Is distortion on an iPhone primarily due to perspective or does lens distortion play a part, and what can be done about it?
I am researching how to correct any facial distortions seen on an iPhone's front camera. Here is my current understanding of the issue:
- close-up photos will enlarge nearer facial features such as nose and forehead, as a short camera distance will magnify the discrepancy in camera distance between different features (as opposed to far-away images in which the distance variation between features is 'diluted' due to the long camera distance)
- even though close-up photos will distort images of 3D objects, there does not appear to be any distortion when photographing a 2D image such as a ruler
- wide-angle lenses (as used in iPhones) can cause a barrel distortion effect on images taken at a close distance. I have tested this out but have not found any noticeable barrel distortion
- what is the root cause for image distortion on an iPhone? Is it solely due to perspective distortion, or does the wide-angle lens also play a role?
- is my reasoning correct as to why there is no noticeable distortion for 2D images, regardless of camera distance?
- what is the minimal camera distance at which point there will no longer be a distortion of facial features on an iPhone's front camera?
- is the focal length different between the front and back cameras of an iPhone?
i think it's just the perspective, smartphone lens do not usually distort the image in a noticeable way. see also: http://stepheneastwood.com/tutorials/lensdistortion/tilepage.htm
I think it is just perspective too. There is no perspective involved on the surface of flat 2D subjects. Barrel distortion can be affected by focal length, but perspective is not. Perspective is only due to the distance where we stand, too close is bad, more is better. Where we have to stand is probably affected by focal length chosen, but it is distance that affects perspective, not focal length. The rule of thumb for proper portrait distance is at least 6 or 7 feet. Some consider more better. You might not mind only 4 or 5 feet, but more is better.
The unflattering faces due to perspective is from being way too close. The nose is significantly closer than the ears. The left ear and right cheek are seen from different angles. That is unlike how faces are normally seen.
back up. Don't take a picture from closer than you would normally view someone.
The size distortions are easily corrected. The effects of seeing what you expect to be parallel features from different angles are not. But having even shadowless lighting removes some of the clues that bring that to your attention.
That's one advantage of having a prime lens (not a zoom lens) on a "social" camera: when I hand it to someone to shoot me, he won't lazily zoom back but will step back instead.
There is software to correct perspective distortion, such as DxO ViewPoint 2. For that matter, Lightroom, and even the free MS Image Composite Editor include pincussion, barrel and other lens and perspective distortion corrections. The free IrfanView can employ 8BF filters, such as the free (again) PhotoPlugins lens corrector.
You can make test shots of a regular grid at various distances and see what settings of the software are the "corrective prescription" for your lens.
software can correct all possible lens distortions, but the perspective distortion is not really a distortion, it is the way objects look in a certain geometrical configuration. software can reproject your photos to make some slanted planes look straight, but it won't do any good with portraits, unless the subject's face is perfectly flat.
To answer the first question, the iphone lens distorts. I am looking to get some sort of correction lens attachment. The people here that are saying it is just perspective are incorrect. Look at a table or a building with your own eyes. And then look at it through the iphone lens. It distorts. It is horrible for portraits in the way that you described.
For the second question, it actually does distort for 2-D images too. Not sure why you would not see it.
It's true that iPhone (and all real-world) lenses do have some degree of geometric distortion, but: A) all of the things the question talks about _are_ due to perspective distortion and B) your test of looking with your eyes and comparing to a photo is not the same, because you are comparing you brain's 3D mental model to an image projected onto a flat surface. It's that projection of 3D to 2D which makes perspective distortion jarring in photographs.
Any lens below 70mm will create a semi fish eye effect on face because wide angle lenses 50mm or below are meant to shot scenery and bend the corners back while blowing up center to fit more of the scene/room. For a face that's bad news because the nose gets big the eyes hallow and face narrow. For portraits photography you want to flatten the distance between the nose to check to eyes which you get with a 70mm and above. I phone is good for scenery and tragic for face med
Focal length isn't the sole characteristic of a lens and your explanation sounds confused at best. For example, the sentence *the nose gets big the eyes hallow and face narrow [for a focal length of 50mm or less]* if only true if you give some information about body and subject distance.