CS2 lens correction affects sharpness - why ?

[Nacoya]

Hi,

Just noticed something i had never picked up on before.

I have an image i took last week

~ here ~

i then used lens correction in photoshop. Straight line to level the image in line with the boat. The result as can be seen is a complete loss of sharpness.

~ straightened ~

nothing else done to the image.

can anyone explain why ?

thanks,

dave

[limaze]

hello, i noticed that too but have no explanation neither. its interesting because the old way of doing this straightening (with the measure tool) doesnt result in that unsharpness. seems we dont know how to use the lens correction tool OR the photoshop guys have another bug to fix

[Nacoya]

hi,

i just tried it with the transform tool/rotate and it's almost as bad, but then i suppose it's the same rotation. It's almost like it's lossy, and i'm working with a tiff straight out of nikon capture nx (for info).

I suspect i'll know once you explain it but what's the method with the measure tool ?

[Photo678]

because of the metrics and voodoo PS uses to do these things.....fix is to apply unsharp mask at about 50

[mistermonday]

Nacoya, that is not a PS Bug. Lens correction is used for optical adjustments and when you change the focal point and depth of field of a lens, a different part of the image is supposed to be in focus. This filter is not intended for straightening and there is a much simpler way to straighten.

Select the Measure tool (its under the Eyedropper tool pull out). Find a straight edge in the image (even though it is not horizontal or vertical yet). Drag the measure tool along that edge, the further you can the more accurate the result will be. Then go Image>Rotate Canvas>Arbitrary. Do not change the value shown, just hit the ok button. Your image should now be straightened. You will probably need to crop it.

Regards, Murray

[duwayne]

I use CS1 but am able to duplicate the out-of-focus. If I do a rotate image, it is out of focus. If I do a rotate canvas, it stays in focus. It's likely that the CS2 lens correction is doing am image rotate. You can see the exposed white slices in the original rotated photo from Nacoya.

The picture is a very low resolution (1000x664 pixels) for the level on details in the photo (i.e. all the water and trees). I increased the resolution by a factor of 4 and then did the rotate image and it stayed in focus
.

[limaze]

Originally Posted by mistermonday

Nacoya, that is not a PS Bug. Lens correction is used for optical adjustments and when you change the focal point and depth of field of a lens, a different part of the image is supposed to be in focus. This filter is not intended for straightening..

i dont know if you are right about this. in the Lens-Correction filter dialog there is that tool (at top left) which is explicitly called straighten-image-tool (or something like that), and its tooltip says this is the tool to use to align an image horizontaly. noone says anything about adjusting anything of the many other features of this filter, its only about that one integrated tool. and if that tool does add some blur to the image, while other methods (measure tool) have much less blur, it is a kind of bug or mismanagement in my opinion at least. maybe someone should tell the adobe guys to implement the measure-tool-method's algorithm at the lens-correction-filter's straighten-image-tool

[mistermonday]

Yes, if you are not modifying any of the lens correction settings and you are just using the straightening tool, it should be the same. However, it appears to be more blurred than the measure tool method. My guess is that there is a lot more going on in the calculations even if the distortion values are all 0. But then, perhaps that is why it is a distortion filter.
In my mind, if all one has to do is straighten, then typing Alt - i - e - a seems the fastest most effective way to do it.
Regards, Murray

[byRo]

Well that's something new.
Really, it does seem that the Rotate Canvas>Arbitrary method does give a less blurred effect. duwayne, seems to have summed it up pretty well.

So, if you're just going to rotate then you should think twice before going to the Lens Correction. However, the Lens Correction does have a lot more stuff to play with, and I will usually also use Vertical / Horizontal distortion, and I just love the edge extension option.

What intrigues me most though is just why Nacoya is rotating this image?
Surely it's the water that is supposed to be horizontal, not the boat?
To me it look as though she's shooting the rapids.

Rô

[Nacoya]

Hiya guys,

just got up in the uk this morning so i'll try to reply in no particular order.

a. byro : tbh the rotation was just me experimenting and not for final output. It was the effect i was using the images to show not the intended output.
b. all : per a. hence i deliberately didn't crop so that the rotation was obvious.
c. all : i applied nothing else at all other than the very slight image rotation is no other distortion, as a number of responses indicate also. And a 2d rotation is not changing the FL or FP.
d. duwayne : the resolution shouldn't matter imo. The reason it is that size is it was resized for web prior to importing to CS2 as capture nx resizes much better in raw than cs2 does as a tiff. (relative sharpness etc)


all : thanks for your input and i will try the other method provided also thanks murray). It was more a post borne out of a. surprise that in the thousands of images i've fed through CS2 i had never noticed it (need my eyes tested LOL) b. to discuss which has provided me with a few alternatives.

thanks very much.

I may go into the adobe forum and link to this thread and see what they say.

cheers,

dave

[Nacoya]

hi again,

i made a small (well 8.5meg) psd with the original and 3 rotations for ref.

measure method, lens correction and rotation prior to photoshop.

if you wish to dl it is ~ here ~

having done a quick google it seems i have been fumbling around oblivious to this for years . It seems that it's the interpolation used for small arbitrary rotations that cause the apparent loss of sharpness.

added : ~ here ~ is some good bumf

[Photo678]

Originally Posted by Nacoya

It seems that it's the interpolation used for small arbitrary rotations that cause the apparent loss of sharpness.

this should have been obvious......photoshop only does the mathematics ,,,,,,,,,,garbage in, garbage out

[Nacoya]

Originally Posted by Photo678

garbage in, garbage out

implying what exactly.

Originally Posted by Photo678

this should have been obvious......photoshop only does the mathematics ,,,,,,,,,,

and the mathematics are bound by the fact that sharpness is a factor of edge contrast and the limitation to that is a pixel is 1x1 and on rotation of less than 90deg or indeed inbetween the 4 poles, the original registered pixel may have to overlap it's neighbours = no defined edge as per original = no contrast = no sharpness.

but thanks for your reply.

[Dm_Cher]

Originally Posted by Nacoya

Hi,

Just noticed something i had never picked up on before.

I have an image i took last week

~ here ~

i then used lens correction in photoshop. Straight line to level the image in line with the boat. The result as can be seen is a complete loss of sharpness.

~ straightened ~

nothing else done to the image.

can anyone explain why ?

thanks,

dave

"Lens correction" is a geometric distortion of your image. So, when you shot using "bad" or specialized lens (for example,"fish-eye lens"), straight line will looks as curve line on shot. "Lens correction" deforms image back in order to straight line on scene will straight line on image. When transformation is executed where is sub-pixel precision calculations and interpolation is needed.
Any interpolation is a low-frequency filter (like lens MTF defocus) and any sharp details are lost.
You can take sharp image and rotate it several times, and your image will smooth without sharp details.