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Forum: Pentax Medium Format 15 Hours Ago  
Lens layout/diagram?
Posted By desertscape
Replies: 10
Views: 288
The 90 LS was called a Takumar, the regular 90mm was a Pentax. I have the cross section of the 55mm 6x7 ( second optical version) if that is the one you want. Let me know Medium4Matt.
Forum: Pentax Medium Format 5 Days Ago  
Post your medium format photos!
Posted By desertscape
Replies: 12,093
Views: 1,973,571
Right lighting (side lit and no harsh shadows), right model.
Forum: Pentax Medium Format 5 Days Ago  
Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro
Posted By desertscape
Replies: 13
Views: 735
Spherical aberration by color has been a challenge for lens designers for years. Most photographic lenses tend to have as many colors as possible focus at the focal plane that emanate from the area 40% out from the center of the optic. They know that the diaphragm can be used to truncate the poorly corrected marginal rays, so the serious spherochromatism at the margin is somewhat ignored. Conventional glass telephotos were afflicted with massive fringing wide open due to spherochromatism but the newer, low dispersion long lenses do much better, mostly because the glass used just does not separate the colors very much in the first place.
Forum: Pentax Medium Format 6 Days Ago  
Post your medium format photos!
Posted By desertscape
Replies: 12,093
Views: 1,973,571
A long forgotten shot of the Cook Islands, 2004, 6x7, just scanned.
Forum: Pentax Medium Format 6 Days Ago  
25mms - visually comparing DFA and DA examples
Posted By desertscape
Replies: 8
Views: 494
A rarely seen internal look at the DA 25mm (a cross section).
Forum: Pentax Medium Format 6 Days Ago  
Grand Lens Test 10: The 645 A* 300mm f/4
Posted By desertscape
Replies: 9
Views: 492
Are you talking spherochromatism here, because it you are, the color correction is improved when stopping down and should not be mistaken for diffraction.
Forum: Pentax Medium Format 6 Days Ago  
Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro
Posted By desertscape
Replies: 13
Views: 735
Okay, I think I found the heritage design that the 120 was based on. It is a Makro-Plasmat, a hybrid between a Double Gauss and Plasmat. Double Gauss in front, Plasmat in the rear. The 120 has a corrector in the middle which is a bit different.
Forum: Pentax Medium Format 06-16-2018, 03:14 PM  
Post your medium format photos!
Posted By desertscape
Replies: 12,093
Views: 1,973,571
Didn't know you were on this site Graham. I recall many of your 6x7 shots on Flickr. Some nice work.

Steve Rasmussen
Forum: Pentax Medium Format 06-16-2018, 03:07 PM  
Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro
Posted By desertscape
Replies: 13
Views: 735
I'm betting that whoever did the sketch, left out the line in the front meniscus. It is probably a cemented achromat, not a single element.
Forum: Pentax Medium Format 06-13-2018, 10:06 PM  
Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro
Posted By desertscape
Replies: 13
Views: 735
The Canon looks like a 7 element Double Gauss similar to a Leitz Elcan but with an achromat at the rear. The 120 macro looks very different to me. I just can't categorize it. It may be unique.
Forum: Pentax Medium Format 06-13-2018, 01:35 PM  
Grand Lens Test 9: Pentax 645 A 120mm f/4 Macro
Posted By desertscape
Replies: 13
Views: 735
The 120 macro has a unique cross section, at least to my eye. I have never seen a lens design that resembled it. Your thoughts?
Forum: Pentax Medium Format 06-12-2018, 02:37 PM  
Pentax 645z gallery
Posted By desertscape
Replies: 16
Views: 2,037
Some very nice "fine art" type shots in there. I found about a third of the shots to be ordinary in composition though. Overall, a success.
Forum: Pentax Medium Format 06-10-2018, 04:56 PM  
Pentax 67 150mm f/2.8 - Additional aperture stop
Posted By desertscape
Replies: 4
Views: 301
I owned one of these for several years and I would not say it was very soft wide open. Slightly maybe.
Forum: Pentax Medium Format 06-05-2018, 11:27 AM  
Grand Lens Test 6: The 45-85 FA Zoom
Posted By desertscape
Replies: 5
Views: 486
The cross sections on the P67 55-100 and the 45-85 are nearly identical. The 55-100 is proven and it sounds like the 45-85 does well also. The 55-100 has performed so well that I sold my 75 f/4.5 and 105 as a result.
Forum: Pentax Medium Format 05-31-2018, 04:21 PM  
Pentax 67 75mm AL underexposes on Pentax 645D
Posted By desertscape
Replies: 6
Views: 405
If your lenses are set to AUTO, does the adapter have the ability to allow wide open metering and stopping down the lens to the shooting aperture? I am doubtful that it will. In most cases, when using an adapter, the lens will have to be set to manual to get an accurate meter reading. Accurate wide open metering is generally not possible when using an adapter.
Forum: Pentax Medium Format 05-29-2018, 05:42 PM  
shutter mechanism problem on Pentax 6x7 MLU
Posted By desertscape
Replies: 7
Views: 495
Mine did this after many years of use. I sent it off for repair. I suggest you do the same, especially after trying to fix it yourself.
Forum: Pentax Medium Format 05-29-2018, 02:33 PM  
Post your medium format photos!
Posted By desertscape
Replies: 12,093
Views: 1,973,571
Excellent lake shot Boris. 645D?
Forum: Photo Critique 05-28-2018, 02:07 PM  
Macro Need your opinion!
Posted By desertscape
Replies: 8
Views: 503
May have been better to shoot it at f/16 to get some more depth.
Forum: Photo Critique 05-28-2018, 02:03 PM  
Landscape Non Branded ND1000 Filter Experiment
Posted By desertscape
Replies: 7
Views: 385
Seascapes with the horizon near the mid point of the frame seldom work well.
Forum: Pentax Medium Format 05-22-2018, 10:40 AM  
Grand Lens Test 5: 645 A 1.4x Teleconverter
Posted By desertscape
Replies: 16
Views: 939
My field experience with Pentax 1.4x converters shows that they do better with long lenses that are not telephoto designs. The ED lenses are not telephoto designs and do well with the 1.4x. The older telephoto designs already magnify the image from the front group, so the addition of a 1.4x to this only magnifies the aberrations more. Using a 1.4x on the telephoto designs, one must use the lens at its sweet spot, otherwise softness results. In the case of the 600 Takumar (67) used at its sweet spot, a 1.4x converter will do very well. Actually, the use of the 1.4 reduces the longitudinal chromatic, as it intercepts the incoming rays long before reaching the film.
Forum: Pentax Medium Format 05-22-2018, 10:15 AM  
Grand Lens Test 3: 645 FA 35/3.5
Posted By desertscape
Replies: 12
Views: 677
So you're saying that the body was designed around the lens design?
Yes, I'm quite aware of mirror box cameras using nearly symmetrical lenses for the normal to slightly long lenses. The Pentax 67 had a number of them-- 90 Takumar LS, 90 Pentax, 105, 135 macro (Dynar) and 165 f/2.8
Forum: Pentax Medium Format 05-21-2018, 10:31 PM  
Grand Lens Test 3: 645 FA 35/3.5
Posted By desertscape
Replies: 12
Views: 677
The fact that the 38mm was used on a camera without a mirror box says that the lens was nearly symmetrical, as opposed to a wide lens with a mirror box which would have to be made very asymmetrical. The symmetrical (or nearly so) advantage is that it corrects lateral color almost completely by design, where the asymmetrical designs have major problems correcting lateral color. You make a good point, although somewhat esoteric.
Forum: Pentax Medium Format 05-19-2018, 04:58 PM  
My grand lens test, coming up
Posted By desertscape
Replies: 17
Views: 1,524
Yeah, the 645 150 and 200 are Ernostar-Sonnar but the 165LS and 200 Pentax for the 67 fit the Ernostar cross section. The original 4 element 200mm for the 67 used an Ernostar-Sonnar but was replaced by a 5 element Ernostar. But yes, similar designs.
Forum: Pentax Medium Format 05-18-2018, 03:37 PM  
My grand lens test, coming up
Posted By desertscape
Replies: 17
Views: 1,524
I've always thought the 165 f/2.8 was a bit too long for a Double Gauss design. On their newer 165, the LS, they went with an Ernostar design, which is more of an accepted type for that focal length. A sharp lens although I don't shoot it at f/4 much. The newer 200mm for the 67 is also an Ernostar. Oddly, this design type is not used for any 645 lens.
Forum: Pentax Medium Format 05-16-2018, 04:47 PM  
Post your medium format photos!
Posted By desertscape
Replies: 12,093
Views: 1,973,571
A classic shot. One of your best. NZ right?
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