Fast lenses and Olympus

[jonoslack]

I was looking at various 4/3 lens specs today, and it occurred to me that Olympus have never made anything faster than f2

Then, thinking back, I remembered someone on DPreview called . . . erm . . Joe Wiznezki - something like that, talking about telecentricity, and that it wasn't possible to make a properly telecentric lens with the size of the 4/3 mount at anything faster than f2.

I don't know if this is true or not, (not being an optical whizzer), but I'm quite sure that Sigma has pretty much ignored the telecentricity rules and simply bunged a 4/3 mount on their existing lenses.

There has been a bit of talk of Olympus relaxing their rules due to better sensor design, so maybe we'll finally see some faster primes.

 

[Vivek]

Joseph Wizniewski- engineer (retired from GM or Chrysler or something like that)/photographer.

Not sure that (f/2) is a hard and fast rule, Jono. It would depend on the focal length and prime or zoom, I would think.

Fast lenses on their own are expensive.

The Sigma 30/1.4 is not a telecentric lens (I have the Nikon mt version).

The 4/3rds and m4/3rds, AFAIK (could be wrong) still require telecentric lenses for optimal results.

Perhaps, in the future, when Panasonic (the sensor maker) incorporates microlenses, they would not need them as much.

 

[jonoslack]

Hi Vivek
I hope you're well

Joseph Wizniewski- engineer (retired from GM or Chrysler or something like that)/photographer.

Not sure that (f/2) is a hard and fast rule, Jono. It would depend on the focal length and prime or zoom, I would think.

I think he was saying that if you were faster than that at any focal length it would no longer be telecentric. Still, the fact that Olympus hasn't made one seems to point in that direction

Fast lenses on their own are expensive.

:ROTFL: That never stopped Olympus, look at the 90-250 f2 and others.

The Sigma 30/1.4 is not a telecentric lens (I have the Nikon mt version).

The 4/3rds and m4/3rds, AFAIK (could be wrong) still require telecentric lenses for optimal results.

Perhaps, in the future, when Panasonic (the sensor maker) incorporates microlenses, they would not need them as much.

I thought that they had pointed out that on the newer CMOS sensors the 'wells' were much less deep, thus allowing some relaxation of the rules?

 

[Godfrey]

The statement isn't so much of telecentricity as of mount design limits, based on the reading I've done (including Joe W's comments).

Summing up the documentation I've read:

- The FourThirds lens mount design incorporates a lens mount diameter to allow an f/1.4 lens design. Faster than that and the size of the lens elements required will result in little effective light transmission increase due to excessive vignetting. This seems to be bourne out: a modest (acceptable) amount of vignetting when wide open is visible with all the available ultrafast lenses ... Sigma 24mm f/1.8, Panasonic/Leica Summilux 25/1.4, Sigma 30/1.4 and Sigma 50/1.4.

- Image quality from *all* digital sensors, even those with shallower photosite wells and regardless of which size/format sensor you're talking about, benefits from more telecentric lens designs. It's the nature of the beast. The latest sensors have shallower photosite wells than the designs available a decade ago when FourThirds was created, which serves to relax the requirement for telecentricity to some degree, but a more telecentric design lens will *always* perform better on a imager sensor (all other factors being equal), at least with any current imager sensor design. No amount of corrective, differential reshaping of the photosite lenses will change that: that technique is simply a way to get around the design problems for existing lenses.

- Making ultra-fast lenses with telecentric optical designs poses additional constraints on size, weight, cost and image quality.

Olympus has concentrated their efforts so far on providing telecentric lens designs with up to f/2 light gathering power to get the best quality imaging possible even wide open, with practical consideration given to size/weight/cost/etc. Only the Panasonic/Leica 25 of the available ultra-fast lenses really performs well wide open, and it's a pretty complex/expensive piece for a normal lens.

I suspect that f/1.4 lenses which can perform well and also be reasonably telecentric will surface for micro-FourThirds. The shorter mount register of micro-FourThirds permits much more latitude for lens design in the focal length range for which such ultra-fast lenses are a practical consideration.

 

[jonoslack]

HI Godfrey
Thanks for chipping in, and for describing it all so succinctly and comprehensibly.

I guess we'll have to wait and see what transpires.

 

[CharlieH]

One thing I'm curious about is whether Olympus was concerned with telecentricity when they designed the Pen-F lenses? Has this always been part of their philosophy or was it originated with 4/3rds? I think they would have an instant hit if they just reissued some Pen-F lenses with micro 4/3rds mounts. Zero development costs and who needs all this electronic lens wizardry anyway?

I found this quote on their web site:

"Brighter lenses by approximately two f-stops with same depth of field as 35mm lenses (plus solutions to provide shallow depth of field if required)"

So if I'm reading that correctly we should be happy with f/2 lenses since they have as much depth of field as an f/4 35mm lens. You always want more depth of field, right? I don't know what they are talking about with the "solutions to provide shallow depth of field".

ch

 

[Jorgen Udvang]

When they launched the 4/3 system, Olympus actually stated that they wouldn't make faster lenses than f/2.0, exactly for the reasons mentioned earlier in this thread.

Not very different from what they did for the OM system: the fastest aperture for 21, 24, 28, 35, 40, 85, 100, 180 and 250mm was also 2.0, in addition to three of their endless range of macros, the 20, 50 and 90mm. The only OM Zuikos that were faster were the 50mm f/1.2, 1.4 and 1.8 and the 55mm f/1.2.

 

[jonoslack]

When they launched the 4/3 system, Olympus actually stated that they wouldn't make faster lenses than f/2.0, exactly for the reasons mentioned earlier in this thread.

Not very different from what they did for the OM system: the fastest aperture for 21, 24, 28, 35, 40, 85, 100, 180 and 250mm was also 2.0, in addition to two of their endless range of macros, the 50 and the 90mm. The only OM Zuikos that were faster were the 50mm f/1.2, 1.4 and 1.8 and the 55mm f/1.2.

Such erudition (and what a memory).
Thanks Jorgen. I was never an OM aficionado, I think it was rebellion, as my father took such wonderful pictures with his. First 'real' camera I had was a Contax 139.

Interesting them saying that they wouldn't go faster than f2, they do appear to have paved the way for doing that now. But perhaps they figure that, with the 4/3 sensor, they aren't ever really going to compete with the small DOF brigade, in which case they may as well avoid the compromises that kind of design inevitably requires.

 

[Robert Campbell]

I thought that I read that the original 4/3 specification imposed a limit of f:2, even if this is now relaxed

 

[Peter Klein]

For what it's worth, I tried using an OM Zuiko 50/1.4 lens on an E-1 several years ago. There was virtually no difference between f/2 and f/1.4, in terms of the amount of light hitting the sensor. I determined this by shooting a white wall at various stops, and checking the pixel values of the resulting pictures.

I did a similar quick-and-dirty light level test of the E-510 when I picked up a used one last year. Same results. So the OM 50/1.4 I already had made a decent f/2 lens, and a much cheaper alternative than the 50/2 macro. But the standard 14-54 zoom outperformed it, stop for stop.

Bottom line--don't buy a 4/3 system to use your fast OM Zuikos wide open. On the other hand, the OM teles, particularly the 100/2.8, work wonderfully. And the 50/3.5 macro is just plain spectacular, from one inch to infinity.

If you want f/1.4 on a (classic) 4/3 camera, right now the Panasonic 25/1.4 is the only truly viable solution.

Sensors just ain't film.

--Peter

 

[Godfrey]

One thing I'm curious about is whether Olympus was concerned with telecentricity when they designed the Pen-F lenses? Has this always been part of their philosophy or was it originated with 4/3rds? I think they would have an instant hit if they just reissued some Pen-F lenses with micro 4/3rds mounts. Zero development costs and who needs all this electronic lens wizardry anyway? ...

Telecentricity in lens design wasn't much of an issue with film since film is utterly insensitive to the incident angle of light hitting it. I doubt very much that the Pen F lenses certainly weren't designed specifically to have telecentric light paths.

However, the Pen F cameras were SLRs and the lenses had to clear the swinging mirror. Lens designs that allow SLR required stand-off from the film plane also tend to be more telecentric than lens designs for rangefinder and compact cameras, which as an aside is why most older SLR lenses in other systems can still work fairly well on "APS-C" sized sensors today. The problems tend to show up on "full frame" sensors and with wide, fast lenses.

The Pen F lens range only went down to a 20mm lens anyway, which is just a wide-normal for FourThirds format, and you usually find the issues creeping in with lenses that provide a wide to ultra-wide field of view and a large lens opening. Adapted SLR lenses in the range that I use a lot ... normal to telephoto ... are the least likely to show up the problems since their light paths are fairly telecentric as a natural consequence of their focal length and geometry.

Much as I love the Pen F lenses, I doubt there would be many buyers for a micro-FourThirds lens line that did not support autofocus, auto-iris control, etc etc. *I'd* be interested, but I think people who fall into the same class with us number in the hundreds, not the tens of thousands needed to support a production line profitably.

 

[Vivek]

Microlenses on the photosites would help. Without this, it would be undesirable to make use of T/S lenses on a DSLR, for example. No telecentricity with a T/S lens.

Also, Olympus never said that their lenses are telecentric but merely "near" telecentric.

 

[Jorgen Udvang]

Bottom line--don't buy a 4/3 system to use your fast OM Zuikos wide open. On the other hand, the OM teles, particularly the 100/2.8, work wonderfully. And the 50/3.5 macro is just plain spectacular, from one inch to infinity.

If you want f/1.4 on a (classic) 4/3 camera, right now the Panasonic 25/1.4 is the only truly viable solution.

Sensors just ain't film.

--Peter

I agree, and only a few telephoto lenses work well wide open on 4/3. The 100/2.8 was my most used lens on the E-1, but it was only really sharp from f/4.0 and up. The 200/5.0 on the other hand, worked perfectly well wide open.

My understanding is that sensor changes is one of the reasons for the more relaxed attitude from Olympus' side. The new sensors from Panasonic apparently allow light from a sharper angle.

 

[Ranger 9]

The strict definition of the term telecentric is much more specific than what we seem to have been using in the context of this discussion, as you'll see if you read the linked Wikipedia article.

I seem to recall that whenever Panasonic has used the term in its discussions of Micro Four Thirds lenses, it has placed it in quotation marks; one interpretation of this would be that they are tryting to tell us that in Four Thirds lens designs they try to follow telecentric design principles, but the lenses don't meet the strict definition of telecentric.

It could be that engineer Wizniewski's comments about an f/2 limit do apply to strict telecentric lenses, but don't apply to "telecentric-ish" lenses designed for camera use.


[Don't believe it? If anybody happens to have an optical bench, presumably it would be easy enough to test a Micro Four Thirds lens or two and see whether or not they comply with this definition from the Wikipedia article: "An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor. This allows the lens to be focused to different distances without changing the size of the image." I'd think a lens with this property would have sufficiently weird photographic results that it would be easy to notice compared to a conventional lens, in which we expect distant objects to be rendered at a smaller size than nearby ones.]

 

[Jonas]

[(...) "An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor. This allows the lens to be focused to different distances without changing the size of the image." I'd think a lens with this property would have sufficiently weird photographic results that it would be easy to notice compared to a conventional lens, in which we expect distant objects to be rendered at a smaller size than nearby ones.]

Hmm. I think you wrote the above in a haste.
They do not mean that distant objects shouldn't be rendered smaller than nearby ones. Rather they assume all rays of light coming out from the rear lens element to be parallel (and thus angled exactly 90 degrees to the sensor). As a result there would be no change in focal length when focusing.

Well, that's what i think about it.

regards, /Jonas

 

[Godfrey]

The strict definition of the term telecentric is much more specific than what we seem to have been using in the context of this discussion, as you'll see if you read the linked Wikipedia article. ...

It's an interesting article. But without wanting to delve into the unwarranted deep optical discussion, suffice it to say that the term telecentric used with regards to Olympus (and Panasonic) FourThirds lenses in their public technical literature is probably more loosely defined than the engineering/optical formal definition.

A telecentric lens in this loose FourThirds parlance simply means a lens that is designed such that the ray path to the sensor meets the sensor at close to a ninety degree angle to the sensor plane. I imagine that as focal lengths become shorter, the lens design begins to resemble more of what might be considered telecentric by the formal optical definition.

It is possible to over-think this stuff... we are gearheads. ;-)

 

[Ranger 9]

Hmm. I think you wrote the above in a haste.
They do not mean that distant objects shouldn't be rendered smaller than nearby ones. Rather they assume all rays of light coming out from the rear lens element to be parallel (and thus angled exactly 90 degrees to the sensor). As a result there would be no change in focal length when focusing.

I didn't write it, I just quoted it. The article explicitly says "This allows the lens to be focused to different distances without changing the size of the image."

I admit that this is hard for me to conceptualize -- but for a lens to be "focused at different distances," it seems clear that the objects focused upon must also be at different distances. And if this can be done "without changing the size of the image," then it must be saying that objects at different distances will be imaged at the same size.

Yes, it sounds crazy -- but no crazier than object-space telecentricity, which allows objects at different distances from the lens to be imaged at the same size on the imager, and which is what makes telecentricity useful for optical-measurement applications.

Clearly the photographic effects of this would be weird compared to what we're used to, so I'm sure that true telecentricity works only under very limited conditions. For example, I read elsewhere (but didn't save the link, unfortunately) than an object-space telecentric lens has to have an entrance pupil larger than the field to be measured. In other words, if you want to set up a machine-vision application to measure parts that are nominally 1 cm long, you need a lens with an entrance pupil larger than 1cm.

That's not a big problem at such a small size... but it implies that if you wanted to design an object-space-telecentric lens for photographing people, you'd need a lens more than 2 meters across! (I guess that explains why we can't just go out and buy one, even though it would be hugely useful -- imagine being able to do sports photography, for example, with a lens that showed a player at the same size whether he was on the near side of the field or the far side!)

If we assume that image-side telecentricity is just the inverse of the same principle, then it implies that the lens' exit pupil would need to be larger than the size of the sensor to be covered.

And that, I suspect, is what led the previously-quoted engineer to write that a truly telecentric lens for Micro Four Thirds couldn't be faster than f/2. I don't know exactly how he computed that, but presumably based on the diagonal size of the 4/3 sensor (22.5mm), the lens mount diameter (41.3-something mm, if I remember correctly) and the flange-to-sensor distance (20-ish mm) a lens big enough to have an exit pupil larger than 22.5mm and mounted far enough away to clear the flange distance would have some of its rear diameter masked by the lens mount.

And you're right, for the most part this is just optical-nerd esoterica. I offered it only as an explanation of how an engineer might determine that a TRUE telecentric lens for Micro Four Thirds couldn't be faster than f/2, while a photographic lens that's only "telecentric" in a marketing sense (i.e., designed along telecentric principles, but not strictly telecentric) could be faster than f/2.

I can definitely state through practical experience that my 50mm f/1.5 Nokton lens on the G1 lets me use a faster shutter speed when set to f/1.5 than when set to f/2, so it's clear that real-world non-telecentric lenses don't adhere to an f/2 maximum aperture limit.

 

[Audii-Dudii]

I admit that this is hard for me to conceptualize -- but for a lens to be "focused at different distances," it seems clear that the objects focused upon must also be at different distances. And if this can be done "without changing the size of the image," then it must be saying that objects at different distances will be imaged at the same size.

I'm no optical expert (or even amateur!), but I know that when I focus my large-format cameras, the size of the image circle projected onto the ground glass typically changes as well and by a large enough amount that I'm often forced to recompose slightly afterward to compensate. Could this be what they mean?

After all, if the image from the lens was being projected at a true 90 degrees to the sensor, the image circle would always remain the same size regardless of how far away from the sensor the lens is positioned.

 

[Vivek]

After all, if the image from the lens was being projected at a true 90 degrees to the sensor, the image circle would always remain the same size regardless of how far away from the sensor the lens is positioned.

Yes. Simple test is 1. change the apertures- should be no change in the image circle. 2. Change the focus- no change in image circle.

Not so simple to discern- area of illumination and area of covered are often different for non telecentric lenses.

 

[jonoslack]

It is possible to over-think this stuff... we are gearheads. ;-)

I don't think it's possible to over-think it (stretching one's brain a little must be a good thing?). I do think it's possible to over-interpret it though.

HI Everyone
really interesting stuff - although I understood the principles of telecentricity I hadn't really thought about it with reference to aperture, and THAT makes my head hurt!

I've always felt that the consistently excellent quality of Olympus 4/3 lenses (even the humble zooms) has been to do with 'telecentricity', of course, I could be wrong (after all, Zuiko lenses have always been good), but whatever it is, I do hope it carries on down to the m4/3 lenses.