Why do many Z lenses have concave front elements?

[capanikon]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

 

[NickZ2016]

You mean the exposed part of the element? Don't the elements fill the lens tube? Z lens tubes are bigger. Lights focused by the centre of the element. I think.

 

[MoreorLess]

At a guess I would say because the small flange distance means they need to get the light rays as close to 90 degrees hitting the sensor as they can.

 

[cosmicnode]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

The elements may be small but they are not concave, concave is "dished " in shape.

 

[briantilley]

Why do many Z lenses have concave front elements?

I wouldn't say it's "many", exactly.

I'm fortunate enough to own seventeen Z Nikkor lenses, and only two of them have a concave front element - the Z 50mm f/1.8 S and the Z 28mm f/2.8 SE.

I don't know the optical reason for either of the design features that you mention.

 

[tjacobs]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

Define many?

Z mount Lenses | NIKKOR Lenses | Nikon Consumer

 

[Joseph S Wisniewski]

The concave front elements help reduce curvature of field, the tendency of a lens to have out of focus corners when the center is in focus.

It's not a new thing, you used to see it a lot in the 60s and 70s, especially in moderate wides and in macros.

And why do many Z have very small front elements, like the 28mm f/2.8?

Because they don’t need to be as big. A 28mm f/2.8 with a single, perfect element would only be 10mm in diameter (28mm / 2.8).

(Compare that to the 28mm f/2.8 F mount).

That's a retrofocus lens. It's built more like a 50mm f/2.8 with a "wide converter" after that to get some distance between the rear element and the sensor.

 

[Wezre]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

The elements may be small but they are not concave, concave is "dished " in shape.

--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."

Actually, the 28mm does have a slightly concave front element:

 

[SecondTimeAround]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

The elements may be small but they are not concave, concave is "dished " in shape.

Actually, the 28mm does have a slightly concave front element:

Yep, it's definitely concave. Not sure why someone would say otherwise.

The Z 50mm f/1.8 has a concave front element, as well:

 

[Joseph S Wisniewski]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

The elements may be small but they are not concave, concave is "dished " in shape.

Exactly. Have a look at the optical construction of the 28mm f/2.8 and you'll see that the front element is quite dramatically dished.

https://imaging.nikon.com/lineup/lens/z-mount/z_28mmf28/spec.htm

Like the front element of the 50mm f/1.8, which was one of the lenses released with the original Z6 and Z7. Reviewers commented on that concave front element.

https://imaging.nikon.com/lineup/lens/z-mount/z_50mmf18s/spec.htm

dpReview notes "In common with other prime lenses we've seen that are specifically designed for mirrorless camera systems, the Z 50mm F1.8 S has a relatively small, concave front element."

https://www.dpreview.com/reviews/nikon-z-50mm-f1point8-s-lens-review

The MC 105mm f/2.8 has one, too.

https://imaging.nikon.com/lineup/lens/z-mount/z_mc105mmf28_vr_s/spec.htm

There are others that aren’t dished, but are closer to "flat" than I've ever seen before in their focal lengths, especially the 85mm f/1.8, 16-50mm f/3.5-6.3 (whose design really has me scratching my head), the 14-30mm f/4 (another head scratcher), and the ghost-written 28-75mm f/2.8.

https://imaging.nikon.com/lineup/lens/z-mount/z_85mmf18s/spec.htm

https://imaging.nikon.com/lineup/lens/z-mount/z_dx_16-50mmf35-63_vr/spec.htm

https://imaging.nikon.com/lineup/lens/z-mount/z_14-30mmf4s/spec.htm

https://imaging.nikon.com/lineup/lens/z-mount/z_28-75mmf28/spec.htm

The 24-70mm f/2.8 is a fascinating example of this. The old F mount version has a very conventionally curved front element, while the Z version is nearly flat.

https://imaging.nikon.com/lineup/lens/f-mount/zoom/normalzoom/af-s_zoom24-70mmf_28g/index.htm

https://imaging.nikon.com/lineup/lens/z-mount/z_24-70mmf28s/spec.htm

It's a bit less surprising, alhough not completely so, in the telephotos like the 100-400mm f/4.5-5.6, 400mm f/2.8, 400mm f/4.5, 600mm f/4, and 800mm f/5.6. I'm going to skip linking all those spec sheets.

--
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[Joseph S Wisniewski]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

Define many?

https://imaging.nikon.com/lineup/lens/z-mount/

21% of the primes on that list.

Sony is similar. The first time I saw their 40/2.5 and 50/2.5 I noted the concave front elements. I haven’t been tracking Canon.

This type of thing is common in extremely well-corrected lenses. The iconic Ultra-Micro-Nikkor 28mm f/1.8 has a concave front element.

 

[Boudewijn van der Drift]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

Because of the imaging process, the overall of all elements must be + (= concave). Otherise you would get no image on your sensor.

Then for most lenses it is desirable to keep them short, which asks for a + (group of lenses) and a - rear (gol). This was the original definition of telefocus design.

The reverse is necessary for (extreme) short focal lengths (especially for SLR's which have a flapping mirror to miss), a retrofocus design.

The sweet spot seems to be 55-58 mm (FF) where the designer can concentrate on correcting the front lens(group) alone. S0 there they splurge with Noctiluxes and the like.

 

[Boudewijn van der Drift]

P.S. I see the 105 mentioned.

That one of course is a macrolens. You don't want circumferal rays to hit the front element too acutely. That would give rise to aberrations.

The lens has to be able to sort of "squint" to see a close subject.

 

[just Tony]

Zeiss has gone there from time to time. My Milvus 85mm f/1.4 in F mount has it. Also the Sony RX 100 versions I and V, probably others as well in that family.

Many box cameras of 50-100 years ago had single element meniscus lenses with concave-forward orientation.

 

[smith-jones]

Sony Zeiss 55 1.8 has a concave front element and with a bit of luck, later today it will be My Z mount concave element lens when my adapter finally arrives.

 

[tjacobs]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

Define many?

https://imaging.nikon.com/lineup/lens/z-mount/

21% of the primes on that list.

Sony is similar. The first time I saw their 40/2.5 and 50/2.5 I noted the concave front elements. I haven’t been tracking Canon.

This type of thing is common in extremely well-corrected lenses. The iconic Ultra-Micro-Nikkor 28mm f/1.8 has a concave front element.

21% is misleading there are 2 versions of the 28mm. The OP asked why many Z lenses have a concave front element, they don't there are only two that do, the 28mm and 50mm 1.8 primes.



I guess I don't consider 2 out of 15 prime lenses to be many.

--
-tj

 

[Joseph S Wisniewski]

And why do many Z have very small front elements, like the 28mm f/2.8?

(Compare that to the 28mm f/2.8 F mount).

Define many?

https://imaging.nikon.com/lineup/lens/z-mount/

21% of the primes on that list.

Sony is similar. The first time I saw their 40/2.5 and 50/2.5 I noted the concave front elements. I haven’t been tracking Canon.

This type of thing is common in extremely well-corrected lenses. The iconic Ultra-Micro-Nikkor 28mm f/1.8 has a concave front element.

21% is misleading there are 2 versions of the 28mm.

Which I counted as one lens. My "three lenses", as I clearly stated, are the 50mm f/1.8, 28mm f/2.8, and MC 105mm f/2.8, which you keep leaving out, despite it being a high seller.

The OP asked why many Z lenses have a concave front element, they don't there are only two that do, the 28mm and 50mm 1.8 primes.

The MC 105mm f/2.8 also has a concave front element. This has been a thing with high performance macro lenses for many decades: my 28mm f/1.8 Ultra Micro Nikkor has a concave front element, as does the 50mm UMN, the incredible Coastal 65mm UV-VIS-IR macro, and the Nikon and Pentax UV macros.

Concave front elements on normal lenses like the 50m f/1.8 and 28mm f/2.8 (which only really shines as a "normal" on the APS bodies) is a pretty new thing, but it's something we're seeing from all the EVIL lens makers.

I guess I don't consider 2 out of 15 prime lenses to be many.

That's not really fair, as you counted the two versions of the 28/2.8 in the total number of prime lenses, but didn't count it in the number of concave front element lenses.

If you add in the MC 105mm f/2.8 that you keep omitting, you've got two possibilities.

• The "modern" and "retro" versions of the 28mm f/2.8 and 40mm f/2 count as separate lenses, in which case there's sixteen primes, four of them with concave front elements for 25%.
• The "modern" and "retro" versions only count as one lens, and now you've got my original fourteen primes, with three having concave front elements for 21.4%

Personally, I don't consider the "retro" versions of the 28mm and 40mm to be separate lenses, and so a total of fourteen primes, three of those (the 28, 50, and 100) with concave front elements. I do consider three of fourteen (21.4%) to be "many", especially when...

• One of those three, the 50mm f/1.8, is the very best selling Nikon Z prime, the only one to have topped 100,000 units, and pretty near twice the sales of the number two lens.
• All three lenses combined account for nearly half of Nikon's Z prime sales (45.8%, if you want to be picky)

As usual, my thanks to Roland Vink for the lens sales numbers.

 

[briantilley]

The MC 105mm f/2.8 also has a concave front element.

Are you sure about that?

The optical diagram that you linked to shows the outer face of the front element as being either flat or very slightly convex, but the inner face is clearly convex.

[pedant hat on]

Technically, to be classed as concave, I think a lens element must have both faces concave, as indeed the Z 50mm f/1.8 S and Z 28mm f/2.8 do. An element with one face concave and one convex is termed a meniscus lens.

Having said all that, I suspect the OP was referring only to the outer face of the front element

 

[SecondTimeAround]

Which I counted as one lens. My "three lenses", as I clearly stated, are the 50mm f/1.8, 28mm f/2.8, and MC 105mm f/2.8, which you keep leaving out, despite it being a high seller.

The OP asked why many Z lenses have a concave front element, they don't there are only two that do, the 28mm and 50mm 1.8 primes.

The MC 105mm f/2.8 also has a concave front element.

Perhaps not a highly accurate method of measurement, but the front element of the NIKKOR Z MC 105mm f/2.8 VR S does not really appear to be concave to me:


NIKKOR Z MC 105mm f/2.8 VR S

 

[ipopov]

105 definitely is not concave.

It's optical scheme is published in the patent WO2022097401A1, where Example 1 matches the released lens. And that example has very slightly convex first surface.

See the diagram on the optical bench here https://www.photonstophotos.net/Gen...alBench.htm#Data/WO2022-097401_Example01P.txt

And the patent is here: https://patents.google.com/patent/WO2022097401A1