> > So prospective distortion could occur even with a non-wide > angle lens, > couldn't it? >
Yes. And it does. But you don't notice it as much. > So the second question is, why is perspective distortion > much stronger and > more usual with a wide angle than a "normal" focal length of > 50mm and above? Nearer objects appear larger than further objects. To fill the frame with a nearer object you typically need a wider-angle lens. Using a tall building as an example, you are closer to the foot of the building than you are to the roof, therefore the foot will appear relatively larger than the roof. The straight lines which connect the foot and the roof must then converge at a greater angle than they would if you were further away. It's the same thing that happens when you photograph railway tracks. It's not a defect, it's a necessary fact of linear perspective. It really isn't anything to do with the lens, except that using a wide angle lens means you can fit more of a near object into the frame. > Third question is, what causes barrel distortion? The > curvature of the lens > surface? There is a technical explanation of it here: http://www.vanwalree.com/optics/distortion.html I must admit I don't really understand it. I don't know what he means when he talks about introducing a stop in the system. I've always thought it was a problem of mapping a 3D world onto a 2D surface, rather like geographical projections, but I guess I'm wrong. -- Bob > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On > Behalf Of [EMAIL PROTECTED] > Sent: 10 November 2007 07:42 > To: [email protected] > Subject: Re: Question - Barrel Distortion vs Perspective Distortion? > > In a message dated 11/9/2007 11:23:06 P.M. Pacific Standard Time, > [EMAIL PROTECTED] writes: > In a message dated 11/9/2007 11:14:38 P.M. Pacific Standard Time, > [EMAIL PROTECTED] writes: > > And anyone who wants to expound more on the difference > between barrel > > distortion and perspective distortion, too, feel free. > > > ============= > A couple more questions, though this is late and I need to > think about it > more. I might phrase it better tomorrow. :-) > > I can see how the sensor is a flat plane, and if I am > shooting upward I am > creating an angle -- the light is coming in at angle to that > plane. If I have > that right. Especially noticeable when shooting buildings > that have straight > lines. > > So prospective distortion could occur even with a non-wide > angle lens, > couldn't it? > > Like I think the leaning lamp post I got with the 50-200 at 63mm was > actually perspective distortion because it was not > perpendicular, but it was not > curving. I was shooting upward a bit. But I will have to go > back and look it may > have been curving a bit too. > > So the second question is, why is perspective distortion > much stronger and > more usual with a wide angle than a "normal" focal length of > 50mm and above? > > I think the barrel distortion I have noticed now and then is > much more minor > than the perspective distortion I have noticed. But maybe I > should find a > brick wall and do some tests. :-) > > Third question is, what causes barrel distortion? The > curvature of the lens > surface? > > Surprisingly enough none of this was ever covered in any > photography class I > have taken. f/stops, shutter speeds, etc., etc., but not distortion. > > Thanks, Marnie aka Doe ;-) > > --------------------------------------------- > Warning: I am now filtering my email, so you may be censored. > > > > > ************************************** See what's new at > http://www.aol.com > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly > above and follow the directions. > > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
