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10-14-2009, 01:16 PM   #46
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QuoteOriginally posted by brosen Quote
I'm looking for an inside/outisde/walk around/all purpose lens, I think the Sigma 30mm will serve that purpose 100%, I like more the Pentax 55mm for the WR properties, but I am concern about the very "narrow" angle of view of an almost 83mm lens, with the Sigma lens the angle of view will be equivalent to a 45mm lens, any previous experience with 83mm lens (equivalent) ?, thanks
The biggest advantage the Sigma will give you is speed. If you plan on working in fairly dark interiors, it may well be your best choice.

You may have checked reviews on this lens, but in case you have not, here is one consideration: the Sigma 30mm has poor performance away from the center of the field. On center, the performance is outstanding, but if you want your subject to the side, or plan to shoot landscapes, this may not be the best lens for you. Here is a reputable review for your consideration:
Sigma 30mm Review for Pentax ala Photozone

10-14-2009, 01:27 PM   #47
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Thanks David, what do you think about the Pentax 55mm f/1.4 ?,
10-14-2009, 01:35 PM   #48
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QuoteOriginally posted by Wheatfield Quote
If you take a picture with (as an example) a 15mm lens, and then, without moving the camera, replace the 15mm lens with a 50mm lens and take another picture, you will be able to crop a section out of the image taken with the 15mm lens that will be an exact match to the one shot with the 50mm lens, with the exception of noise or grain due to having to magnify one image to match the other one.
Precisely - thanks for that. This instantly gives lie to the notion that different focal lengths have different "perspectives", at least in the usual sense of that word. There's actually a decent demonstration of this in the Wikipedia entry on perspective distortion; see about a third of the way down where there is a picture of a scene taken from one spot with three lenses of very different focal lengths. The views look very different, of course, but then they crop out the same section from each picture, and the differences vanish entirely:

Perspective distortion (photography) - Wikipedia, the free encyclopedia

The actual subject of the article touches on the definition of "normal" field of view in a pretty useful way, too, as it makes clear the importance of print size and viewing distance in the equation.

Not that any of this really is relevant to the actual subject, though. Whether or why a given focal length is called "normal"or "standard" or not is beside the point; either you want a prme in that focal length and need it to be faster than f/2.8, or you don't.
10-14-2009, 02:22 PM   #49
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The differences do not vanish entirely if you consider the background though.
I know that a lot of photographers do not care about what is in the background of their photos as long as the subject is in the right perspective, but I do.

10-14-2009, 02:53 PM   #50
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QuoteOriginally posted by brosen Quote
Thanks David, what do you think about the Pentax 55mm f/1.4 ?,
I have no personal experience with it, but everything I have seen (both on the lens and pictures taken with it) are excelent. If weather-resistance is manditory, it is definately a good option.

There is a pretty big difference between how/what you shoot with a 30mm and 55mm. The horizontal angle of view changes from 43 degrees to 24 degrees. If you think you want candid shots of people outside, the 55mm will allow you to be less invasive; if you want to shoot natural landscapes, you might want a wider lens.

If you have a kit zoom, I would walk around with it and see what focal range you are drawn to the most. I would bet you will prefer either 55mm or 30mm.
10-14-2009, 03:53 PM   #51
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Well I know this, there is no camera lens out there...anywhere, that can render the same angle of view & perspective as the human eye/s

The angle of view in binocular (stereo/both eyes) vision is about 120-130 degrees while focused on a distant fixed object. Actually, the angle of view for single eye would be 180 degrees if not for the bridge of your nose, which reduces it to about 140 degrees.

As you can see, for a camera, this would be a very wiiiiddddde angle lens with load of perspective (distortion), however, in our vision, the perspective is "corrected" by the primary visual cortex in your occipital lobe.

Vision is an absolute marvel of biological optics..Imagine the auto focus camera & lens that could pull pinpoint focus as fast as the human eye...it will never happen

Last edited by ramair455; 10-14-2009 at 07:16 PM.
10-14-2009, 06:08 PM   #52
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I don't think "normal" has anything whatsoever to do with fov. It's strictly to do with depth perception. A wide angle makes the background seem farther away, a telephoto makes it look closer, a normal lens makes it look normal.
10-14-2009, 06:56 PM   #53
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QuoteOriginally posted by Shashinki Quote
The differences do not vanish entirely if you consider the background though.
Yes they do. Did you check out the link? Proves conclusively there is no difference in perspective whatsoever. Difference in DOF or bokeh, maybe, but that's also lens and aperture dependent,and in any case, is a totally separate matter.

QuoteQuote:
I know that a lot of photographers do not care about what is in the background of their photos as long as the subject is in the right perspective, but I do.
Oh, I agree backgrounds are important - but backgrounds don't magically get different perspective when shooting the same distance with difference lenses any more than subjects do.

10-14-2009, 07:12 PM   #54
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QuoteOriginally posted by Marc Sabatella Quote
the effect you are showing here is not dependent on focal-length, but *distance to subject*. The dog looks weird because you shot from an abnormally close distance.
I agree that it is partly because of distance to subject - but it is also because of the lens used.

If I had used a lens with a longer focal length from the same distance, the image would have looked more like a macro image (eg. perhaps the entire frame would have been filled with a pair of dog nostrils).

The bottom line is that through my eyes, no matter how close I get to my dog, he never looks like he does in that image taken with the DA15.
10-14-2009, 07:21 PM   #55
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QuoteOriginally posted by ramair455 Quote
Well I know this, there is no camera lens out there...anywhere, that can render the same angle of view & perspective as the human eye/s

The angle of view in binocular (stereo/both eyes) vision is about 120-130 degrees while focused on a distant fixed object. Actually, the angle of for single eye would be 180 degrees if not for the bridge of your nose, which reduces it to about 140 degrees.

As you can see, for a camera, this would be a very wiiiiddddde angle lens with load of perspective (distortion), however, in our vision, the perspective is "corrected" by the primary visual cortex in your occipital lobe lobe.

Vision is an absolute marvel of biological optics..Imagine the auto focus camera & lens that could pull pinpoint focus as fast as the human eye...it will never happen
Yes, but the human eye only has one small spot on the retina (the fovea) that allows pin sharp vision. The wide field of sharp vision you appear to have is an illusion. The fact is that peripheral vision is actually quite poor and the only reason our whole field of view appears sharp and clear is because our eyes are always darting around - 'drawing' a clear image with the fovea - that is then stitched together in the brain to form 'the big picture'.

Oh and the eye doesn't focus that quickly at all. Try this exercise in a dark(ish), large room: Bring your finger as close to your nose as you can focus on it. Then snap your focus back and forth from the finger to the distant wall. You may find it is not as fast as you thought - and not that much different to an autofocus camera.

Last edited by Spock; 10-14-2009 at 07:30 PM.
10-14-2009, 08:17 PM   #56
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QuoteOriginally posted by Spock Quote
I agree that it is partly because of distance to subject - but it is also because of the lens used.

If I had used a lens with a longer focal length from the same distance, the image would have looked more like a macro image (eg. perhaps the entire frame would have been filled with a pair of dog nostrils).
Yes, but the *perspective* would be the same. Only the field of view would differ. So in other words, the image from the longer focal length would look *exactly* like a crop from the 15mm shot.

QuoteQuote:
The bottom line is that through my eyes, no matter how close I get to my dog, he never looks like he does in that image taken with the DA15.
Actually, if you closed one eye and put the other right where the lens was, it would look *exactly* like the DA15 shot. Perspective is completely determined by position and position only. If this weren't true, the last 500 years of art history would not have been possible.
10-14-2009, 11:06 PM   #57
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QuoteOriginally posted by Marc Sabatella Quote
Yes, but the *perspective* would be the same. Only the field of view would differ. So in other words, the image from the longer focal length would look *exactly* like a crop from the 15mm shot.

Actually, if you closed one eye and put the other right where the lens was, it would look *exactly* like the DA15 shot. Perspective is completely determined by position and position only. If this weren't true, the last 500 years of art history would not have been possible.
You are quite right of course, but as the Wikipedia article on perspective (that you linked above) said:

Photographs are ordinarily viewed at a distance approximately equal to their diagonal. When viewed at this distance, the distortion effects created by the angle of view of the capture are apparent. However, theoretically, if one views pictures exhibiting extension (wide angle) distortion at a closer distance, thus widening the angle of view of the presentation, then the phenomenon abates. Similarly, viewing pictures exhibiting compression (telephoto) distortion from a greater distance, thus narrowing the angle of view of the presentation, reduces the effect. In both cases, at some critical distance, the apparent distortion disappears completely.

In the end, theory aside, when we view captured images from a 'normal' distance, we can easily determine if they look 'normal'.

I think audiobomber put it well:

QuoteOriginally posted by audiobomber Quote
I don't think "normal" has anything whatsoever to do with fov. It's strictly to do with depth perception. A wide angle makes the background seem farther away, a telephoto makes it look closer, a normal lens makes it look normal.
10-14-2009, 11:37 PM   #58
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QuoteOriginally posted by Spock Quote
Yes, but the human eye only has one small spot on the retina (the fovea) that allows pin sharp vision. The wide field of sharp vision you appear to have is an illusion. The fact is that peripheral vision is actually quite poor and the only reason our whole field of view appears sharp and clear is because our eyes are always darting around - 'drawing' a clear image with the fovea - that is then stitched together in the brain to form 'the big picture'.

Oh and the eye doesn't focus that quickly at all. Try this exercise in a dark(ish), large room: Bring your finger as close to your nose as you can focus on it. Then snap your focus back and forth from the finger to the distant wall. You may find it is not as fast as you thought - and not that much different to an autofocus camera.
Well, here we go...the numbers I stated for field of view are quite accurate, however, I did not mention anything about acuity through the whole field of vision. You are right about the fovea have the greatest acuity. This "stitching together" of the visual fields that you refer to is a function in of the tectum in the mid brain (superior colliculus), but is not a function of the angel of view.

Night vision is a function of rods in the retina and cones are for color. As for focus speed at night, let your rods adapt to the dark for about 20 minutes, the time it take to restore rhodopsin, then check you focus speed. But more importantly, focusing on you finger, then to a distant wall is "accommodation" and is a function of lens flexibility and its ability to change from concave to convex, which we lose as we get older (presbyopia).

I don't have to look up this info, because I teach it in undergraduate Neuroanatomy.
10-14-2009, 11:49 PM   #59
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QuoteOriginally posted by ramair455 Quote
I don't have to look up this info, because I teach it in undergraduate Neuroanatomy.
Well that's got to be more fun than digging trenches!

Anyway back on topic - after much deliberation, here was my solution to the "Best Prime/Standard Lens for K7 (28mm to 35mm)"


Pentax A28 F2 (circa 1984-88) on K7

Here are two quick pics taken with this lens:


1/125s and F3.5 (400 iso)


1/50s and F5.6 (400 iso)
10-15-2009, 12:07 AM   #60
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I also believe that the best prime lens for this APS-C format is 28mm...it just looks and feels right...
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