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How to Correct Perspective Distortion in Photography?

Perspective has an important role to play in photography. Perspective distortion correction is also a vital activity in the post-photography phase. You would come across many topics discussing the same. Most of them contain hard to understand technical jargons and complex explanatory illustrations which make the topic more confusing. However, this article delves into the concept in a simple manner.

Perspective: What it is?

Linear perspective is commonly used in the domains of photography and art. It is indicative of the way the lines are drawn for capturing an object’s dimensions. This facilitates demonstrating the object in its perceived dimensions within the allocated space while translating the depth of real-world object into the flat surface of the image.     

Linear Perspective

Human eyes can discern the depth in a scene with perspective. The thing viewed by us through one eye is perspective. It is monocular as the combined action of both eyes is not involved. The angle at which parallel lines are located within the scene can be referred to by perspective. As the parallel lines start converging or coming closer to each other, the depth is visible to the eyes. Just stand on a straight road and look down. The road that is farthest from your location would appear to be narrowed. 

Vanishing point and the horizon line are two integral aspects of Linear Perspective. The observer’s angle of viewing is represented by the horizontal line. The points over the horizon line where the convergence of parallel lines occurs are called vanishing points. On the straight road, the horizon line is the line along which both sides of the road seem to converge at the far end. 

In the scene containing Linear Perspective, edges and vertical lines stay vertical. When you try to exhibit a pyramid or triangular shape in the scene, the vertical lines have to be brought together. For example, imagine the shape of a mobile tower which is triangular at the top.     

Right from the beginning of photography, manufacturers of cameras and lenses have tried to simulate the visual cues related to Linear Perspective. This is done to add realism to the clicked images. Contemporary camera lenses are rectilinear in nature, i.e., straight lines of any scene are captured without alterations in the photograph. For all lens, the lines are kept straight for various types of devices like iPhones, wide angled cameras, DSLRs, etc. This allows human eyes to discern depth in a 2-dimensional scene. In fisheye type non-rectilinear lens, straight lines are represented in a curved manner from view’s farthest field. 

Perspective Distortion

Keystoning or perspective convergence is a major type of perspective distortion in photographs. You would commonly come across this distortion in photographs related to traveling, architecture, and street. The distortion becomes apparent when falling or leaning effect is visible for tall buildings in an image.

Since the distortion is commonplace, people simply tend to ignore it within the images shot. Some also consider it as focal length distortion which is inherently present in photography. The vertical lines converge to a large extent in some images. 

 

Many photographers consider distortion and vertical convergence undesirable and nasty as they fail to comply with cues of Linear Perspective in which vertical lines stay vertical. The scene appears different than what we perceive it to look like. The distortion can be put to creative use but most people prefer the corrected images.

Reason for Occurrence of Distortion

This is because of a simple reason. Modern lenses have designs which are meant to display straight lines. The lens behaves in this manner only when you point the camera straight in line of the object which is being captured. This is due to the fact that that the distance between the object and camera always stay the same. When the camera is held in a tilted manner, the distance also changes. 

 

For example, if you are trying to click photos of a tall structure directly in straight line with the camera, you can only capture the bottom part. This is because the verticals lines prevent the remaining of the building from being captured. When the camera is angled higher, the entire building is captured within the frame. But, the building’s sides i.e. the vertical lines would start to converge. This is because the uppermost part of the building becomes farther away from the lens as compared to the bottom section. This is similar to the example of the road we have discussed earlier.

 

For real estate image editing, the picture of the tall building has to be captured without causing this distortion. This can take place in some ways. The best one is to change the viewpoint. Make your position higher than the current level so that more portion of the building’s front can be captured. The mid-point can be called as the optimum position for capturing the maximum portion of the building. You could also reach the window of any structure which lies in the neighborhood of the building to be shot. You can also hire a crane for capturing a stellar image while avoiding perspective distortion.

 

If the possibility of getting physically higher snap is not possible, the viewpoint can be changed in other forms. You can take the help of shift lenses or you can resort to lens perspective comparison software. 

What are Shift Lenses?

Since the time photography was invented, the concept of shifting the lens position has been around. During the 1840s, view cameras were developed. Bellows system was used in them for allowing photographers to shift the lens position with respect to the film. The shifting took place in a manner similar to a mini elevator in which the viewpoint of the photographer was moved higher or in bottom direction.

In the historical context, shifting the lens position for the purpose of correcting perspective convergence had been an integral part of photo capturing process. Alongside this, aperture, shutter movement, and focus are also adjusted. Shifting occurred as the lens occupied a significant amount of circular view field compared to the film. This is similar to the functioning of contemporary shift lenses.

Architecture photographers commonly use contemporary shift lenses for limiting vertical convergence in tall structures. DSLR or cameras of medium format can only use the shift lenses. The lenses are also very costly. 

Correction Through Software

The lens shifting process can be replicated by digital technology which has evolved significantly in recent times. The viewpoint of the photographer can be artificially changed. Top software packages which can do this trick include Adobe Photoshop, Lightroom, Gimp, etc. The pixels in the image are reworked for straightening the vertical lines for the purpose of lessening distortion. 

For correcting perspective distort in Photoshop or other software, the image is forced back to the right position with the help of sliders or with ‘corner select and drag’ process. One of the drawbacks of such correction is that lots of adjustments mandate image resampling. This reduces the sharpness of the image in parts. You may choose to ignore minor perspective distortions. However, perspective correction can make a mediocre looking architectural or urban image look good.

Distortion Adjustment with Photoshop

The distortion photography techniques are many. Similarly, fixing of lens can be done in numerous ways. In Photoshop, you can accomplish the same with ease. With the help of plug-ins, you can correct the distortions in Lightroom also. Adobe Photoshop is a powerful software with many top-notch and cutting-edge features. You can save little by little to buy it later on.

Import the image in Adobe Photoshop. From the toolbar on the left-hand side, choose the ‘crop’ tool. Alternately, you can hit the ‘C’ on the keyboard to do the same. After selecting, the crop tool is to be dragged over the complete image. 

After the crop overlay covers the picture, ensure to select the ‘Grid’. This option is better compared to ‘Rule of Thirds’ since lining up of the distortion correction can be done quickly. In Grid, a small box is present about which most people are not aware of. If the box is not selected, only standard crops can be made when corners are dragged or sides are moved up and down. After selecting the box, the corners can be dragged for correcting perspective lens distortion. 

Grab the top corner and use the columns on right and far left as a guide. This would cause the grid to line up with respect to the angle of columns. Once you do this, Photoshop would take care of the rest for the adjustment needed. After the crop overlay is in place, you need to press enter and wait for the correction to happen. 

Conclusion

After you have applied the aforesaid technique, the portions of the structure which suffered from perspective distortion would become straight in both top and bottom directions. You may be required to carry out lens correction slightly. You can place the portions of the structure far from the main building inside the frame so that they can serve as the guide later on in Photoshop. There are many such tricks and you can gradually learn them. You can also seek the specialized help of perspective correction services offered by leading companies like WBS.