Converging verticals.

This is something I've thought about asking before, but Womble's post in another thread has prompted me.

a) why does this happen?

b) how do I avoid or minimise it, without paying out huge lumps of money for fancy lenses? Is it up to my skill ( yeah right! ) to frame the shot to minimise this effect, or does it need backroom processing to get rid of?

Or maybe I just need to learn to like it.

a) Because of the physics of the normal lens
b) Keep the camera back parallel to the subject (which may mean you have to go higher)

or... you can correct using the Photoshop distortion correction filters (or Silkypix has vertical & horizontal correction controls too).

or... you can get a "Tilt / Shift" lens at a huge price You may find this helpful: http://en.wikipedia.org/wiki/Perspective_control_lens

Convergence can be used for creative effect - to create more diagonals in the image, for example. Basically, pointing the camera slightly upwards makes verticals converge at the top, and pointing slightly downwards makes them converge at the bottom of the frame. The same thing happens with horizontals, incidentally, if you are not pointing perpendicular to the subject.

Matt

Converging verticals are correct.

Think of it like this. If you looked at a train track receding off into the distance it would converge to a point. We expect that, it's a fact. And we don't try to correct it! These are converging horizontals and our eyes and minds accept this as normal.

Now look upwards. Straight lines in buildings do the same thing - they converge to a point in the distance. We expect them to and it looks right. Unfortunately when we look at a photograph our minds rebel and we want then convergence "corrected" - in fact try correcting converging horizontals and see how weird it looks.

How to "correct" converging verticals is to use a Shift Lens, or use Photoshop. Transform>Skew or Transform>Distort will both do the job. Just pull the image into the shape you want and then recrop to eliminate the bits of the image you can no longer see (but which are stiil there using up memory). Obviously there's a limit to how far you can go with this.

The final way is to keep the camera back vertical, use an ultra wide lens and crop out the excess foreground.

Thanks guys

The other alternative is to use a view camera which has independent movements of both the lens board and the film plane but that is a whole other field of photography which the only the insane embark upon.

In terms of taste, dramatic converging verticals deliberately done can look great. Slightly converging verticals where the camera has been tipped up a little to get the top of the roof in are less appealing to my eye.

Cheers, Kris.

They converge, but will also be distorted and even a shift lens won't help with the distortion. The only proper solution is Lens Corrector software such as PTLens or Proxel Lens Corrector. Amazing what can be done with the right tools. A lot of editing programs also allow correcting converging verticals, but it can be quite difficult due to the distortions and you usually have to make do with a compromise of not quite fully correcting the convergence.

shim

shim, what distortion are you referring to?

With an image from an ultra wide angle lens, it can be quite satisfying just to partially correct the convergence, then the perspective looks more like what we see with the human eye.

However, sometimes you might want to go the other way for artistic effect and exaggerate the convergence by tilting the camera more than you need to.

John, have a look on the two sites.... there's even the Pentax kit lens and you can see the before and after. This shot's unbelievable.... how straight the lines are..... even more unbelievable when you consider the original was from a cheap Russian fisheye.

shim

Distortion? What distortion? Do you mean barrel or pincushion? They are none.

Unfortunately no one makes a shift lens for Pentax DSLRS. I have the 28mm f3.5 shift which is a fabulous big expensive piece of glass which needs to be used on a tripod with spirit levels and a grid focusing screen, you cant use the split image/microprism sort because the centre turns black as soon as you start to shift. It also has a pre-set manual iris since there can be no actuator. The front element protrudes so it has built in yellow, orange and UV filters with a gel filter clip behind the rear element - welcome to the world of archtectural photography.

Sadly on digital there isn't much call for a shifting standard lens.

I usually shift to the 'correct' position and back off one 'click stop'.

In PSE the procedure is Image/Transform/Perspective and drag either of the top corner handles.

I have proxel but as far as I can see it corrects for lens distortion (pin cushion, barrel etc.) not converging verticals which is an entirely different thing. The shot you link to has clearly diverging verticals because the camera has been pointed downwards to include more of the tracks.

Best wishes, Kris.

PTLens definitely corrects converging verticals, I've not used Proxel since last year and can't remember how I did it. I tried correcting in an editor (not Photoshop, can't remember what it was... it wasn't my computer but it had some weird stuff for correcting verticals and skewing buildings round) but when I fully corrected the convergence other things started to lean the other way, can't remember if I just ran that through Proxel or started again and corrected the original image. The good thing about Proxel is that the lens doesn't have to be in the database and you just photograph a door or similar for it to pick up what should be straight lines and calculate the lens correction. PTLens will put your lens in the database if you send them some test shots.

Gartmore, PTLens does handle shift lenses.

shim

Lens distortion is a totally different subject to converging verticals, which is an optical effect that just exists.

Lens distortion of the barrel or pincushion variety is commonly found in wide angle and telephoto lenses respectively but is a defect of the lens.

Converging verticals could I suppose be called a defect of the subject, or maybe of Physics! Of course, it isn't a defect at all.

Found out what it was Zoner Photo Studio and the tool was called Collinearity. It was actually quite good 2 vertical lines that you lined up with 2 verticals pressed accept and it would even Autocrop the result to a rectangle. If you pressed the tool button again you could drag one of the vertical bars around and check other items to see if they were all vertical. Also had a similar tool for perspective i.e. if you photographed a noticeboard from an angle you could make the corners square with all the sides parallel. It even had a tool for aligning horizons just line a bar up with the horizon and it corrected the tilt and offered to Autocrop. That could be another problem with converging verticals.... the camera might not have been level, making it look worse than it actually is.

shim

Just checked my copy of proxel and it fixes distortion but not converging verticals. The Zoner thing sounds very interesting. I have never had much luck using the photoshop tools. Never can get it looking quite right, hence the Horseman.

Best wishes, Kris.