[repaap]

Well actually even on that lenstips review there is construction image, and there is clearly an AL element drawn in that picture.

edit: link

here is link to that page.

[Digitalis]

Well actually even on that lenstips review there is construction image, and there is clearly an AL element drawn in that picture.

The diagram is incorrect - from what I have seen in reviews and from my own impressions of the lenses optical characteristics, especially the bokeh it creates is that of a strictly spherical design. Pentax has always designated the lenses they make with aspherical used in their optical design the "AL" designation. E.G, DA16-45mm f/4 ED AL, DA15mm f/4 ED AL, FA31mm f/1.8 AL Limited, Pentax DA 18-135mm f/3.5-5.6 AL DC WR, DA 17-70mm f/4 AL [IF] SDM, DA*16-50mm f/2.8 ED AL [IF] SDM , DA 21mm f/3.2 AL Limited - are you seeing the pattern there?

[repaap]

Yes, I see pattern.
Could be that what I saw from that picture is false, and you are absolytelly right. But if I'd believe these drawings I could have an impression that it is an AL lens, even if it was not written on the lens. It could also be that Ricoh made lenses brakes Pentax made lens pattern. Even that I can't see that.

The diagram is incorrect - from what I have seen in reviews and from my own impressions of the lenses optical characteristics, especially the bokeh it creates is that of a strictly spherical design. Pentax has always designated the lenses they make with aspherical used in their optical design the "AL" designation. E.G, DA16-45mm f/4 ED AL, DA15mm f/4 ED AL, FA31mm f/1.8 AL Limited, Pentax DA 18-135mm f/3.5-5.6 AL DC WR, DA 17-70mm f/4 AL [IF] SDM, DA*16-50mm f/2.8 ED AL [IF] SDM , DA 21mm f/3.2 AL Limited - are you seeing the pattern there?

I don't want to argue with you.

[lytrytyr]

I looked at the optical construction of it the 20-40mm and like the FA77 there are no aspherical elements used in its construction.

The FA lens of reference for the DA 20-40 is the FA 20-35 AL, not the FA 77.

The diagram is incorrect - from what I have seen in reviews and from my own impressions of the lenses optical characteristics, especially the bokeh it creates is that of a strictly spherical design.

This is the diagram quoted on the Optyczne/Lenstip sites:



It is the diagram published on the Ricoh website

HD PENTAX-DA 20-40mmF2.8-4ED Limited DC WR / Standard-Angle Lenses / K-mount Lenses / Lenses / Products | RICOH IMAGING

Pentax has always designated the lenses they make with aspherical used in their optical design the "AL" designation.

Before I saw the diagram,
I also thought that there was no aspheric element,
for this very reason.

But Ricoh (who now have the say in all things Pentax)
may well have decided that HD, ED, DC, and WR
are more than enough acronyms for a mere 2x zoom to carry.

[Digitalis]

The FA lens of reference for the DA 20-40 is the FA 20-35 AL, not the FA 77.

The FA77 is the only modern non ASPH all spherical lens design lens I really can compare it to - I never liked the FA20-35mm, to me it was a rather pointless lens.

But Ricoh (who now have the say in all things Pentax) may well have decided that HD, ED, DC, and WR are more than enough acronyms for a mere 2x zoom to carry

But if that was the case they would have mentioned the incorporation of an aspherical lens element in the lens literature which is curiously absent. There are three ways of making an aspherical element: Hybird, GMO, and Ground aspheric - the ground aspheric being by far the most expensive, which would explain the cost of this particular lens. A glass molded (GMO) Aspherical lens is cheaper and easier to mass produce, a molded aspheric is employed in the highly regarded FA31mm f/1.8 ASPH - but there is no mention of either type of aspheric lens being used.

When I look upon the Bokeh and the reviews that cover issues like corner-to-corner sharpness, coma and spherical aberrations from this lens, there is no indication that there is an asperical element in this lens - they tend to have a particularly distinctive impact on these areas of IQ.

[Zygonyx]

Interesting mystery...

[lytrytyr]

A glass molded (GMO) Aspherical lens is cheaper and easier to mass produce, a molded aspheric is employed in the highly regarded FA31mm f/1.8 ASPH - but there is no mention of either type of aspheric lens being used.

IIRC, Pentax have stated that the 08 wide zoom for the Q has a molded aspheric element.
I suspect that this is what they are using in the DA 20-40,
as opposed to the hybrid elements used in the DA 15 and FA 24,
which are clearly marked on the published diagrams.

[cfraz]

But if that was the case they would have mentioned the incorporation of an aspherical lens element in the lens literature which is curiously absent.

The lens diagram on Ricoh's website clearly shows an aspherical lens element, mentioned by name. So either the text is incomplete, or the diagram is wrong (or both).

[lytrytyr]

The lens diagram on Ricoh's website clearly shows an aspherical lens element, mentioned by name. So either the text is incomplete, or the diagram is wrong (or both).

BTW, it is the second element which is shown on the diagram as aspherical,
within the front group of three elements that is very similar to the arrangement on the FA 20-35.

For comparison with the DA 20-40 diagram I posted earlier,
here is the FA 20-35 diagram (from Boz Dimitrov):

[Digitalis]

I suspect that this is what they are using in the DA 20-40, as opposed to the hybrid elements used in the DA 15 and FA 24, which are clearly marked on the published diagrams.

The point is that Pentax mentioned the use of these aspherical elements in the lens literature - even if they were Hybird aspherics, which are known to be inferior to all other types.

it is the second element which is shown on the diagram as aspherical, within the front group of three elements that is very similar to the arrangement on the FA 20-35.

Similar but not the same, From what I see the FA20-35mm has better flat field correction than the DA lens. Just because the older FA lens has aspherical elements in it doesn't mean the modern DA version has them: they are two different lenses. ED glass can correct for some of the issues that aspherical lenses are designed for, however the fundamental problems that arise due to using a purely spherical lens design cannot be completely eradicated by using exotic glass types alone.

[lytrytyr]

Hybird aspherics, which are known to be inferior to all other types.

Hybrid aspherics are superior to resin-molded aspherics,
most notably in applications like the DA 15
where the front lens surface is the glass side of a hybrid element.

[Digitalis]

Hybrid aspherics are superior to resin-molded aspherics,

But not by much. When making a hybird aspheric lens, the aspheric surface is plastic - the glass surface it is bonded to is slightly spherical. The difficulty in making these effective is to align the two elements perfectly but also to make sure all materials have exactly the same refractive index: As a consequence, a compromise has to be made on all three components used in making the individual lens:. The effectiveness of a hybrid aspheric is reduced considerably by this requirement. And optically the DA15mm f/4 ASPH is inferior in corner sharpness when compared to the by the Sigma 8-16mm f/4.5-5.6 @ 15mm at every aperture. To the best of my knowledge the Sigma 8-16mm has lens has several glass molded aspherics used in its design.