P67 Optics

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As an avid user of this camera and having a background in optics, I would be happy to answer as many questions as possible from other P67 advocates. Did you know that SMC ia a seven layer coating or that all lenses since at least 1984 have been computer aided designs? Steve Rasmussen

-- Steve Rasmussen (srasmuss@flash.net), January 31, 1998

Answers

Benson, since both lenses use a slightly modified Double Gauss design, they should be about the same price. It seems that lens prices are related to the square inches of glass that must be ground. In this case, they should be within $50 dollars of each other. I own a 105mm and it is super sharp. SR

-- Steve Rasmussen (srasmuss@flash.net), February 01, 1998.

Be forewarned, the use of the term APO by certain manufacturers is misleading. Consumers believe it is a guarantee of quality in telephoto lenses. It can be, but all too often, the companies that don't use the designation are considered to make inferior telephotos. This black or white thinking is what many optics marketers want you to think. What is important is the ratio of secondary spectrum to the focal length. A 1000mm lens with a secondary spectrum of 1/20mm would have a ratio of 20,000 to 1 and would be considered to have excellent correction of Longitudinal Chromatic Aberration. Manufacturers know these numbers but will not share them with consumers; much to our detriment. There are many telephotos with no designation that out perform the ones that do. The P67 600mm f/4 is one such example. No ED glass, no aspherical elements and no APO claim but it is at least APO and possibly Superachromatic(the next step beyond APO). Its secondary spectrum must be very small, given its sharpness and yet Pentax does not hype or brag at all. They let performance speak for itself.

-- Steve Rasmussen (srasmuss@flash.net), February 02, 1998.

Dear Ambiguous: My opinion about why Pentax has such old fashioned macro capabilities has more to do with their redesign cycle than their optical philosophy. The 300mm is an old design, so they have an excuse on that one. The new 200mm you'll notice, has a 1.5 meter close focus vs the older 200 with 2.5 meter focus. So, they are changing. Pentax's redesign cycle is so long that the lenses that are last to be redesigned are painfully limited. (Try the 600mm on hummingbirds-only with the extension tube set.) It seems like they are aware of the problem. SR

-- Steve Rasmussen (srasmuss@flash.net), February 03, 1998.

Diffraction- When the wave nature of light is disturbed by objects (iris diaphram, dust)in the optical path. This is why aperture settings of around 5mm and less are detrimental to the image quality.

Dispersion- The separation of light into colors after passing through a refractive optical medium (lens). Low dispersion glass separates colors very little.

-- Steve Rasmussen (srasmuss@flash.net), February 21, 1998.


Robert: I have not used both lenses, just the takumar which is ten years old, so I can't give you a direct comparison. Be assured in knowing that this design (Double Gauss)is quite old and there haven't been major changes to it in decades. At this level of perfection, the older lenses are therefore quite good. The only changes I've heard of have to do with the correction of spherical aberration with the use of modern glasses. My 105 Takumar is a great lens. SR

-- Steve Rasmussen (srasmuss@flash.net), February 22, 1998.


Gary: You shouldn't have any problems with motion due to shutter/mirror shake if you take the proper precautions (even though this lens on the 645 is 10x magnification). I haven't used this lens myself but can speculate on its performance given its optical cross section. There is a serious attempt toward a symmetrical design here. This is good because several aberrations can be corrected automatically, leaving the designer with more freedom to concentrate on the other aberrations. Longitudinal Chromatic Aberration and Spherochromatism(change in spherical aberration with color)are the major concerns with long lenses. The 800 ED has a quadruplet group up front which can be an indicator that the lens is corrected for four colors (superachromatic). The old 800 also has a quadruplet up front but is not as well corrected in the rear group. My guess as to the difference between the old and new 800s is that the ED version will be much sharper wide open(due to less spherochromatism) and that you probably couldn't tell the difference between the two at f/8 through f/45. I have not seen any evidence of design change with the 800ED. My guess as to the best aperture is f/16. Not sure- I personally would look into the 800mm f/4 used, before spending that kind of money. You might check to see if outer bay lenses (800 f/4)can be mounted on the 645. Steve

-- Steve Rasmussen (srasmuss@flash.net), February 28, 1998.

Distortion- Differential image magnification at the film plane caused by the lens. Pincushion type- The image in the marginal areas of the focal plane are magnified more than the central areas. Barrel type- The image in the central area of the focal plane is magnified more than the marginal areas.

Asymmetrical designs have the biggest problem with distortion. This is why so many wide angle and telephoto lenses have distortion. Also, there is a trend to make both of these lens types symmetrical. German lenses are famous for this.

-- Steve Rasmussen (srasmuss@flash.net), March 12, 1998.


Joel: I don't own a 55mm lens so I'm at a little disadvantage. In general, Pentax will not change a lens unless they can improve it. I don't have the cross section of the f/3.5 but I can tell you that there have been three 55mm lenses that I know of. There is one f/3.5 version and two f/4.0 versions. The older f/4 was a nine element design that was very asymmetrical and had the typical negative elements in the front group which is common on wide angle lenses that need to clear the mirror box. (The effective focal length is 55mm but the distance from the front element to the film is much more). The newer f/4 is an eight element design that is still asymmetrical but there is a major change in design. It is more of a classic reversed telephoto. The older f/4 uses some thick elements while the newer version uses none. Both designs can produce sharp images though. Both f/4 versions have enough optical surfaces to correct all aberrations fairly well. My vote would have to be for the eight element(latest version) f/4 as the best designed 55mm. Test data from people who have different versions would help. Steve

-- Steve Rasmussen (srasmuss@flash.net), March 31, 1998.

I thought I'd share something with everyone concerning Pentax optics. I called the Pentax U.S. corp office in Colorado about a problem with one of my telephotos and had asked them for some technical info about this lens (600mm). They were very tight lipped about it and said that Japan does not like to give out information concerning optical aberrations or correction methods used. This is understandable due to the competative nature of the business. Steve

-- Steve Rasmussen (srasmuss@flash.net), April 15, 1998.

I thought I'd share a fact about color correction with the group since we spend so much on optics. Any lens over 150mm or so must consider correction of more than just two colors. The shorter lenses are generally corrected for blue and red. The longer the focal length, the more separation of light into colors will occur and the need for more drastic measures to compensate for it. It is important to remember that when a designer corrects for colors in the lens using different glasses, they can only correct those colors in one zone of the lens. Zones are imaginary areas(rings) that when you look into the front of the optic, emanate from the optical axis outward. These zones are often refered to in percent. Most older telephotos are corrected in the 40% zone while the newer ones are corrected in the 70% zone.(The 40% zone is 40% of the way from the optical axis to the edge of the lens.) Light passing through areas that are not a part of the corrected zone will cause image degredation. This is why you must stop down telephotos for best results. The diaphram truncates the partially corrected outer rays. So, an APO lens is not APO across the entire lens surface, only in one zone. Steve

-- Steve Rasmussen (srasmuss@flash.net), April 24, 1998.


Sheldon: The older nine element German-looking design has a close focus of .4 meters while the newer eight element Japanese-like design has a close focus of .35 meters. You can see this on the lens barrel. Also, the written designation on the lens itself, telling you the focal length, focal ratio, Pentax and SMC is in a different location on the newer eight element version. It is now on the front lip OUTSIDE not on the inside like the older 55mm f/4. BTW the 45mm design is very similar to the nine element 55mm f/4 design. It makes me wonder if they are going to upgrade the 45mm to a Japanese-like design as well. Sometimes, old designs perform so well that they don't need upgrading. The 75mm f/4.5 is based on a 1950 design called the retrofocus by Angenieux. The lens is Tack Sharp. Steve

-- Steve Rasmussen (srasmuss@flash.net), September 12, 1998.

Brian, good point. Manufacturers that use the term APO are mostly adding to the confusion among consumers about color correction in long focal length lenses. I believe Canon gave up on using that term and I don't recall Pentax ever using it. Having an industry standard for APO, achromatic and superachromatic would not solve the problem. The reason is that there is still too much latitude in each category to give the consumer an exact idea of the degree of correction. Two different APO lenses could be very different in performance. Even though these two would have three colors come to a focus at the film plane, it is the degree of correction of the remaining colors that is critical to performance. BTW I'm no optcal expert either. SR

-- Steve Rasmussen (srasmuss@flash.net), October 01, 1998.

A trivial question, but I was wondering why the current 90mm costs considerably more than the 105mm? It has one more element, true, but is slower. I now own a SMCP 90mm but haven't seen any results yet nor have I used a 105.

-- Benson (btw@vnet.net), January 31, 1998.

I'd be curious to know, if there is an simple explanation in layman's terms, why so many of the Pentax lenses have such lousy minimum focusing distances. For example, the 165 f2.8 is 5.3 ft, and the 300 f4 is a very crummy 16.4 ft! What are the design tradeoffs that have steered the Pentax designers to these l-o-o-n-g minimum focusing distances?

-- Ann Honimous (absolutely@nospam.com), February 03, 1998.

Steve , I own an older Takumar 105mm how doe's this lens compare with the newer generation of the same lens.

-- Robert Huebner (huebner@harborside.com), February 21, 1998.


Steve, You are just the man I'm looking for. I have a chance to buy the 800ED lens complete as sold in 1991. I intend on using it on my 645. Will combination fall prey to the camera-shutter-mirror shake syndrome? (assuming big solid tripod and rests) Past that how sharp is this lens? Has the design changed since it was produced? And lastly, what apeatures does it like. Bottom line: should I buy it or get my head examined? Many thanks. Gary

-- Gary M. Younkin (yf92525@navix.net), February 28, 1998.

Steve, Thanks so much for the fast reply. The lens I questioned about is used, but not much--and is mint-(cosmetics). Your suggestion is well taken but, the 800 f/4 wieghs just under 40lbs! It is true that one can be had for about 1/2 the price of the ED lens but the 800ED 6.7 although more expensive and 2 stops slower; wieghs 14lbs. I almost said "only". I think my 600ED 5.6 wieghs too much(10.5 lb) and it has a handle! 40 pounds is just too much to bear at least for me. I read your field test of the 800 f/4 and I understand that this is an excellent glass and was hoping that the 6.7 would be the same or better. I am absolutely satisfied with your answer. Gary

-- Gary M. Younkin (yf92525@navix.net), March 01, 1998.

Steve,

Perhaps you have a few commetns on the 55mm Pentax 67 lenses. I've read conflicting comments on the old (3.5) and new (4) versions of this lens. There seem to be some diehard fans of the older design, but others say the new glass is better. From an optical enginering perspective, do you have any comments on their designs? Better yet, have you used either of these lenses? Which one would you recommend? One drawback I see for the 3.5 lens is the 100mm filter size.

Thanks for your input.

Joel Collins http://www.mindspring.com/~jwc3

-- Joel Collins (jwc3@mindspring.com), March 31, 1998.


Steve, regarding the three versions of the 55mm:

You said that you would recommend the 8-element version over the older 9-element version. When looking at the used market, are there any physical clues on the lens that will let me know which one is which (focusing ring, serial #)? Thanks in advance.

-- sheldon hambrick (shambric@us.ibm.com), September 11, 1998.


I'm far from being any kind of an optics expert but I do know that there is no ANSI or other industry-wide standard for determining when a lens is an "APO" lens and when it isn't. A lens is an APO lens if the manufacturer decides to call it an APO lens. Therefore, as someone else has pointed out, the "APO" designation, by itself, means very little.

-- Brian Ellis (beellis@gte.net), September 30, 1998.

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