Photoshop

Fireman

In P/S I use the transform tool sometimes to make an object bigger and it also gets rid of unwanted part of the photograph, But the question I would like answered, do I lose some quality of the photo even though it is still the same frame size

Regards Fireman

DaireQuinlan

Sort of. You're effectively doing the same thing as cropping to whatever composition you want and then resizing to the original size of the image, either which way PS has to take a smaller area of the image and expand it to fill the available space. You're not actually LOSING information as such (except obviously the bits you cropped out) but the resulting picture won't be as sharp or as detailed as if you had shot it that way in the first place.

Damned Thing

You do lose quality.

What you're doing is making less image data fill the same space.
What Photoshop does to facilitate this is 'guess' what extra data is needed to expand the image.

Depending on the original file size and what the final use of the image is going to be will determine how 'noticeable' the loss of quality is.

For example:
say you start off with a 10mb image file and select a 90% portion of the image to expand to fill 100% of the space. If the end result is a 5x7 inch print I doubt you'll notice any loss of quality. On the other hand if you take a 20% portion and expand it to fill 100% of the space Photoshop has to do significantly more guesswork and quality will be severely compromised.

Look at it this way, if you have a 2 pixel image - one red, one yellow - and want to increase that to a three pixel image Photoshop will probably guess the middle pixel to be something like orange; something neither red or yellow but a best guess. Images have millions of pixels, the more guesswork that has to be done the more the original data is compromised.

Fireman

Thanks for replying to my thread, I was thinking that you would loose quality, but I heard somewhere that P/S would add more pixels (obviously not )


Regards Fireman

5uspect

Fireman wrote:

Thanks for replying to my thread, I was thinking that you would loose quality, but I heard somewhere that P/S would add more pixels (obviously not )


Regards Fireman

Photoshop interpolates. If you go to image>image size and increase or decrease the resolution of your image there you will see an option at the bottom that says interpolation method. The standard method is bicubic.

This tends to smooth sharp gradients of a few pixels. Usually you should apply an unsharp mask after an interpolation. The bicubic interpolation somewhat is better than simply averaging the pixels as it does add some information based on neighbouring pixels in a non linear fashion.

EDIT: Here's an example


The top left dot is a 6 x 6 pixel circle. Not much information there. The middle dot is the free transformed dot. Photoshop automatically interpolates it so you get those blurred edges. The biggest dot I enlarged using the image size dialogue and then sharpened the right side of it heavily with unsharp mask. Information is added but its just a curvefit between points.

5uspect

Damned Thing wrote:

Look at it this way, if you have a 2 pixel image - one red, one yellow - and want to increase that to a three pixel image Photoshop will probably guess the middle pixel to be something like orange; something neither red or yellow but a best guess. Images have millions of pixels, the more guesswork that has to be done the more the original data is compromised.

Yes but with a large image you have more data points to interpolate between so the guess should be reduced. The only problem is sharp edges or when you significantly increase the size of an image. Usually it is recommended that you resize in steps of 100% rather than one say 500% jump reducing the interpolation blur, sharpening between steps to correct.

Zillah

Fireman wrote:

but I heard somewhere that P/S would add more pixels (obviously not )

Well no, it would indeed add more pixels. But as said, those pixels are added by photoshop, they weren't part of the original image. So if you add a lot of them then it will look kinda crappy because photoshop can't magically return to the source and take a bigger photo, it has to guess what they should be based on whats there already.