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01-13-2015, 10:06 AM   #46
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QuoteOriginally posted by Digitalis Quote
look at the shape of the MTF curves at wide open Vs stopped down to f/8 the Sagittal and Meridonial lines should ( with an ideal lens) both be perfectly straight, but here we see that there is a slight shifting in the focal plane caused by field curvature (...).
The decrease in the MTF values towards the edges of the full frame format can have many causes: coma, spherical aberration, lateral chromatic aberration, etc. and isn't directly linked to field curvature / shift in focal plane

QuoteOriginally posted by Digitalis Quote
(...) Also the divergence of the Sagittal and Meridonal lines towards the right edge of the MTF chart is strongly indicative of astigmatism.
Indeed such divergence would be indicative of astigmatism but it is very limited in the chart presented by Samyang.

01-13-2015, 10:49 AM   #47
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I think Samyang got their colors mixed up. The red lines should be 10 lines/mm and the blue ones 30 lines/mm
01-13-2015, 05:25 PM   #48
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QuoteOriginally posted by Mistral75 Quote
Indeed such divergence would be indicative of astigmatism but it is very limited in the chart presented by Samyang.
And if the MTF chart is a computer simulation of lens performance (what canon publishes on every recent lens they have made) than the chart is next to useless when one tries to glean the optical properties of this lens.

QuoteOriginally posted by Mistral75 Quote
The decrease in the MTF values towards the edges of the full frame format can have many causes: coma, spherical aberration, lateral chromatic aberration, etc. and isn't directly linked to field curvature / shift in focal plane
to identify field curvature I'm not looking at the edges of the MTF - i'm looking at its entirety. Look at the shape of those curves - at wide aperture there is a clear standard arch shape, but at a stopped down setting the shape flattens out with a slight concave bend in the middle - that suggests that the lens has a non-planar focus field. Also how close the samyang is to 1.0 is suspicious to me, even the Carl Zeiss 135mm f/2 Apochromatic cannot achieve a perfect MTF*.

*And from my personal experience on the Nikon D800 the Zeiss 135mm f/2 is an astonishing lens, I hardly bother with sharpening images from it, in a similar fashion to the Carl Zeiss Otus 55mm f/1.4.
01-13-2015, 08:25 PM   #49
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QuoteOriginally posted by Digitalis Quote
look at the shape of the MTF curves at wide open Vs stopped down to f/8 the Sagittal and Meridonial lines should ( with an ideal lens) both be perfectly straight, but here we see that there is a slight shifting in the focal plane caused by field curvature ( The focus field isn't flat, which is bad for macro or close up work).
First, the 135/2.0 isn't a macro lens, and it is unlikely to be meant for flat field close up work.

Second, the divergence between the meridional and the saggittal planes looks like the textbook approach to achieving a flat field (leaving the saggittal plane slightly under corrected) so I'm not sure why you are suspecting the field to be not flat. Where are these additional MTF curves for different apertures you are looking at? So far I've only seen the one diagram for one aperture posted here.

Third, note that the FA 77/1.8 was deliberately designed to not have a flat field so that in itself wouldn't even necessarily be a problem, if it was the result of a compromise to achieve better rendering as opposed to better measurement figures.

Fourth, and most importantly, the proof of the pudding is in the eating. If the lens produces gorgeous images -- and it does from what I've seen -- it does not really matter what may or may not be inferred from the MTF curves.


Last edited by Class A; 01-13-2015 at 08:48 PM.
01-13-2015, 10:10 PM   #50
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Answering your points in reverse order:

QuoteOriginally posted by Class A Quote
If the lens produces gorgeous images -- and it does from what I've seen -- it does not really matter what may or may not be inferred from the MTF curves.
A fair point. the FA77 isn't apochromatic, at apertures of f/1.8 it suffers from visually distracting LoCa, PF, and lowered contrast - but it is a superb lens, I own one.

QuoteOriginally posted by Class A Quote
divergence between the meridional and the saggittal planes looks like the textbook approach to achieving a flat field (leaving the saggittal plane slightly under corrected) so I'm not sure why you are suspecting the field to be not flat.
under-correcting those planes is one approach to eliminating astigmatism, as is introducing some distortion to counteract the shift in the focal planes is another method. However, focus shift is a common symptom of under-correcting the focal planes, along with field curvature. As you mentioned the FA77 was designed in this fashion - but the FA77 is a wider lens, it also has FREE* incorporated into the optical design**

QuoteOriginally posted by Class A Quote
First, the 135/2.0 isn't a macro lens, and it is unlikely to be meant for flat field close up work.
It does use floating lens elements which is more common amongst close focus/macro lenses. While this lens may not be explicitly called a macro lens, it appears to share features more common amongst the more popular 100mm+ macro lens designs.

*Fixed Rear Element Extension.
**to correct spherical aberration at close focusing distances.
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