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05-16-2009, 10:00 PM   #106
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If I understand what you're trying to say by sliding f ratio (variable aperture on zooms), I think thats actually done by the 6 aperture connectors that change depending on when in the zoom range you are. And just to slightly clarify, older bodies with aperture ringless lenses don't shoot wide open, they shoot all the way stopped down (much less useful). Not nitpicking, just think that a rumor thread should be full of lots of random semi-useless non-rumors, to make it worthwhile

05-16-2009, 10:45 PM   #107
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I still maintain that it is physically impossible to make an F1.0 lens on a PK mount, and so far I have not seen anybody provide information that suggests otherwise.

An F-number is, in basic terms, the degree to which a lens can gather the light that is in front of it onto the focal plane. So, to make things easier to understand, think of an "idealized" lens that exists at the mount plane. Your maximum diameter (D) is going to be the maximum diameter of the mount, and your focal length (F) is going to be the register.






Again

f = F/D



since for PK mount F=45.5mm and D=45mm, an f1.0 lens cannot exist. The point here is that this diagram is as basic and efficient as it gets. There's nothing you can "add" to this simple diagram that will magically make the F-stop smaller, while maintaining the focal distance.

Now, several people mentioned some fast primes with small rear elements. This can easily be explained. What you forget is that rear elements protrude. So you don't need a rear element with a diameter that would be suggested by the simple formula above.





Case in point. Here's the rear element of my fastest lens, Zuiko-S 50mm f1.2






As you can see, the diameter is around 34mm. The OM mount has a register of 46mm, so to get an f1.2 lens, you'd think you need a 46/1.2=38mm rear element, right?

Nope, because the rear element protrudes:





This protrusion is around 6-7mm, making the distance from the rear element to the focal plane around 40mm. What's 40mm divided by 1.2? 33.3mm, just about the diameter of the rear element in the Zuiko.

Do the same calculation with your 31/1.8 (I did), comes out about the same.



So, you may ask, all you need is a rear element that protrudes far back enough without hitting the mirror and maintaining a large diamter, and you should get an F1.0 lens, right? Say, a 40mm rear element that protrudes 5mm behind the PK mount? Hey, what about a mirror-less design? Then you can REALLY have the lens close to your sensor plane! Should make an f1.0 lens a sinch!


The answer is NO, because regardless of how big the rear element is or how close you get it to the sensor plane at some point during the travel of light, it had to go through the PK mount opening, which has a 45mm diamater, at a distance of 45.5mm from the focal plane. So regardless of whatever voodoo you conduct in front of, or behind the mount, at the very best, what you are going to get is the idealized "single lens at the mount plane" situation, and because of that, the largest aperture you can get on a PK mount is limited to 45.5/45 = f1.01.


If someone wants to prove me wrong, just give me one example of an F1.0 lens on a mount/system that has a longer register than the mount diameter.


Couple of points:

(1) There is an exception, which is macro lenses. There is, e.g., an M42 Carl Zeiss Jena f0.75 lens, strictly for macro. This is possible, obviously, because the focal distance is, since it's a macro lens, much shorter than the register distance.

(2) Somebody mentioned the Noct-Nikkor. That was a 58/1.2, not an f1.0.
05-17-2009, 12:47 AM   #108
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QuoteOriginally posted by RawheaD Quote
So regardless of whatever voodoo you conduct in front of, or behind the mount, at the very best, what you are going to get is the idealized "single lens at the mount plane" situation, and because of that, the largest aperture you can get on a PK mount is limited to 45.5/45 = f1.01.
did you mean to type f1.01? for most manufacturers (and photgraphers) thats more than close enough to label as f1.0.
05-17-2009, 01:02 AM   #109
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well thought out argument there Rawhead...I was going to go through the math myself but My mind wasn't up to it( I've had this headache for three days damn it all). but have you taken into consideration of making a lens under the DA specification? it certainly wouldn't have to cover the full 36X24mm frame area so surely it would be possible to make faster lenses.

Any pentax 50mm f/1.2 wipes the floor with the noct nikkor when it comes to flare resistance and resolution. It's one of the very few nikkor lenses I have used and on digital it suffers from a few problems associated with legacy lens designs (it suffers from veiling glare rather badly fall off isn't too good either) but is is very well corrected for coma, but it still suffers from it depending on your focus distance. curiously, the Noct nikkor 55mm f/1.2 includes a hand ground aspherical element. Like many f/1.2 lenses is has it's own particular way of rendering OOF areas.


Last edited by Digitalis; 05-17-2009 at 01:12 AM.
05-17-2009, 01:54 AM   #110
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QuoteOriginally posted by illdefined Quote
did you mean to type f1.01? for most manufacturers (and photgraphers) thats more than close enough to label as f1.0.

You're right. But there, I think the problem becomes how difficult it is to achieve the "theoretical maximum". When Canon made the EF mount, they didn't go for a mount diameter of 44mm, which, with EF's 44mm register, will give them a maximum theoretical f-stop of 1.0, they went with a whopping 51.2mm opening (theoretical maximum f0.85), probably because it makes it that much easier (relatively speaking, that is) to make an f1.0 lens.

Similarly, the reason why f1.0 and below lenses are fairly common (again, relatively speaking) in C-mount cine lenses is because of their very, very short registers (17.5) compared to their mount diamater (25mm); ditto for Leica M-mount.

So while f1.01 is the theoretical maximum for PK mount, in reality, I think f1.1 would be the "ridiculous" lens (akin to Canon's F1.0L lens) on PK. From what I hear about the IQ of the Canon f1.0, it might make sense that Pentax stick to their f1.4s and maybe f1.2s :-)
05-17-2009, 02:06 AM   #111
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QuoteOriginally posted by Digitalis Quote
well thought out argument there Rawhead...I was going to go through the math myself but My mind wasn't up to it( I've had this headache for three days damn it all). but have you taken into consideration of making a lens under the DA specification? it certainly wouldn't have to cover the full 36X24mm frame area so surely it would be possible to make faster lenses.
Thanks.

See, that's the part where my understanding of optics gets a little fuzzy; my gut feeling is that while it's "easier" to make faster lenses with smaller image circles, you are still confined to this theoretical limit. Thus, the "difficulty level of making fast lenses" curve for APS-C starts out really low compared to FF at larger maximum apertures, but rises dramatically as the maximum aperture increases, and will hit the same asymptotic line at f1.01 as on FF.

In other words, it's much easier to make an f2.8 lens on APS-C than FF. Still very easy to make f2.0 lens on APS-C than FF. For f1.4, it's really not much different (that might explain why Pentax hasn't released a DA 50/1.4) and for f1.2, it is almost just as hard to make it for APS-C as it is for FF. And finally, regarding f1.0, it is as impossible to do on APS-C as it is on FF. Do you understand what I'm trying to say? :-)

Again, that's my (not very) educated guess.
05-17-2009, 02:33 AM   #112
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0,95 on micro4/3rds comparison

I don't know, haven't been following the discussion here with much concentration, but just stumbled across the following link, which I thought may be of some interest in this thread. Not knowing if it may support the one or the other point from someone here...
Here we go:
0,95 lenses on DMC-G1

Last edited by MMVIII; 05-17-2009 at 10:47 AM.
05-17-2009, 04:10 AM   #113
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QuoteOriginally posted by RawheaD Quote
If someone wants to prove me wrong, just give me one example of an F1.0 lens on a mount/system that has a longer register than the mount diameter.
RawheaD, I very much welcome your post. It perfectly explains why a f/1.0 lens for PK mount is so hard to make. So hard that it actually make little sense to try.

However, for the sake of completeness of your argument, let me add that retrofocus designs can escape your limit.

Mathematically, your argument only holds true for a single "thin lens" element (where optical rays are bended only once). And even then, only for a zero diameter image circle.

Basically, retrofocus designs start with a short focal length at the front of the lens and a long focal length at the end of the lens. So, you could bend the rays twice: on entry into the lens, and where it passes the mount opening.


This is why, in some previous post, I said that it is hard to make. Because an f/1.0 lens which is sharp wide open is hard in the first place. Add to this a retro-focus design and you may find that it isn't worth the trouble at all.

05-17-2009, 07:34 AM   #114
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QuoteOriginally posted by MMVIII Quote
I don't know, haven't been following the discussion here with much concentration, but just stumbled about the following link, which i thought it may be of some interest in this thread. Not knowing if it may support the one or the other point from someone here...
Here we go:
0,95 lenses on DMC-G1
Regarding micro-4/3, the register is even shorter than M-mount, which means it can mount a Leica f0.95/f1.0 lens no problem with an adapter. Furthermore, the register is very close to that of C-mount, and since it's a mirror-less design, there are C-mount to micro-4/3 adapters that sink into the G1 a few millimeters, making the mounting of C-mount lenses with very fast apertures (like the Xenon 25/0.95) possible.

These cases don't prove my hypothesis correct, but supports it, because, like rangefinders and C-mounts, micro-4/3 has a much larger mount diameter (44mm) compared to it's register (less than 20mm, don't know exact number).k
05-17-2009, 07:40 AM   #115
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As we all know ISO and shutter speed is not the only reason to use a large aperture. One of the good things about a fast 50 was you could use it to isolate the subject with DOF. Now for me more often then not I am looking for more DOF not less but I do see the point in less DOF sometimes. If a 30mm is going to be the new fast 50 then it needs to be even faster. Even at f/1 it cant isolate the subject as well as a 50 but it is a lot closer. When you take into account that most lenses need to be closed down 1-2 stop to get the best out of them then having a 30mm f/1 is even more desirable and that is why some will pay premium prices.

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05-17-2009, 07:56 AM   #116
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QuoteOriginally posted by DAZ Quote
As we all know ISO and shutter speed is not the only reason to use a large aperture. One of the good things about a fast 50 was you could use it to isolate the subject with DOF. Now for me more often then not I am looking for more DOF not less but I do see the point in less DOF sometimes. If a 30mm is going to be the new fast 50 then it needs to be even faster. Even at f/1 it cant isolate the subject as well as a 50 but it is a lot closer. When you take into account that most lenses need to be closed down 1-2 stop to get the best out of them then having a 30mm f/1 is even more desirable and that is why some will pay premium prices.
So, with a 50mm f/1.4 on a Pentax dSLR (assuming circle of confusion of 20 m) at a distance of 6 feet, you've got about 2.9" in focus. A 30mm lens will give you the same framing at 3.6', and a depth of field at f/1.4 of about 2.9". (In fact, it's exactly the same.)


The assumption about how stopping down to get better performance works has some flawed thinking, too, I'm afraid.
05-17-2009, 08:15 AM   #117
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I still maintain that it is physically impossible to make an F1.0 lens on a PK mount, and so far I have not seen anybody provide information that suggests otherwise.



Maybe in FF or 35mm film SLRs but a 30mm F1.0 for the smaller sensor DSLRs should not be a problem. Olympus has F2.0 zooms like the 14-35mm & 35-100mm and in FF or 35mm film SLRs that would be impossible.




05-17-2009, 08:18 AM   #118
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QuoteOriginally posted by DAZ Quote
As we all know ISO and shutter speed is not the only reason to use a large aperture. One of the good things about a fast 50 was you could use it to isolate the subject with DOF. Now for me more often then not I am looking for more DOF not less but I do see the point in less DOF sometimes. If a 30mm is going to be the new fast 50 then it needs to be even faster. Even at f/1 it can’t isolate the subject as well as a 50 but it is a lot closer. When you take into account that most lenses need to be closed down 1-2 stop to get the best out of them then having a 30mm f/1 is even more desirable and that is why some will pay premium prices.

DAZ
I think it's a case of diminishing returns and cost/benefit.

According to DOFmaster On the K20, subject distance of 10':
50mm @ f/1.4

Depth of field
Near limit 9.67 ft
Far limit 10.4 ft
Total 0.68 ft

In front of subject 0.33 ft (48%)
Behind subject 0.35 ft (52%)

Hyperfocal distance 290.2 ft
Circle of confusion 0.02 mm
30mm @ f/1.4

Depth of field
Near limit 9.13 ft
Far limit 11 ft
Total 1.91 ft

In front of subject 0.87 ft (45%)
Behind subject 1.05 ft (55%)

Hyperfocal distance 104.5 ft
Circle of confusion 0.02 mm
30mm @ f/1.0
Depth of field
Near limit 9.37 ft
Far limit 10.7 ft
Total 1.35 ft

In front of subject 0.63 ft (47%)
Behind subject 0.72 ft (53%)

Hyperfocal distance 147.7 ft
Circle of confusion 0.02 mm

That means you'd be paying a massive premium to shave off a whopping 6" of depth wide open (to clarify I mean a 1.0 versus a 1.4 lens).

@ a subject distance of 20', the theoretical 30mm/1.0 is doing better, but still not in thesame league as the 50mm.
50mm/1.4
Depth of field
Near limit 18.7 ft
Far limit 21.5 ft
Total 2.75 ft

In front of subject 1.28 ft (47%)
Behind subject 1.47 ft (53%)

Hyperfocal distance 290.2 ft
Circle of confusion 0.02 mm
30mm/1.4
Depth of field
Near limit 16.8 ft
Far limit 24.7 ft
Total 7.91 ft

In front of subject 3.2 ft (40%)
Behind subject 4.71 ft (60%)

Hyperfocal distance 104.5 ft
Circle of confusion 0.02 mm
30mm/1.0
Depth of field
Near limit 17.6 ft
Far limit 23.1 ft
Total 5.49 ft

In front of subject 2.38 ft (43%)
Behind subject 3.12 ft (57%)

Hyperfocal distance 147.7 ft
Circle of confusion 0.02 mm


Best bang for the buck seems like it would be a 33mm/1.2. Not quite as fast as the theoretical lens but closer to the same angle of view as a true fast fifty on the crop sensor, easier to manufacture based on the analysis in this thread, and right around double the DOF of the 50/1.4, and slightly less than the 30mm/1.0:
@ 10 feet
Depth of field
Near limit 9.38 ft
Far limit 10.7 ft
Total 1.32 ft

In front of subject 0.62 ft (47%)
Behind subject 0.7 ft (53%)

Hyperfocal distance 150.3 ft
Circle of confusion 0.02 mm
@ 20 feet
Depth of field
Near limit 17.7 ft
Far limit 23.1 ft
Total 5.39 ft

In front of subject 2.34 ft (43%)
Behind subject 3.05 ft (57%)

Hyperfocal distance 150.3 ft
Circle of confusion 0.02 mm


mattdm replied after I wrote this, and I didn't feel like doing another chart for 5', but a quick check will show that the relationships are fairly linear.
05-17-2009, 08:50 AM   #119
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QuoteOriginally posted by mattdm Quote
So, with a 50mm f/1.4 on a Pentax dSLR (assuming circle of confusion of 20 m) at a distance of 6 feet, you've got about 2.9" in focus. A 30mm lens will give you the same framing at 3.6', and a depth of field at f/1.4 of about 2.9". (In fact, it's exactly the same.)
I see what you are saying you can change 3 parameters to change DOF (keeping it apples to apples with things like coc the same) and you are saying just change distance. I was thinking that to keep the FOV about the same of a 50mm on FF then you would use something like a 30mm (more like 33mm) on an ASP-C keeping the distance the same. So if you wish to keep the distance the same (like a person at a conversation distance) you would use a 30mm and close the aperture more to keep the DOF isolation. Yes you could just move closer but this brings up things like more facial distortion and personal space issue. This is why I tend to use my 31mm like I used my 50mm on film.

DAZ
05-17-2009, 08:59 AM   #120
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QuoteOriginally posted by nixcamic Quote
If I understand what you're trying to say by sliding f ratio (variable aperture on zooms), I think thats actually done by the 6 aperture connectors that change depending on when in the zoom range you are. And just to slightly clarify, older bodies with aperture ringless lenses don't shoot wide open, they shoot all the way stopped down (much less useful). Not nitpicking, just think that a rumor thread should be full of lots of random semi-useless non-rumors, to make it worthwhile
Features and Operation of the Kaf Mount
The less obvious of the two is the addition of a seventh body-lens contact, next to the six that were already added by the the KA mount. The new contact is used for exchanging digital information, and in the KAF mount supports the exchange of the following lens paramters: focal length, lens-to-subject-distance, exact absolute value of the selected f-stop (even for variable-aperture zooms), and lens size. This information is mainly used for making flash operation easier and less error-prone.
Oh heck more from Mr. Dimitrov.
The KAF mount features the same six electric contacts as the KA, and thus remains fully compatible with the older mounts. However, in addition to those (electric) contacts, it features a seventh — electronic one. Using a yet-unknown serial protocol, the body uses this contact to communicate with a digital chip contained inside the lens housing. The chip provides at least the following information:

* Focal length of the lens. This allows more efficient flash operation as well as calculating the longest hand-holdable shutter speed. In addition, it is also one of the parameters taken into account when the body is in one of the "smart" picture modes and has to choose a program curve. Prime lenses provide one fixed value, while zooms provide an approximate value that changes as the focal length changes. The focal length information is precise enough to distinguish between 35, 28 and 24 mm lenses, as evidenced by F-flashes that trigger a warning when they cannot cover the field of view of the lens.
* Lens-to-subject distance. This is used by the more advanced bodies for calculating if the flash is powerful enough to illuminate the subject that is in focus. In addition, this is one of the parameters taken into account when the body is in one of the "smart" picture modes and has to choose a program curve.
* Exact selected aperture. The KA mount supports displaying the picture-taking aperture when the "A" aperture setting is engaged. The KAF mount supports displaying the aperture even when a numeric f-stop is selected. The displayed value is quantized by half-stops, but is correct even when a variable-aperture zoom lens is used.
* Lens dimensions. The lens also gives the body enough information for determining if the lens is going to cast a shadow on the subject when the camera's built-in flash is used. This parameter takes into account the size changes due to zooming and focusing too.

The lenses contain a ROM chip that holds the program curve (brightness/shutter speed/aperture) for prime lenses or up to three program curves for zooms at their different focal length settings. The distance information is hardwired on the lens barrel. A number of tiny parallel conducting/non-conducting strips are glued to the rotating part of the lens. The same number of contacts are attached to the fixed part of the lens. As the lens barrel turns during focusing, different combinations of the strips are detected by the contacts. There are four parallel strips in the F 50/1.7, resulting in at most 16 different distance settings. But for that particular lens not all combinations are used.

A similar technique must be used for the focal length.
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