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02-26-2013, 08:27 PM   #1
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645 600mm A*. Why so many elements?

The title asks the question. The 645 600mm A* has 12 elements in 11 groups . Compare that to some other Pentax lenses: 35mm 600mm A*, 8/6; DA 560mm, 6/5; 67 M* 800mm, 9/8.
Any thoughts?

02-26-2013, 08:59 PM   #2
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sounds like a question for desertscape...........fascinated to hear an answer.........
02-26-2013, 09:34 PM   #3
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02-26-2013, 09:51 PM   #4
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QuoteOriginally posted by Thomas Quote
The title asks the question. The 645 600mm A* has 12 elements in 11 groups . Compare that to some other Pentax lenses: 35mm 600mm A*, 8/6; DA 560mm, 6/5; 67 M* 800mm, 9/8.
Any thoughts?
Each lens design is different. The simple design of the 560mm means that the lens has to physically be longer, and there are fewer ED elements, which will result in more fringing. However, it's also sharper because it has fewer elements.

Essentially there is always going to be a trade-off, but modern lenses typically have more elements than older lenses, as it allows for better correction of aberrations and distortion, and generally more compact designs.

02-27-2013, 01:58 PM - 1 Like   #5
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The short answer is that Pentax wanted better correction for this lens. Why? Only they know. With lenses in this focal length, color correction is of paramount importance. Not only are lateral and longitudinal chromatic important but also the combination of longitudinal chromatic and spherical aberration ( AKA spherochromatism). If the designer really wants to have the shooter use their 600mm lens wide open, it needs to be highly corrected. The A* 600 uses 3 ED elements in the quadruplet up front (not counting the optical flat used for protection). This is a bit of overkill unless you are seeking perfection. The lack of a negative group in the rear signifies that this is not a true telephoto design. Pentax has gotten away from that in most of their ED designs. The negative rear group would magnify aberrations from the front groups but it had the advantage of shortening the lens length. Pentax obviously decided against that compromise. Besides the front and rear groups, this lens uses two middle groups as well. This was probably done to maintain the color fidelity from the front group. The rear group looks like it has 4 elements and little if any magnification. Since the designer uses each element surface to correct aberrations, the more surfaces there are, the better the correction ( to a point). It looks like this lens was developed to be the ultimate in not only color correction but also for the other remaining aberrations as well.

The older 600mm Takumar for the 6x7 used conventional glass and has 6 elements and would fringe so badly wide open that it could only be shot there in low contrast lighting. It would not fringe when stopped down and was sharp from f/13 to f/32. Its fringing was from spherochromatism. The A* 600 has solved this problem.

Last edited by desertscape; 02-27-2013 at 02:23 PM.
02-27-2013, 03:50 PM   #6
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Thanks desertscape, that's what I expected. A related question might be why weren't similar corrections necessary or made to other star lenses e.g., the 67 400mm M*, 800mm M* and the 35mm 600mm A* (which at least externally) resembles the 645 600mm?
02-27-2013, 05:11 PM   #7
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The M* 400 and 800 are fairly old designs for ED lenses. The difference in performance between the early and recent ED lenses is small. The recent M* 300ED and older M* 400ED have similar performance. I don't feel it was necessary to upgrade to the degree of the 645 *A 600.

Last edited by desertscape; 02-27-2013 at 07:24 PM.
03-02-2013, 01:54 AM   #8
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I'll tell you even with all the effort that Pentax put into the 645 A* 600mm a small amount of fringing can still be seen in extreme contrast when viewed at 100%. Of course this is easily correctable. On the other hand images I have seen from the K A* 600mm fringe significantly more and the old 67 600mm fringes beyond repair.

Personally I prefer to use my 67 M* 400mm over the 645 A* 600mm. The performance is comparable even with a 1.4x converter. It does fringe slightly more but the option of having a 400mm 4.0 and 560mm 5.6 is a key advantage. In addition, it is about the same weight and length. Of course the other reason I prefer the M* 400mm is that I have a drop in polarizer for it. I have yet to ever find this item for sale individually for the 645 A* 600mm and have come to the conclusion that if I want it I will have to have somebody fabricate it specifically for me.

The ergonomics of the 645 A* 600mm are where the pentax design really shines.

03-02-2013, 05:24 AM   #9
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Atlnq9, a 49mm B+W slim polarizer fits the 600mm; of course you need to remove it for rotation. If you need another for the 400
Pentax 67 CPL Filter for 67M 400mm F 4 67M 800mm F 6 7 | eBay
03-02-2013, 08:38 AM   #10
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You think that is excessive? The Canon EF 70-200mm f/2.8L IS has 23 elements in 18 groups, the Pentax A*600mm is very well corrected optically, the more elements a prime lens has typically the better the degree of optical correction it will have - the only drawback to having many elements in a lens is the increased flare potential.
03-02-2013, 03:51 PM   #11
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It should be noted that even with the use of low dispersion glass, long lenses will have some color aberrations (tertiary spectrum). The only way to eliminate them is to use mirrors. But mirror lenses have their problems as well. The use of Fluorite would be an improvement over the typical ED glasses currently used but there is the problem with cost, difficulty of production and fragility.

Given that the M* 400 (P67) and A* 600 (645) are both ED designs, it makes sense that the 400 would have slightly better performance color wise. There is simply less distance for the colors to separate.

Last edited by desertscape; 03-02-2013 at 03:59 PM.
03-03-2013, 04:14 AM   #12
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QuoteOriginally posted by Thomas Quote
Atlnq9, a 49mm B+W slim polarizer fits the 600mm; of course you need to remove it for rotation. If you need another for the 400
Pentax 67 CPL Filter for 67M 400mm F 4 67M 800mm F 6 7 | eBay
Yep, have the 49mm slim, it was commented on in my review of the lens on this site. Basically the polarizer is too much effort when trying to get everything right without a drop in polarizer that has an external rotation wheel. Since I have the 400mm with the polarizer I just use it if I need a polarizer, this lens turns into a 560mm f5.6 with comparable IQ.

Ever tried photographing moving subjects with a polarizer you have to pull out, hold next to your eye to get the right polarization, put back in and find the subject has moved too much causing you to have to repeat for the angle. Not to mention refocusing and framing. Framing is the big issue because with the amount of stabilization this lens requires you have to use a lens support bracket and a fourth leg... Too much work that has to happen to quickly.

BTW that is a fantastic price on the 400/800mm polarizer. If you were ever thinking about this lens buy that right now.
03-03-2013, 10:21 AM   #13
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atlnq9-- My 400mm Takumar (6x7) uses a rear filter and had many problems using it that way. There were internal reflections, it was hard to use and the focus would be slightly changed. I went with a multicoated filter; still wasn't a solution. I ended up using a B+W 105mm polarizer adapted to fit over the lens hood. The thread size of this lens is 108mm so no filter would screw in. The front filter is way better than the rear.
03-04-2013, 07:10 AM   #14
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If anyone is interested there's a copy on ebay:
Pentax A 645 600mm F 5 6 Ed If Lens | eBay

Last edited by Thomas; 03-04-2013 at 08:11 PM.
03-04-2013, 10:01 AM   #15
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Desertscape,
I have seen no issues with using a polarizer in the drawer. No flaring and I always focus with the filter in the camera (I have never tried to focus with the filter drawer removed from the camera to know if there is a focus shift issue). A front filter is not practical with this lens. It is 128mm so like you won't be availible for sale. Putting a makeshift option inside the lens hood would not be practical because there is no way I could be looking through the viewfinder and rotate the polarizer at the same time. Holding it in front of the hood would be a big flaring/glaring risk as I only use a polarizer when the sun is out... But luckily as I say there is no optical downside to the drawer I have seen. The 400 with drop in polarizer is superb and the only down side to the 49mm in the drawer on the 600 is ergonomics...

On a side note what is also very interesting on this lens is the depth of the hood a good +/- 25cm, in case you can't tell from the pictures that lens hood is double extending. Something I have never seen on a pentax lens and could be unique to the 645 A* 600mm 5.6. They were either going overboard in the ergonomic design of the lens or had good reason to keep any/all stray light out. A hood of this depth I have only ever seen from one other manufacture, Nikon. And those do not of the ergonomics of being built in like the Pentax.

The 645 A* 600mm is an ergonomic beauty if only I could either find a machine shop/watchmaker/camera repair shop that could make that polarizer for me or find that once in 5yrs one for sale.
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