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09-17-2011, 12:40 PM   #1
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Pentax M 100 2.8 vs f4: which is sharper?

Thought provoking results from old magazine tests with MTF curves...the 40lp/mm contrast of the M100/2.8 outstrips that of the M100/4 and the Tamron SP 90/2.5.

Saw this the other day and thought I would share:
Pentax K fit 100mm Telephotos Photo Gallery by Steve Flynn at pbase.com

09-17-2011, 02:49 PM   #2
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Yet it is not the sharpest Pentax lens in this focal length. I would give the price to the K 105/2.8.
09-17-2011, 03:55 PM   #3
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QuoteOriginally posted by Douglas_of_Sweden Quote
Yet it is not the sharpest Pentax lens in this focal length. I would give the price to the K 105/2.8.
Stopped down I think there is not much difference, but at f/2.8 the M100 2.8 is a lot sharper than the K105 2.8. I have both. No doubt the K has appeal as a portrait lens wide open, but sharp it isn't. A stop or two in and it sharpens up dramatically.

Tim
09-17-2011, 04:41 PM   #4
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In my opinion, the K105/2.8 is the softest of the three lenses mentioned wide open, I have the K105 as well as the M100/4 macro which is infinitely sharper, and also being a macro is not limited by close focus, I can't comment on the M100/2.8 as I passed on this to get the K105

09-19-2011, 01:30 AM   #5
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I still hanker after the M100/4. I think it will eventually end up in my collection, when the wife is not watching. I had the 50/4. It was a touch too short for frequent usage, but some of the photos I shot with it were super and looking at them again, I realise that the background blur was divine. I am guessing this is going to be a quality of the 100/4 which will trump the 100/2.8. I am interested to see the difference in the lens diagrams of the Tamron 90/2.5 and the Pentax 100/4. It looks like the Pentax is going to be less well corrected given it has about half the elements in it. Was printing some photos at the weekend, and a lens which really pleases me is the M135mm. Some portraits shot with it at about 5m distance and at f8 are absolutely tack sharp. Unfortunately on Acros 100, and I dont have a scanner...
09-19-2011, 06:31 AM   #6
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in comparing the lenses, you need to consider that the 100/4 is a macro lens, designed to be flat field, and focus close, for portrait use, it still performs well, but is limited in artistic approaches due to the F4 maximum aperture.

I have used the M100/4 for portraits in the past and wedding photos for the one wedding I shot all the way back in 1986, and the results when printed were excellent.

but as a general purpose telephoto, and for portraits the 100/2.8 would be better suited in my opinion. It is also a LOT smaller since it does not need a focusing helix that goes to 1:2
09-19-2011, 08:09 AM   #7
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QuoteOriginally posted by Lowell Goudge Quote
in comparing the lenses, you need to consider that the 100/4 is a macro lens, designed to be flat field, and focus close, for portrait use, it still performs well, but is limited in artistic approaches due to the F4 maximum aperture.

I have used the M100/4 for portraits in the past and wedding photos for the one wedding I shot all the way back in 1986, and the results when printed were excellent.

but as a general purpose telephoto, and for portraits the 100/2.8 would be better suited in my opinion. It is also a LOT smaller since it does not need a focusing helix that goes to 1:2
Very good points Lowell. The close focus of the M f4 is .45 M while that of the M 2.8 is 1 M which is due to the helicoid.
09-19-2011, 08:11 AM   #8
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QuoteOriginally posted by Douglas_of_Sweden Quote
Yet it is not the sharpest Pentax lens in this focal length. I would give the price to the K 105/2.8.
Also left out was the A 100/2.8 and the A 100/2.8 Macro lenses, granted the later is a 1:1 lens.

09-20-2011, 02:16 AM   #9
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Any comments on how these Pentax lenses compare to the Kiron 105mm? I'd guess SMC coating gives an advantage in contrast, but otherwise?
09-20-2011, 07:21 AM   #10
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QuoteOriginally posted by hoojammyflip Quote
I still hanker after the M100/4. I think it will eventually end up in my collection, when the wife is not watching. I had the 50/4. It was a touch too short for frequent usage, but some of the photos I shot with it were super and looking at them again, I realise that the background blur was divine. I am guessing this is going to be a quality of the 100/4 which will trump the 100/2.8. I am interested to see the difference in the lens diagrams of the Tamron 90/2.5 and the Pentax 100/4. It looks like the Pentax is going to be less well corrected given it has about half the elements in it. Was printing some photos at the weekend, and a lens which really pleases me is the M135mm. Some portraits shot with it at about 5m distance and at f8 are absolutely tack sharp. Unfortunately on Acros 100, and I dont have a scanner...
from the super takumar forward to the M100/4 macro all the lenses have the same optical formula, just advances in coatings. Same is true of the 50mm macros.

One thing I am not sure about, and I asked this question once before but never got an answer, is the following.

Some nikon lenses had multiple versions, where the elements got thinner and the weight changed, but otherwise the optical formula remained the same. Did pentax do the same? Are M lenses for example, lighter than the same optical formula K lenses because pentax in this case, learned how to grind reliably thinner glass. THinner glass would have 2 benefits, lower weight and cost is obvious, but also thinner glass would have lower CA because the light travells through the element over shorter distances, and therefore reduces the color separation.

Anyone???
04-05-2012, 11:31 PM   #11
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Reviving this thread, I believe the thinner elements in the Nikon lenses could have been allowed by different glasses being employed:
LEICA Barnack Berek Blog: OPTICAL GLASSES

Its interesting to know that Pentax were pioneering with their thoriated lenses, and to see the technology in the achromat/apo Takumar 85mm...its a shame manufacturers are not more explicit with which glasses they employ, as it makes more difference to IQ than any bells and whistles such as auto focus or waterproofing.
04-06-2012, 01:30 AM   #12
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I recently got the M 100/2.8 and @2.8 it's as sharp as my M 50/1.7 @1.7
04-06-2012, 03:59 AM   #13
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QuoteOriginally posted by whojammyflip Quote
Reviving this thread, I believe the thinner elements in the Nikon lenses could have been allowed by different glasses being employed:
LEICA Barnack Berek Blog: OPTICAL GLASSES

Its interesting to know that Pentax were pioneering with their thoriated lenses, and to see the technology in the achromat/apo Takumar 85mm...its a shame manufacturers are not more explicit with which glasses they employ, as it makes more difference to IQ than any bells and whistles such as auto focus or waterproofing.
But it also tells the competition too much of their optical design and basisk for price (from material cost)

It is much better that the competition reverse engineer things, as that takes time, as opposed to telling them how to do it. Although today many makers tell the photo reviewers pretty well exactly what they do, on the basis that market turn around is too long to lose much usiness once the design is released, it wasn't that way years ago when lens designs lasted a very long time
04-06-2012, 10:19 AM - 1 Like   #14
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QuoteOriginally posted by timo Quote
Stopped down I think there is not much difference, but at f/2.8 the M100 2.8 is a lot sharper than the K105 2.8. I have both. No doubt the K has appeal as a portrait lens wide open, but sharp it isn't. A stop or two in and it sharpens up dramatically.

Tim
Yoshihiko’s lens resolution tests have the K105/2.8 sharper than the M100/2.8 wide open and at all apertures.Same goes for the K85/1.8 over the M85/2.


http://www.takinami.com/yoshihiko/photo/lens_test/pentax_tele.html

Phil.
04-06-2012, 11:57 AM   #15
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QuoteOriginally posted by gofour3 Quote
Yoshihiko’s lens resolution tests have the K105/2.8 sharper than the M100/2.8 wide open and at all apertures.
Those tests are based on a perceived resolution,
which also factors in contrast to a large extent.
Thus a contrasty lens like the FA*24/2 is favored,
while a sharply resolving lens like the M100/2.8 looks poorer,
because it is less contrasty.

I have tried to set the record straight in my review of the M100/2.8:
SMC Pentax-M 100mm F2.8 Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
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