Converging Lines

Posted by on November 17, 2010 in Blog, Editing & Workflow | 6 comments

Converging Lines

A good rule of thumb to follow in any photo, is that if there is a vertical or horizontal line in the image, they should be just that… vertical or horizontal. For instance, the horizon should be straight, not sloping. If there was a flag pole, or lamp post, these should be straight up, i.e vertical. This is all well and good if there is only one horizontal or one vertical line within a scene. Through careful framing or by means of  a quick rotation in Photoshop, we can ensure these lines look straight and the scene looks “natural”. But what if there are multiple vertical lines or multiple horizontal lines? What happens if you straighten one it leads to the other becoming even more off axis? Welcome to the problem of converging and diverging lines :!:

 

 

Looking at the diagram above, on the left hand side we have a “building” that we are looking at from a front on view. Next to this, we can see a profile view of this building, where we now introduce a camera into the scene. The camera is actually photographing the front of the building. In this case, the camera, lens and film plane are all exactly parallel to the front of the building. So we can see that the top of the building is the same distance to the film plane as the bottom of the building. In general, anything closer to the film plane appears magnified in an image and anything further away appears smaller. However, as the top and bottom are parallel to the plane, they are both exactly the same distance away from it, and as such are subjected to the exact same magnification factor. This means the building will photograph exactly as seen (the capture on the right, appears identical to the original scene on the left).

But what happens if we tilt the lens upwards :?: The effect of this tilt means that the front of the building is no longer parallel to the film plane. In fact, the bottom is closer meaning it appears larger in the frame and the top of the building is further away, meaning it appears smaller. The leads to converging lines, where the edges of the building appear to approach each other as they get near the top. The greater the degree of tilt, the greater this effect becomes.

A similar thing happens when you tilt the lens downwards :!: As you tilt it down, the top of the building becomes closer to the focal plane and appears larger, whilst the bottom moves further away, appearing smaller in the frame. Regardless of whether you are tilting the lens up, or down, the tilt introduces this “distortion” into the image. As mentioned already, the degree of tilt, your focal length and indeed how close you are to the subject, all play a roll in how much or indeed how little this convergence effects your image.

On my recent shoot with Raphaella, I really struggled with converging lines, particularly when I was shooting her on the stairs (as you can see from the image above). For this set, I needed to use a reasonably wide focal length so as to ensure I could include the whole scene. My shooting position was the top of the stairs, so had I kept my lens parallel with the wall/bannisters etc. I would only have been able to photograph the top of Raphaellas head. So in this case I had no choice, but to tilt the lens downwards.

The problem is though, with so many obvious verticals within the scene, this had the effect of causing the lines to converge towards the bottom of the scene, leading to a lot of distortion. An ideal fix for this would have been a tilt-shift lens, which allows you correct this distortion by shifting the focal plane so that it is back parallel to the edges in the scene. Unfortunately though, I don’t have one and couldn’t really justify the cost of one for the very rare occasion I’d use it.

Thankfully Photoshop, within their Raw Converter offers a feature to correct for this sort of distortion. Effectively what it does is counteracts the effect by reducing and magnifying the top or bottom of the image by inverse amounts so that it brings everything back to how it should be. Obviously it is no where near as accurate as doing it optically in camera with a real tilt-shift lens, but for us paupers it does the job quite well.

But be warned :!: Once the effect is applied, some cropping is required to trim off the blank edges, this means you will lose parts of your image which were included in the original frame. This can be a real pain, especially if you go to great lengths to ensure you are including detail in the shot at the time of capture, which may then be cropped out later :oops: In addition to cropping, I found I also had to do a lot of cloning to the final edit, to “bring back” areas within the image that were cropped out.

Whilst by no means is this is a perfect result, it is one I find far more pleasing than the original capture. Whilst photographers specialising in architecture and interiors are all to familiar with this problem, it really hasn’t been a consideration for my photography in the past. This is because I generally shoot at longer focal lengths, with a pretty large separation between model and camera, so the distortion was never noticeable to the degree that it was a distraction within the image. But it is something I will pay more attention to in the future. Or maybe I just need to get myself a tilt-shift lens after all ? ;)

6 Comments

  1. Great article Ciaran and well explained in your usual exemplary way … Lovely shot too. Looking forward to seeing more from this shoot. C

  2. Ah! the good old “but I NEED one of them…” excuses for gear. i’ve used it many times, and I manage to convince myself every time I try it. :-)

    Nice shot!

  3. Good article Ciaran, and a useful guide for anyone out there wanting to do architectural or interior work especially. Very concise and well explained. Could do with a ts lens myself but damn they’re pricey! Lovely shot by the way…

  4. Cheers Rob. Yeah TS lenses come at a madness price :!: And being honest, even if I could afford one, the problem would be choosing the best one? They are all primes, so in reality you’re fixed to a specific focal length and I’m more of a zoom guy myself.

  5. Small correction Ciaran. It is the shift function of a T/S lens that helps a photographer deal with converging lines. The tilt alters the focal plane, giving you more DOF at wider apertures (or less at small apertures). The tilt also allows you to make those stupid fake-miniatures that everyone seems to be doing these days (but most do it in Photoshop).

    You can see in this shot how I used the tilt on my TS-E 24mm T/S lens to control the plane of focus, keeping the side of the building in focus, while throwing the rest completely OOF (this is the opposite to how tilt is normally used by landscape photographers). This is an example of sever tilt though, much more than you would normally use.

    http://www.flickr.com/photos/mikealex/5028054114/

    Shifting the lens moves the image circle up or down (or left/right if you rotate the lens), allowing you to effectively change your perspective while keeping the sensor parallel to your subject. Shift is largely a replacement for carrying around a ladder. I used shift in this shot to get the perspective I wanted and keep the columns straight.

    http://www.flickr.com/photos/mikealex/5111806881/

    To be honest, I don’t think a T/S lens would have helped you in your shot. To keep the verticles verticle, you would have had to keep the camera parallel to the pillars and window frame. But then you wouldn’t be pointed down the stairs anymore, and wouldn’t see the steps or your subjects feet. Shifting the lens wouldn’t fix that. If you were shooting up the stairs, then a T/S lens would have helped a lot.

    I do think you should buy a T/S lens though. I’ve had mine since late September, and have never had as much fun with a new lens as I’m having with this one.

  6. Thanks for the clarification Mike. I have a few more items on my wish list before I get round to a TS lens, if in fact I ever do. Looks like you are having a lot of fun with yours though :)

Care to share your opinion?