if you want to enlarge an image.

So, it's best to leave things like sharpening, contrast and saturation dialed back a little from defaults in a camera's menus, depending on the camera model, so that you have more control over this process using software later, as not to have an "overprocessed" look to your images if larger sizes are desired.

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