01-29-2020, 03:39 PM
(This post was last modified: 02-03-2020, 06:22 PM by Brightcolours.)
(01-29-2020, 01:50 PM)Mistral75 Wrote: Considering a complex optical system with several lens elements moving independently one from another, there is no direct relation between minimum focusing distance, maximum magnification and focal length. Other parameters have to be considered.
Anyway, most people speaking of variation in the angle of view, and not in focal length, when they talk about focus breathing, one can simplify things and consider each of the two lenses as if it were made of one single lens element. In this case, there is a direct relation between minimum focusing distance (MFD), maximum magnification (MM) and focal length at minimum focusing fistance (FL@MFD):
FL@MFD = MFD x MM.
HD Pentax-D FA 70-210 mm f/4 ED SDM WR: MFD = 0.95m MM = 0.32x ==> FL@MFD = 304mm. The angle of view decreases with the focusing distance.
HD Pentax-D FA★ 70-200mm f/2.8 ED DC AW: MFD = 1.2m MM = 0.13x ==> FL@MFD = 156mm. The angle of view increases when the focusing distance decreases.
Even if you "consider" the lenses to be single element lenses, your calculation is not correct. If you have a single element 200mm lens, the FOV will narrow moving focus closer to the lens. The reason is that you are not looking at the projected FOV from the focal point, but from a certain distance from the focal point ( where the image is actually projected). The formula you use is incorrect to try and calculate focal length.
In short: when the focal length remains the same, the closer you focus, the more narrow the FOV becomes.
Because the distance of the lens to the imaging plane increases, while focal length remains the same (so the FOV that gets captured decreases). My estimate of under 110mm at MFD is much closer than what the faulty formula you use gives.
01-29-2020, 03:54 PM
(This post was last modified: 01-29-2020, 03:54 PM by Kunzite.)
Mistral75: Of course, if you are into video - but then, Pentax isn't such a great choice for video... (and would you use a 70-200?)
That's precisely why I'm saying the Tamron doesn't have a focus breathing issue and the Pentax D FA* has it.
Ephotozine tested the D FA 790299 f/4 with unsurprising results.
https://www.ephotozine.com/article/pentax-hd-pentax-d-fa-70-210mm-f-4-ed-sdm-wr-review-34615
Yes, focus breathing aside, the D FA* is optically superior (though the 70-200 f/4 is also very good).
In my boredom I glanced on the old site ..... Rice High's Pentax blog ....... I was still wondering about the different resolution results between the brands ..... of the Tammy and the K mount version
.... then interestingly, according to a Chinese lens review it shows how Tamron's optical elements were positioned slightly differently to accommodate Pentax's AF SDM ring motor in their FA 70-210mm F4 lens....... it is also claimed that the same optical elements were used ... just moved slightly ....... also the coatings are seemingly different ....
That would certainly explain ephotozine's differing resolution results between the two brands and Cameraville's findings .......
Rice High's site:
http://ricehigh.blogspot.com
....... a shame that Pentax had to fiddle with the formula just for their SDM motor at the price of sharpness at the long end ......
Hmmm ......
Changing the optics to accommodate Pentax' SDM ring motor doesn't make any sense, as that is a standard "ultrasonic" ring-type motor. Those are made by various companies, it's not a Pentax exclusive.
Even more, since the Tamron already uses such a motor changing it to a "Pentax" part.... why on Earth would they do that?
Cameraville and RiceHigh? 
RiceHigh used to be a very active member of this forum, we was always frustrated about Pentax, yet he seems to be always a loyal Pentax user...
Wonder what happened to Roland Mano from Sweden, he was a die hard Pentax fan
Loyal... like an aggressive stalker, perhaps?
It looks like he lost some of his anger though.
Roland Mabo, OTOH... I haven't seen him around.
(05-02-2020, 09:43 PM)Kunzite Wrote: Changing the optics to accommodate Pentax' SDM ring motor doesn't make any sense, as that is a standard "ultrasonic" ring-type motor. Those are made by various companies, it's not a Pentax exclusive.
Even more, since the Tamron already uses such a motor changing it to a "Pentax" part.... why on Earth would they do that?
Cameraville and RiceHigh?
I don't know if you completely read the review Kunzite ...... but it was very very comprehensive (Google translate apart) ..... and actually the reviewer was very kind to the Pentax version "throughout" ......
....... it was only at chapter 6 when comparing to the Tamron that he found differences .......
The review stated:
"At aperture F4.0, the resolution of 70mm focal length and 210mm focal length DFA is not as good as that of SP lens, but the performance of 100mm and 135mm DFA and SP are basically comparable."
take that as you will but that is pretty much in line with Ephotozines test results (they tested and reviewed both lenses btw) ....... and Cameraville's findings .....
So, whether the lenses are the same or not ..... we still wait for a copy that can match the Tamron ......
........... sorry but the result is too messy !!
05-03-2020, 01:05 PM
(This post was last modified: 05-03-2020, 01:06 PM by thxbb12.)
It's difficult to compare a single sample of each lens.
The IQ difference at both ends could be related to copy-to-copy variation and nothing else.
If someone were to compare the average of 20 Pentax samples vs 20 Tamron samples, then we could draw some proper conclusions (as what lensrentals.com does).
One sample is just not enough to be sure of anything, beside copy variation.
05-03-2020, 01:22 PM
(This post was last modified: 05-03-2020, 01:25 PM by Brightcolours.)
(05-03-2020, 01:05 PM)thxbb12 Wrote: It's difficult to compare a single sample of each lens.
The IQ difference at both ends could be related to copy-to-copy variation and nothing else.
If someone were to compare the average of 20 Pentax samples vs 20 Tamron samples, then we could draw some proper conclusions (as what lensrentals.com does).
One sample is just not enough to be sure of anything, beside copy variation.
I mostly disagree with that. Lenses can vary in AF accuracy, but that did not get tested. Lenses can vary in centring success, but these lenses are not decentered. For the rest, lenses do not vary very much, that is why reviews do make sense, also from OL and Lenstip (who do not average 20 copies).
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