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02-23-2013, 11:18 PM   #1
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Optical construction comparison of normal primes

When searching on info why the Pentax DA* 55mm F1.4 got its quite unique specs and I started looking for optical constructions as comparisons to break it down. What I came up with is that normal primes are pretty much the same construction-wise no matter the brand or the price tag, with a few exceptions.

Lets start with the boring group as they all look the same no matter the specs or price. They have the same construction as old primes and nothing really added. The difference in performance has to be glass quality and slight dimension differences in the lenses. One interesting thing is that all of them seem to only use regular lenses and glass, no special things at all.


Canon EF 50/1.4 USM - Very standard


Sony/Minolta AF 50/1.4 - Nothing out of the ordinary


Pentax FA 43/1.9 Limited - Yep, the same configuration but with a different focal length and aperture.


Pentax FA 50/1.4 - Same old same old.


Carl Zeiss Planar T* 50/1.4 - Did anybody expect this retro lens to be different?



Now on to lenses with some modern changes to the optical construction. Why are these lenses generally more expensive? Lets see.

Pentax DA* 50/1.4 - An extra very slightly convex lens in the back and a slightly concave one in the middle. Just looking at it the changes are really slight but judging from reviews they seem to improve the corner sharpness. They can also be responsible for the extra fringing some reports compared to the FA 50/1.4 as they aren't aspherical nor two cemented lenses and I cant help to wonder if the performance would have been greatly increased if at least the lens in the back would have been aspherical. The extra space needed for the lenses is probably why it became a 55mm and an extra 200€ on the price tag. The question is, did they only go halfway with the construction?


Canon EF 50/1.2L - Why the high price? The back part of the lens is the reason (of course with the huge aperture). It's the only lens from the big camera makers with an actual aspherical lens in the back and together with the fact that the next element actually is two lenses instead of one explains its impressive performance at big apertures. It's the most advanced construction considering the aspherical element its price is easily explained.


Nikon 50/1.4G - The description mentions improved optical construction and they have added one lens second from the back. So they actually split the rear element in two to improve the construction. The interesting thing is that they don't really take any extra money for it so perhaps they can use cheaper lenses instead?


Sigma 50/1.4 EX DG HSM - Many seems to dislike Sigma but they have been creative with their 50/1.4 prime at least, compared to other makers. It starts with an aspherical element similar to what Canon does with their 50/1.2 but the second group is really interesting as it almost looks like it mounted backwards compared to others. This explains both why it is reported to be quite different to other lenses and why Sigma is so proud of it ant wants to sell it as a serious alternative to the the OEM lenses.



So the conclusion is that most look pretty much the same as quite old fast 50's but some makers have recently tried to improve the old optical constructions, Pentax is one of them. Also this shows the importance of good glass and lens craftsmanship as this is pretty much the difference between the lenses in the first group.

Feel free to add more lenses in the thread!


02-24-2013, 01:13 AM   #2
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QuoteOriginally posted by VisualDarkness Quote
Pentax FA 50/1.4 - Same old same old. Carl Zeiss Planar T* 50/1.4 - Did anybody expect this retro lens to be different?
Actually they are different, on the Zeiss planar the rear surface of the 4th element is shaved flat, but in the Pentax 50mm f/1.4 is is stull curved - this causes the Zeiss planar 50mm f/1.4 to have slightly higher resolution at infinity - but the pentax FA 50mm f/1.4 lens has better correction of spherical aberrations.
02-24-2013, 01:14 AM   #3
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One thing to notice is that the old designs from Canon, Sony, Carls Zeiss has a flat surface between the lenses in the inner group. On Pentax, this surface is curved. According to Mike Johnston at "the online photographer", this curved surface reduced the sharpness a little bit, but improves bokeh.


Edit. A few seconds later than Digitalis, it seems :-)
02-24-2013, 01:45 AM   #4
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QuoteOriginally posted by StigVidar Quote
One thing to notice is that the old designs from Canon, Sony, Carls Zeiss has a flat surface between the lenses in the inner group. On Pentax, this surface is curved.
If the diagrams on Boz Dimitrov's site are to be believed,
there's a similar, although less extreme, distinction within the Pentax camp:
The curvature of the corresponding surface on the M50/1.4
is less than on the K or A series equivalents.

02-24-2013, 01:47 AM   #5
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I know and see what you are saying, still I don't feel that the change is big enough to separate them from the "standard" double-gauss lens design. All of them have different degrees of concave and convex curvature to be slightly different but it's still pretty much the same design just slight variations. The elements are still together in a almost similar group even though it will probably shift the optical path slightly. For me the lens have to be more different than just different degrees of curvature or elements put together creating a standard-looking group.

Do you agree with this as a limit? I wanted a pretty hard limit and the lenses in the lower section are all different enough. If you start talking smaller variations you will get many groups with just very simple variations of the standard double-gauss type.

Last edited by VisualDarkness; 02-24-2013 at 01:52 AM.
02-24-2013, 01:56 AM   #6
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QuoteOriginally posted by VisualDarkness Quote
I know and see what you are saying, still I don't feel that the change is big enough to separate them from the "standard" double-gauss lens design. All of them have different degrees of concave and convex curvature to be slightly different but it's still pretty much the same design just slight variations. The elements are still together in a almost similar group even though it will probably shift the optical path slightly. For me the lens have to be more different than just different degrees of curvature or elements put together creating a standard-looking group.

Do you agree with this as a limit? I wanted a pretty hard limit and the lenses in the lower section are all different enough. If you start talking smaller variations you will get many groups with just very simple variations of the standard double-gauss type.
Yes, I agree with this limit :-) Thanks for your OP, anyway. It is always interesting to read about.
02-24-2013, 02:06 AM   #7
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Feel free to add more lenses in this thread and I'll copy paste it in the list.
It's quite amazing how old many of the lens designs we still use are!
02-24-2013, 02:08 AM   #8
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OK, staying within the general scheme of things,
does anyone have an optical diagram for the "standard" 6x7 lens:
SMC Pentax 67 / Takumar 6x7 105mm F2.4 Reviews - 67 Normal Primes - Pentax Lens Reviews & Lens Database

02-24-2013, 02:24 AM   #9
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QuoteOriginally posted by lytrytyr Quote
OK, staying within the general scheme of things,
does anyone have an optical diagram for the "standard" 6x7 lens:
SMC Pentax 67 / Takumar 6x7 105mm F2.4 Reviews - 67 Normal Primes - Pentax Lens Reviews & Lens Database
It would be interesting but I can't seem to find one.
02-24-2013, 03:36 AM   #10
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Here's a thread with a mamoth posting of "double gauss" designs (some Planar, some Biotar/Xenon, some Ultron, which is usually what we see) that will be helpful.

Double Gauss Designs

This is also helpful:

List of lens diagrams: triplets, planars & hybrid lenses

Actually, I was quite interested in this matter a few days ago because of the new Sony/Zeiss 55/1.4 lens that was announced. It (besides having two asperical lenses) is listed as having 8 elements in 5 groups, which is very rare for the double gauss designs. The only other 8/5 standard lens I can think of the Leica Noctilux-M 50/0.95, which has both internal groups cemented (i.e. Biotar/Xenon type), a cemented rear group, and an extra element at the very front. I rather suspect the Sony will have the extra lens towards the rear instead, perhaps used for focusing (as it is internal-foucusing, as I recall). I would assume the two aspherical elements will be the two uncemented ones, so the front element and the one towards the rear.

Last edited by macTak; 02-24-2013 at 05:26 AM.
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