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MTF - The English Translation

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This post contains absolutely no mathematics. Explaining MTF without math is sort of like doing a high-wire act without a net. It's dangerous, but for any number of reasons is more likely to keep the audience interested.

Why Am I Doing This Again?

I wrote an article on reading MTF charts several years ago. It focused on deciphering the MTF maps that many lensmakers publish when they release a lens, like the one below. But I get a lot of emails asking me asking me how to compare the MTF graphs we use in testing to the manufacturer's MTF charts. Or asking me to show an MTF frequency chart (and if I show it, then lots of emails asking what it means). So I thought I'd write a quick post about the different types of MTF data and charts.

So first, an overview of the common types of MTF charts.

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Yet Another Sigma 50mm Art Post

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I suspect I'll never be on any manufacturer's "early review copy" list for new lenses. There are already plenty of good early lab reviews on the eagerly awaited Sigma 50mm f/1.4 Art lens at SLRGear, Lenstip, and DPReview, among others. So when we received our first Sigma 50 f/1.4 Art lenses for rental stock I really didn't plan on posting about it.

But I always think it's worthwhile to review multiple copies of a lens bought off-the-shelf from retailers. Plus, we'll have our new MTF bench installed in another week or two, and I want to do some comparisons using the bench versus Imatest, so when we got our first seven copies of the Sigma 50mm f/1.4 Art in yesterday I compared them with Imatest. I decided I might as well post the results, even though I'm a bit late to the party.

The comparison most people are making is with the Zeiss 55mm Otus, because the Sigma Art and the Otus are both newly designed, excellent lenses. I added the Canon 50mm f/1.2 into the mix, since I suspect some 50mm f/1.2 shooters are wondering if they should migrate to the new Sigma. I choose a beat up and battered two-year-old Canon for the photo below, just to emphasize that it goes into this contest like a prize-fighter coming out of retirement for the fifth time. True, it was released in 2006, but even that was something of a comeback. The optical design dates back to the early 1980s.

As always, this isn't a complete lens review, just lab testing and comparisons on multiple samples.

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A Look at Stopping Down

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We all know that aberrations affect points of light off-center, making them blurred. We all know that some aberrations are worse the farther away from center we go. And we know that some aberrations are improved when we reduce the aperture.

Some of us even know the various rules-of-thumb for what makes an aberration worse or better. Astigmatism, field curvature, and distortion get exponentially worse as you move away from the center of the lens. Coma and lateral chromatic aberration get worse away from center, but not exponentially, and spherical aberration isn't worsened at all as you move away from center. Reducing the aperture dramatically improves spherical aberration and coma, reduces field curvature and astigmatism to a lesser degree, but doesn't have much effect at all on distortion or lateral chromatic aberration.

It gets pretty complex, doesn't it? And since different lenses have very different amounts of the various aberrations, none of us really have any idea exactly how much improvement to expect when we stop down a bit. We do some trial and error (well, most of us do) and decide where the "sweet spot" for a given lens is. I know to shoot my Zeiss 50mm f/1.4 at f/5.6 if I want sharp images away from the center, for example, just because I've played with it and figured that out. On the other hand, I can shoot my Sigma 35mm at f/1.4 -- it doesn't really seem to get much sharper at f/2.8.

Since we've been using OLAF to look at how lenses render points of light off-axis, I thought it might make a fun demonstration to see how moving across the field of view affects how the lens "sees" a point of light, and how stopping down improves it. Continue reading

Introducing Lenscap+

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In 2006, Lensrentals was the very first photographic rental company to offer an optional damage waiver program to its customers. Since then, we have continued to develop innovative options for our renters like the Lensrentals HD subscription shipping program.

Since the introduction of the damage waiver, one of the most common requests we’ve received is to offer additional levels of coverage that protect against theft and situations where a total loss occurs. Unfortunately, it just wasn’t financially possible for us to offer such a program at a price that made sense for our customers, but we never stopped working on it.

Finally, after years of trying, we are thrilled to announce the Lenscap and Lenscap+ protection plans! In addition to providing our renters the additional coverage options they want, we think we’ve succeeded in setting the prices so they’re an absolute value for everyone.

Meet Lenscap and Lenscap+

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OLAF's Lens Art

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This is a Geek Article, with very little practical information. But there are pretty pictures that non-Geeks might like. (Not the construction pictures, the ones further down.)

First, I should explain why I haven't posted much lately. Lensrentals was able to expand into some adjacent space, which was desperately needed. But the testing and repair departments were moved and expanded into the new space. Over the last 10 days the second testing area went from this . . .

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I Sing of OLAF . . .

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I have a couple of things to talk about today. First, is to announce the winner of our name the new machine contest. Second is to answer about 1,000 people's questions regarding optical testing and adjustment.

The New Machine's New Name

First, if you don't known what we are naming then you can find out about it here. Second, as so often happens, I was unprepared for the number of responses. We received nearly 700 suggestions here, not to mention dozens more via Petapixel, DPReview, and the Imaging Resource.

 

We quickly realized that, once again, we'd started a contest without establishing any criteria for winning.

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Roger's New Toy Needs a Name

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First and foremost, if you aren't at least a little Geeky, this post is not for you. Unless you're one of those people who thinks 'just take some pictures, dammit' when I write some article about resolution testing.  In that case, you might like this article because we're taking pictures to test lenses. Sort of.

For the last 18 months or so I've been on a Holy Quest, trying to find better ways to optically analyze and adjust bad lenses. Why? Partly because we need to. Factory service just can't seem to return some lenses to proper optical adjustment. Partly because some smug people told me I could never learn how to do that, which, of course, made me really, really want to.

I can use any of several tools we have to generate MTF charts that tell me the lens is decentered. But those don't tell me in what way the lens is decentered. More importantly, when we optically adjust a lens, those MTF charts tell me it's better or worse, but not exactly how it's better or worse.

We discussed things with some very high-power optical consultants who said exactly what many photographers have been saying all along. The numbers don't tell you everything; you need a picture. That led to some meetings with the fine engineers at Optikos, who manufactured a machine to do what we wanted. Continue reading

The Heights and Depths of Nadar: TL;DR Version

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Nadar Rotating Portrait. Circa 1865, Bibliothèque Nationale de France

 

"He is a man of wit without a shadow of rationality . . . His life has been, still is, and always will be incoherent."  Charles Philipon, describing Felix Tournachon.

Photographers are the most fascinating people. I'm sure there are some that simply spent their life just taking great photographs, but I tend not to notice those. There are too many more who were great photographers and a whole lot more. I write about the 'whole lot more' photographers. It can be the photographer who invented strobes and took pictures of atom bombs. Or the photographers that made the fastest automobile of their day. Or the photographer who invented the telegraph, or the drunken geologist that invented the telephoto lens.

It just seems that the most interesting photographers had a lot of other things going on. Today's subject had more going on than most.

He was at least the first great photography marketer, if not the greatest self-promoter ever. He also was arguably the best portrait photographer of his day, the first to routinely use electric lights, and the first aerial photographer. In his spare time he filed dozens of patents, was the model for the main character in a Jules Verne novel, wrote over a dozen books and hundreds of articles, was the top editorial cartoonist in Europe, and sort of established the first airmail service.

That stuff is pretty great, of course, but my boy Felix was way more interesting than even those things would suggest. He hung out with people like Jules Verne, Sarah Bernhardt, and Victor Hugo. He openly despised Napoleon III when that was politically incorrect in a much more dangerous way than we use the term today. He gave the Impressionist guys (Monet, Renoir, Cezanne and others) their first exhibition largely because he knew it would upset all of the Paris art critics. He sued his brother. He made several fortunes, but was always out of money. 

Unfortunately, while there is an out of print biography of him, there are only snippets of information here and there online. I'm a blogger, I know the drill: 1,000 words is all people will read these days. But 1,000 words just can't tell the story sometimes. So I wanted to gather all that information together in one place.

I'll warn you, though, it's a long story. But it's a good story. Nadar was so awesome that I already look forward to reading this post in a few years when I've forgotten that I wrote it. (My wife says it won't take a few years. A month or two is sufficient for me to forget almost anything.)

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The Sky is Falling and the Light is Leaking: the A7r Anti-Massacree

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Anti-Massacree - A humorous anti-war movement from the 1960s, suggested in the Arlo Guthrie song Alice's Restaurant. The song, like many of my posts, was criticized for being overly long.

Believe it or not, I'm mostly a lurker in online forums. I read the hysteria of the day mostly for my own amusement. Sometimes I type a response but I almost always delete it. Interjecting facts into one of the daily hysterical rants would be about as welcome as a cat at a dog show. Usually I don't even go that far. I just think there's a lot of people online without much to do and go back to work. Continue reading

A 24-105 Comparison

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Sigma has been releasing one great lens after another for a while now. Some, like the 35mm f/1.4 and 18-35mm f/1.8 zoom, have created feeding frenzies soon after (or even before) their release. Others, like the 24-105 f/4 OS, haven't created a whole lot of fuss. So I thought we would do a little optical testing and make the logical comparison between the classic Canon 24-105mm f/4 IS and the new Sigma 24-105 f/4 OS.

As usual, this is not a complete lens review, just a simple resolution test on several copies of each lens. One thing we'll do with this test that we haven't been doing a lot -- we'll test the lenses both with Imatest and also on an optical bench so we can compare performance both at infinity and at closer focusing distances. Continue reading