Vertical Perspective correction, degradation of image.

[bjtmiguel]

I am using high quality Nikon, Canon and Zeiss lenses around the 21-24mm focal length to photograph churches, convents and other buildings with high towers etc.

My problem is using photoshop to correct the converging verticals present in these images.

For example, the church cross at the top of the image is always softer than the lower part after straightening the verticals in photoshop. The sharpness of the cross is acceptable being near the edge of the image circle before using perspective correction, but unacceptable thereafter.

I do appreciate that photoshop will degrade the top of the image if I stretch the pixels, it is good, but not good enough because clients are commenting on the slight softening of the image tops.

I have tried Perspective Warp, Lens vertical correction, Free transform, Transform skew, Transform perspective and transform in ACR, but they all seem to produce identical results.

Can you please tell me which photoshop method is the least destructive, if there is one?

I could resort to using Canons 17 and 24mm tilt and shift lenses but thats an expensive outlay which may not produce the answer?

For the moment I am using ACR's adjustment brush and brushing in some extra sharpening where required, not the best solution, but I can't find a better one at present.

Any advice you can offer will be greatly appreciated.

Regards

Michael

[davescm]

Any pixel transformation is going to reduce the sharpness to a degree and if you are starting with an area just on the limit then it could quickly stray into the unacceptable.

This discussion summarises some of the issues. Unsurprisingly it comes out strongly in favour of T&S lenses as it is by Rodenstock but it does show the issues well.

http://www.rodenstock-photo.com/Archiv/Perspective%20Control.pdf

Dave

[bjtmiguel]

Hi Dave,

Thanks a lot for that excellent article from Rodenstock, its heavy reading but I am sure

it will answer my question.

Regards

Michael

[D Fosse]

Obvious point, perhaps - but instead of expanding the top, shrink the bottom. Yes, you lose total resolution, but with the newer high resolution sensors that shouldn't be a big problem. And the end result may still look better.

If you need very high resolution, there's always stitching. Same principle: reduce the base image a bit before running the align, so that you avoid too much stretching in the edge frames.

[bjtmiguel]

It seems like there are limited answers to the pronlem.

1- buy shift lenses, very expensive but if its necessay, then it has to be, I am not sure how Canons 17 and 24mm

lenses will preform, any experience here?

2-Double up my sensor size and leave ample space in order to pull in the bottoms of the image.

3- Even more expensive, go for larger format with shift facilities.

Can you successfully stitch vertical images that have not been shot on a shift lens? any tips?

[D Fosse]

bjtmiguel  wrote

Can you successfully stitch vertical images that have not been shot on a shift lens?

In principle there's no problem. Take one horizontal base shot with no perspective distortion, and put that at the bottom of the stack. The others, that do have perspective distortion, will be automatically corrected to that.

If you then scale down this base shot first, there won't be stretching at the top. All resampling is down, not up.

However, for optimal results you need to avoid parallax error. That means the camera must be rotated around the optical center of the lens (that's where the diaphragm appears to be when you look into the lens from the front). This is easy enough horizontally with a focusing rail, a bit more tricky vertically. I raise and lower the center column by a predetermined amount, marked on the column.

Turn on lens correction in ACR! You need good rectilinear correction before merging.

Oh, one more thing. Don't use Photomerge. Run Auto-align first, then Auto-blend. Once the frames are aligned, you may need to nudge layers a couple of pixels. Sometimes Auto-align offsets individual layers a bit, I don't know why. Go to 100% and check for perfect alignment.