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10-15-2009, 12:14 AM   #61
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It occurred to me that people will ask "How is the A28/2 wide open?" - so here are two I just took at F2:


1/320s, F2 and 400 iso


1/125s, F2 and 200 iso
Note the shallow depth-of-field - the eye is in perfect focus but not tip of the nose (where was the cat when I needed him!)

10-15-2009, 03:28 AM   #62
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QuoteOriginally posted by Marc Sabatella Quote
Anyone can do this test right now. Look straight ahead, fixating on a particular object in front you. ...
The property of being "normal" for a lens has nothing got to do with FOV. FOV is almost impossible to compare between two eyes + brain vs a lens.

QuoteOriginally posted by Marc Sabatella Quote
Choosing 50mm as somehow best representing the "natural viewing perspective of the human eye" is just plain arbitrary.
No, it isn't. 50 is close to the diagonal of the FF diagonal (43.27mm, 50mm were easier to make sharp).

A lens with a focal lens of the sensor/film diagonal (~28mm for APS-C) will result in an image that when viewed at the standard/typical viewing distance of the print diagonal will appear without perspective distortion.

This is not a fuzzy or vague or arbitrary definition.

QuoteOriginally posted by Marc Sabatella Quote
(that term has relevance only with respect to making prints of a "typical" size and then viewing them at a "typical" distance.
What do you mean by "only"? There is no other way to avoid perspective distortion. Note that it has got nothing to do with "typical size". Only typical (or adequate) viewing distance.


QuoteOriginally posted by Marc Sabatella Quote
This instantly gives lie to the notion that different focal lengths have different "perspectives", at least in the usual sense of that word.
Marc, that's wrong again. Your experiment only proves that cropping is equivalent to increasing the focal length. Both change the perspective distortion.

You are rigtht, only distance to subject defines the perspective the photographer has on a certain subject. But if you view a print of an image of that scene at a distance that corresponds to the print's diagonal then only a normal lens will recreate the same perspective. Any other lens will result in perspective distortion (exaggeration of angles for wide angle lenses, compression of perspective for long focal lenses).

Cropping the image taking with a normal lens is equivalent to using a longer focal lens. It results in a flattening of the perspective. One can correct the latter by viewing the print with a larger viewing distance.

BTW, Pentax calls the "50s" normal because they are still referring to FF normals.

FWIW, the Sigma 28/1.8 has very good close focusing capabilities. Subjects can almost touch the front lens.
10-15-2009, 03:35 AM   #63
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QuoteOriginally posted by brosen Quote
I am undecided between:

- Pentax SMC DA* 55mm f/1.4

- Sigma 30mm f/1.4 EX DC HSM
These are rather different focal lengths suggesting different applications.

The Pentax would be useful for portraits. It won't be useful as a normal lens.

The Sigma is close to being a normal lens on APS-C and you could use it for documentary photography or other applications where you don't want the focal length to leave an imprint on the image.

Normal lenses like that are a bit of a challenge for a photographer since they don't add "zing" to an image but only give you a neutral perspective. But this is why they have been (and still are) recommended for photography beginners, since they will teach you how to make an interesting image without relying on perspective effects.
10-15-2009, 01:40 PM   #64
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QuoteOriginally posted by Spock Quote
In the end, theory aside, when we view captured images from a 'normal' distance, we can easily determine if they look 'normal'.
Yes, but again, my point is, it isn't just as simple as being a matter of one FOV matching human vision; it's wrapped up in the specifics of print sizes and viewing distances.

BTW, regarding the fovea - I wonder if one can say with any precision what angle of view it generally covers?

10-15-2009, 02:00 PM   #65
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QuoteOriginally posted by Class A Quote
The property of being "normal" for a lens has nothing got to do with FOV.
Well, that's what started this little digression. The quote from Pentax lens catalog about "natural viewing perspective of the human eye" (or whatever it actually read) implies it does, and that is of course a common way that people have tried to define what "normal" might mean. I'm not really concerned with whether it is valid to try to define "normal" in this way; I'm simply saying that *if* one wishes to define 'normal" this way, the actual statement in the catalog is far too imprecise to be meaningful. You need to get into the whole bit about typical print sizes and viewing distances - as you did below - in order for it to actually yield a specific answer.

But it is true that one can choose to reject the association between "normal" and human vision entirely, as you are implying here, and define it solely in terms of the diagonal of the sensor. I like that definition because it's totally objective, but it leaves open the question of *why* one would care to invent a special term for lens whose focal length is the diagonal of the sensor (eg, why not call it "normal" if it's focal length is the width of the sensor, or the height, or the average between the two, or the sum of the two, or the whole perimeter, etc). The *reason* why this metric has any significance obviously has something to do with human vision and perspective and all that - and in particular, it turns out to have to do with how the FOV of the image compares with the angle subtended by a "typical" sized print viewed from a "typical" viewing distance.

QuoteQuote:
FOV is almost impossible to compare between two eyes + brain vs a lens.
Absolutely, which is why I observed that the statement in the Pentax lens catalog that referred to "natural viewing perspective of the human eye" or words to that effect was hopelessly imprecise.

QuoteQuote:
A lens with a focal lens of the sensor/film diagonal (~28mm for APS-C) will result in an image that when viewed at the standard/typical viewing distance of the print diagonal will appear without perspective distortion.

This is not a fuzzy or vague or arbitrary definition.
I agree. *Your* definition here is very clear. It's the one in the Pentax lens catalog I was quibbling with.

QuoteQuote:
What do you mean by "only"? There is no other way to avoid perspective distortion. Note that it has got nothing to do with "typical size". Only typical (or adequate) viewing distance.
Had to go back and look up what i had written to make you write this. What I meant by "only" here is that what I was referring was *only* relevant for discussing "perspective distortion" - not for discussing *perspective* in general (see below). And actually viewing size *does* matter, as when you get to extremely small or extremely large prints, the typical viewing distances don't end up necessarily resulting in the print subtending a "normal" AOV any more.

QuoteQuote:
Marc, that's wrong again. Your experiment only proves that cropping is equivalent to increasing the focal length. Both change the perspective distortion.
That's why I included the phrase, "at least in the usual sense of the word" - by which I meant, "linear perspective", not "perspective distortion". And linear perspective is not affected by focal length.

Last edited by Marc Sabatella; 10-15-2009 at 02:07 PM.
10-15-2009, 02:20 PM   #66
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QuoteQuote:

BTW, regarding the fovea - I wonder if one can say with any precision what angle of view it generally covers?
Nice question, the fovea is the area of the retina with the lowest ratio of photorecptors: neurons, which means "greatest acuity." or the sweet spot so to speak. This is the area we use pinpoint focus on a object at a distance, or focus on type when reading, but, it is not used in night vision, which is low acuity.

About its angle of view, or how wide this sweet spot of our vision is....it is extremely narrow. If you could graph acuity (10 max/1 minimum) as a function of angle of vision, 120 degrees (60 to the left field/60 to the right field). Max acuity (10) is only about 3 degrees in either direction. Another 10 degrees either direction, and acuity falls to about 3-5. So you can see the sweet spot in out vision is extremely narrow.
10-15-2009, 07:49 PM   #67
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QuoteOriginally posted by ramair455 Quote
About its angle of view, or how wide this sweet spot of our vision is....it is extremely narrow. If you could graph acuity (10 max/1 minimum) as a function of angle of vision, 120 degrees (60 to the left field/60 to the right field). Max acuity (10) is only about 3 degrees in either direction. Another 10 degrees either direction, and acuity falls to about 3-5. So you can see the sweet spot in out vision is extremely narrow.
3 degrees in each direction, meaning 6 degrees total arc? So it would take something longer than a 200mm lens on APS-C to more or less approximate this?
10-15-2009, 09:05 PM   #68
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You got! But, we can pan that total visual field and pull pin point focus in milliseconds, the younger you are the faster change focus fields. And, this would be the "darting" around the other poster Spock is referring to. Then we also have "visual gaze reflexes" which are faster than conscious voluntary changes in pinpoint focus.

Vision is a truly amazing special sense.

10-16-2009, 07:50 AM   #69
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QuoteOriginally posted by Spock Quote
It occurred to me that people will ask "How is the A28/2 wide open?" - so here are two I just took at F2:

Note the shallow depth-of-field - the eye is in perfect focus but not tip of the nose (where was the cat when I needed him!)
Thanks for the images, it looks pretty good wide-open. Hope I can find this lens one day!
10-16-2009, 09:33 AM   #70
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One more 'Perspective'

Thank you everyone for your posts on 'Normal.' I have found it edifying, and even a bit motivational. I went to the Wiki reference, checked the footnote and tracked it down: a bit of writing from View Camera Technique by Leslie D. Stroebel (1999). Seems like a good read, and I will have to pick up the book some time.

--edit-- After posting, I noted that this description is in agreement with what Class A wrote (and Marc agrees with). What follows is an elaboration of that definition. I needed to get a better understanding for myself, and I hope it gives some clarity to other folks.--edit--

The link from Wiki only takes the reader to a section indirectly related to the definition of normal lenses, but gives a bit more background on the definition put forth by Class A:

For this perspective on the issue, forget about aperture, FOV, and the angle of view from the human eye. 'Normal' means that the photo *when printed and viewed* at a natural distance, gives a natural look. As the book puts it, the objects in the photograph "subtend the same angle to the eye as the original objects would have subtended to the eye placed in the same position of the camera, and the linear perspective will appear normal."

With a normal lens, more distant objects recede (shrink) as we would expect them to if viewed with our own eyes. You could stand where the camera was and hold up the photo at a ‘natural viewing distance’ and it could nearly substitute for what is behind it. Consider a picture with a tree in the foreground and the moon in the background. Holding an 8X10 photo shot with a telephoto lens at a natural distance (about 12"), the moon seems unnaturally large in relation to the tree; with a wide-angle, the moon seems unnaturally small in relation to the tree. With a normal lens, the moon will look "just right," and would just cover the circle of the moon if we are standing at the spot where the camera took the picture.

So, the focal length of a normal lens *can* be calculated, given a few assumptions. First, I’ll assume a 'natural' viewing distance of a photograph is about 12in. or 300mm (other distances can be used, but the author uses 12”, a bit close IMO).

The correct focal length to achieve ‘normal’ is the viewing distance (300mm) divided by the magnification factor of the print (width of a 8X10 print divided by the width of the sensor or 254/23.4)=>10.85. Calculating it this way, for a K20D and an 8X10 print, a normal lens is 28mm (300/10.85). A smaller photo (say 4X6) would tend to be held closer, around 8”, resulting in the same “normal” lens.

I hope this wasn’t too garbled. It is a fascinating bit of writing. Here is a link:
Reference to the Wiki footnote

Last edited by DavidWasch; 10-16-2009 at 10:40 AM. Reason: Clarified that this definition was already discussed, in part
10-16-2009, 10:31 AM   #71
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I just want to say that I got my FA 35mm f/2.0 brand new from B&H earlier this year for $299. NYAH, NYAH!


Last edited by audiobomber; 10-17-2009 at 09:16 AM.
10-16-2009, 01:38 PM   #72
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QuoteOriginally posted by ramair455 Quote
You got! But, we can pan that total visual field and pull pin point focus in milliseconds
Yes, I've noticed this phenomenon. Caught me by surprise one day stuck in traffic behind a truck with a turn signal on both his side mirror and the rear of the car. I could see the lights were not in sync and was trying to figure out which was flashing faster (like I said, stuck in traffic...). I realized that while I could easily "see" them together, I couldn't quite focus on them both fully without darting back and forth between them, which interfered with my ability to answer the question of which was flashing faster. I quickly ascertained the field of view encompassed by those lights was a little bit wider than that of a 100mm lens on APS-C, and kind of assumed I was right on the edge of being able to focus on both, and thus concluded that somewhere around 100mm FOV was probably about what I could focus on fully. Looks like I was overly optimistic in my estimate, but probably on the right track.

QuoteQuote:
Vision is a truly amazing special sense.
No kidding. The aspect that has been fascinating me lately is our ability to perform the equivalent of "automatic white balance" - somehow perceiving the local colors in a scene even under fairly strongly colored lighting.
10-16-2009, 01:40 PM   #73
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QuoteOriginally posted by DavidWasch Quote
For this perspective on the issue, forget about aperture, FOV, and the angle of view from the human eye. 'Normal' means that the photo *when printed and viewed* at a natural distance, gives a natural look. As the book puts it, the objects in the photograph "subtend the same angle to the eye as the original objects would have subtended to the eye placed in the same position of the camera, and the linear perspective will appear normal."
I should get extra credit for having used the word "subtend" earlier in this thread in this same context too.
10-16-2009, 01:54 PM   #74
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I've narrowed down the search to two lenses:

- PENTAX SMC DA* 55mm f/1.4
- PENTAX SMC DA 40mm f/2.8 Limited

I will be using the lens mainly inside for pictures, weddings, meetings, etc., I am not an expert in Photography, do you think f/2.8 is enough for inside photograhy with artificial light ? or the f/1.4 would be better for that purpose ?, thanks
10-16-2009, 02:54 PM   #75
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QuoteOriginally posted by Marc Sabatella Quote
No kidding. The aspect that has been fascinating me lately is our ability to perform the equivalent of "automatic white balance" - somehow perceiving the local colors in a scene even under fairly strongly colored lighting.
Astute observation on your part!! Current theories conclude this natural AWB occurs in the primary visual cortex. Cones (3 types S,M,L) in the retina are responsible for color vision, with each type stimulated by specific wavelenghts of light, with some overlap.

It is thought when a certain cone is over stimulated, example a S cone with blue light (open shade), or a L cone with tungsten light -- this is ignored -- so proper color perception is still perceived. Its as if the brain know the Kelvin temp of any given light source and makes the appropriate correction.

Amazing....
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