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12-08-2017, 08:56 AM - 1 Like   #16
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QuoteOriginally posted by clackers Quote
It's Full Frame, Alex, this has been known for years and reported on this forum.
That's good to know. Too bad that B&H, Ricoh, and even PF's own Lens Finder tool still lists it for APS-C. As we all know, any FF would work fine for APS-C, but not the reverse (usually) and listing it this way does a disservice to the number of FF available for the K-1.

Telephoto Lenses / K-mount Lenses / Lenses / Products | RICOH IMAGING

12-08-2017, 10:11 AM - 1 Like   #17
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Alex, that list also has the DA* 200 and 300 as APS-C, like the DA 50/1.8.

But you can go to the "K-1 lab"
PENTAX K-1 Laboratory | PENTAX K-1 Special site | RICOH IMAGING
to see sample images of the DA 50/1.8 in FF mode on the K-1.
12-08-2017, 01:08 PM   #18
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QuoteOriginally posted by Alex645 Quote
That's good to know. Too bad that B&H, Ricoh, and even PF's own Lens Finder tool still lists it for APS-C. As we all know, any FF would work fine for APS-C, but not the reverse (usually) and listing it this way does a disservice to the number of FF available for the K-1.

Telephoto Lenses / K-mount Lenses / Lenses / Products | RICOH IMAGING
We have our own thread here, Alex, which has an extensive list of lenses and real reports.

As usual, this place provides accurate information versus 'what's out there'. I wouldn't trust B&H and Tony Northrup or whoever to get anything right.

As a generalization, any normal or tele DA prime works, but no DA zoom.

Even in the case of the zooms, they often work fine at the long end (my DA12-24 f4 is great from 17mm on, which becomes attractive to anyone who finds the DFA15-30 too expensive or big) or in the case of the DA*60-250, removing an inner baffle makes a real difference to vignetting.

Going full frame can be less expensive than APS-C owners think, especially with Pentax.
12-08-2017, 02:06 PM   #19
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QuoteOriginally posted by hadi Quote
front element size - does size matter?
Not by itself, though it is (obviously) something that is inherent in the design.

Comparison of the Super-Takumar 28/3.5 v1 and v2 might be a good example. The earlier lens is fairly bulky to accommodate a big front element (filter size = 58mm). The later version is much smaller (quite tiny, actually) with a modest-diameter front element (filter size = 49mm). It should be noted that both lenses have identical FOV and "see" the same amount of light at the surface of the front element to convey to the focal plane. I have owned both lenses and while the larger lens is not bad, the smaller is much better. Here is the v1 attached to my SV and a comparison with the v2 (S-M-C actually) pictured at lower left next to my Spotmatic II.


Super-Takumar 28/3.5, early version


S-M-C Takumar 28/3.5 (same design as S-T 28/3.5 v2) pictured with Spotmatic II

Another, more drastic, comparison might be the Vivitar 28/2.5 (Kiron) with a 67mm filter ring as compared to the Tamron 28/2.5 (02B). Not only is the Tamron's filter ring only 49mm, but the front element is fairly tiny at about 2cm diameter. Comparison for optical properties would be a little unfair considering that the Tamron cost almost 3x more new, it is enough to say that the Vivitar performs considerably less well than its smaller-fronted, Komine-made, stablemate (not pictured) and not in the same league as the Tamron. Here are the two lenses mounted to similar-sized bodies for comparison...


Vivitar 28/2.5 (Kiron) vs. Tamron 28/2.5 (02B), front element size

Again, both lenses "see" the same FOV and same amount of light within that FOV.


Steve

12-08-2017, 02:31 PM - 1 Like   #20
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QuoteOriginally posted by Alex645 Quote
The DA 50mm f/1.8 is designed to create a smaller image circle for APS-C sensors, whereas that lens would vignette with a larger FF sensor.
QuoteOriginally posted by clackers Quote
It's Full Frame, Alex, this has been known for years and reported on this forum.

There is a thread where I posted pics of a day in a park where I used only the K-1 and the DA50 and DA35.

Before the K-1 was even released, I had used the DA50 on my Sony A7.

These plastic fantastics are a great pair of sharp, cheap primes Pentax can be proud of.

They of course have 35mm heritage in the M50 f1.7 and FA35 f2.
Not wanting to directly contradict, but I tested the DA 50/1.8 for vignette against the A 50/1.7 on 35mm film and the difference was significant (about 2 stops wide open).* This was obvious on direct examination of the negative as well as on scans (auto expose disabled). Evaluative metering tends to balance the vignette against the scene brightness such that it is less evident on other than a fully manual digital capture, but it is there none-the-less.** As for heritage, aside from being planar-derived with similar element/group count, the resemblance to any previous Pentax f/1.7 or f/1.8 fast-50 is limited.

It is a great little lens, is high value, and is usable on a FF camera, but I would not purchase it with that in mind.***


Steve

* White foam board target, exposure placed to zone IX, and apertures from wide-open to f/5.6. Scans were done using manual settings to approximate zone IX for on-screen brightness.

** The same is true for RAW import into tools such as Lightroom. Initial brightness and contrast settings tend to minimize vignette.

*** The DA 50/1.8 is on my "no sell" list simply because it is so much fun to shoot with and punches well above its weight in terms of performance for price. I recommend it without hesitation for APS-C shooters interested in trying a fast 50mm prime.

Last edited by stevebrot; 12-09-2017 at 01:59 PM.
12-08-2017, 02:45 PM   #21
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QuoteOriginally posted by UncleVanya Quote
I can't find the info now but the site I used for information claimed that pre-A series lever distance traveled was proportional to aperture diameter, whereas lever distance traveled in the A series is proportional to aperture area.
I have read this as well, though it is hard to demonstrate or prove. What is certain is that the lens is stopped down to minimum aperture at full travel. What's more, attempts to spoof the A-contacts usually fail due to the mechanical action being dissimilar to that on a true A-contact lens.


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12-08-2017, 03:45 PM   #22
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QuoteOriginally posted by clackers Quote
We have our own thread here, Alex, which has an extensive list of lenses and real reports.
Yes, and since May 2015 when I joined PF, I agree. But if it wasn't for customer service at B&H telling me they couldn't answer a lens hood question and that I should inquire with PF, I then discovered this as a real deal resource. I've never seen before or after another forum that is so active and helpful.

But until Ricoh themselves, or the major Pentax retailers lists accurate information, I don't think the general public would come here for information. Most folks rely on the manufacturer or retailer's web site or customer service.
12-08-2017, 04:08 PM - 1 Like   #23
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QuoteOriginally posted by Alex645 Quote
.

But until Ricoh themselves, or the major Pentax retailers lists accurate information, I don't think the general public would come here for information. Most folks rely on the manufacturer or retailer's web site or customer service.
Correct. That's how lucky we are.

We should appreciate the efforts of our fellow forum members for all their assistance, and become financial supporters to look after the place that makes it all possible.





12-08-2017, 04:20 PM   #24
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QuoteOriginally posted by Alex645 Quote
Yes, and since May 2015 when I joined PF, I agree. But if it wasn't for customer service at B&H telling me they couldn't answer a lens hood question and that I should inquire with PF, I then discovered this as a real deal resource. I've never seen before or after another forum that is so active and helpful.

But until Ricoh themselves, or the major Pentax retailers lists accurate information, I don't think the general public would come here for information. Most folks rely on the manufacturer or retailer's web site or customer service.
If your point is that the lack of good info is hurting K-1 sales... well, perhaps you are correct in a few cases but honestly the sales have been so strong I'm not sure that's true. Some buyers may not be aware of the site and may have trouble finding out that information - but I'm not sure how anyone could get Ricoh to change their stance. They have obviously set some criteria (border/corner sharpness, vignetting, combo of the two) that governs which lenses available today are considered Full Frame. They also likely factor in costs and profits and since the FA 50 f/1.4 exists and they may not be entirely happy with the DA 50 f1.8 on FF they don't recommend the latter. The same may also be true of the DA 40 since the FA 43 exists alongside the FA 31 and FA 35 f/2. Similarly why tell people to buy the DA 70 when the FA 77 is still sold?

Just between us Forum folks - I can't understand how they keep their website so out of date and inaccurate. For example under K mount Telephotos they list the FA* 645 300mm... sigh.
12-09-2017, 02:17 AM   #25
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QuoteOriginally posted by UncleVanya Quote
I can't find the info now but the site I used for information claimed that pre-A series lever distance traveled was proportional to aperture diameter, whereas lever distance traveled in the A series is proportional to aperture area.
You can find this on this page:
Features and Operation of the Ka Mount
12-09-2017, 10:04 AM   #26
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Thanks guys - that was helpful info and helpful to find that link again.
12-09-2017, 11:10 AM   #27
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Focal length and maximum aperture determine the minimum size of the front element because those two variables determine the diameter of the physical opening of the lens. No 300mm f/2.8 lens can have a front element smaller than 300/2.8 = 107 mm.

Yet often the front element must be larger than this minimum size to accommodate a stack of corrective optical elements in front of the aperture iris. Each of those elements is progressively larger the further it is from the aperture and the wider the angle of the lens. That is why high-quality wide angle lenses have such large front elements.

Generally speaking, high-quality, large aperture lenses have big front elements although that does not imply that all lenses with big front elements are high quality.
12-09-2017, 01:57 PM - 1 Like   #28
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QuoteOriginally posted by photoptimist Quote
Generally speaking, high-quality, large aperture lenses have big front elements although that does not imply that all lenses with big front elements are high quality.
Well said and the inverse is true as well. A smallish front element is not an indication of low quality or lack of sophistication.


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12-09-2017, 02:01 PM   #29
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Optical physics tells us that a larger diameter optic will outperform a smaller one. This is the main reason why larger telescopes will out resolve smaller ones. This is proven; however in photographic optical design, a more important factor is the degree of aberration correction. Aberrations determine performance more than diameter.

Wide lenses (Retrofocus, reverse telephoto, etc) do not fit in the typical "focal length divided by diameter" computation for focal ratio because of the use of a negative lens group in front of the stop. This negative group reduces the size of the entrance pupil. That entrance pupil must be used to figure the focal ratio, not the diameter of the front element. Since the negative group reduces the entrance pupil size, it also reduces the light, therefore the front element must be made larger to compensate.
12-10-2017, 05:50 AM - 2 Likes   #30
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QuoteOriginally posted by stevebrot Quote
Not by itself, though it is (obviously) something that is inherent in the design.

Comparison of the Super-Takumar 28/3.5 v1 and v2 might be a good example. The earlier lens is fairly bulky to accommodate a big front element (filter size = 58mm). The later version is much smaller (quite tiny, actually) with a modest-diameter front element (filter size = 49mm). It should be noted that both lenses have identical FOV and "see" the same amount of light at the surface .
The explanation is simple.
The speed of a lens is not given by the diameter of the front element nor by the diameter of the "hole" inside the objective (with the iris wide open).
It is given by the diameter of FL / "entrance pupil", both expressed in the same unit (usually millimeters).
The enlarging power of the elements in front of the diaphragm comes into play. That power of course can be negative (which is what happens with retrofocus designs, IIRC).
You can easily check with any lens fitted to a standard leaf shutter. All large format lenses set in a leaf (or central, if you prefer) shutter, and all the Schneider repro and enlarger lenses which use standard barrels are built the same way: there are two cells, one in front and one behind the iris/shutter, that can be easily unscrewed.
If you remove the front cell and measure the max aperture with a ruler, you have a number.
If you screw the optical cell back, keep the diaphragm wide open, and place a ruler in front of the objective, you can look (one eye, in line with the optical axis) that the diameter of the "apparent aperture" (read, the entrance pupil) is often different from the measure you've taken before. Most of the times the difference is minimal, because most LF and enlarger lenses are almost symmetrical double gauss.
Other designs, like the wide angles for SLR cameras, would show substantially different measures.
So yes, the design of the lens makes a difference

Apart from the basic laws of optics, there is one thing that was true 160 years ago, when my oldest lens was manufactured, and is still true today: there is no perfect lens. The blanket is always too short, if you cover your head, your feet will get cold.
We can't have cheap, fast, small and sharp at the same time. Even if we forget about cheap, we still have one term too many.
Over the years the compromises have gotten better and better. They are still compromises though, the name says it all...

Cheers

Paolo

Last edited by cyberjunkie; 12-10-2017 at 06:09 AM.
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