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Author Topic: progessive stretch  (Read 13903 times)
peter.s.
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« Reply #40 on: May 19, 2010, 06:21:57 PM »
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Hi Marc,

I have done the "progressive stretching" or shrinking on quite a few wide angle shots in the past. Not AP shots but mainly shots involving people where you can't back up anymore and are deliberately shooting with a too wide lens to be able capture the entire scene. As everyone know people will be progressively stretched out as the a closer to frame.

After thinking long and hard about it I came to the conclusion that the filter > distort > spherize can do the same thing better and with less work.

The process for a AP shot like the ones you have above would be to:

1) do the perspective correction and "regular" stretching.

2) increase the size of the canvas to 200% with an empty area either on top or below but not both. You can do it interactively with the crop tool or by doing image > canvas size and then setting the height to 200% and clicking on one of the up or down arrows.

3) Go to filter > distort > spherize and set mode to "vertical only". Adjust the preview size and change the amount to taste.

4) Remove the extra canvas again with the crop tool or image > canvas size set to 50%.

Regards,
Peter

PS. I use CS2 so newer versions might have other options as well.
« Last Edit: May 19, 2010, 06:26:19 PM by peter.s. » Logged
KevinA
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« Reply #41 on: May 20, 2010, 06:31:57 AM »
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This is an effect that has always been there and I can remember at College being told it's sometimes better to under correct to keep it natural at the top. I also thought it unnatural to stand so close to a tall building and expect it to look straight. I assumed that a camera and lens worked like my eyeball does. Well it just went outside to catch some sun and was thinking about the points raised here. I looked at my house, I tilted my head up down and around, I expected to see the house change shape just like it would if I looked through a camera  and moved it up and down. I've not done this before with my eye to look for changes, I was surprised to see no change in verticals or shape to the building, it was more like moving an enlarged image on screen around, I've never noticed that before. I did try it with a camera and the effect is completely different, I'm not sure why that should be, is it a brain or an optical design thing?

Kevin.
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BartvanderWolf
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« Reply #42 on: May 20, 2010, 07:35:39 AM »
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Quote from: KevinA
I did try it with a camera and the effect is completely different, I'm not sure why that should be, is it a brain or an optical design thing?

Hi Kevin,

A camera uses a so-called rectilinear projection. It projects the optical image on a flat plane. At extreme projection angles this will cause anamorphic distortion (think about text/numbers painted on a road). The distortion will not be noticeable when viewed from the correct position.

Cheers,
Bart
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KevinA
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« Reply #43 on: May 20, 2010, 07:46:27 AM »
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Quote from: BartvanderWolf
Hi Kevin,

A camera uses a so-called rectilinear projection. It projects the optical image on a flat plane. At extreme projection angles this will cause anamorphic distortion (think about text/numbers painted on a road). The distortion will not be noticeable when viewed from the correct position.

Cheers,
Bart

I assumed my eyeball worked the same way, obviously not. Strange after 40 years looking through a camera you think I would of noticed the difference.

Thanks,

Kevin.
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Kevin.
Blendenteufel
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« Reply #44 on: May 31, 2010, 02:39:58 AM »
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Hi,

maybe the topic is already considered closed, but in case anybody is interested:

I am not sure what algorithm DXO uses but it might be worth a try to use it for architectural shots, if the elimination of anamorphic distortion is what you need. You can run a trial and import tiff, jpg or dng files afaik

http://www.dxo.com/us/photo/dxo_optics_pro...ns/anamorphosis

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Kirk Gittings
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« Reply #45 on: May 31, 2010, 12:10:24 PM »
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Quote from: Blendenteufel
Hi,

maybe the topic is already considered closed, but in case anybody is interested:

I am not sure what algorithm DXO uses but it might be worth a try to use it for architectural shots, if the elimination of anamorphic distortion is what you need. You can run a trial and import tiff, jpg or dng files afaik

http://www.dxo.com/us/photo/dxo_optics_pro...ns/anamorphosis

Unfortunately, I think with serious AP we are talking about distortion in combination with movements (shift, rise, fall etc.) and DXO's lens correction profiles, via discussions I had with them last year, does not deal with either T/S lenses or shifted MF lenses.
« Last Edit: May 31, 2010, 12:14:34 PM by Kirk Gittings » Logged

Thanks,
Kirk

Kirk Gittings
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JeffKohn
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« Reply #46 on: June 06, 2010, 12:18:50 PM »
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Quote from: KevinA
I assumed my eyeball worked the same way, obviously not. Strange after 40 years looking through a camera you think I would of noticed the difference.
The image is not projected onto a flat surface in the case of the eyeball, the retina is concave.

That said, the keystoning effect does happen with human eye sight, just to a much lesser degree compared to a camera. You don't see the convergence when looking at your house, but if you stand at the base of 30-story building you'll definitely see it (but it still won't be as severe as when looking through a camera viewfinder with a wideangle lens mounted).
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JeffKohn
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« Reply #47 on: June 06, 2010, 12:20:05 PM »
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Quote from: Kirk Gittings
Unfortunately, I think with serious AP we are talking about distortion in combination with movements (shift, rise, fall etc.) and DXO's lens correction profiles, via discussions I had with them last year, does not deal with either T/S lenses or shifted MF lenses.
PTLens can do it. It's kind of klunky, and not 100% precise but it does work. They have PS actions that will expand the canvas to the size of the full image circle. Then you drag your photo to the correct position on the canvas and it does the correction.
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Abdulrahman Aljabri
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« Reply #48 on: July 30, 2010, 11:27:08 AM »
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Quote from: marc gerritsen
hi jim
i uploaded a rudementary iphone movie of a photo i worked on
chose a different photo then the other examples as this one can really demonstrate that the "progressive stretch" actually works
i realized i can not use it very much in architecture but for interiors it works wonders
again this is only to describe the concept and anyone can apply their own ways to it
cheers
marc

https://download.yousendit.com/bFFNbGtESEJreEJFQlE9PQ


Thanks Marc for this great topic. This link is not working anymore, is the video available anywhere else?
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David Klepacki
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« Reply #49 on: July 30, 2010, 09:34:07 PM »
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Quote from: KevinA
I assumed my eyeball worked the same way, obviously not. Strange after 40 years looking through a camera you think I would of noticed the difference.

While it is true that the eye's cornea is comparable to a lens with the pupil acting as a diaphragm, the human eye works VERY differently in all other respects.

Only about 0.02% of the human retina is capable of acute resolvable vision.  This area of the retina is called the macula lutea.  You eye muscles basically move your eyeball rapidly across your field of view to focus portions of the object of interest onto this tiny area of the retina.  Your brain then pieces this retinal information together to give you the perception of a single larger image.  During the brain's reconstruction process, it accounts for the "optical distortion" in a very complex manner, unlike any camera photographic process.

If you had to make a comparison to a camera, I guess it would be closer to doing a huge amount of  complex stitching with a very sophisticated tilt-shift lens.
« Last Edit: July 30, 2010, 09:35:55 PM by David Klepacki » Logged
OldRoy
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« Reply #50 on: July 31, 2010, 11:03:59 AM »
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An extremely interesting thread.

I am a relative amateur - although I earn modestly from time to time shooting (mostly) interior VR panos for websites. In addition I usually shoot some conventional solo shots, usually interiors, for my clients - frequently using the Nikon 14-24 mm 2.8 at its widest. So the issues discussed here are familiar to me, even though up to now I have only looked longingly at T/S lenses, much less view cameras. What I'd like to ask, since the contributors here show such a high level of expertise, concerns the Nikkor 14-24 although it obviously applies to other rectilinear wide-angle lenses. Perspective distortion aside, the peripheral stretching seen when using the wide end of this lens has always bothered me, although just how much it bothers me is determined by the nature of the subject matter.

At one point I downloaded an evaluation copy of the DxO software which offers volume anamorphosis correction:
http://www.dxo.com/us/photo/dxo_optics_pro...ns/anamorphosis
which seems to me to be an extremely useful feature. I have to confess that I have not purchased a copy of the application as in most other respects I'm quite happy using NX2 (bugs and eccentric UI notwithstanding.)

So my question is, how to effect the same correction of a single shot? I use PTGui quite a lot for both VR and other stitching projections and it seems possible that it may be useable for this purpose? I've managed to get reasonable results using "distortion torture" in PS but it feels very haphazard.

Apologies if this is a little off-topic. Another apology - I hadn't read the immediately preceding posts.

Roy
« Last Edit: July 31, 2010, 11:07:34 AM by OldRoy » Logged
Ray
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« Reply #51 on: July 31, 2010, 11:18:52 AM »
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Marc, this is an interesting topic that deserves more analysis and discussion. I would like to see some examples of the same scene shot from the same position using both a wide-angle normal lens, and a less wide-angle shift lens, after the normal shot has been adjusted and corrected.

The 'free transform' controls in Photoshop allow for a variety of different types of perspective-type adjustments, including perspective, distort, and warp, which are the ones I use most.

What looks best is a matter of taste.

I recently returned from a major holiday in Europe and Russia where I took lots of photos of churches and various buildings at 14mm, using the Nikkor 14-24 on my D700. When attempting to correct what is clear to my eyes is an abnormal distortion, I encounter all the problems mentioned in this thread.

I simply don't agree that an image of any tall architectural structure, taken from a close distance (on the ground) with a wide angle lens, looks natural. Some correction is necessary.

Below is a shot I took in Cologne, of the Gross St Martin. I show the full, uncorrected image first, followed by the corrected image using free transform (perspective, distort and warp). I prefer the corrected version.

Does anyone prefer the uncorrected version? If so, why?

[attachment=23425:Great_St...d_6_0871.jpg]      [attachment=23426:Great_St...d_6_0871.jpg]
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David Klepacki
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« Reply #52 on: July 31, 2010, 01:02:43 PM »
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Quote from: Ray
I simply don't agree that an image of any tall architectural structure, taken from a close distance (on the ground) with a wide angle lens, looks natural. Some correction is necessary.
Exactly right.  As I mentioned above, it can never look "natural" as the eye/brain is unable to reconcile a wide angle perspective.  No amount of correction can fully account for this.

Quote from: Ray
Below is a shot I took in Cologne, of the Gross St Martin. I show the full, uncorrected image first, followed by the corrected image using free transform (perspective, distort and warp). I prefer the corrected version.
The corrected version is less offensive to the eye, but it still has serious discrepancies making it difficult for the brain to reconcile the reality of the image.  For example, the apparent size of the fountain in the foreground in relation to the buildings in the background at first suggests that the fountain must be approximately one story high and having the width close to that of one of the buildings.  Yet, we know that it cannot be this large based on the proximity of the ground in the image and our experience with the size of actual buildings.  In addition, the building on the right still looks like it is falling backwards, while the top of the building on the far left appears to be bending to the right.  All of this is very disturbing to the eye, since it is not at all what we encounter when we simply look at things.
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JonathanBenoit
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« Reply #53 on: July 31, 2010, 01:33:26 PM »
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Quote from: David Klepacki
In addition, the building on the right still looks like it is falling backwards, while the top of the building on the far left appears to be bending to the right.  All of this is very disturbing to the eye, since it is not at all what we encounter when we simply look at things.

The picture on the right still isn't properly corrected, this is probably why you think it looks like it is still falling backwards.
The waterwall looks larger because it is closer to the lens.
« Last Edit: July 31, 2010, 01:35:16 PM by JonathanBenoit » Logged

David Klepacki
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« Reply #54 on: July 31, 2010, 05:00:49 PM »
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Quote from: JonathanBenoit
The waterwall looks larger because it is closer to the lens.
That's precisely my point of what makes the image look unnatural in comparison to one's visual perception.  The near-far relationships of objects in a wide-angle photograph do not correlate with how the eye renders the same imagery directly.   The more planar the content in the photograph, the less objectionable this becomes (in the sense of creating "natural looking" images).  I do not mean to imply that wide angle perspective is in any way inferior, as it does provide a unique way of "seeing" things that our eyes cannot do on their own; and, of course, many photographers enjoy working with these lenses for this reason.  
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Diapositivo
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« Reply #55 on: July 31, 2010, 05:15:03 PM »
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Quote from: Ray
Does anyone prefer the uncorrected version? If so, why?

I prefer, between the two, the uncorrected version, the reason being that in the corrected version the tower looks much "fatter" than it is in the original and, I do presume, in reality. Trying to correct keystoning in software in my limited experience stretches and deformates objects by making them lower and fatter.

So after having done what you have done to the image, one might apply a second-pass of deformation by re-stretching again the image so that the tower looks again having the same height, and the same height-width proportions, as in the original image. But by then one will have applied two rounds of stretching, quality will certainly have suffered somehow, and the final result might look not so good anyway.

Generally speaking I tend to try to correct perspective only when there is a tall element (such as this tower) at one side of the image, clumsily converging toward the "empty" centre of the image. When the tall element is in the center of the picture I often find that even with a 24mm equivalent the converging lines are good-looking and overall don't give an unnatural rendering of the object matter. Wide-angle lenses can generate images which are up to a certain extent unnatural, but the more wide angle become widespread the more the casual observer will get a trained eye for that and will consider the "distortion" as a natural characteristic of the medium, a bit like black-and-white pictures, that don't exist in reality but are not perceived as "unnatural".

As a final consideration I would add that even when using a PC lens I normally don't correct keystoning completely, as IMO leaving some convergence of lines makes the picture more natural, while making parallel those lines that, in the reality of human vision, are not parallel would make the picture, in general, slightly unnatural.

But overall I tend to leave a lot of this kind of images totally "uncorrected".

Cheers
Fabrizio
« Last Edit: July 31, 2010, 05:15:34 PM by Diapositivo » Logged
JonathanBenoit
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« Reply #56 on: July 31, 2010, 05:50:18 PM »
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Quote from: Diapositivo
As a final consideration I would add that even when using a PC lens I normally don't correct keystoning completely, as IMO leaving some convergence of lines makes the picture more natural, while making parallel those lines that, in the reality of human vision, are not parallel would make the picture, in general, slightly unnatural.

But overall I tend to leave a lot of this kind of images totally "uncorrected".

Cheers
Fabrizio

Are you saying that a building doesn't have straight sides? When I look at a building I see the sides as parallel to each other. Even if my eyes showed convergence, in reality we know that the sides are parallel. It's the job of a photographer to capture reality - or at least it's the architectural photographer's job.
« Last Edit: July 31, 2010, 05:50:59 PM by JonathanBenoit » Logged

craigwashburn
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« Reply #57 on: July 31, 2010, 09:54:09 PM »
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Quote from: Diapositivo
I prefer, between the two, the uncorrected version, the reason being that in the corrected version the tower looks much "fatter" than it is in the original and, I do presume, in reality. Trying to correct keystoning in software in my limited experience stretches and deformates objects by making them lower and fatter.

So after having done what you have done to the image, one might apply a second-pass of deformation by re-stretching again the image so that the tower looks again having the same height, and the same height-width proportions, as in the original image. But by then one will have applied two rounds of stretching, quality will certainly have suffered somehow, and the final result might look not so good anyway.

Generally speaking I tend to try to correct perspective only when there is a tall element (such as this tower) at one side of the image, clumsily converging toward the "empty" centre of the image. When the tall element is in the center of the picture I often find that even with a 24mm equivalent the converging lines are good-looking and overall don't give an unnatural rendering of the object matter. Wide-angle lenses can generate images which are up to a certain extent unnatural, but the more wide angle become widespread the more the casual observer will get a trained eye for that and will consider the "distortion" as a natural characteristic of the medium, a bit like black-and-white pictures, that don't exist in reality but are not perceived as "unnatural".

As a final consideration I would add that even when using a PC lens I normally don't correct keystoning completely, as IMO leaving some convergence of lines makes the picture more natural, while making parallel those lines that, in the reality of human vision, are not parallel would make the picture, in general, slightly unnatural.

But overall I tend to leave a lot of this kind of images totally "uncorrected".

Cheers
Fabrizio

The same effect will happen with a view camera if using a wide angle w/ movements, and there are objects nearby.

I used to have a book on scheimpflug published by Sinar that had a tilt degrees point past which you should consider *not* using movements (or using less, and tilting up some) because the result may be unnatural.  Human vision does start to keystone after a certain tilt of the head.  I wish I could remember the exact amount, but usually I notice it in the viewfinder anyway.

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Ray
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« Reply #58 on: August 01, 2010, 02:11:05 AM »
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Quote from: David Klepacki
Exactly right.  As I mentioned above, it can never look "natural" as the eye/brain is unable to reconcile a wide angle perspective.  No amount of correction can fully account for this.


The corrected version is less offensive to the eye, but it still has serious discrepancies making it difficult for the brain to reconcile the reality of the image.  For example, the apparent size of the fountain in the foreground in relation to the buildings in the background at first suggests that the fountain must be approximately one story high and having the width close to that of one of the buildings.  Yet, we know that it cannot be this large based on the proximity of the ground in the image and our experience with the size of actual buildings.  In addition, the building on the right still looks like it is falling backwards, while the top of the building on the far left appears to be bending to the right.  All of this is very disturbing to the eye, since it is not at all what we encounter when we simply look at things.

Quote
......the building on the right still looks like it is falling backwards, while the top of the building on the far left appears to be bending to the right.

Quite right. I agree. And that's what disturbs me about the image. It needs more work in this regard.

However, I'm puzzled by your following comment.

Quote
...the apparent size of the fountain in the foreground in relation to the buildings in the background at first suggests that the fountain must be approximately one story high and having the width close to that of one of the buildings. Yet, we know that it cannot be this large based on the proximity of the ground in the image and our experience with the size of actual buildings.

I don't see this size issue as a disturbing problem. The brain is well-used to assessing distances by the relative size of objects. When a large building appears small in relation to a small object that appears big, we assume that we are very close to the small object, and relatively far from the big object. So it is the case with the fountain and statues. The statues were one of the main features in the composition, if not the main feature, and I was very close indeed when I took the shot using a 14mm lens.

There is nothing in front of the fountain to cause confusion about its size and everything that is behind the fountain looks, to my eyes, appropriately diminished in size (approximately), indicating its greater distance from the viewer.

However, I agree there is something disturbing about the height (or perspective) of the fountain and that I believe is the result of an impression created by the parallel walls of the buildings, rather than their size. There's a sense that the viewer is about 6 metres high (that is, the shot was taken from across the street from a window about 6 metres off the  ground, in another building). Such an impression might be in conflict with the perspective one gets of the two statues on the fountain. If the shot was taken from a 6 metre height, then the statues must be very high off the ground because we are not getting a perspective of the top of their heads in the photo.

What I'd like to know is, if this effect can be avoided using a shift lens. I did consider buying the new Canon 17mm TSE for this latest trip, but decided against it on weight grounds. The D700 with 14-24/2.8 is heavy enough. The Canon 50D with EF-S 17-55/2.8 was a lighter option as a second camera than a 5D with 17mm TSE plus a 24-105/F4 zoom. I might have made a mistake, though.  

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Ray
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« Reply #59 on: August 01, 2010, 04:17:28 AM »
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One fascinating aspect of this issue in this thread, of how best to create a natural perspective in archetecture, is the other issue of the true focal length of the average human eye.

If one does a Google search on this question, one gets a variety of answers ranging from 17mm to 50mm. What the heck's going on here?

So I picked up my D700 with 14-24 attached, and 50D with 17-55 attached, and stood in the corner of my lounge, about 1.5 metres from my laptop, chair and small table.

The chair was the closest to the camera, and the front door, diagonally at the opposite corner of the room about 8 metres away.

I stared at the door, then raised the camera to my eye, several times at various focal legths using both cameras.

My impressions are, at 14mm (full frame 35mm) the view through the finder appears to approximate to my eye's angle of view including the extreme peripheral vision which has totally lousy resolution and which is only good for movement detection. In fact the peripheral vision through two eyes might be slightly greater than a 14mm angle of view, full frame DSLR, but I have no doubt which photo would be sharper towards the edges, comparing the D700 at 14mm with a snapshot of my brain's impression.

What's also very obvious is that, at 14mm, my front door appears much smaller through the viewfinder than it does when not looking through the camera. The effect of 'smallness' of an object just 8 metres away continues at higher focal lengths until we reach about 45 to 50mm.

At 50mm, when I toggle between gazing through the viewfinder and staring straight at the door, I see no noticeable change in the size of the door. However, a 50mm angle of view crops off most of that peripheral vision.

I get the impression (just a rough impression) if I were able to take a recordable snapshot of my brain's impression of the view of the front door, then crop off all the peripheral fuzzy stuff (and some of that peripheral stuff is a lot fuzzier than the out-of-focus areas of a shot with a 50mm lens at F1, including the worst corner and edge degradation of any lens every made), then I would get a fairly decent analogy with a photo taken with a 50mm lens.

The attraction of wide-angle lenses is that the camera is able to record in fine detail scenes which the human eye cannot encompass from one, fixed perspective. The eye can of course move around and take in a very wide scene in all its detail, but not in one fixed stare, even if the eyeballs are moved.

That's why I find the following shot interesting, taken recently in Vienna. There's no perspective correction here, by the way. This is straight out of the camera, uncropped. D700 with 14mm at F16. (Sharp from big toe to infinity   ).

[attachment=23430:DSC_1387.jpg]
« Last Edit: August 01, 2010, 04:20:05 AM by Ray » Logged
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