[DAZ]

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

Oh I am definitely NOT trying to justify cost/benefit. Nor am I saying it is even possible/practical. I am just trying to point out that it maybe desirable for other then ISO/shutter speed reasons. Yes at the mythical f/1 a 30mm is only half as good at DOF isolation as a 50mm at f/1.8 but it is still a more DOF then a 30mm f/1.8.

DAZ

[mattdm]

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.

Of course — that's what's meant by the whole "focal length equivalent" thing.

I think in general that as you get wider and wider, using wide apertures becomes more difficult, because you're more likely to have elements in the photograph which are near the plane of focus but still blurry. With narrower focal lengths, it's easy to get just the subject and background (and the background can then by very blurry).

Anyway, at "normal" focal lengths at "conversational" distances with a 30mm lens on APS-C, f/1.4 barely gives enough depth of field for a subject's nose and ears to both be in focus. I don't think an f/1 lens — possible or not — would be useful in enough situations to justify the huge increase in size, weight, complexity and cost. I'm not saying it's useless, just extreme.

Me, I'd be very happy with a DA★ 30mm f/2.0.

[RawheaD]

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.

Note, again, that Olympus 4/3 has a very large mount diameter (50mm) compared to its register distance (38.67mm), making the theoretical maximum aperture f0.77, according to my calculation. Thus, I would actually contend that that is the reason why it's easier for them to make fast lenses, *not* the smallness of the sensor size (although having a small sensor size is probably correlated with having a short register---my guess is that it is difficult to design a large sensor/lens combination when the register is very small; this is probably the reason why we haven't seen a full frame Leica M-mount digital camera. It's not as simple as throwing a full size sensor on the film plane of an M-8).

[RawheaD]

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.

Here, again, my understanding of optics is limited, so I have nothing definitive to say; however, I can perfectly understand how the retrofocus design will allow you to have a lens with a focal length that is shorter than the register distance (indeed, you *need* a retrofocus design for that), I fail to see how it can overcome the limitation I presented. Here's my reasoning:


With a retrofocus design, you have a converging lens element at the rear, which is more or less stationary near the mount plane (with a certain amount of protrusion, as I mentioned above).


Whatever amount of light that is gathered in front of that element has no bearing on the f-stop of that lens. This is an important concept, because the f-stop is not dicated by the "amount" of light that passes through––if that were the case, any lens will become a super fast f0.1 lens by pointing it at the sun ;-)––but the degree to which it can take the light in front of it, and "concentrate" it on the focal plane.


So let's say you have a ginormous primary element, which gathers tons and tons of light, and the light travels through the barrel of the lens and it hits the rear element of your retrofocus lens. At that point, regardless of how much light has been brought there, it now lies squarely on that very final element to cocentrate that light on the focal plane, and the f-stop of that lens is dictated, again, by that very simple formula

f=F/D

Where the F is going to be a number close to the register distance sans the protrusion and D will be its diameter. So, again, the very last element and its diameter is going to be the bottleneck, and, in turn, the diameter of the rear element is constrained to the diameter of the mount.


I mean, think about it, if a retrofocus design makes it easier to achieve fast lenses, (a) wouldn't everybody make all their lenses retrofocus, even longer lenses, and (b) shouldn't we have more 30/1.4s and 24/1.2s, etc?

[falconeye]

f=F/D

If only things were this easy ...

[jogiba]

Note, again, that Olympus 4/3 has a very large mount diameter (50mm) compared to its register distance (38.67mm), making the theoretical maximum aperture f0.77, according to my calculation. Thus, I would actually contend that that is the reason why it's easier for them to make fast lenses, *not* the smallness of the sensor size (although having a small sensor size is probably correlated with having a short register---my guess is that it is difficult to design a large sensor/lens combination when the register is very small; this is probably the reason why we haven't seen a full frame Leica M-mount digital camera. It's not as simple as throwing a full size sensor on the film plane of an M-8).

Explain exactly how a 50mm F1.2 is not a problem with the K-mount on FF/35mm DSLR/SLR cameras but a 30mm F1.0 is on the much smaller APS-C .

[ytterbium]

Maybe because 50 mm is more than ~ 45mm register distance for K mount.
So 30mm needs a retro focus optics, because it cannot be well placed at its "native distance".
Designing F1.2 lens is already a challange. Designing so complex F1.0 lens, might be unreasonable (too expensive/heavy/many drawbacks).

---

Last edited by ytterbium; 05-17-2009 at 01:34 PM.

[jogiba]

Maybe because 50 mm is more than ~ 45mm register distance for K mount.
So 30mm needs a retro focus optics, because it cannot be well placed at its "native distance".
Designing F1.2 lens is already a challange. Designing so complex F1.0 lens, might be unreasonable (too expensive/heavy/many drawbacks).

The APS-C format should be much more easy to go one extra F stop for the same angle of view lens like the F2.0 zooms for 4/3rds format. 30mm is a normal lens on APS-C DSLRs just like 50mm is normal on FF DSLRs. Were are the low cost 26 mm - 676 mm (26x) ultra-zoom lens for FF DSLRs like on $360 ultra-zoom cameras ?

[jogiba]

Maybe because 50 mm is more than ~ 45mm register distance for K mount.
So 30mm needs a retro focus optics, because it cannot be well placed at its "native distance".
Designing F1.2 lens is already a challange. Designing so complex F1.0 lens, might be unreasonable (too expensive/heavy/many drawbacks).

How about a 20mm F1.4 for K-mount FF/35mm DSLR/SLRs ?


K 20/1.4 AL

[RawheaD]

If only things were this easy ...

Thanks, but, with all due respect, you're not addressing my point; can you give me a formula that does explain how the rear element's diameter is not the limiting factor? This is a sincere request, because I fail to see how it's possible, and would love to be proven wrong.

You also have not addressed my point about retrofocus designs; if retrofocus designs, by definition somehow increases the aperture, why is there not a single 35mm f1.2 lens in existence? Why did Canon have to design a mount with a much larger diameter than their FD mount to make the 50/1.0 a possibility, rather than make a retrofocus design 50mm lens?

[RawheaD]

Explain exactly how a 50mm F1.2 is not a problem with the K-mount on FF/35mm DSLR/SLR cameras but a 30mm F1.0 is on the much smaller APS-C .

I can't believe I'm reading this. My previous several posts did, or at least tried to do, this very thing.

Why don't you do this for me; Explain exactly how a smaller sensor makes designing a faster lens for it so much easier, and specifically, how it would make designing an f1.0 lens possible, when at full frame it was not.

If it's so much easier to make faster lenses for APS-C, why are all of the Pentax DA lenses, including limited lenses, slower than f1.8? The only DA lens that is 1.4 is the DA* 55/1.4, and that's a monster of a lens.

Again, bringing in examples from four thirds and micro fourthirds doesn't support your hypothesis, because, as I've said several times over, 4/3 and MFT both have much shorter registers than their mount diameters, which actually supports my hypothesis.

[falconeye]

can you give me a formula that does explain how the rear element's diameter is not the limiting factor?

I just wanted to make a comment that things may be more complicated.

Now, since you ask again, I need to dig into it a bit.

First of all, you are right, a retro-focus or tele design (retro-focus is an inverted tele design) do not by itself overcome your limit (cf. the Wikipedia animations).


(Retro-focus lens design. H is the thin lens principal plane)


(Tele lens design. H is the thin lens principal plane)

But then, if I see lens constructions like the following (taken from a telecentric design), I am not sure that the rear element's diameter is always that limiting ...


As far as I understand, the mount hole diameter must be larger than the exit pupil's diameter (let's forbid protruding rear elements for a second).

The f-stop is the focal length divided by the diameter of the entrance pupil.

Now, if exit pupil >= entrance pupil, then you are perfectly right. But simply quoting f/D may not be enough to proove it. I know that equality of exit and entrance pupil does not always hold true. In the Wikipedia animations shown above, they are equal, actually.


So, I made my "if only things were this easy ... " comment

[Aristophanes]

And soft
Some facts and history of the 50mm f1.0 and why it may really not be necessary:
That's the Canon 50mm f1.0 lesson...: Open Talk Forum: Digital Photography Review
john p vansteenberg wrote:
> I
My sources say there was just one production run, and it took Canon 15 years to clear them all out of the warehouse.

Aaaahhhhh.......

Depth of Field meet depth of market. Latter wins, hands down.

There will be no 30mm. It's off the roadmap because it would:

1) Cannibalize or make redundant sales of other Pentax lenses (FA 31mm/DA 35mm)

2) Market is too small for primes (see above); focus is mostly on zooms.

3) Sub f/1.4 = $$$$$ Pentax has made it totally clear that their market is mid and low-end.

Al the tech speculation is interesting, but misses the most important metric of all:

Follow the money for the truth. Haven't we learned anything in the last 12 months?

[mattdm]

Aaaahhhhh.......
1) Cannibalize or make redundant sales of other Pentax lenses (FA 31mm/DA 35mm)

There's definitely room for a weather sealed normal.

2) Market is too small for primes (see above); focus is mostly on zooms.

Then why DA 15mm Limited? Or the DA 35mm Macro Limited, for that matter.

3) Sub f/1.4 = $$$$$ Pentax has made it totally clear that their market is mid and low-end.

Now this one, I agree with.

[jogiba]

How about a 240mm F1.2 ?



On the 6x7 Pentax !