Depends on your line per inche colour screen preference when you are doing colour separation. I doubt the quality will be that bad. I do magazine and brochure design work and have done newspaper colour ads. 4 colour printing for magazines and newspaper are very difference. I have often use 175 lines ( we don't really talk megapixel or just plain pixel) when we do production...well at least not me and the suppliers I work with. Magainze or any 4 colour printing job in general uses screen work to overlay the 4 CMYK colours to form the picture. So how sharp you get also depends on the screen work. Not all magazine will print at very high screen work as it is costly for productions. Take a magnifying glass and look at a typical magzine and you will see what I mean by screen and then you look at your pciture on your PC screen and then at what you print from your computer printer. You will notice you don't look at all of them in the same way and more importantly NOT AT THE SAME DISTANCE. I have shot pictures for my company ads and then made 20feet by 20 feet banner to put outdoor..and it is at only 30dpi ! Because you are looking at it from a distance and not close up. So 300 dpi is redundant is impossible. Imgaine what kind of freak camera you need to use just to get you true 300dpi to capture all the details.
Same predicament with magazine stuff and other printed medias. You can get away with some enlargment to your picture used in an artwork. I am not sure why someone here will equal everything with "300dpi" for all size of artwork and that to use a large picture you need a larger camera megapixel since it has to be 300dpi? That might be true in some situation but there is a lot of elbow room in between.
Want to know more about this..don;t ask the photographer. Talk to the colour separation company, they are the best people to tell you what can really be done and what can't. You would be surprise how many graphic designers don't even know those fundemental and "over scale stuff" which translate to over size data file then needed, cost, time and processing resource to get it done.
Try Genuine Fractals to increase image size before sending for printing. I have been using it for a while now and I am very impressed with the quality of the resultant print. Email me if you want some help with this.
You can use a film camera with a large frame size, of course. But all too often, film images aren't really that sharp either. The film grain masks the lack of sharpness to a certain extent. Looking at older and not-so-old magazines and photo books, I am amazed how many grainy and blurred pictures there are.
With respect to the original question and depending on the particular picture, I would be more concerned about JPEG compression artifacts than resolution.
I do magazine and brochure design work and have done newspaper colour ads. 4 colour printing for magazines and newspaper are very difference. I have often use 175 lines ( we don't really talk megapixel or just plain pixel) when we do production...well at least not me and the suppliers I work with
Depends on the lens used. Some lenses can only resolve to 3 pixels on a D70. Which means that the optics resolution is the deciding factor. In this case, all you need to do is to resample the image to 18"x300dpi x 24"x300dpi pixels, perform some unsharp mask and it should be ok.
If the lens can resolve to better than 2 pixels, then the sensor resolution is the limiting factor, using a sensor with a higher pixel count may resolve more detail. However, it should still be ok to blow the print to 18"x24", only that it may not be as sharp as it could have been.
That will have to also depend on the type of offset printing machine use and what kind of quality of print you are looking at. Some magzine company might want to keep price cheaper and sort after printing firm using more older or lower quality printing machine to print their editorial or magazine for them. There are a few print screen uses to mesh together to form the colours and images you see on print medias like magazine, newspaper..etc.
There are some higher screen magzine out there but many are oversea publication but more are starting to make it to SIngapore printing company who are willing to pay the higher price to purchase those machine. A full colour printing machine that let you do the standard four colour printing PLUS maybe another 1-2 spot colour printing is not cheap. They can be anything from half a million to a few million for those heavy duty ones. A printing machine can last a few decade and at that price not many will upgrade for no reason. Also how many people will be able to tell the resolution or care that much about it when reading a magazine? Of course there are those with more fine screen used and usually those magazine are about art, design or something that would look great when seen in higher print screen format. But none of them can beat even your 6 or 12 megapixel camera in my view when you are talking about printed media that is about the size of A4 to even the size that was mentioned here AND take a look at their colour graduation with a magifying glass (the finished printed magazine) and you see the screen work is alot more coarse unlike your phtographs you see on your PC or those you print out with your computer printer. I have even submited artwork for magazine publication in jpeg format!!! heheh.. And it still comes out well. So, it is very useful to ask the magazine for production specification because doing so will help you sometime save time and not try to make waste time doing some high resolution work when at the end of it, the quality is degraded in the printing process.
Can, and software to 'increase' resolution like Genuine Fractals will be good. If the object has smooth straight lines and not much fine details, certainly not a problem to print to 24" wide.
I have a poster of orchids hanging on my wall at 24" by 36" taken with my S2pro.
Printers need 175dpi images to start with, but the final print will be about 300dpi worth. This is how CMYK offset printing works. Plenty of info on the net, do a yahoo.
Film is not equivalent to 20MP. It is just that film grain and noise, when blown up large is soft and round, more pleasant and most people can accept it in prints. On Jpegs or even tiffs, the pixellation is terrible. 135 format film (used for 35mm cameras) is about 6MP when it comes to resolving details like a photo shot of newspaper classified adverts.
Magazines with needs for very high quality images will only accept medium format (120 format) film. 35mm just not good enough, although very protable.