[MossyRocks]

The other answer: increase pixels. Either by upgrading the sensor (The k-1 gives 36m) or stitching multiple images to increase pixels.

Even without upgrading the sensor or using a longer lens and stitching there is the option to do a super resolution image. I have played around with the technique a number of times and with a scene that is static over the course of a few seconds can produce some very good results. This is the method I learned and use that has worked well however it assumes photoshop. Instead I use Hugin to produce the aligned images and the will use the hugin_stacker command line tool or enfuse command line tool to merge them. Usually I will do several merges of the remapped shots from Hugin usually doing hugin_stacker in the modes of average, median, and sigma and also enfuse with contrast blending as if one were doing a focus stack.

[DeKay]

Do you have any examples or articles showing this? What program or method is used to do the expansions?

I picked this tip up in a book from way back in 2003 called “The Photoshop book for digital photographers.”
I’ve been using it ever since. Although not so much since I have a camera that produces large enough images now.

Anyway, I went looking on the internet for some info about this technique and it looks like it was reprinted into another photoshop book in 2005.
Here is a link to the extract from the book showing how it’s done The Cool Trick for Turning Small Photos into Poster-Sized Prints | Cream of the Crop: cropping and resizing | Peachpit
If you have photoshop or any other editing app you can give it a go and see the results for yourself.

[longbo]

You will get better results if you just increase the photo size by 10% each time repeatedly until you get to the desired size, rather than jumping up to full size in one hit.

I'm intrigued by this technique. Can you explain how it works?

[mmjoshi]

Enlargement software like gigapixel can greatly help with this (Topaz Gigapixel AI Review - Introduction | PentaxForums.com Reviews).

The conventional way is just to up sample the image and apply some sharpening, which would be less detailed of course, but might work depending on the viewing distance for the print.

The viewing distance should be around 3 feet considering that this will be part of an exhibition. There is an option in Gimp for image scaling. Never tried that. I guess that should work.

---------- Post added 07-31-22 at 02:30 AM ----------

Very roughly calculated, you're aiming for 300dpi but you'll have about 157dpi using an image straight from the camera. As others have noted, you'd either have to use an upscaling program or you'd need to shoot a group of images, stitch those in post to create one image.

Be that as it may, another thing to be considered is viewing distance: With other words, if you look at your 30x20" print from up close, it will be obvious that it's a 157dpi image, if printed without any AI work done to it, but if your image is likely to be viewed from some distance, then the resolution becomes less important.

You may want to print a sample image(of a part of the image) and see if the quality matters at the viewing distance you envision. Good luck!

This photo will be part of an exhibition. I guess the viewing distance should be around 3 feet. Will it matter if I do not upscale the image? Another option is use the Gimp Scale image option to upscale. Never tried it though.

---------- Post added 07-31-22 at 02:36 AM ----------

Using the software that you've got available, my preferred option would be the Lanczos upsampling in RawTherapee. Save the photo that you want to print as an uncompressed tiff file, then open the tiff in RawTherapee -- don't worry, it's capable of working with tiff and jpeg as well as raw. Turn off all processing options except resizing, choose Lanczos upsampling, and set the pixel dimensions to 6000x9000 (20"x30" at 300dpi). Then save the upsampled photo as a new tiff (or jpeg if that's what your printer demands).

Lanczos scaling is the best quality you'll get without investing in AI upsampling software like Topaz Gigapixel, and at normal viewing distances the quality will be fine. The sort of people who like to examine prints from three inches away might feel that it's not up to their standards, but honestly who cares what people like that think.

At the risk of going off topic, a thing that will make a much bigger difference to how the print looks at normal viewing distances is soft proofing. You'll find plenty of helpful guides online showing you how to do that in GIMP after you've resampled to the right size.

I did turn on the soft proofing in Gimp and gave the correct color profile of the printer. I was wondering whether the Image|Scale image option of Gimp is the right way to go. I am using open source software for all my photography needs. digiKam for cataloging, RawTherapee for RAW image processing and Gimp for advance image editing. However, have not tried to print an image so far. Considering that the viewing distance will be about 3 feet, would Scale image of Gimp be adequate?

---------- Post added 07-31-22 at 02:39 AM ----------

Even if you can't tell the difference some printers ask for 300dpi images.

That's true. My printer is insisting on a 300 dpi file. I am in India BTW.

[Dartmoor Dave]

I picked this tip up in a book from way back in 2003 called “The Photoshop book for digital photographers.”
I’ve been using it ever since. Although not so much since I have a camera that produces large enough images now.

Anyway, I went looking on the internet for some info about this technique and it looks like it was reprinted into another photoshop book in 2005.
Here is a link to the extract from the book showing how it’s done The Cool Trick for Turning Small Photos into Poster-Sized Prints | Cream of the Crop: cropping and resizing | Peachpit
If you have photoshop or any other editing app you can give it a go and see the results for yourself.

Well I'll be darned. . .

I've just given it a try and somehow, amazingly, almost beyond belief. . . it works! It shouldn't work, and I can't even begin to imagine why it does work, but it does.

I'll have to try some comparisons against Topaz Gigapixel tomorrow, but for now consider my mind blown. Thanks for the link to the how-to.

[mmjoshi]

Given you selection of tools (GIMP + RawTherapee) I would suggest using the GIMP resynthizer plug-in as an option as it does have a resize option. Another GIMP plug-in to consider is the GIMP-ML plug-in. Both of these do some form or synthesis instead the nearest neighbor, or interpolation of linear, cubic, or lanczos scaling techniques. When the new pixels are made up instead of smoothed it shouldn't result in a perceived degradation in image quality like what one sees with interpolation methods.

I do have the resynthesizer plug in installed. So far have used only the heal selection option. Is the resize option the same as Filters|Ejhance|Enlarge & sharpen option? When I tried that with a scale factor of 1.25 seems to be taking ages! Had to kill Gimp.

The GIMP-ML plugin seems to be interesting. Will download it. I guess super-resolution is the option I should be using.

A question, as compared to Gimp's Image|Scale image option, how do these 2 alternative compare? The scale image option also seems to have multiple interpolation methods such as Cubic, linear, NoHalo and LoHalo. Have no clue on which one is better. Have you tried any of these?

[JohnMc]

There is no pat answer for which method/type/amount will work best given that they are image dependent when one is pixel peaking. Select a small sample area maybe 1/4 of the orig copy to a new document and do some tests.

[eric1965]

What is the quality of the files, were they taken at base ISO have these been heavily postprocessed?
Who is your printer? JPG files at 300 ppi is fairly generic. What printer does he use?
You have 3136 x 4864 Pixels available (around 15Mp), a 300dpi print will need roughly a 54Mp sensor.

You want a 20"x30" print roughly (50cm x 75cm)
On what media? Glossy, mat, structured paper or canvas?
If your printer knows his job you can have a fine print out of 15Mp sensor, let him do the final upsampling and sharpening if he knows his job, contact him, visit him.
These are fairly expensive prints, you want something perfect, hanging on your wall for at least a decade...
A couple of years ago I printed on my Epson SC-P5000 17" from a 10Mp camera onto A2 paper (16.5"x 23.4") on Epson enhanced matte paper. I had to lower the output resolution quite a bit but the print came out without problems and even at close viewing distance the result was above my expectations.

[DeKay]

Well I'll be darned. . .

I've just given it a try and somehow, amazingly, almost beyond belief. . . it works! It shouldn't work, and I can't even begin to imagine why it does work, but it does.

I'll have to try some comparisons against Topaz Gigapixel tomorrow, but for now consider my mind blown. Thanks for the link to the how-to.

Thanks, I’ve been using this for years and never really understood how it worked, just knew that it did.
I would be happy to hear how your tests go against the Topaz tools. Please report back with the results.

[MossyRocks]

I do have the resynthesizer plug in installed. So far have used only the heal selection option. Is the resize option the same as Filters|Ejhance|Enlarge & sharpen option? When I tried that with a scale factor of 1.25 seems to be taking ages! Had to kill Gimp.

I used it once and you are correct it is slow. That is the nature of python code. I left it running for over an hour when I tried it. I usually don't use tools like that but then I am usually running a K-3, K-3ii or K-3iii so I often don't need it extra pixels, or if I think I will I will do a super resolution set of shots and fill the buffer then let the camera grind away.

I really like the resynthesizer plug-in and use it frequently for astro shots to I can split out the stars into their own layer allowing better processing of the background and of the stars. Even there it can grid away for a long time.

[PePe]

Many printers routinely ask for 300 ppi just to be on the safe side. However in real life it is not this straightforward, you can often get excellent results also with less pixels. This is very image dependent, and of couse the original image has to be a really good one. I have made a 1m x 1,5m print for gallery use out of a K-10 10Mpx file, and it looked just fine. This was really pushing it beyond its limits, and took guite a lot of tweaking in PS. I did not upsample the image, but did add some very slight texture to it. Upsampling did not really work with this image. It really depends a lot on what you have in the image. You probably need to make a couple of test prints to find out what works best in your case.