Focal Length and Perspective
 

Some people say that the focal length of the lens in use effects perspective. Others say 'not so,' it's the subject distance only that determines perspective. What do members think about this?

I caught a bit of flak last time this subject came up John. So I am going to lurk on this thread for a bit and see others point of view. Lots more members now. :)

Don

OK Don, thanks for coming in. John

My understanding is that, for a given "image in viewfinder" size, focal length does alter perspective but it doesn't change depth of field.

This article http://luminous-landscape.com/tutorials/dof2.shtml is primarily about depth of field but I think it also illustrates the perspective issue.

There is an interesting article HERE
about photographic perspective. I don't think you can argue with the logic or the maths that it is Distance to subject only.

However as photographers we know that there is the right lens/focal length for the job. If you are a portrait photographer for example, you wouldn't use say a 35mm lens and simply get closer to fill the frame with your subject. No, I would hope you would use a much longer focal length 100mm plus and move further back in order to fill the frame.

Why is this? To my mind it is because the longer lens alters the whole dynamics of the image to the viewer. Although it may show the same area of the subject, the viewers eye sees the subject somewhat differently and generally in a more pleasing and flattering fashion. Not only this but the area behind the subject is different too. If you want to photograph a subject and crop out as much of the background as you can then I would use a long lens, whereas if I wanted to photograph the subject with a building behind I would bring the subject closer to the camera and use a wider lens. Simple photo technique that any photographer knows but the typical Japanese tourist doesn't seem to grasp :D

Off to the races here again John. I understand the effect a lens has on elements within a scene and thought that to be perspective. So I agreed with Gidders.

Stephens link got me a bit confused as it says, [ 1. Lens focal length does not affect perspective. 2. Film size does not affect perspective. 3. Location of the camera does affect perspective. ]

Now to my simple mind in no.3 the lens attatched must affect perspective, if perspective is the relationship between objects in the scene.

So I am now trying to find a definition for ' PERSPECTIVE '.

I do not mind owning up to lack of formal education before I get ripped apart again. :D

Don

Link to a US Navy training manual for a definition of perspective http://photoinf.com/General/NAVY/Perspective.htm

Don

Don, I am inclined to agree with Stephen on this one. When I have finsished mowing and weeding I will post a very simple sketch and mathematical proof that focal length does not affect perspective. There is an illussion that it does because it causes the photographer to shoot from a different postion and it is this change in subject distance that causes the change in perspective.

This will be a long one Rob, John has been ages typing. :rolleyes:
I'll go and get a coffee.:D

Don

Hope you enjoyed your coffee Don. I can't say I enjoyed the weeding or the mowing.
The attatched diagram shows two light rays one from the top and one from the bottom of the image which are helping to form the image on the sensor. In practice the actual passage of the light is very much more complex than this but the diagram will serve its purpose.

Suppose now that we have two subjects A and B one of height Ha at distance Da from the lens and the other of height Hb at distance Db from the lens.

Then the image heights are given by:-

ha = Ha x f / Da and
hb = Hb x f/ Db

The ratio ha/hb (ie perspective) will be ha/hb = Ha x Db / ( Da x Hb)

Notice that focal length f has vanished from the equation and therefore, does not affect the ratio ha/hb (ie perspective) whereas the subject distances remain and do affect perspective.

And I wish I had never bothered!