Discussions about lens design

[Travis Butler]

Mod edit: I moved these posting into their own thread, so they get more attention.

I agree, the results are good (also nearly "good enough" for me), but it is nevertheless frustrating when I compare the images of my 10 year old FZ1000 (value ~800) to the images of my new S5ii+28-200 combo (value ~2500) and cannot see really differences. At bit like SoJuJo said:

Yes, there are differences in low light situations, in action scenes etc, but, honestly, 10 years further, sensor area is about 8 times greater, and 3 times the price of my old bridge camera, and many images do not look any better (nor sharper)...? That is at least somehow disappointing (for me).

The simple answer is that it’s Really Really Hard to make zoom lenses that cover that wide of a focal range - and that doesn’t change if you’re a bridge camera or a full-frame. Well, no - it gets much harder for full-frame.

I’ve mentioned my interest in lens design in another thread, and frankly it’s hard to explain without at least a little understanding about how it works. I’ll try to summarize: lenses work by taking the light waves coming in and bending them using shaped pieces of special glass - lens elements - to come to a sharp focus, to produce the desired focal length. There are many classic designs for doing so, and all of them introduce one or more aberrations - optical errors like barrel distortion or astigmatism. Making a good lens requires a lot of fine-tuning, and usually some corrective lens elements to fix aberrations.

And that’s just for a single focal length. For a zoom lens, you have to do this over and over again for every focal length the zoom covers, moving the same set of elements into new positions to give a new magnification. The aberrations multiply as you increase the zoom range, asking the same elements to do more and more jobs; you have to either live with the aberrations, or keep adding more and more corrective elements.

So a high-quality zoom lens either covers a short focal range, or has lots and lots of corrective elements that make the lens big, heavy and expensive. That’s just the laws of optics. A larger sensor makes all this harder, because the light rays have to be controlled over a wider area. Back in the film days, a classic pair of zooms that most companies made were the 35-70 and 70-210; together they covered a wide focal range, and one was only 2x and the other 3x, which were manageable challenges. The 28-200 is a 7x zoom, more than the two classic zooms combined. It’s amazing they could do it all, really - and doing it requires accepting that it won’t be perfect at any focal length, and averaging out the aberrations and sharpness across the entire focal range.

Hopefully I haven’t made any serious mistakes or bored anyone. ^^;;

 

[dirk]

So a high-quality zoom lens either covers a short focal range, or has lots and lots of corrective elements that make the lens big, heavy and expensive. That’s just the laws of optics. A larger sensor makes all this harder...

This is where MFT excels. It is a smart move from Panasonic to use these two different sensor sizes, fullframe and MFT. Maximum benefit for the users.

 

[GeorgeHudetz]

The simple answer is that it’s Really Really Hard to make zoom lenses that cover that wide of a focal range - and that doesn’t change if you’re a bridge camera or a full-frame. Well, no - it gets much harder for full-frame.

I’ve mentioned my interest in lens design in another thread, and frankly it’s hard to explain without at least a little understanding about how it works. I’ll try to summarize: lenses work by taking the light waves coming in and bending them using shaped pieces of special glass - lens elements - to come to a sharp focus, to produce the desired focal length. There are many classic designs for doing so, and all of them introduce one or more aberrations - optical errors like barrel distortion or astigmatism. Making a good lens requires a lot of fine-tuning, and usually some corrective lens elements to fix aberrations.

And that’s just for a single focal length. For a zoom lens, you have to do this over and over again for every focal length the zoom covers, moving the same set of elements into new positions to give a new magnification. The aberrations multiply as you increase the zoom range, asking the same elements to do more and more jobs; you have to either live with the aberrations, or keep adding more and more corrective elements.

And then there are manufacturing tolerances that come into play.

So a high-quality zoom lens either covers a short focal range, or has lots and lots of corrective elements that make the lens big, heavy and expensive. That’s just the laws of optics. A larger sensor makes all this harder, because the light rays have to be controlled over a wider area. Back in the film days, a classic pair of zooms that most companies made were the 35-70 and 70-210; together they covered a wide focal range, and one was only 2x and the other 3x, which were manageable challenges. The 28-200 is a 7x zoom, more than the two classic zooms combined. It’s amazing they could do it all, really - and doing it requires accepting that it won’t be perfect at any focal length, and averaging out the aberrations and sharpness across the entire focal range.

Hopefully I haven’t made any serious mistakes or bored anyone. ^^;;

Not at all. I'd enjoy reading more about your journey of learning about lens design.

I'd be curious about this point: Looking at lens resolution graphs across many lenses (I like Lenstip.com) it seems that nearly all lenses need to be stopped down one or two stops to reach peak resolution (i.e., minimize most aberrations). Not all, but most.

If you apply that "rule of thumb" to the 28-200 - which is f/7.1 at the long end - that means it should be stopped down to f16 or so to get peak performance. Of course, that puts you deep into diffraction territory. So, it's a configuration that has some unescapable compromises built in.

Of course, it's the lightest 2x-200 FF lens on the market today, so it doesn't compromise there. Or, at least, it a set of compromises that may, or may not, work for any given photographer.

Anyway, I decided to test my "f16" theory by taking a set of shots at different apertures at 200mm, and then using Capture One's diffraction correction to try & overcome diffraction at anything smaller than f8. I was able to convince myself that the diffraction-corrected f16 image had more detail and contrast (when pixel-peeping) than the f7.1 or f8 images. For what that's worth & if anybody cares.

 

[Travis Butler]

Not at all. I'd enjoy reading more about your journey of learning about lens design.

Thank you! I cribbed most of what I know about lens design from a series of articles on LensRental's blog; I linked to one above. Unfortunately, most of them were written before they changed blogging systems, and a lot of the internal links are broken. A search for "History of Photography" on their blog will pull up most of them; here's a few in particular that I think make good starting points.

• Lens Genealogy Part 1 and Part 2 (Part 2 is the part that covers zoom lenses.) This is a later set of articles that doesn't go into the details about optical laws or lens history, but it's a good overview of the different kinds of optical designs in the lenses we use.
• A Brief History of Early Lenses Part 1 Another overview, this one focusing more on the history of lens design. The comparison at the end shows just how many lenses in the last 40 years date back to just one basic design from the start of the 20th century.
• The Seven Deadly Aberrations - This one gets more into the optical theory stuff, what optical aberrations are, why they occur and what can be done to fix them.
• Reflections on Reflections - An explanation of lens coatings and how they enabled the foundational development of modern lenses: removing the element cap. Before coating breakthroughs in the last 50-60 years, internal reflections limited the number of elements you could add to a lens; that limited the number of aberrations you could fix and made good zoom lenses fairly impractical. Coatings changed all that, allowing both effective zooms and perfectly-corrected elite lenses like the Lumix S Pro 50/1.4.
• From Petzval's Sum to Abbe's Number: Here we're really getting into the details of how lenses originated and how they work, and some of the mathematical formulas that can be used to design them.
• The Schott Heard Round the World follows on from Petzval, and covers how optical glass development changed lens design. Also explains why Zeiss had such a huge role in the history of lenses.
• Cooking With Glass continues the story, going into detail on the Cooke Triplet and touches on the foundation of zoom lenses.
• Who Invented the Telephoto Lens? The next step in the story, a.k.a. "Where did all those birding lenses come from?"
• The Development of Wide-Angle Lenses: Completing the story of prime lenses. (The end of the article teases a future article on the history of zoom lenses, but I haven't been able to find it.)
• A Clear History of Glass: Even more geekery on the foundation of lenses, glass.
• Finally, The Minolta 40-80mm f/2.8 Gearbox Zoom; The Clockwork Lens just because that's a really cool piece of gear and I'd love to own one.

(Hm, Dirk? Think this stuff on lens history and design would be of general interest and worth breaking out into a separate post? If so, where would you suggest I put it?)

I also picked up some from the discussions on the Adapted Lens Talk forum on DPR; some of the people there like to try and adapt lenses from anything, including scanners and movie projectors; in the process, they often talked about lens design and how that applied to the particular lens they were adapting. Some of them claim to be able to identify particular lens designs just by certain characteristics they produce in photos, Sonnar lenses in particular.

I'd be curious about this point: Looking at lens resolution graphs across many lenses (I like Lenstip.com) it seems that nearly all lenses need to be stopped down one or two stops to reach peak resolution (i.e., minimize most aberrations). Not all, but most.

The basic reason is actually pretty simple - most aberrations have their strongest effect at the edges of the lens elements, and stopping down cuts out the light from the edges, so it reduces those aberrations. Ran across this article while doing the LensRentals blog deep dive, it might have some further stuff of interest for you.

 

[dirk]

Hm, Dirk? Think this stuff on lens history and design would be of general interest and worth breaking out into a separate post? If so, where would you suggest I put it?)

Done

 

[Travis Butler]

Done

Thanks! Hope people find them helpful.