Sensor size v Pentax focal lengths

clutch · 10-10-2010, 01:42 PM

Originally posted by Class A

It is a physical fact that an image viewed at the distance of the image diagonal (commonly regarded as the natural viewing distance) will show a scene with natural angles if the image was taken with the focal length of the sensor/film diagonal.

Could you elaborate on that? I understand that the diagonal of the film/sensor is equal to the focal length of a "normal" lens, but I don't understand how viewing distance is involved. What units are you talking about? An "image viewed at the distance of the image diagonal" would have to be viewed at a distance of 28mm for APS-C or 43mm for FF. Or are you talking about the diagonal of the print? So, a 13"x19" print should be viewed at a distance of 23"?

What are "natural angles"? Do you mean that the perspective distortion will appear natural, i.e. no expansion or compression?

brecklundin · 10-10-2010, 03:21 PM

Originally posted by Class A

Originally posted by brecklundin

fyi rumor has it, the dude, Hirakawa, who designed the 43ltd, 77ltd and a bunch of other classic Pentax glass has apparently been asked to take his lunch break elsewhere, retired, kicked to the curb, pick your most salacious terms and insert here....

Maybe he will go work for Canon since his cohort in that period of lense design is apparently now with Nikon...so they could compete head to head so to speak. Or the guy is gonna take his cash and enjoy life at just 48yrs old...lucky bastage!! Nothing but golf and Hello Kitty Porn...

Personally I think the pink K-r lenses and bodies broke the guy...I mean a person has to have limits, right? And the thought of a camera with Notre Dame of, almost as bad, 'Frisco Junts (San Francisco Giants for those w/o a sense of humor) colors on a camera make me wanna buy a sledge hammer and a blow torch!!

This rumour has no basis. No one knows anything. Some usual suspects used the retirement of Hirakawa to paint gloom and doom on Pentax. Maybe he has retired himself to enjoy life? The DA* 55/1.4, his latest creation, wasn't that great a lens anyhow, was it?

I think I'd prefer the K55/1.8 over the DA* 55/1.4. I'm happy to be proven wrong.

Ummmm...you realize you contradict your own comment right? A thing cannot be with basis then used as part of the argument itself....right?

hehehehe...

I think I know what you are saying but I think you missed that I was being playful in my observation there as well as wrote exactly what you claim is the case...I mean geeze, I thought all of this is all for fun isn't it? I am on limited time now as such I may see things differently but lemme tell you, this is not important enough to argue over...trust me. And there is something I may be reading into the tone of your comment, but it's sort of a pressing need to refute something that is apparently true only the source may chafe for some reason. I dunno but it's sort of not the normal tone I read in your observations and comments. But if this is over the source, life is waaaay to short, something I am well aware of first hand.

Perhaps with all the confusion with Pentax right now we all need to lighten the heck up. I thought I was...maybe not I guess.

Impartial · 10-12-2010, 04:14 AM

Originally posted by RioRico

There's a simple explanation for the progression: the square root of two, ~1.4.

A 135/FF frame has ~twice the area of an APS-C or 135/HF frame, which has ~twice the area of an m4/3 frame; just as an f/1.4 iris opening has twice the area of an f/2 opening, which has twice the area of an f/2.8 opening, etc. Those frame diagonals are actually the diameters of the image circles needed to cover those frames. To double a circle's area, multiply the diagonal | diameter by the square root of two, ~1.4. No magic, no coincidence, just geometry.

A more telling progression is that of common focal lengths. Here is a table of that progression, and the ratios:

012mm 0.00 (like my Vemar 12)
016mm 1.33 (like my Zenitar 16)
021mm 1.31 (like my Tokina 21)
028mm 1.33 (like any 28)
037mm 1.32 (like my Mir-1)
050mm 1.35 (like any 50)
065mm 1.30 (there's a hole here)
085mm 1.31 (like a portrait 85)
105mm 1.24 (like a macro 105)
135mm 1.29 (like any 135)
180mm 1.33 (like my Meyer 180)
240mm 1.33 (like my Enna 240)

If we go from 55 to 70mm, the ratio is... 1.27, close to the above ratios. Each common focal length is roughly 1/3 longer than the previous shorter FL. This suggests that changing the FL by ~1.3 gives a different enough AoV | FoV to serve a different purpose.

Personally, I prefer to look at lens focal length in terms of the same geometric ratio as apertures. So, for 35mm film with a (socially accepted) 50mm normal, some significant lengths were:

24mm
35mm 1.46
50mm 1.43

20mm
28mm 1.40
40mm 1.43
55mm 1.38

77mm

Impartial · 10-12-2010, 04:23 AM

Originally posted by RioRico

There's a simple explanation for the progression: the square root of two, ~1.4.

A 135/FF frame has ~twice the area of an APS-C or 135/HF frame, which has ~twice the area of an m4/3 frame; just as an f/1.4 iris opening has twice the area of an f/2 opening, which has twice the area of an f/2.8 opening, etc. Those frame diagonals are actually the diameters of the image circles needed to cover those frames. To double a circle's area, multiply the diagonal | diameter by the square root of two, ~1.4. No magic, no coincidence, just geometry.

A more telling progression is that of common focal lengths. Here is a table of that progression, and the ratios:

012mm 0.00 (like my Vemar 12)
016mm 1.33 (like my Zenitar 16)
021mm 1.31 (like my Tokina 21)
028mm 1.33 (like any 28)
037mm 1.32 (like my Mir-1)
050mm 1.35 (like any 50)
065mm 1.30 (there's a hole here)
085mm 1.31 (like a portrait 85)
105mm 1.24 (like a macro 105)
135mm 1.29 (like any 135)
180mm 1.33 (like my Meyer 180)
240mm 1.33 (like my Enna 240)

If we go from 55 to 70mm, the ratio is... 1.27, close to the above ratios. Each common focal length is roughly 1/3 longer than the previous shorter FL. This suggests that changing the FL by ~1.3 gives a different enough AoV | FoV to serve a different purpose.

Personally, I prefer to look at lens focal length in terms of the same geometric ratio as apertures. So, I can find two separate geometric series that account for many of the conventional prime lengths, to wit:

24mm
35mm 1.46
50mm 1.43
70mm 1.40
100mm 1.43
135mm 1.35

20mm
28mm 1.40
40mm 1.43
55mm 1.38
77mm 1.40
105mm 1.36

And I have been torn as to which series to "complete". Maybe I need to try both. :-)

P.S. on the topic of this thread, it looks as though many of the Limiteds were a try at a third series:

15mm
21mm 1.40
31mm 1.48
43mm 1.40
60mm 1.40 -- the missing Pentax prime
85mm 1.42
120mm 1.41

:-)

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10-10-2010, 01:42 PM	#16
clutch Inactive Account Join Date: Sep 2010 Location: Birmingham, AL Posts: 28	Originally posted by Class A It is a physical fact that an image viewed at the distance of the image diagonal (commonly regarded as the natural viewing distance) will show a scene with natural angles if the image was taken with the focal length of the sensor/film diagonal. Could you elaborate on that? I understand that the diagonal of the film/sensor is equal to the focal length of a "normal" lens, but I don't understand how viewing distance is involved. What units are you talking about? An "image viewed at the distance of the image diagonal" would have to be viewed at a distance of 28mm for APS-C or 43mm for FF. Or are you talking about the diagonal of the print? So, a 13"x19" print should be viewed at a distance of 23"? What are "natural angles"? Do you mean that the perspective distortion will appear natural, i.e. no expansion or compression?

10-10-2010, 03:21 PM	#17
brecklundin Veteran Member Join Date: Jun 2009 Photos: Albums Posts: 1,674	Originally posted by Class A Originally posted by brecklundin fyi rumor has it, the dude, Hirakawa, who designed the 43ltd, 77ltd and a bunch of other classic Pentax glass has apparently been asked to take his lunch break elsewhere, retired, kicked to the curb, pick your most salacious terms and insert here.... Maybe he will go work for Canon since his cohort in that period of lense design is apparently now with Nikon...so they could compete head to head so to speak. Or the guy is gonna take his cash and enjoy life at just 48yrs old...lucky bastage!! Nothing but golf and Hello Kitty Porn... Personally I think the pink K-r lenses and bodies broke the guy...I mean a person has to have limits, right? And the thought of a camera with Notre Dame of, almost as bad, 'Frisco Junts (San Francisco Giants for those w/o a sense of humor) colors on a camera make me wanna buy a sledge hammer and a blow torch!! This rumour has no basis. No one knows anything. Some usual suspects used the retirement of Hirakawa to paint gloom and doom on Pentax. Maybe he has retired himself to enjoy life? The DA* 55/1.4, his latest creation, wasn't that great a lens anyhow, was it? I think I'd prefer the K55/1.8 over the DA* 55/1.4. I'm happy to be proven wrong. Ummmm...you realize you contradict your own comment right? A thing cannot be with basis then used as part of the argument itself....right? hehehehe... I think I know what you are saying but I think you missed that I was being playful in my observation there as well as wrote exactly what you claim is the case...I mean geeze, I thought all of this is all for fun isn't it? I am on limited time now as such I may see things differently but lemme tell you, this is not important enough to argue over...trust me. And there is something I may be reading into the tone of your comment, but it's sort of a pressing need to refute something that is apparently true only the source may chafe for some reason. I dunno but it's sort of not the normal tone I read in your observations and comments. But if this is over the source, life is waaaay to short, something I am well aware of first hand. Perhaps with all the confusion with Pentax right now we all need to lighten the heck up. I thought I was...maybe not I guess.

10-12-2010, 04:14 AM	#18
Impartial Senior Member Join Date: May 2010 Photos: Gallery Posts: 208	Originally posted by RioRico There's a simple explanation for the progression: the square root of two, ~1.4. A 135/FF frame has ~twice the area of an APS-C or 135/HF frame, which has ~twice the area of an m4/3 frame; just as an f/1.4 iris opening has twice the area of an f/2 opening, which has twice the area of an f/2.8 opening, etc. Those frame diagonals are actually the diameters of the image circles needed to cover those frames. To double a circle's area, multiply the diagonal \| diameter by the square root of two, ~1.4. No magic, no coincidence, just geometry. A more telling progression is that of common focal lengths. Here is a table of that progression, and the ratios: 012mm 0.00 (like my Vemar 12) 016mm 1.33 (like my Zenitar 16) 021mm 1.31 (like my Tokina 21) 028mm 1.33 (like any 28) 037mm 1.32 (like my Mir-1) 050mm 1.35 (like any 50) 065mm 1.30 (there's a hole here) 085mm 1.31 (like a portrait 85) 105mm 1.24 (like a macro 105) 135mm 1.29 (like any 135) 180mm 1.33 (like my Meyer 180) 240mm 1.33 (like my Enna 240) If we go from 55 to 70mm, the ratio is... 1.27, close to the above ratios. Each common focal length is roughly 1/3 longer than the previous shorter FL. This suggests that changing the FL by ~1.3 gives a different enough AoV \| FoV to serve a different purpose. Personally, I prefer to look at lens focal length in terms of the same geometric ratio as apertures. So, for 35mm film with a (socially accepted) 50mm normal, some significant lengths were: 24mm 35mm 1.46 50mm 1.43 20mm 28mm 1.40 40mm 1.43 55mm 1.38 77mm

10-12-2010, 04:23 AM	#19
Impartial Senior Member Join Date: May 2010 Photos: Gallery Posts: 208	Originally posted by RioRico There's a simple explanation for the progression: the square root of two, ~1.4. A 135/FF frame has ~twice the area of an APS-C or 135/HF frame, which has ~twice the area of an m4/3 frame; just as an f/1.4 iris opening has twice the area of an f/2 opening, which has twice the area of an f/2.8 opening, etc. Those frame diagonals are actually the diameters of the image circles needed to cover those frames. To double a circle's area, multiply the diagonal \| diameter by the square root of two, ~1.4. No magic, no coincidence, just geometry. A more telling progression is that of common focal lengths. Here is a table of that progression, and the ratios: 012mm 0.00 (like my Vemar 12) 016mm 1.33 (like my Zenitar 16) 021mm 1.31 (like my Tokina 21) 028mm 1.33 (like any 28) 037mm 1.32 (like my Mir-1) 050mm 1.35 (like any 50) 065mm 1.30 (there's a hole here) 085mm 1.31 (like a portrait 85) 105mm 1.24 (like a macro 105) 135mm 1.29 (like any 135) 180mm 1.33 (like my Meyer 180) 240mm 1.33 (like my Enna 240) If we go from 55 to 70mm, the ratio is... 1.27, close to the above ratios. Each common focal length is roughly 1/3 longer than the previous shorter FL. This suggests that changing the FL by ~1.3 gives a different enough AoV \| FoV to serve a different purpose. Personally, I prefer to look at lens focal length in terms of the same geometric ratio as apertures. So, I can find two separate geometric series that account for many of the conventional prime lengths, to wit: 24mm 35mm 1.46 50mm 1.43 70mm 1.40 100mm 1.43 135mm 1.35 20mm 28mm 1.40 40mm 1.43 55mm 1.38 77mm 1.40 105mm 1.36 And I have been torn as to which series to "complete". Maybe I need to try both. :-) P.S. on the topic of this thread, it looks as though many of the Limiteds were a try at a third series: 15mm 21mm 1.40 31mm 1.48 43mm 1.40 60mm 1.40 -- the missing Pentax prime 85mm 1.42 120mm 1.41 :-)