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11-06-2018, 11:05 AM   #61
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QuoteOriginally posted by amstel78 Quote
The Pentax-D FA 50 is one of my favorite lenses. I use it more than my Pentax 24-70. It has a level of sharpness that I've just never seen before in a 50mm lens designed for an SLR or DSLR. The only problem I've ever had with it, and it's more me and the camera rather than the lens, is that it can be sometimes difficult achieving critical focus at wide open apertures. For instance, focusing on a model's eye, if the camera or the model moves even minutely, can result in the eyelashes being tack sharp with the pupils being slightly less so. It becomes evident on a 4k monitor looking at 100% pixel resolution, but for casual prints, not so much.

What surprises me even more however is the lens' resolving power at the edges wide open. It's just silly sharp at the top, bottom, left, and right edges of the frame. I'd love to see a 24mm or 35mm prime from Pentax with the same design ethos.
I use it for portraits too and yeah it can be hard to catch critical focus wide open. Other than that, I love the lens.

12-04-2018, 04:54 AM - 1 Like   #62
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With F/1.4 and 80cm distance from subject DOF will be 1cm Now imagine you're shooting handheld composing frame, simple breath may cause OOF. Besides, your model is living organism too
12-07-2018, 07:25 PM   #63
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QuoteOriginally posted by Medex Quote
Imatest: If the results of the ZEISS Milvus were impressive (50.1 lp/mm), then the HD PENTAX-D FA ★ 50mm F1.4 SDM AW they are generally stunning today (2018). The maximum result on the open aperture is 69.4 lp / mm!
The 69.4 lp/mm figure is inflated due to sharpening. One can clearly see the overshoot in the edge profile charts.

Lens resolution measurements are extremely sensitive to sharpening and clearly ACR performs some sharpening (presumably even when sharpening is set to zero).

I don't want to take away anything from the D FA* 50/1.4 -- I'll get a copy myself as soon as a good opportunity arises -- but the measured lp/mm figure is not trustworthy.
12-14-2018, 07:58 AM   #64
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QuoteOriginally posted by normhead Quote
OK, how are you seeing the Milvus is better, according to these numbers the DFA out resolves it by about 40%.

Especially since the DFA is better in the red channel. The red channel is definitely the most difficult to resolve, The top end for blue channel is around 100 MP for APS-c. Yellow green is middle of the pack. IN the red spectrum, if memory serves me well, red spectrum can only theoretically produce around 40mp on FF or 16 MP on APS_c.. So basically, you can cheat to make the lens have higher test chart resolution by making it more sensitive to blue and fudging the red. which is what from this analysis what the Milvus has done. That however does not produce a more a pleasing image. It just works better on a test chart.

If memory serves me well, theoretically,the red channel is maxed out on a 16 MP APS_c or 36 MP FF camera, (Which explains in part the need for an FF sensor to get to the 51 MP and the disadvantage to going to 100 MP on FF. 100 MP on FF if the theory is correct is only capturing at most 60 MP resolution in the red channel, while may capture 100 in the blue and possibly the yellow/green. You can go a long way because most things are not uniform red, there may be enough green or blue detail in them to provide the illusion of sharpness in the red channel, and that works until it doesn't.

In short, a focus on lens clarity in the red channel is probably the most important thing you can do to create the impression of a sharp lens, in real images where as a lens that emphasizes the blue channel will produce better test results, by reducing the impact of those muddy reds with the really wide wave length that are much more likely to cross pixel borders than those skinny little blue ones.

This also relates back to "lenses for the way people take pictures, not for the test charts" Made for the test charts could mean the lenses were made blue spectrum heavy to increase test scores, where as a more natural looking lens optimized for reds, might score slightly lower. So to me, just the fact it has both a higher lw/ph plus better reds, makes it even more of an accomplishment. Higher test scores and better reds.. how do you beat that? IN terms of "the way people take pictures" better reds is already an improvement, even without the higher test scores.

It also explains my feelings about all ZeIs lenses. They look sharp, but the colours don't look right to me and there's something un-natural about the look of the images. I'm guessing they all sacrifice sharpness in the red channel to increase lw/ph in the blue channel, for better test scores.

It makes me wonder coupled with kenspo's comment in another thread if Pentax is the only company that has noticed the way to increase real world resolution (not test chart resolution) is to have the best results in the red spectrum.

The best test chart resolution would be produced by blocking yellow, blue , green with the colour cast of your lens and just exposing for blue, but it would be a crappy picture in terms of realistic appearance.
This has got to be the most interesting statement about Pentax lens design since the famous (at least to me) design philosophy document about the FA Limiteds.

Lessons from a Legendary Lens Designer | Photographic Ideals, Basic Principles | The Northcoast Photographer

Pentax for the FA limiteds design for real world rendering that rather than looking at optical test charts looks at the overall rendering. This explains a lot of how modern lenses look sharp but feel weird. Our eyes are sensitive to red and not blue and so I wonder how lens manufacturers prioritize one design idea (blue channel sharpness) over another once we switched to digital sensors.

12-14-2018, 01:14 PM   #65
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QuoteOriginally posted by y0chang Quote
This has got to be the most interesting statement about Pentax lens design since the famous (at least to me) design philosophy document about the FA Limiteds.

Lessons from a Legendary Lens Designer | Photographic Ideals, Basic Principles | The Northcoast Photographer

Pentax for the FA limiteds design for real world rendering that rather than looking at optical test charts looks at the overall rendering. This explains a lot of how modern lenses look sharp but feel weird. Our eyes are sensitive to red and not blue and so I wonder how lens manufacturers prioritize one design idea (blue channel sharpness) over another once we switched to digital sensors.
If I recall correctly our eyes aren't very sensitive to red. They are more sensitive to yellow/green than anything but blue sensitivity is a little better than red typically. This is especially true at low light levels.

https://www.nde-ed.org/EducationResources/CommunityCollege/PenetrantTest/Int...htresponse.htm
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