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Field Curvature and Stopping Down

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Ignorance more frequently begets confidence than does knowledge. Charles Darwin

What I know that I don't know doesn't cause me much trouble. I don't know the formulas for 7th order optical aberrations for example. I don't know enough math to calculate those formulas even if I did memorize them. But I've got shelves of books on optics and friends who know way more optical theory than I do. So if I need to find out something about 7th order aberrations (mercifully, to date that hasn't been necessary) I can go find out about them.

What I don't know that I don't know causes me all kinds of trouble. I had yet another learning experience last month relating to field curvature (more accurately, the plane of focus curvature). Like everyone else I knew that when I stopped the aperture down on a lens the image got sharper and the depth of field got larger. I knew that the center might sharpen up more than the corners, at least at first. But I assumed that if the corners were getting sharper, and the center was getting sharper, than everything in between was getting sharper, too. It turns out that isn't always the case.

I'd also never really thought about what happens to field curvature when you stop down.  If I thought about it at all, I probably assumed it would flatten out. Or maybe stay the same but the increasing depth of field would make it less noticeable. Turns out that isn't always the case either. So, because I made assumptions and didn't know what I didn't know, I wasted a lot of time. Weeks of time. Continue reading

The Itsy Bitsy Spider

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We spend a lot of time here at Lensrentals getting dust out of lenses. Dust doesn't affect an image, except in very rare circumstances, but people want their rental lenses to look nice and clean inside and out, and our inspectors check the inside of every lens with spotlights and send any dusty ones over to the repair department.

Yesterday one of the inspectors sent over a Canon 135mm f/2 L lens with a fairly unique note: "Contains a large dust chunk and the internal elements are scratched." Now, we see a lot of scratched elements, but that really is pretty limited to the front and rear ones. Scratching internal elements is, well, unusual. When we took a look inside the lens, though, we were pretty impressed. There was indeed a big chunk deep in the center of the lens, and what at first glance appeared to be multiple scratches on the internal elements. Continue reading

Just the Lenses: Canon and Nikon Mount 35mm f/1.4s

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As I discussed in the last post, it's impossible to accurately compare lenses from different mounts using any form of computerized target analysis testing (methods like DxO or Imatest). Target analysis tests an entire system (camera and lens). That's a very practical thing, of course, but it has some limitations.

Directly comparing the lenses on an optical bench without any camera involved is interesting for several reasons. The most important is that the optical bench gives some information about a lens that is difficult or impossible to get using Target Analysis methods.  Field curvature is a good example of information that is easy to get on an optical bench, but nearly impossible to obtain with Target Analysis. Plus, optical bench testing shows performance at infinity, rather than the closer focusing distances used for target analysis. Continue reading

Just The Lenses: Canon vs Nikon Zooms at 70mm

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It's impossible to accurately compare lenses from different mounts using any form of Computerized Target Analysis testing (methods like DxO or Imatest). Target analysis tests an entire system (camera and lens). People try to, of course, but it's not accurate since you always have the added variables of camera sensor, microlenses, in-camera image processing, etc. Some people try to use adapters to test different lenses on the same camera, but then you have added variables from the adapter and sometimes from sensor stack thickness.

In general, it doesn't really matter. If you shoot Nikon you aren't really interested in Canon lenses, and vice-versa. Still, directly comparing the lenses without any camera involved is interesting to some people, I think. Some people are thinking of changing brands - they know the other brand's camera has higher resolution, but aren't sure if the other brand's lenses are as good. More people than ever are shooting lenses across brands using adapters, and they're sometimes curious, too.

It is, of course, possible that nobody but me is interested in direct cross-brand lens comparisons. But since I am interested and since I have to test all of the lenses in our inventory anyway, I thought I'd show some of that data. If others find it interesting, too, I'll write up some more comparisons like this. Continue reading

Some M-Mount Field Curvatures

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I haven't posted very much lately. We've had some new equipment installed and we've been doing a LOT of testing as we develop our new database of lenses on the optical bench. As the database fills out I'll be posting more than ever, just because a lot of this stuff is just fun. Today's post is largely for fun, but will have some additional interest for those who shoot Leica or shoot M-mount lenses adapted to other cameras.

One thing that optical bench testing gives us that is hard to find elsewhere is a clear map of field curvature. We had a client interested in determining field curvatures for a several M-mount lenses and thought there would be a few among you who also wanted to see them. Continue reading

HandeVision IBELUX 40mm f/0.85

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Image credit Lensrentals.com

Preconceptions

I try to start these articles by putting my preconceptions out there first. Every reviewer or blogger has them, they affect our opinions, and you have a right to know them. So I'm writing this introduction the day before our first copies arrive.

The HandeVision IBELUX 40mm f/0.85 is designed by IB/E Optics GmbH in Germany and manufactured by Kipon (aka Shanghai Transvision Photographic Equipment Co. Ltd). IB/E has developed a number of lenses and adapters for the Cinema world and other optics, so I figured the design would be good; probably a telecentric lens with a built-in Speedbooster-type element or group. Kipon is known as a lens adapter company, although Shanghai Transvision has also manufactured and distributed video and photo accessories. They are rumored to manufacture lenses for other brand names, so they have some lens manufacturing experience.  But, I have to say, my expectations for build quality weren't great. I expected a lot of variation between copies. I don't know if I even had any expectations regarding image quality.

Okay, so much for what I expected. There are now five new copies sitting on my desk so let's take a look. Continue reading

A Useful Metabones EF to Black Magic Pocket Camera Article

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Way back in the old days, before everything was plug and play, you could buy a computer and accessories from one manufacturer and be certain it would all work together. The cool kids, though, would mix-and-match different pieces and end up with a better system that could do more things than any one manufacturer's system would, and for less money. They realized they might have to spend some time getting this to work with that, and once in a while things wouldn't work at all. That's the price you pay for being a cool kid and getting more for less money.

The people who had problems bought the same thing the cool kids did, but just got irritated when things didn't all work together right out of the box. The cool kids laughed, felt even cooler, and made some money debugging the other kids' systems for them.

We do the same thing today with cameras. It's simple to use a rig, lenses, monitors, whatever of the same brand. It will probably cost a little more and you may have a few less capabilities, but it's just about guaranteed everything will work fine with everything else. But if you want to save some money and expand your capabilities, you mix-and-match systems, often using an adapter or two.  The problems come when people don't realize that not everything is going to play nice with everything else. The problem is compounded because the people who make the widgets just don't have the capabilities of testing everything with everything else.

A few weeks ago Metabones sent me one of their new EF lens to Black Magic Pocket Camera Speedboosters to play with. I did the usual optical testing and let some of the video guys shoot with it, like all the other bloggers do. I planned on writing a piece saying how awesome the optics were (they are) and how many cool things it lets you do with a Pocket Camera (it does) like all the other bloggers do. But in our testing we found a few things that didn't work well together, and it occurred to me that rather than adding yet another "The EF to Pocket Camera Speedbooster is Really Great" blog post, I could do something useful and actually list what it works well with and what it doesn't.

Continue reading

Of Course We Took One Apart

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A Look Inside the Canon 16-35 f/4 IS

This is a Geek Article. Many of you don't understand the term 'Geek' properly, so perhaps this will help. As the graph shows, if you aren't both intelligent and obsessed with photo gear, you won't enjoy this article. 

I've tried hard to find whom to credit for this, but haven't been able to. If you know, please let me know so I can credit this brilliant work.

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Canon Wide-Angle Zoom Comparison

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Left to right: Canon 16-35 f/2.8 II, 16-35 f/4 IS, 17-40 f/4. Can you spot the one with the wrong hood? The intern obviously couldn't. 

 

As is so often the case, I bit off more than I wanted to chew when I came back from vacation. The Canon 16-35mm f/4 IS lens had just been released, a few copies were in stock, and I thought I'd do a nice quick test. But one of the reasons I'd wanted an optical bench was because I don't trust Imatest results with wide-angle lenses. At 16mm, even with the very largest test charts, we're testing at about 4 feet shooting distance.

So I after I did our standard Imatest on the 16-35 f/4 IS, I wanted to repeat the results on our optical bench. Of course, I don't have a big database of optical bench results to compare against like I do with Imatest. So I had to do optical bench tests on some other wide zooms for comparison purposes. Then I had to do some more comparison with other lenses to see if the variations we were seeing on the optical bench were simply a new, higher resolution testing method, or if they were telling me something about variation with wide-angle zoom lenses. (Both things were true.) Anyway, the testing I thought would take a week has taken three.

I realize some of you just want to see the usual Imatest results on a group of these lenses since that's what you're used to seeing. Others are also interested in the optical bench results showing how the lenses resolve at infinity, rather than just close up. And of course there are a few of you who want all the gory details of Geekiness that the optical bench reveals. So I'll try to present this in three parts: the Imatest results first, the optical bench optical test results second, and the geeky stuff third. It's a buffet; just grab what appeals to you.

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Save 15% This Summer

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Get 15% off this summer when you reserve your order a week in advance. Just place your order at least 7 days in advance of the arrival date and save!

Enter the code SUMMER15 during checkout and get 15% off on your order.

  • Order must be placed 7 days in advance
  • Eligible arrival dates: 7/28/14 - 8/29/14
  • May not be combined with other offers