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09-26-2008, 12:55 AM   #16
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Thank you for all the answers, I do understand it now.
That two images were a big help to visualize it.

09-26-2008, 08:33 AM   #17
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QuoteOriginally posted by Wheatfield Quote
As soon as you move, perspective changes. As you move farther, the near/far relationship of the camera to objects as various distance also changes.
A couple of steps with a long lens generally will show far less perspective shift than a very short one, also, providing there are objects at varying distance within the scene.
That depends on how you measure that shift. If you measure to the way I suggested: actually taking a picture with lenses of the same focal length, then cropping them to show the same FOV - you will find no difference whatsoever. Couldn't possibly be any other way unless the laws of physics have changed sometimes since the days of Brunelleschi. However, if you measure the sift by some sort of calculation of how far a given point moved moved within the frame as a percentage of the frame size or something like that, than sure, you're making focal length play a role in the calculation.
09-26-2008, 10:23 AM   #18
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Another way to convince yourself that focal length doesn't change perspective is to put a zoom lens on your camera and zoom it. It'd be pretty funky if the perspective was changing when you zoomed.
09-26-2008, 01:57 PM   #19
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DOF article.

QuoteOriginally posted by k100d Quote
check this out, very useful DOF2
I find the article rather misleading. Normally when checking the effect of changing a parameter, you keep all other parameters the same, to isolate the effect.
In this article they change TWO of the DOF parameters.
Throw in aperture as well, and you can show images where DOF actually INCREASES with increasing focal length. Doesn't prove much imo.

09-26-2008, 02:32 PM   #20
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QuoteOriginally posted by baw Quote
I find the article rather misleading. Normally when checking the effect of changing a parameter, you keep all other parameters the same, to isolate the effect.
In this article they change TWO of the DOF parameters.
Throw in aperture as well, and you can show images where DOF actually INCREASES with increasing focal length. Doesn't prove much imo.
this test was to see the change of DOF if the size of the object was the same. by changing only one of distance / focal length, that does not help answer the question
the test basically assumes that as you go farther, you choose a proportionately longer focal length
09-27-2008, 12:50 AM   #21
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DOF article.

QuoteOriginally posted by k100d Quote
Do Wide Angle Lenses
Really Have Greater Depth of Field
Than Telephotos?
QuoteQuote:
Most photographers accept the common belief that short focal length lenses have greater depth of field than do long lenses. A wide angle lens, in other words, will give greater depth of field than will a telephoto. Right?

Sounds about right, but it's not the case.
These quotes from the article certainly give the impression that the author is stating that wide angle lenses do NOT have greater DOF than telelenses.
He tries to proof his point by changing focal length AND focussed distance.
As stated above, when changing aperture as well you can show that telelenses have more DOF than wide angle lenses. Not a very realistic approach imo.
09-27-2008, 01:12 AM   #22
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QuoteOriginally posted by baw Quote
These quotes from the article certainly give the impression that the author is stating that wide angle lenses do NOT have greater DOF than telelenses.
He tries to proof his point by changing focal length AND focussed distance.
As stated above, when changing aperture as well you can show that telelenses have more DOF than wide angle lenses. Not a very realistic approach imo.
I see what you are saying, but I think the idea there was that he was trying to hold constant the FOV, eg, reduce number of changing variables. Of course this necessitated changing the focus distance... you have to choose at some point what to hold constant. If he kept the same subject distance then people would complain that the picture looks totally different etc.
09-27-2008, 08:28 AM   #23
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Question for Stevebrot

Stevebrot, early in this discussion, referring to your 2 posted pics, you state "You can tell which one was taken with the longer focal length by comparing the DOF, but that is another confusing discussion." Then, just a few posts later, you state this, "The link is about DOF, not perspective, and it is absolutely correct."

The link about DOF concludes that: "In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field."


Can you explain your two above statements in light of the conclusion on focal length and DOF published at Luminous Landscape? Thanks.

09-27-2008, 08:46 AM   #24
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.

85mm does not = 55mm! 85mm does not = 55mm! FOV be damned! DOF trumps all!

(just thought I'd throw a little non-sequiter on the fire.)


09-27-2008, 09:02 AM   #25
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Luminous Landscape

QuoteOriginally posted by sewebster Quote
I see what you are saying, but I think the idea there was that he was trying to hold constant the FOV, eg, reduce number of changing variables. Of course this necessitated changing the focus distance... you have to choose at some point what to hold constant. If he kept the same subject distance then people would complain that the picture looks totally different etc.
I'm fine with a statement like: For a given FOV DOF is the same for different focal lengths. At least it seems to be so.
My point is that the article clearly implies that DOF is independant of focal length, which is nonsense.
I'm a bit surprised by this article. I had a high regard for Luminous Landscape....
09-28-2008, 06:07 PM   #26
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QuoteQuote:
Jewelltrail Question for Stevebrot

--------------------------------------------------------------------------------
Stevebrot, early in this discussion, referring to your 2 posted pics, you state "You can tell which one was taken with the longer focal length by comparing the DOF, but that is another confusing discussion." Then, just a few posts later, you state this, "The link is about DOF, not perspective, and it is absolutely correct."

The link about DOF concludes that: "In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field."


Can you explain your two above statements in light of the conclusion on focal length and DOF published at Luminous Landscape? Thanks.
Hey Steve, was really looking forward to your discussion on this one.
09-28-2008, 06:19 PM   #27
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QuoteOriginally posted by Jewelltrail Quote
Hey Steve, was really looking forward to your discussion on this one.
The subject size (on the sensor) in his two photos was not the same. He resized the photos to make them look the same (which is what you want to do for the comparison he was making).
09-28-2008, 06:30 PM   #28
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Back on topic and my 2 cents. In plain English I would say that FOV = what you are seeing, Perspective = how you are seeing it.

And Depth of Field is very important when the farmers are deciding what to plant.
09-28-2008, 06:35 PM   #29
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QuoteOriginally posted by Jewelltrail Quote
Stevebrot, early in this discussion, referring to your 2 posted pics, you state "You can tell which one was taken with the longer focal length by comparing the DOF, but that is another confusing discussion." Then, just a few posts later, you state this, "The link is about DOF, not perspective, and it is absolutely correct."

The link about DOF concludes that: "In fact, if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field."


Can you explain your two above statements in light of the conclusion on focal length and DOF published at Luminous Landscape? Thanks.
As noted above and in one of my other posts, the original image size on the sensor for the two pictures was different. Both were cropped to the same composition and outputted to the same final size. The process was similar to if you were enlarging from film.

The cutting board pictures were originally posted on another thread where the equal size/equivalent DOF was also demonstrated. Here are the DOF pictures...the challenge was to tell which was taken at 50mm and which at 85mm.





BTW, if anyone is not convinced by the above or by the Luminous Landscape article, here is a link to a calculator page where you can prove it to yourself without even touching a camera or lens:
Julian's Lens Calculator

Steve

Last edited by stevebrot; 09-28-2008 at 06:43 PM.
09-28-2008, 06:37 PM   #30
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QuoteOriginally posted by Jewelltrail Quote
Hey Steve, was really looking forward to your discussion on this one.
I finally got back to this thread today. See my note above. Thanks Sewebster for pinch-hitting with the explanation.

Steve
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