science

Perspective control

Perspective control is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Perspective control rather than just read about it. In short: Perspective control, also known as perspective correction, is a procedure for composing or editing photographs to better conform with the commonly accepted distortions in constructed perspective. The control would: make all lines that are vertical in reality vertical in the image.

Key takeaways

Perspective control belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Perspective control to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Perspective control from memory before moving on to harder problems.

Reference excerpt

Perspective control, also known as perspective correction, is a procedure for composing or editing photographs to better conform with the commonly accepted distortions in constructed perspective. The control would:

make all lines that are vertical in reality vertical in the image. This includes columns, vertical edges of walls, and lampposts. This is a commonly accepted distortion in constructed perspective; perspective is based on the notion that more distant objects are represented as smaller on the page; however, even though the top of the cathedral tower is in reality further from the viewer than base of the tower (due to the vertical distance), constructed perspective considers only the horizontal distance and considers the top and bottom to be the same distance away; make all parallel lines (such as four horizontal edges of a cubic room) cross in one point. Perspective distortion occurs in photographs when the film plane is not parallel to lines that are required to be parallel in the photo. A common case is when a photo is taken of a tall building from ground level by tilting the camera upward: the building appears to fall away from the camera.

At exposure

Professional cameras where perspective control is important control the perspective at exposure by raising the lens parallel to the film. There is more information on this in the view camera article. Most large format (4x5 and up) cameras have this feature, as well as plane of focus control built into the camera body in the form of flexible bellows and moveable front (lens) and rear (film holder) elements. Thus any focal length lens mounted on a view camera or field camera, and many press cameras can be used with perspective control. Some interchangeable lens medium format, 35 mm film SLR, and Digital SLR camera systems have PC, shift, or tilt/shift lens options which allow perspective control and, in the case of a tilt/shift lens, plane of focus control, but only at a specific focal length.

In the darkroom A darkroom technician can correct perspective distortion in the printing process. It is usually done by exposing the paper at an angle to the film, with the paper raised toward the part of the image that is larger, therefore not allowing the light from the enlarger to spread as much as the other side of the exposure. The process is known as rectification printing, and is done using a rectifying printer (transforming printer), which involves rotating the negative and/or easel. Restoring parallelism to verticals (for instance) is easily done by tilting one plane, but if the focal length of the enlarger is not suitably chosen, the resulting image will have vertical distortion (compression or stretching). For correct perspective correction, the proper focal length (specifically, angle of view) must be chosen so that the enlargement replicates the perspective of the camera.

During digital post-processing Digital post-processing software provides means to correct converging verticals and other distortions introduced at image capture. Adobe Photoshop, Affinity Photo and GIMP have several "transform" options to achieve, with care, the desired control without any significant degradation in the overall image quality. Photoshop CS2 and subsequent releases includes perspective correction as part of its Lens Distortion Correction Filter; DxO Optics Pro from DxO Labs includes perspective correction. Affinity Photo 2 includes all the main distortion corrections. GIMP (since 2.6) has its own perspective tool (Tools/Transform Tools/Perspective). RawTherapee, a free and open-source raw converter, includes horizontal and vertical perspective correction tools too. Note that because the mathematics of projective transforms depends on the angle of view, perspective tools require that the angle of view or 35 mm equivalent focal length be entered, though this can often be determined from Exif metadata. It is commonly suggested to correct perspective using a general projective transformation tool, correcting vertical tilt (converging verticals) by stretching out the top; this is the "Distort Transform" in Photoshop, and the "perspective tool" in GIMP. However, this introduces vertical distortion – objects appear squat (vertically compressed, horizontally extended) – unless the vertical dimension is also stretched. This effect is minor for small angles, and can be corrected by hand, manually stretching the vertical dimension until the proportions look right, but is automatically done by specialized perspective transform tools. An alternative interface, found in Photoshop (CS and subsequent releases) is the "perspective crop", which enables the user to perform perspective control with the cropping tool, setting each side of the crop to independently determined angles, which can be more intuitive and direct. Other software with mathematical models on how lenses and different types of optical distortions affect the image can correct this by being able to calculate the different characteristics of a lens and re-projecting the image in a number of ways (including non-rectilinear projections). An example of this kind of software is the panorama creation suite Hugin. However these techniques do not enable the recovery of lost spatial resolution in the more distant areas of the subject, or the recovery of lost depth of field due to the angle of the film/sensor plane to the subject. These transforms involve interpolation, as in image scaling, which degrades the image quality, in particular blurring high-frequency detail. How significant this is depends on the original image resolution, degree of manipulation, print/display size, and viewing distance, and perspective correction must be traded off against preserving high-frequency detail.

In virtual environments Architectural images are commonly "rendered" from 3D computer models, for use in promotional material. These have virtual cameras within to create the images, which normally have modifiers capable of correcting (or distorting) the perspective to the artist's taste. See 3D projection for details.

See also Anamorphosis Distortion (optics) Keystone effect

References

External links

Illustrations Perspective correction, Panorama Tools wiki Transform perspective while cropping using the Perspective Crop tool, Adobe Photoshop help Transform perspective while cropping, Adobe Photoshop help (for CS3/10.0) (archived) Correcting perspective using the Open Source Hugin software

Illustrations

Worked examples

Example 1 — a first encounter with Perspective control

Start with the simplest possible case. Write down what Perspective control claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Perspective control before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Perspective control ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Perspective control

In research: Perspective control appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Perspective control in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Perspective control is common in secondary-school and first-year university syllabi. It links to neighbouring topics Composition in visual art, Perspective projection, Photographic techniques, so understanding it makes those chapters shorter.
In everyday life: Look for Perspective control outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Perspective control in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Perspective control means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Perspective control out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Perspective control in simple terms?

Why does Perspective control matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Perspective control?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Perspective control.