The first post I made on sensor-stack thickness wallowed deeply in PhotoGeekery. This one is meant to be of practical use so I'll try to leave the Geek stuff out. We'll start with the simple facts.
1) There are several pieces of glass right in front of the sensor of every digital camera.
2) The thickness of this layer varies from less than 1mm to slightly more than 4mm depending upon the camera.
3) The thickness of the stack can affect the optics of a lens mounted to that camera.
There is some confusion on when this stuff matters so I'm going to attempt to accomplish two things with this post. First, we'll do a general summary of when it might matter. Second, we'll start a database of information that's not readily available so those who are interested can come back to this page and find out if a certain camera-lens combination might have a problem. Continue reading →
Single glass piece from the sensor stack of a Canon (left) and Micro 4/3 (right) camera. Image credit Aaron Closz.
NOTE: This is a Geek Post. If you aren't into geeky photo measurements, or into adapting lenses from one brand of camera to another, you'll not be interested.
A year or two ago, I wrote a blog post where I basically showed lenses shot on adapters on other cameras aren't acceptable for testing. If you run them through Imatest the results aren't accurate. I suggested that reviewers shouldn't test lenses on adapters, although obviously adapters are a great way to use interesting lenses to take pictures. Continue reading →
I doubt any of you remember it, but 1995 was the coming out party for Digital SLRs. There had been digital cameras before then. There had even been digital SLR cameras of a sort - modified cameras tethered to a hard drive -- before then. But in 1995 camera makers released digital SLR cameras for the masses. Well, sort of for the masses. I think the masses were a few hundred people. Continue reading →
Well before our Imagemaster optical bench was delivered, I knew what I wanted to do first: compare some Leica lenses with the best SLR lenses on a level playing. I did one comparison involving Leica lenses a long time ago, but Leica lenses are difficult to test in our Imatest lab. It's nothing to do with Imatest or the lenses; simply that mounting and aligning rangefinder cameras in a testing setup designed for SLRs is difficult and very time consuming.
That was a couple of weeks ago. Usually I have the write up done within a day or two of testing. But I've never processed and graphed this much data before -- the optical bench generates a lot more numbers than Imatest. I tried a few things. I hated them. I tried some more. Finally I asked the good Geeky people who tend to read this blog to help out. I'm pleased to say over 40 people are currently working with another set of data, trying to help me find a good way to overcome my mathematical and graphical limitations to handle all of this data. But that's going to take some time.
In the meantime, I still have a bunch of information about those 50mm lenses I want to tell you about. So I've decided to go ahead and post using the fairly crude methods I've come up with so far. Consider this a very preliminary effort since I hope the next set of tests we do will have a far better presentation. Please take it for what it is - a beta trial of a new technology we're learning to work with.
OK, when I introduced our new optical bench, I mentioned that it generates a lot of data. We gathered data for a nice comparison test (I mean who wouldn't want to know how the Zeiss Otus and Sigma 50mm f/1.4 Art stack up against some nice Leica 50mm lenses). Since then I've spent about 20 hours wading through data and trying different ways to present it. I'll wallow through that article, but I can't say I'm totally happy with my efforts.
A number of people have left posts that if I'd furnish them some raw data they'd take a shot at graphing and presenting it. Which seems like a wonderful idea, especially after I went cross-eyed over the holiday weekend trying it myself. So this morning Aaron collected a small data set that anyone who wants to play with is welcome to.
Note: This is a Geek article. If you aren't into geeky stuff, you won't be into this.
"We take a step back so that we may leap further." African proverb
I wrote a post a couple of weeks ago about our first copies of the Sigma 50mm Art lens, and promised to follow up when we got more samples. Unfortunately, for the first time in history, Tyler just can't get any copies, so I've been unable to look at sample variation any further.
It's always been my practice to not write anything when I don't have anything to write about -- somewhat unusual for a blogger, I know. But it's been so long since I posted that several people have asked if I was OK. So I thought I'd show you why we've been so busy, and why we should start posting some very cool things quite soon.
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.
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.
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 →