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!

OK John I have saved your diagram so that I can view it while reading the explanation.

Maybe a couple of drinks in order to lubricate the ' little grey cells '. :D

Don

Firstly let me say John education on the African plains was easier than this.

I am still trying to get to grips with the term ' Perspective ' in this discussion.

I have been compiling a reply in note pad that I have just read. Got totally confused and deleted it. :confused:

Being a simple soul I would look at subject A and decide how I wanted the background to impact on the final image. I may move closer with a wide angle to give depth to the background or move further away and use a telephoto to draw the background up. The relationship of other elements within either image would be quite different.

The question is then is that affecting perpective as I understand it. I would say YES.

Don

PS Phew :D

Do you mean like this Don. I have been trying to get my head round this thread. It just so happens we were at Newports national sports centre today which has a very long drive. I was to lazy to put my 50mm lens on as we had been walking a while. The first shot was taken by Mario at a 100mm his shortest length and the second was taken by me on the same spot at 170mm my shortest length.
Added an extra photo I didn't realise Mario took one at 400mm

Got it in one Christine. I did not even have to open the thumbnails to see the effect on the distant trees.

Well posted, thanks.

Don

Christine

The logic is that if you walked up the drive with the 100mm until the people were the same size as in the shot at 170mm the trees in the backgound would also be the same size as in the 170mm shot. Therefore the perspective effect is due to the distance not the focal lenght.

It's dead easy to check in about 2 seconds with a zoom lens on your camera - find two subjects that are at different distances but which appear the same size as one another. Now zoom your lens one way or the other and you'll see that they are still the same size as each other. QED

AHHHHHH! Mario's pocket calculator has just exploded trying to work that one out.

My opinion but Christines pictures show this effect very well, all be it at the telephoto end.

My question still stands. Is this not a case of using lenses to control perspective. If not how should this effect be described ?

Don

Adey

You would have to move as well to keep the magnification factor the same. Because as you zoom and remain in the same spot you are changing the magnification factor. The factor which made the 2 objects the same size has now changed and therefore they will appear different in size.

The size of a near and distance object is due to the magnification factor. In the equation for magnification focal length plays no part in it. It is ratio of the lens to film/sensor distance to the lens to subject distance.

Adey,

If I did this with my 28 to 105 lens then I would change magnification and the effect on the background would also alter. Wide angle to medium telephoto. Using a 24 -200mm would have an even more pronounced effect.

Its dark outside so I can't do an actual.

Don

Try it and see...you're defying the laws of physics if you think you can move the size relationships around without moving your position

Don, re your post number 12.

I think you are talking about the compression of planes that you get with long lenses. This is known as ariel perspective. But perspectve as I was using the term relates to the size of objects relative to each other. Christine's pictures illustrate this admirably. Or have I misunderstood you. What I was trying to give was a mathematical knock down proof that the relative size of objects in a picture depends only on subject distance and not at all on focal length. That is why I looked at the ratio ha/hb.

John

John,

My interpretation of the effect of changes in presective was as in Christine post.

May well be wrong. Brain getting a bit fried. :D

One thing I will say, you have livened up our evening. I will go through this again tomorrow with a clearer head and daylight.

Don

The point is that if christine had walked up the drive with the 100mm lens and stood in the correct spot you would not be able to tell the difference between the 100mm and 170mm shots. The same applies to DOF.

DOF Don, now there's another story; but I haven't the nerve to start another thread.
John

Good idea Don then you will get everything in the right perspective!

Yes Rob, I understand and agree with that. From my angle it is knowing the correct, if there is such a thing, term to describe the effect as in Christines shots, if it is not perspective.

Don

Don once you start moving closer or farther away you are changing perspective. It is not the change of focal length that is doing it, that is only to fit the image on the screen.

John

Don

The effect in Christine shots is changing perspective. But stickly speaking it is due to changing the magnification factor which is a by-product of changing the focal lenght. I know this may sound pedantic but it becomes important when trying to understand the science. If this effect was purely a function of focal lenght then moving closer would never give the same view.

That's right - it's the moving closer (or farther away) that changes the perspective. Focal length won't - can't - alter the perspective.

Morning all,

Brain has had time to refresh. :D

I guess that I only ever consider the change of focal length with a change in camera position. ie wide angle move closer, gives greater depth. Telephoto, move back as it compresses the scene and draws up the background.

So it is always a combination of 2 things. Camera position and focal length.

Surely if only focal length were considered then final image magnification should be taken into account as well. The crop from a 50mm lens image in comparison to that from a 150mm lens to give the same final output size, will show significant image degredation due to the increased magnification.

Don

Morning Don

Wheres that glorious sunshine gone ??

OK Don look at it from this point of view. You can achieve exactly the same perspective using any focal lenght providing you take the shot in the required position.

If you use a telephoto and move back far enough it will give the same effect as a wide angle. Likewise if you use a wide angle and move in close enough it will give the same effect as a telephoto.

The thing is that we don't normally use a lens it that way. Think about some of the photography in wildlife where they take you into the world of an insect or use small models of buildings instead of lifesize scenery in films.

With you on that explanation Rob.

Weather .... well, dull and rain on the way.

Like John I spent yesterday gardening as it was a great day for that ....... somethings you just have to do. ;)

I think that when we have managed to get some pics together it is worth posting the effect of changing your lens to obtain a particular effect. I think lots of people just use the zoom function to fill the frame, without considering how focal length choice can have a significant impact on the final image.

Stephen described it well in his post 5, but as they say, a picture is worth a thousand words, and if you are new to photography this is not something that has necessary been considered.

Christines pics were a good example of the effect a telephoto can have on the background.

Just read John's post 25 ............ Mmmmmm :rolleyes:

Don

I have just run some tests with the 100-400 lens @ 100mm, 150mm, 200mm, 250mm. All were at the same F stop and speed. I can see how the background is affected by the changes in distance and focal length. I am still finding this very hard to take in.

It's only affected by the changes in the distance from the subject, not the focal length

Adey can I have a brain transplant it may work better.

It's ever so easy, really! (honest, Guv!). Perspective is entirely governed by viewpoint, even without a lens! The different focal lengths just 'select' part of the scene in front of you and, provided you don't move, everything will be the same relative size to each other no matter which focal length you choose.

It's impossible for a lens to alter the relative sizes of objects at different distances in the frame - only by moving yourself, either backwards or forwards, can you achieve this.

I understand in a sort of way but I think I will stick to just taking photo's and if I like what I see in the viewfinder, I will take a picture and leave this side of things for people younger with fresh brains and who can understand.

Well said Christine. I wish more people would think the same.

Duncan

I can't argue with that - in post #16 of this thread I stated that you can check it with a zoom lens in two seconds yet I've still managed to let myself be dragged into the debate! I don't think I qualify for the 'younger' bit, though ;)