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hst Jan 31, 2004 8:57 PM

Will more MegaPix make printed images worse?
As cameras increase in their MegaPix will printed quality get worse on small 4x6 prints. Typically when you try and squeeze things down with software you get to a point where you start loosing details and resolution. If you trying to make big prints lots of MP make good sence but I'm wondering about small prints.

slipe Jan 31, 2004 9:30 PM

Doesnít work that way. The printer spooler, whether commercial or inkjet, has to upsample or downsample. It is rare that you can get exactly what they need.

There seems to be very little quality loss downsizing, and it is the same loss whether you downsize 10% or 300%. The same isnít true for upsizing, but you are talking just of downsizing. Whether you start with 8Mp or 4Mp it looks about the same printed 4 X 6.

JimC Feb 1, 2004 7:57 AM

In most cases, the printer is going to upsample the images (because modern printers are using extremely high dpi).

I have read that you can get slightly higher quality by using a PPI count that divides evenly into the printers DPI count, but I have not personally noticed any difference at all.

For example: for an Epson model that prints at 1440 dpi, use an image with either 240PPI or 360PPI for the desired print size (both divide evenly into 1440).

I am aware of one Software Package (QImage Pro) that uses this approach (optimizes output for a printer model). Here is a page about this technique. Note that this is an older description, and QImage Pro now prefers a different interpolation algorithm:

Again, I can't see any difference trying this approach (optimizing output for the dpi of the printer being used). The sampling algorithms on modern printers are very good.

slipe Feb 1, 2004 11:02 AM

Printers make a lot of dots to reproduce a pixel. A few years back I was reading numbers as high as 14 or 16 dots to reproduce a pixel. Recently with the printers making much smaller dots and requiring more for coverage Iíve seen references to 8 X 8 arrays of dots to produce one pixel. As printers have made better pictures by using smaller dots they havenít required especially larger inputs.

There is a dearth of information from the printer manufacturers about exactly how many dots they are actually using to represent a pixel. This guy after some cajoling says he managed to get recommendations from Epson and HP. They both said you need at least 240PPI to get a good print from a top photo printer. Neither mentioned a higher number and I suspect that represents the highest resolution the printer needs to do its best quality. Since he makes reference to 5660DPI I presume this is fairly current:

I have a quality photo printer and have a point of greatly diminishing returns around 180 PPI for input. I absolutely canít see improvement over 240 PPI. I would wager there is not an inkjet photo printer that will show improvement over 300 PPI and the best is probably still in the 240 PPI range.

Photoshop until recently said it was sending a 300 PPI image to the printer no matter how high the resolution you were working with. It was probably a burden on their tech support so you no longer get the message. I wouldnít be surprised to find they are still sending only 300 PPI as that is probably more than any photo printer can use.

If you had a printer with the resolution the same in both vertical and horizontal, and if you could find a white paper with the exact number of dots your printer uses to make a pixel at best photo quality, then you might be able to match the input so the spooler doesnít have to interpolate. I have read posts and articles by people who THINK they are matching input to output, but I have never seen one from anyone who had the necessary information to even have a chance at actually doing that. The manufacturers have been tight-lipped with information on exactly how many dots they are using for a pixel, and of course printers like Epson that have different vertical and horizontal resolution make it absolutely impossible to avoid spooler interpolation as image editors output symmetrical images.

If you are using a 3Mp image with minimum cropping to produce a 4 X 6 print you are sending any printer more information than it can use. If you start with an 11Mp image you are also sending more information than the printer can use. It has to resample in both cases and the print is going to be the same (all other things being equal of course).

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