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Perspective and Focal Lengths
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PostPosted: Thu Nov 06, 2008 5:51 pm    Post subject: Perspective and Focal Lengths Reply with quote

Messages copied and pasted from another thread:


PostPosted: Thu Nov 06, 2008 5:51 pm    Post subject: Reply with quote

Rob Leslie wrote:
The Tamron Adaptall 2 34mm is a very good lens.
The perspective on these shots does like it was the 50mm lens, 24 would give more separation apparent distance/depth
I'm sure you could test this with a couple of shots from the 24 and 50 and maybe post them to prove me wrong!

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sichko wrote:
I agree with Rob. My first "impression" was that the pics were taken with a 35mm. But then I've got a crop camera and the pics are on film - so the impression, such as it is, fits.

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peterqd wrote:
After being corrected by Rick a long while ago, and still being sceptical, I checked out this perspective thing. I don't have the pics any longer, but I took two shots looking along my road. The camera was about 5m from a parked car and there was another car parked about 100m further on. The first shot, using a 135mm lens showed the classic distance-shortening perspective of tele lenses and the second, with a 20mm, made the distant car look much further away.

However, when I cropped the wide-angle shot to the same limits as the tele shot, they did indeed become identical. It's an optical illusion.

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Rob Leslie wrote:
peterqd wrote:
After being corrected by Rick a long while ago, and still being sceptical, I checked out this perspective thing. I don't have the pics any longer, but I took two shots looking along my road. The camera was about 5m from a parked car and there was another car parked about 100m further on. The first shot, using a 135mm lens showed the classic distance-shortening perspective of tele lenses and the second, with a 20mm, made the distant car look much further away.

However, when I cropped the wide-angle shot to the same limits as the tele shot, they did indeed become identical. It's an optical illusion.

There are few people who have this illusion; I have never experienced it myself.
As this article explains Perspective is about subject to camera/viewing distance. Even without a lens and a camera one can see Perspective change as we change our viewing distance.

Below I quote from the experts and Link to the full article
http://hobbymaker.narod.ru/English/Articles/perspective_eng.htm

Many people are firmly convinced that all lenses reproduce perspective in the same way. Their reasoning is simple. They suggest taking several pictures with the help of various lenses while standing on the same spot. If we allow for the natural differences in content, we will see that perspective will be practically the same in all such photos. With the help of the same arguments, they demonstrate that perspective cannot depend on the size of the film frame (or digital sensor). If we simply cut out the central part of the frame, why should its perspective change? they ask. But when saying this, they just demonstrate that they do not understand the kernel of the problem. Both the focal distance of a lens and the size of the film frame affect the perspective through the same mechanism, namely through the change of the distance between the camera and the foreground. To see the changes in perspective, a photographer has to move closer to (or farther from) the foreground in order to allow the foreground to fill the same share of the image. When we say that different lenses show perspective differently, we imply that we have to change the distance between the camera and foreground to make an adjustment for the changed viewing angle. Only under such a condition, perspective will be completely determined by the viewing angle of the lens plus frame system.

The simple practical implications of this are few people use a wide lens and then just crop the centre and it doesnt work anyway. Try cropping the centre of a normal distance for the lens Zenitar shot and tell me its the same Perspective as a 35-50mm lens! This is why when we want to portray a different Perspective we change lenses but we are always aware that the subject matter further away will appear even further away.
I know and Im sure you do that you could not post a normal landscape type photo taken with a 200mm lens and claim it was taken with a 21mm lens! Everybody would see something odd!

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peterqd wrote:
Rob Leslie wrote:
Try cropping the centre of a normal distance for the lens Zenitar shot and tell me its the same Perspective as a 35-50mm lens! This is why when we want to portray a different Perspective we change lenses but we are always aware that the subject matter further away will appear even further away.
I know and Im sure you do that you could not post a normal landscape type photo taken with a 200mm lens and claim it was taken with a 21mm lens! Everybody would see something odd!

Here it is - I found my test pic on a CD! Smile

If I hadn't labelled each shot I can't see any difference to indicate the FL of each lens used. Try it for yourself, please!


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sichko wrote:
Peter - After being confused, for a short while, by the extra white van in the second shot, I've looked long and hard. And I can't see any difference. My earlier comments were based on what I might expect to see from an uncropped picture at a particular FL. However your experiment is very interesting. I think I need to try this myself - so that I can relate the pictures to a familar situation - and so that I can look at them before cropping.

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peterqd wrote:
sichko wrote:
Peter - After being confused, for a short while, by the extra white van in the second shot, I've looked long and hard. And I can't see any difference. My earlier comments were based on what I might expect to see from an uncropped picture at a particular FL. However your experiment is very interesting. I think I need to try this myself - so that I can relate the pictures to a familar situation - and so that I can look at them before cropping.

I used to think the same John. When you see a shot with a tele lens, especially from an elevated position, you get a strong sense that distances are foreshortened, but it really is an optical illusion. Rick Oleson asked me to test it for myself and I was really surprised to find I was wrong.

Sorry about the white van - it appeared while I was changing lenses! Smile Here's the uncropped pic:


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Rob Leslie wrote:
sichko wrote:
Peter - After being confused, for a short while, by the extra white van in the second shot, I've looked long and hard. And I can't see any difference. My earlier comments were based on what I might expect to see from an uncropped picture at a particular FL. However your experiment is very interesting. I think I need to try this myself - so that I can relate the pictures to a familar situation - and so that I can look at them before cropping.

At present I am enjoying reproducing some early 1900s photos of local places. When doing Todays view I try to line up points to the left and right to get the same FOV as the old shot but this simply doesnt work as the perspective in the photos is always different so I also have to make choice of lens. It is usually easy, either being 28mm or 50mm (On crop DSLR) but the difference between the perspectives of those two lenses is easily seen and it cant be reproduced or changed by moving backwards or forwards, or making a crop. One can't reproduce the same FOV and perspective with any one lens.
If you wish to do a practical test of Perspective take a picture of a scene with a wide angle lens and then try to recreate that same picture with the same FOV with a more Normal FL lens. You will always end up with the wide angle image having the foreground objects larger/nearer.
If you still think it is wrong then by all means shot all you landscapes with the widest lens you have, or ignore the use of different FL for perspective control.
If you want a simple test of perspective look at you monitor from 12 inches away and then look at it from 4 foot away. Isn't it small. Yes I know that is to do with the distance to the subject but that is what Perspective is all about
Using perspective is one of our best creative tools so it is worth fully understanding it. Art books have good explanations which apply to lenses as well as our eyes and a paint brush.
You are fooling yourself by making a centre of the frame crop and missing the point of Perspective completly. Anybody can make a point by cheating it.
Please follow the link I bothered to post, for a full detailed and simple explanation

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Rob Leslie wrote:
Practical vs Tests, opinions and theory. You don't have to make tests just take photos.

I took both of these shots only yesterday. So they are handy.

50mm on crop DSLR Full Frame


The shot I want from the above cropped etc


24mm on DSLR Full frame. The view is covering roughly the same area


The question is Can I pass the crop 50mm shot off as being taken with 24 or even a 35mm lens?
If you think I can with all respect you should find another hobby!

Which brings me back to the original post. Do I think those cityscapes are 24mm or 50mm?

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peterqd wrote:
Rob, I'm not going to reply to you here because:

a This is spiralcity's thread about his Tamron 24mm

b Your two last messages are probably the most condescending I've ever read. I take exception to your overbearing manner and at the moment I don't think I'm in the right frame of mind to reply calmly.

Later when I have time I am going to try to move the off-topic messages in this thread to their own new thread and reply to you there, by which time I hope you will have given some thought to how you might be more respectful and more receptive to other people's viewpoints.

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Rob Leslie wrote:
Sorry you feel that way it was not my intention to upset anybody but I admit I often do and I'm not bothered about agreeing with somebody just for the sake of it, one never learns anything like that and I once again have learned something.
I always have a respect for I am receptive to other people's viewpoints. In fact I welcome it everyday. Most people know far more than me about something and more important they can often give me food for thought or encourage me to try something.

Now surprise You are right and I had hopped somebody would give the explanation to the different View points Your test and post is actually the facts and mine is a viewpoint based on practical use.
Im amazed nobody hasnt already picked up on it.

I shall post because Im sure some people want some sort of conclusion and also I do think it has a bearing on the original post and the question were they taken with a 24mm lens

There are two FACTS at play here (Not viewpoints or opinions)

Fact 1 All lenses do in fact have the same DOF and 'Apparent' perspective irrelevant of their FL.

Fact 2 Perspective is relative to the position of the camera lens but is unaffected by the focal length. The focal length has no effect on the perspective and only determines the size of the image. No matter what size camera/sensor we use, we just have to position the lens in the same place, perspective only changes when we move relative to the subject.

At first glance that is very clear but in practice is it?
Perspective is relative to the position of the camera lens? And Perspective only determines the size of the image. Yet FL has no affect on Perspective?

I must admit these do seem to be almost conflicting facts.
If perspective is unaffected by FL and in Practical usage the choice of FL determines our position and the size of our FOV which one do we take any notice of.

I cant change the Facts. But do any of us go around taking photos with one lens at one regular shooting distance and then making heavy centre crops to give us the perspective we wanted?

IMO both facts should be studied and consider and although they are Facts (Laws of optics) we have to consider the practical everyday implications. Do we not constantly change our subject to camera distance, or do we stay in one place and just crop the picture to suit?

To sum up Perspective is not an optical quality it is a product of the position of the camera relative to the subject. Simple a near object appears closer/bigger.
What is a Near object is the crux of the question. If we change FL we often in practice also change what is near and far.
The link I first posted explains it better.
We have lots of FACTS that can't be changed, but they can always be looked at in a practical way and different conclusions arrived at.
For example try telling a 35mm FF user a 50mm lens or any other lens has the same DOF on his camera as it does on ANY crop frame one.
Yes as a FACT it does, but in practical use?

Again my apologies for upsetting you but I always thought forums were for thrashing out and hopefully resolving tricky topics.

P S The original post is in Manual lenses and the Topic is 24mm Tamron. Do correct me if I am wrong but doesnt that mean it was posted to discuss the qualities of the lens? Isnt the perspective of a 24mm lens (mistakenly ! ) one of the properties some of use one for?


PostPosted: Thu Nov 06, 2008 6:33 pm    Post subject: Reply with quote

Rob Leslie wrote:
Sorry you feel that way it was not my intention to upset anybody but I admit I often do and I'm not bothered about agreeing with somebody just for the sake of it, one never learns anything like that and I once again have learned something.
I always have a respect for I am receptive to other people's viewpoints. In fact I welcome it everyday. Most people know far more than me about something and more important they can often give me food for thought or encourage me to try something.


Rob, apology accepted. I wasn't expecting you to agree with me "just for the sake of it." But if one disagrees with something there are far more polite ways of saying it. And, without any knowledge of my experience in working with perspective, you absolutely rubbished what I said. I won't elaborate, but try making a perspective sketch of a new building in its existing surroundings to present to a client (without photographs to help) and you'll have some idea. If you've learnt something today as you say, I hope it includes giving a little more respect to other people, even if you know for certain they are wrong.

You found the flaw in your argument. We were discussing whether one can tell from the perspective of a single photograph what focal length lens was used. I think we now agree you can't. Of course you will see a difference in perspective if you move your viewpoint and change lenses to match the field of view, but as you say, it is the new position, not the lens, that makes the difference. But then you'll have two pictures from different viewpoints to compare, not one. The scene in front of a camera has no more perspective than a flat canvas, and different focal length lenses just change the field of view.


PostPosted: Thu Nov 06, 2008 7:28 pm    Post subject: Reply with quote

At the risk of starting this all up again I have to point out that I am not agreeing that you cant Have a good idea of the FL of the lens by the photo. With many photos given the format one can.
Even looking at a painting one can see (If the artist is good) a relative photo lens FL. A point that has been studied with Art that is suspected of being created with the camera obscure.
I posted my example shots and asked the question could I pass the crop shot of as being taken with a 24mm?
IMO I could not. Although I havent tested the idea I doubt I could even pass it off as a heavily cropped 24mm lens shot.
The perspective is wrong!
Why is it wrong?
If the shot would have been done with a 24mm the distance would have been half and still require the same crop. The change of shooting distance would drastically change the perspective of the benches and if the shot would have been taken with 24mm at the same distance as my original 50mm shot it would be unusable Of course if the shot would have been taken to fill the frame with a 24mm that perspective would be even more pronounced. This is why I made the point about the facts being almost conflicting and the need to translate them into real world practical usages.

Again I point anybody interested in this to follow my original link which
I quote from
If we simply cut out the central part of the frame, why should its perspective change? they ask. But when saying this, they just demonstrate that they do not understand the kernel of the problem.

Many of use buy and use wide lenses especially ultra wide ones for the perspective rather than the wide FOV. Of course the perspective is a result of the wide FOV in respect that it shortens the viewing distance to foreground objects.
How many here only buy wide and ultra wide just to fit more in?


PostPosted: Thu Nov 06, 2008 8:34 pm    Post subject: Reply with quote

another sample like Peter test pic
on tripod
flek 20mm shot

85mm shot

crop of the above 20mm to emulate the 85mm


with a wide you can 'emulate' a zoom by cropping
with a zoom you can 'emulate' a wide by stitching


PostPosted: Thu Nov 06, 2008 8:50 pm    Post subject: Reply with quote

Wow, what a great place. How old is the castle?


PostPosted: Thu Nov 06, 2008 9:04 pm    Post subject: Reply with quote

Peter wrote:
How old is the castle?

the acropole of Lindos that you see on the top right of the castle is from IV bc
the walls have been made XIII ac


PostPosted: Thu Nov 06, 2008 9:19 pm    Post subject: Reply with quote

I have to admit, gentlemen, that my brain is smouldering now.

I have learned that a crop of a wide angel shot shows the same picture like a tele shot. Hedgecoe explains this well in his books.
But I think there are diverging definitions of the term "perspective" in use which triggers some confusion. Confused


PostPosted: Thu Nov 06, 2008 9:41 pm    Post subject: Reply with quote

Thanks poilu - that's a great demonstration.


PostPosted: Fri Nov 07, 2008 12:47 am    Post subject: Reply with quote

Is there anybody here who uses a wide angle lens for their classical head and shoulder portrait shots?
Why not if nobody can tell the difference between the perspectives of shots done with different FL, or do they crop their portrait shots to get the perspective right!
Is there anybody here who has ever used a wide lens and cropped more than half the frame to change the perspective? But why crop if there is no difference in perspective between lenses used in a Normal (Whatever that is) everyday shooting situation.
Everybody can clearly see the different perspective of the FF poilu shot. When that perspective is cropped out of course it isnt there anymore, but is that how we take our photos?
What would happen if there was no distant subject. What if the entire frame was relatively near objects say between 6 and 20 feet away, I dont think that would be an unusual everyday shot. Would the crop trick work then?
Has anybody followed the link and read the expert opinion? Laughing


PostPosted: Fri Nov 07, 2008 7:38 am    Post subject: Reply with quote

Rob Leslie wrote:
Is there anybody here who uses a wide angle lens for their classical head and shoulder portrait shots?


It's only a matter of distance between the focal plane and the subject. Many "environmental portraits" by famous photographers were taken with 35mm or even 24mm lenses. If the camera is less than 1.5m from the subject, then some stretching will appear (what is often incorrectly called "wide angle distortion"). This effect can add some "punch" to the pictures and it doesn't only happens with wide angles. Here's a picture of my son taken at short distance with my Helios-40 (85mm lens, equivalent to a 130mm when mounted on a crop format camera):



Cheers!

Abbazz


PostPosted: Fri Nov 07, 2008 9:28 am    Post subject: Reply with quote

To put it in easy words:

The perspective is influenced by the distance to an object, not by FL or aperture.

The DoF is influenced by the distance to an object and the aperture, not the FL. Although in everyday use the FL "influences" the DoF because of the way we shoot and compose with different FLs.


PostPosted: Fri Nov 07, 2008 9:30 am    Post subject: Reply with quote

Rob Leslie wrote:

Everybody can clearly see the different perspective of the FF poilu shot. When that perspective is cropped out of course it isnt there anymore, but is that how we take our photos?


Perspective can't be cropped. It's the field of view that is cropped.

Perspective has nothing to do with focal length. Actually it has nothing to do with lenses or cameras. Your perspective will always be exactly from where you stand in relation to objects you see. Changing the focal lenght doesn't change that Laughing


PostPosted: Fri Nov 07, 2008 10:00 am    Post subject: Reply with quote

Riku wrote:
Rob Leslie wrote:

Everybody can clearly see the different perspective of the FF poilu shot. When that perspective is cropped out of course it isnt there anymore, but is that how we take our photos?


Perspective can't be cropped. It's the field of view that is cropped.

Perspective has nothing to do with focal length. Actually it has nothing to do with lenses or cameras. Your perspective will always be exactly from where you stand in relation to objects you see. Changing the focal lenght doesn't change that Laughing


It's just change "how much" (alas angle) you can capture of that perspective.


PostPosted: Fri Nov 07, 2008 11:33 am    Post subject: Reply with quote

LucisPictor wrote:

The perspective is influenced by the distance to an object, not by FL or aperture.


Right.

LucisPictor wrote:

The DoF is influenced by the distance to an object and the aperture, not the FL. Although in everyday use the FL "influences" the DoF because of the way we shoot and compose with different FLs.


At that point it becomes a little more complex. The aperture number is the relation of aperture diameter to focal length. However, the circle of confusion and depth of field don't depend on that ratio, but on the absolute aperture diameter - which is smaller on a shorter lens. So there is indeed a difference in DoF between a picture taken with a 35mm lens cropped to 28 angle of view and one taken with a 85mm lens.


PostPosted: Fri Nov 07, 2008 12:23 pm    Post subject: Reply with quote

Sevo wrote:
So there is indeed a difference in DoF between a picture taken with a 35mm lens cropped to 28 angle of view and one taken with a 85mm lens

True, it is the reason why p&s have more dof; the smaller sensor is like a crop

Another interesting fact about dof
LL wrote:
In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field

push here to read more about


PostPosted: Fri Nov 07, 2008 1:09 pm    Post subject: Reply with quote

LucisPictor wrote:
To put it in easy words:

The perspective is influenced by the distance to an object, not by FL or aperture.

The DoF is influenced by the distance to an object and the aperture, not the FL. Although in everyday use the FL "influences" the DoF because of the way we shoot and compose with different FLs.

Please correct me if I'm wrong......

I always understood that DOF was a combination of aperture, distance from subject AND focal length. Whereas the latter is not strictly true.

In real usage, focal length and FF/film v crop sensor may cause the photographer to change his distance to subject. Therefore it is this that is responsible for the difference in DOF (if not corrected by stopping down).

I hope that made sense.


PostPosted: Fri Nov 07, 2008 2:57 pm    Post subject: Reply with quote

First of all I have to say that this is the first time I have ever seen a heated technical argument on this forum, and this was resolved peacefully. This is high praise indeed, I can mention several other forums where this just could not have happened Smile Stand proud, all of you. Sometimes (a lot of times, really) people take stuff (and themselves) to seriously. Embarassed This of course happens to me too, but I try to impose a strict line between facts and opinion for myself, and I try to be as open as possible for other ways of expressing the same thing. (Other ways than mine... that is) The important thing is the general understanding, not how we express ourselves.

Now... Martin - as I understand it, and this answer is mainly a way how to express things: Smile

FL only influences APPARENT DoF. When I say apparent, I mean that the definition of ABSOLUTE DoF is as in:
"The rubber duck that's 2m behind the garden gnome statue (that is on the plane of focus) is affected by a blur radius that is exactly as big at it's own body length".
Using the same aperture, the absolute defocusing amount of the duck will always be the same regardless of what focal length you use - the blur radius will be the same as the duck's own body length.

BUT! - Regarding APPARENT DoF : in a 28mm lens the "blur radius" will be almost three times smaller IN THE PICTURE (or, you can also express this as in "amount of pixels wide" if you shoot digital) than if you used an 85mm lens. (the 85mm has a viewing angle of 24 horisontally, almost three times narrower than the 28mm's 65)

This is my way of explaining or visualizing this, I might be wrong or unclear, take it for what it is... Smile[/b]


PostPosted: Mon Nov 17, 2008 11:36 am    Post subject: Reply with quote

Sorry to drag this up again, but I'm trying to grasp which is correct. I looked at the link that Attila posted which gives examples of different focal lengths showing the same DOF.

However, here http://www.cambridgeincolour.com/tutorials/depth-of-field.htm they have a DOF calculator (scroll down a bit) which shows DOF changing as either FL or sensor size changes. Are we saying that this is incorrect?

Thanks


PostPosted: Mon Nov 17, 2008 12:25 pm    Post subject: Reply with quote

Attila is wrong, and that site is right. The focal length enters the equation to the square, while all other parameters (f stop, diagonal) are linear or constant.

The formula for calculating the hyperfocal distance (the easiest calculation, but the same goes for all others) for a constant angular circle of confusion is:

f/(a * (d/c)) - with f the focus length, a the aperture, d the diagonal and c the angular factor for the circle of confusion (usually assumed as 1500, this being the resolution limit of the average human eye).

This gives a hyperfocal distance of around 64m at f/2 for a normal lens (f=d) at 6x6cm, and 33.25m at f/2 for 24x36mm.

Sevo


PostPosted: Mon Nov 17, 2008 2:20 pm    Post subject: Reply with quote

Oh.. I missed this whole thread.. and still dont have a time to go through each and every line of this great thread..
But I will try to touch bot issues,

For both the point, Perspective and DoF... You need to consider 2 things as base:
1) Using the same sensor size
2) The main Subject size will remain same.

In all the examples, we have discussed, although the sensor size remained same, but not the subject size.
The point is, you dont capture the scene by reading the focal length of the lens but try to capture the subject.

Lets take a case of portrait. You will always try to capture head-shoulder shot (a classical example, nothing hard and fast). With 200mm focal length, you move few meters backwards and its gives the illusion of compression (different perspective). For 20mm focal length, you come near (really near) to subject. and this created bulging effect and different perspective. Nice britannica link,
http://www.britannica.com/EBchecked/topic-art/457963/3120/Effects-of-using-lenses-of-different-focal-lengths

Some other good discussions,
http://photography-on-the.net/forum/showthread.php?p=6648466
http://reviews.davidleetong.com/?p=531

Same with DoF, if you keep the subject size same, the DoF doesnt change with Focal length. For the above discussed portrait example, if you will keep same camera sensor (size) and head-shoulder subject size... with same f-stop, the DoF will remain same. Some nice links on this subject,

http://www.luminous-landscape.com/tutorials/dof2.shtml

And for geeks, (mathematically can be proved),

http://www.normankoren.com/Tutorials/MTF6.html (my fav site... My gateway to digital photography... root of my Picture Window pro usage... Wink )

And about all the posted example here in this thread, when you crop the pic, you changes the sensor/film size. That why you are not seeing the effect of perspective and DoF differences....

( just found, poilu already posted one of the link... I think its will be oK to refresh the stuff Wink)


PostPosted: Mon Nov 17, 2008 2:36 pm    Post subject: Reply with quote

Quote:
Same with DoF, if you keep the subject size same, the DoF doesnt change with Focal length.


Yes, that is the inverse consequence - on the same image area and with the same magnification of the object at the focal plane, the relation of the focus distances is proportional to the relation of focal lengths, and the change drops out of the main DOF equation. There is a difference regarding the front and rear DOF zone (a shorter focal length gives more DOF towards infinity, and less towards the front, from the plane of perfect focus), but as far as the total distance covered by the zone in DOF is concerned, they are equal.