INTERPOLATION. Help Please.
 

Dear all,

I need help here.

I use PS7 and on occasion would like to enlarge an image, a crop, without loosing quality.
Could a kind subscriber hold my hand and guide me though the subject of interpolation.
Please be patient this is a new adventure about which I know nothing.

Regards.
G B-S.

Hi G B-S

I don't use Photoshop 7 anymore, I now use CS2, however, I believe the method of changing the size of an image is still the same in all versions.

First open your preferred image, click on Image on the Tools Menu, select Image Size from the drop down menu.

Now if you want the proportions of the photo to remain the same but just want to increase the size, make sure you have Constrain Proportions and Resample Image boxes ticked.

In Document Size enter the new size you would like the photo to be, (only change one dimension).

If you are resizing for the web leave the resolution at 72, if you are resizing for printing, then resize to a minimum of 200 pixels/inch, preferably 300 pixels/inch.

If you don't want to keep the original proportions then you will need to untick Restrain Proportions and enter both sets of dimensions in Document Size.

When you have done this your file size will have increased also, make sure you have enough space on your disk, then save your new sized photo with a new name, that way you won't lose your original file.

Hope this helps.

nirofo.

It just needs adding that the amount of interpolation you need will affect the final quality of the image.

There are software programs available such as 'Genuine Fractals' that claim to make a better job of it than the basic interpolation in PS but all must, by definition, lose a bit of quality somewhere as they're all sort of 'stretching the original image and adding bits in' to make it bigger.

I agree there will be some loss as you are adding pixels that did not exist in the origional and these are only an estimate by the program of what it thinks should be there.

I have used Genuine Fractals and with my very poor knowledge of pp at the time managed results that amazed me. This is a link to an earlier posting in the RAW or JPEG thread. http://www.worldphotographyforum.com...p?t=662&page=8 Post 73

I have tried increasing size in standard imaging program in 5% increments with good results but that takes ages as lots of steps involved. All a long time ago now, so I would have to revisit the subject afresh to be any real help.

Don

Nirofo and Adey,

Thank you for the reply,

I have had a go following your instructions. The results were interesting, as you say a large file and a degradation in the quality.

I used the bicubic option as it was the default. Presumably this is best.

Genuine Fractals is new to me. Have either of you used this software?

Finally off topic – Nirofo is your avatar a Sea Eagle?

Cheers Gordon.

Yes - the 'cubic' bit means it's taking information from all the adjacent pixels to form the new pixels.

I haven't used GF (although I've got a free sample CD somewhere that allows 30 free goes before it cuts out - and no doubt then keeps reminding you to buy the damned thing;) ).

'Amateur Photographer' did a test some time back and GF came out best with the 'bit at a time' method described above by Don coming in second and the basic bi-cubic third, though with the quality of magazine reproduction I don't think there was that much difference visible to the reader - you have to trust the comments of the reviewer.

Whichever method you use, the best results will come when you use it to make a larger print than normal - big prints are usually viewed from a greater distance where the loss of resolution won't be too visible! When you try to enlarge a cropped area you're starting with the resolution 'stretched' to begin with and you'll probably be looking at it from the same distance as your full-frame images.

Hi Gordon

Interesting that you think your results so far are interesting, large file and a degredation of image quality. What increase in size are you talking about, I quite regularly scan my slides at approx 28mb and interpolate to 40mb plus using either Photoshop CS2, or Genuine Fractals 4, personally I can't see the difference in quality only in file size? These files are sent to various publishers and Photo agencies without comment, other than most now prefer to receive submissions from digital cameras rather than scanned images!

On another tack, I was asked recently to supply a photograph urgently of a male Blackcap feeding young in the nest for inclusion in a glossy brochure, the only image I had available immediately was a scan I had done for the web some time ago, file size was 193kb jpg. I opened this image in Photoshop, changed the size to 10x15cm x300 pixels/inch, now = 5.99mb. This image was used commercially and was more than adequate for reproduction purposes. This of course far exceeds anything needed for the web.

Attached is the image in question.

My avatar is a flight shot of a Griffon Vulture, taken in Andalucia, Spain using a Tokina 75-300 zoom at 300 f5.6 on a Nikon F90X, film was Fuji Sensia 100 pushed to 250ASA and processed at 200ASA.

nirofo.

Dear all -
Thank you for all the trouble you are going to, and the time.
After reading your letters again I have had more success and I am now going to attempt to attach a photograph - the original picture and a crop of the eagle interpolated to 800 pixels.
Comments and further advice please!
Cheers - Gordon.

Gordon

What may of helped in this case before you enlarged was to us a noise reduction program such as Picture Cooler or Neatimage.

Below is an example from your first image and enlarge 100%. So on your original file the result should be quiet good.

Gordon,

Having seen what Rob has done it may be worth posting a similar crop from the origional image with as little compression as possible.

Lots of clever people here who can then play with the image and give best suggestions.

Don