Understanding basic Interpolation

[sanfairyanne]

I realise that if I have a 6 mp camera, my pixel dimensions would be 3000/2000, therefore I can divide those figures by 240 or 300 ppi to give me my finished size print. Printing at 300ppi I'd have an optimal sized print of 10x6.7''. However, using Photoshop I can use interpolation to increase the pixels to give me a larger print.

My question is: how far can this be pushed before quality is impaired. I assume there must be a point where quality is noticeably impaired and I just wonder what kind of percentage this might be.

Many thanks.

[Paul2660]

I think you will find the answer is not too much.  6MP is going to limit you on final output.  Epson needs 360 dpi for ideal output and Canon 300 dpi.  You can try any of the specialized software tools and uprez your shot, like:

Photozoom Pro
Genuine Fractals, now part of On-One photo suite
LR's uprez
CC 2014 uprez that preserves details.

If you don't use LR for the print (which does a nice job of uprezing to 360 dpi in the print module), then any of the the tools I listed will work better than allowing the print driver to make the uprez.  If you are printing to Epson, and send a 300 dpi image, the driver will still uprez the image to 360 and the driver is using nearest neighbor, possibly the worst algorithm there is.   Another option is Qimage (PC only) as again, you have an very good tool that can provide the correct uprez when making the final print.

A lot also depends on the camera and lens that were used and how sharp the image is.  Is it a macro or a image with a lot of small details.  I believe you will find a macro shot will give you a larger overall uprez opportunity. 

The largest I have been able to get a 6MP output from a Nikon D2x 6MP camera is about a 20 x 30 and that was pushing the file quite a bit.  The D2x had a strange layout in pixels so you were penalized from the start. 

I also feel that the printed results will improve with the best possible 6MP output and raw conversion, not working with an in camera jpg. 

Paul

[Alan Goldhammer]

It also depends on the viewing distance.  If you are preparing something for wall viewing it can be larger than if you are holding a smaller print in your hand.  Obviously the image needs to be in focus.

[ErikKaffehr]

Hi,

One rule of thumb that you need 180 PPI for a very good print. With 6 MP that would be around 11"x16". You can stretch this quite a bit, depending on subject and viewing distance.

Personally, I print A2 (about 16"x23"). I would say 12 MP is quite OK for that size, good enough for a decent "wov" factor. Side by side 24 MP may have a benefit at that size. Ctein, who has been named as the best printer in the world by "Eastman Kodak", seems to say that 16 MP is good enough for A2 and that is in line with my observations.

Best regards
Erik

I realise that if I have a 6 mp camera, my pixel dimensions would be 3000/2000, therefore I can divide those figures by 240 or 300 ppi to give me my finished size print. Printing at 300ppi I'd have an optimal sized print of 10x6.7''. However, using Photoshop I can use interpolation to increase the pixels to give me a larger print.

My question is: how far can this be pushed before quality is impaired. I assume there must be a point where quality is noticeably impaired and I just wonder what kind of percentage this might be.

Many thanks.

[sanfairyanne]

Thanks Paul, Alan and Erik,

I guess I forgot to mention viewing distance, macro photography never entered my mind so thanks for that Paul, my question was more a hypothetical one. I was copying the 6mp data from a magazine article. I'm fortunate enough to shoot with a larger megapixel camera. It did interest me to realise that optimal sizes for 300ppi are as follows:

6mp 10x6.7''
36.3mp 25.4x16.3''
80mp 34.4x25.8''

Clearly the bigger your print the further you're going to stand back from it anyway. It's interesting food for thought.

Thanks.

[highway0691]

I have used interpolation to push photos massively and I've found what's as important as anything else is how good the photo is in the first place. A well lit, exposed, focused photo taken with a good lens will do well. I have stitched panoramas from 3 photos I took with a 6mp camera to over 2 metres in length and they look great.

[sanfairyanne]

Highwayman I would think in the case of a pano you're creating more actual real pixels rather than having software create adjacent pixels so I don't think the pano' argument is quite the same.

As always I write these comments in a questioning manner rather than stating a fact.

[enduser]

Using Qimage, my old 6MP 300D Canon could easily enlarge up to and beyond 24 inches x 36 inches.

[Tony Jay]

Highwayman I would think in the case of a pano you're creating more actual real pixels rather than having software create adjacent pixels so I don't think the pano' argument is quite the same.

As always I write these comments in a questioning manner rather than stating a fact.

To create such a large panoramic print from three 6 MP images as a base would require substantial interpolation I would think.

In answer to your initial question there is no one correct answer to how much interpolation is too much interpolation.
It is actually a horse-for-courses scenario that depends on an individual image.

My suggestion to you is to print a particular image at several different sizes using appropriate interpolation for the printer in question and then view the results.
If you do a similar test on several different images you will likely find different limits for each image as to what appears acceptable to you.
Ultimately the issue devolves to a matter of aesthetics rather than hard science.

Tony Jay

[highway0691]

Highwayman I would think in the case of a pano you're creating more actual real pixels rather than having software create adjacent pixels so I don't think the pano' argument is quite the same.

As always I write these comments in a questioning manner rather than stating a fact.

Not at all. The photos once stitched come out at 240dpi at around 60cm. To print to 2 metres - there's a bit of interpolation required. That in fact is trebling the native resolution, and a bit.

It has worked when the photo has been taken in good light and with a good lens. I actually sell a lot of a particular panorama (3 photos overlapping 25% and then cropped) taken with a compact 3mp pocket camera 11 years ago. The photo is such good quality that it enlarges perfectly to 150 x 40 cm. Interpolation is one of the greatest things if used correctly.

[sanfairyanne]

Highwayman I'm totally missing something then. If I use the example of a pano I took a while back, I shot with the 5d2 a 5 shot pano, the resulting file once processed into a pano far exceeded the pixel dimensions of a regular 5d2 single image. I assumed that was because my pano' software had simply joined the image files at the overlap to create the pano'. I also assumed this is a true increase in image data rather than taking a single image and upsizing it via interpolation.

[Paul2660]

Highwayman I'm totally missing something then. If I use the example of a pano I took a while back, I shot with the 5d2 a 5 shot pano, the resulting file once processed into a pano far exceeded the pixel dimensions of a regular 5d2 single image. I assumed that was because my pano' software had simply joined the image files at the overlap to create the pano'. I also assumed this is a true increase in image data rather than taking a single image and upsizing it via interpolation.

Yes it is a huge increase in available data the dpi may be the same 240 or 300 but the resolution is much higher. 

It's a simple test, take a single landscape shot with your camera then take the same shot in three verticals. Merge the verticals into one shot.  With the same dpi on both images set to 300 the output size of the image stitched from the vertical images should be considerably higher. Even with the overlap.  You will have more resolution to work with and if needed interpolate to a larger image.

Until I moved to a digital back, I stitched almost everything on my a Canons using a zork adapter which allowed for 18mm of shift horizontally.  3 vertical to create 1 horizontal.

Paul

[sanfairyanne]

Paul your explanation confirms my understanding, thanks very much.

[highway0691]

Highwayman I'm totally missing something then. If I use the example of a pano I took a while back, I shot with the 5d2 a 5 shot pano, the resulting file once processed into a pano far exceeded the pixel dimensions of a regular 5d2 single image. I assumed that was because my pano' software had simply joined the image files at the overlap to create the pano'. I also assumed this is a true increase in image data rather than taking a single image and upsizing it via interpolation.

Of course - it's a much larger file size because it's joining 3 images. At 75% x 3 = 225%  or 2.25 x resolution minus any data lost from cropping of a single image. But interpolation can stretch that even further - and the success of that depends on image quality out of the camera.

[sanfairyanne]

Thanks Highwayman initially I thought you were telling me that a pano only interpolated, I do understand it all now. Thanks for your time.