Like a lot of photo history buffs, I’ve been quite excited about Lomography’s new iteration of the Petzval lens in 85mm focal length. For those of you who don’t know about the Petzval lens, I wrote about it a few years ago. It really has a rather a fascinating story.
Since writing that article, I’ve been rather obsessed by this lens. I own several of them, made in the late 1800s, but I haven’t been able to adapt them to work on a modern camera. Now Lomography has reproduced the Petzval lens in a nice brass housing, for either Canon or Nikon mounts. Our first copies arrived yesterday and I grabbed them for a bit before they headed out the door.
The new Petzval is quite a handsome bit of kit.
It truly is the classic design, complete with Waterhouse stops instead of an iris, and rack-and-pinion focusing instead of a rotating ring. None of your fancy focusing helicoids or 16-bladed aperture rings for this lens.
Now most of you, when you get a shiny new lens arriving, run outside and start taking pictures. But that’s not how we roll here at Lensrentals.
What follows is just wrong.
But just wrong seems so right sometimes. We decided some new 175 year-old technology needed to meet some new 6-months old technology.
So the first Petzval got put up on the Trioptics MTF bench.
While the other one got slapped on OLAF, our 5-micron pinhole tester.
Obviously, this 175 year-old design isn’t supposed to complete with modern lenses for resolution or aperture. It provides a classic dreamy look. But hey, we test lenses, so guess what we’re going to do?
OLAF actually is more fun here, showing what this lens is about. The spherical aberrations should make the out-of-focus areas smooth and pretty.
In case the built-in aberrations aren’t enough for you, Lomography includes, in addition to the standard Waterhouse stops at various apertures, some very fun cut-out aperture stops: a teardrop, hexagon, and a star.
They have some interesting effects on OLAF’s pinhole light. I can’t wait to see what they do with actual photographs.
The hexagonal aperture isn’t all that strange.
The star aperture should make nice star points from light sources in night shots.
Star aperture with lens in focus.
But it should also make for some interesting effects with out-of-focus highlights.
Star aperture defocused.
The teardrop aperture looks much like a decentered lens on OLAF when properly focused.
But becomes more interesting in out-of-focus areas.
Of Course I Tested It
As I’m sure you know, the indispensible Kingslake’s History of the Photographic Lens points out that the Petzval design “uses overcorrected astigmatism to flatten the tangential field . . . giving excellent definition in the center of the image deteriorating rapidly towards the edges.” I was quite pleased that the MTF bench showed that the new version does exactly that. Note that once you get away from the middle 1/3 of the image astigmatism is, well, pretty damn impressive.
Red–10lp/mm; Green–20lp/mm; Blue–30lp/mm – you get the drill.
What the MTF charts suggest we’ll get is exactly what Petzval lenses are supposed to deliver; a fairly nice centered portrait with the outer half of the image significantly blurred. Even in the center, though, the lens doesn’t resolve very well by modern standards wide-open. The frequency graph of the center of the lens emphasizes this.
I know that most of you, at this point, are thinking, “Sure, Roger, we expected these MTF results. But can’t you please show us the field curvature, too?” Fear not, my friends. I can and I will. As Kingslake said, the astigmatism of the lens flattens the tangential lines pretty well, but the sagittal lines have some wicked field curvature. Although in this case that’s a good thing, since a major purpose of the lens is to blur everything off-center.
Field curvature. Red shows the sharpest area. “0″ on the vertical axis is the plane of focus.
Yes, I Sort of Took Some Pictures
It’s ugly and rainy here, and the two copies we’ve received are on their way out the door today, so I had no chance to exhibit my superb photographic talents. (Which is good, because I really don’t have much in the way of superb photographic talent. I’m a Photo Geek, not a Photo Grapher.)
But I did discover a few useful things. First, you are not going to want to shoot landscapes with this lens. It’s so soft at infinity that at first we thought it wasn’t reaching infinity focus. Not to mention that all the King’s Photoshop Horses can’t make the edges sharp.
But one thing I found while trying this is that the odd shaped apertures really mess up the camera’s autoexposure (on a Nikon D3x at least). The image on top is with the f/4 Waterhouse stop in place, the one on the bottom with the Star Aperture, otherwise they were identical although both autometered.
The effect can be kind of cool, though, close up. For the closer shots below, the top image is shot wide-open, the second with the Star aperture (can you tell I like that one?) and exposure bumped up in post about half a stop. I should also mention that I’d shoot raw with this lens. Color seems to change with aperture a bit and almost every shot either needs to be white-balanced individually with the Waterhouse aperture in place, or corrected in post.
Wide open. Even shrunk down for web display, the softening of the image away from center is quite apparent.
The major use of this lens, of course, is fun. But it originally was called the Petzval Portrait Lens, so a portrait seems in order. Since my usual swimsuit models and studio lighting weren’t available at 7 a.m., I made do with Corey, the only person who managed to be at work on time, lit by the soft, romantic glow of a test chart.
Now, I’ll have to end my little post, as the packers have come and ripped the last copy from my hands. I’ll mention a couple of things that those of you interested in this lens might want to know before we close, though.
The rack-and-pinion focusing is quite accurate, but rather clumsy to do while you’re looking at the LCD to Live-view focus. I’d really recommend using a tripod if at all possible. Or if you want, just set it at the estimated hyperfocal distance and shoot away. It’s not going to be razor sharp no matter how well you focus.
I’d really avoid any crosshatched backgrounds. Or maybe go find them. The astigmatism makes them look odd, but whether it’s a good-odd or a bad-odd is probably in the eye of the beholder.
I mentioned earlier that the lens is pretty soft at long distances, but it seems to do quite well close up.
You aren’t going to replace any of your current lenses with this one. But for some of you, it might be a fun addition that gives your portraits some really different looks.
Oh, and one last link for the overly serious among you who want to lecture me about how this lens is about taking pictures and not about lab values.
Well, I have to admit this has been a fun series. I’ve learned a whole lot. That’s what makes this so fun — I get some results I don’t understand, get some help figuring out what is going on, and before I know it, I’ve learned something that explains other things I haven’t been able to understand.
In the second part of this series, we started a database of sensor stack thickness and exit pupil distances, hoping that it would help people decide which lenses would adapt best to which cameras. (And, of course, determine which lenses would not adapt well to which cameras.) A number of people have added information to the database since it was first posted — enough to make it pretty useful.
Since the database is now large enough to be useful, I thought it would be a good idea to make a summary of what we know about lenses and sensor stacks. The best thing about all this, for me at least, is that it lets us make some generalizations about which lenses would be expected to have problems on which cameras.
To summarize, there are three main factors that determine if a given lens will have problems when used on a different camera than it was designed for:
Aperture (wider aperture has more problems)
Exit pupil distance (shorter exit pupil distance has more problems)
Difference between sensor stack thickness the lens is designed for, and the sensor stack thickness of the camera it’s being used on.
As always, Brian Caldwell has been kind enough to furnish graphs of the theory, which really helps in visualizing things.
Aperture and Stack Difference On-Axis
Brian’s first graph demonstrates two important points.
Graph courtesy Brian Caldwell
The graph demonstrates the on-axis (center) MTF of a theoretically perfect lens designed for no sensor stack filter (orange line), and the effects of adding a sensor stack equivalent to 1,2, and 4mm of glass. The first takeaway message is that for apertures of about f/2.8 and smaller (higher f number) there is really no significant effect on-axis. (Emphasis on the ‘on-axis’ part. This doesn’t mean the corners will be great, although they may be.)
The second takeaway message is that the effect is proportional to the difference between the sensor stack thickness the lens was designed for and actual thickness of the camera it’s being used on. For example, adding 1mm of extra glass in the path doesn’t really affect things until f/1.4 or so, while a 4mm difference is quite apparent at f/1.8. Once the effect begins, though, MTF decreases exponentially with increasing aperture.
The summary, for on-axis effects, is that shooting a lens at f/1.4 on a camera with a sensor stack different by 2mm or more from what the lens was designed for will probably reduce MTF even in the center. Stopped down, though, you’re unlikely to see any difference on-axis.
Exit Pupil Distance from Sensor
Brian’s second graph shows the effect of a 2mm sensor stack on an f/2.0 lens designed for no sensor stack. If you go back to the graph above, you’ll see that at f/2.0 a 2mm sensor stack difference has really no effect on-axis. This graph shows the off-axis effects out to the edge of the sensor.
Graph courtesy Brian Caldwell
As you can see, the off-axis effect shows a mild decrease in MTF and increased astigmatism even with a 100mm exit pupil distance. Would you see this in real-world photography? You might, if you compared an image taken with the adapted system to an image taken with the lens native camera system. But overall, you’d probably be pretty happy with the lens adapted to a different camera.
With a 25mm exit pupil distance, the effects are very severe, even just a short distance from the center. An exit pupil distance of 50mm definitely has some effect, too, although as expected it’s not as bad as 25mm.
Sensor Stack Thickness
Our database for sensor-stack thickness remains incomplete (although I’m hoping for more contributions soon). Also remember that physical measurement of thickness and optical thickness may be different. If the glass used has a high index of refraction, it would have an optical effect greater than what is measured physically. For example, we might measure a physical thickness of 2mm on two different cameras, but if one has low-refractile glass and the other high-refractile glass, the optical measurement made might be 2mm and 2.5mm.
The bottom line is all of the sensor stack thickness measurements we have are guesstimates, probably accurate to 0.5mm or perhaps 0.25mm. However, since a difference of 1mm should have only a minimal effect that’s probably accurate enough.
Our optical bench measurements with f/1.4 lenses seem to support this. When we measure lenses with a difference of 1mm of optical glass in the path we see almost no change on-axis and usually just a mild astigmatism change off-axis. A 2mm difference usually causes significant problems, though.
The summary regarding sensor stacks seems pretty clear. Leica cameras (with possible exception of the M240) have less than a 1mm stack. Most SLRs are around 2mm. Micro 4/3 cameras, with the exception of the Black Magic cameras, are about 4mm.
We should expect the following to give problems:
1. Using a lens designed for film on an SLR would give a 1.5 to 2.5mm stack difference, and we should notice a performance drop-off when using wide-aperture lenses with shorter exit pupil distances.
2. Using a lens designed for an SLR on a micro 4/3 camera would give a 2mm stack difference, and we may notice a performance drop-off on wide-aperture lenses with shorter exit pupil distances.
3. Using a lens designed for film on a micro 4/3 camera would give a 4mm difference, and if the other factors (exit pupil distance and wide aperture) are present we will almost certainly notice a performance drop-off.
On the other hand, using a Nikon lens on a Canon camera, or either of those on an NEX or Fuji camera shouldn’t give major problems since all of those sensor stacks are similar.
This should answer one question that several people have asked: Do third-party lens makers have to alter a lens optically when making a Canon versus a Nikon mount? I wouldn’t think so; the sensor stack thickness is very similar. It may (I’m just guessing) also answer an unasked question: Did Zeiss decide to make Tuitt lenses for Sony and Fuji, but not m4/3 because of the sensor stack difference? I don’t know the answer, but Sigma makes the same mirrorless lenses for both m4/3 and NEX. There may be compensating optics in those, but also none have wider aperture than f/2.8, which might minimize the optical difference.
Rangefinder Wide-angle Lenses
As the graph above shows, lenses with an exit pupil distances of less than 50mm are significantly affected by a sensor stack difference. Those with exit pupil distances of less than 25mm are greatly affected. Glancing at the database, it’s obvious that most SLR lenses have an exit pupil distance of 50mm or more. Many rangefinder lenses are less than 50mm and some (especially wide-angles) are around 25mm.
The lens database has gotten fairly large, so I’ve changed it to be sortable, which should help you find what you’re looking for. A couple of lenses do stand out that, according to theory at least, should make bad adapter candidates (at least when shot wide open).
A number of other rangefinder lenses have exit pupil distances of less than 50mm, but particularly notable are the Voigtlander 35mm f/1.2 and 50mm f/1.5, both of which have wide apertures in addition to short exit pupil distances. Obviously, stopping these lenses down will help minimize the effects of sensor stack if you just have to shoot them on an m4/3s camera.
Most SLR lenses have exit pupil distances of 50mm to 100mm. One thing that might be lost in the table is that the Sigma 18-35mm f/1.8 Art lens has a nice, long exit pupil distance, especially at the side end where it’s 150mm. This should make it a superb lens to use on cameras with different sensor stack thickness. Other ‘long exit pupil distance’ lenses include the Nikon 35mm f/1.8 DX (136mm), Nikon 24-70 f/2.8 AF-S G (116mm at 24mm), and Tokina 11-16 f/2.8 (100-117mm depending on zoom distance), and Canon 50mm f/1.2 L (103mm). The Canon 17mm f/4 TS-E (91mm) and 24mm f/3.5 TS-E (86mm) also have fairly long exit pupil distances. All of these should give good performance even out to the edges (assuming a good adapter, of course).
While I don’t have a mathematical formula to predict how well a given lens will work on a given camera, it should be apparent that wide-angle rangefinder lenses designed for film are going to struggle, especially on m4/3 cameras, but to some degree on other cameras. Longer focal length rangefinder lenses tend to have longer exit pupil distances and should do better. Stopping down to f/4 or f/8 should reduce the problem significantly, although it isn’t guaranteed to eliminate it near the edges of the frame.
SLR lenses may also have some troubles on m4/3 cameras, since there’s nearly a 2mm difference in stack thickness. They shouldn’t have as much of a problem as rangefinder lenses, though, since they are designed for a 2mm sensor stack and they tend to have longer exit pupil distances. Older lenses designed for film cameras will tend to have more issues, of course, since they were designed for no sensor stack.
Obviously, these are generalizations and suggestions. There are going to be exceptions. But, if nothing else, hopefully we can help a few people stop trying one adapter after another, hoping to make their 15mm rangefinder lens look great on the m4/3 camera.
A couple of weeks ago I got an email asking if we would be willing to take some lenses, remove the electronics, fix the aperture wide-open, and permanently lock them at infinity focus. It seems the person who needed this done was having trouble finding a legitimate repair shop or service center that was willing to do it.
Well, illegitimate is our specialty, so I started negotiations about just how exorbitant a fee we would charge for this work. We quickly arrived at a fair price (no money, but we get to take pictures) and yesterday received brand new copies of the Canon 100mm f/2 and Sigma 35mm f/1.4 Art to work on. If you’re the kind of person who slows down to view car wrecks or spent $200 on fireworks for the 4th of July holiday, you might like this.
(For those of you who aren’t American, the 4th of July is when we celebrate our Independence by getting sunburned, making burnt offerings of animal parts in our backyards, and then eating said offerings. During the entire day, we drink massive quantities of American beer and once it gets dark we shoot off massive quantities of Chinese fireworks. All too often, the results of mixing alcohol and explosives prove that Darwin was correct — but hey, that’s what celebrating is all about, right?)
If torn apart camera lenses make you squeamish, then you won’t like this, and I suggest you not read further. You won’t miss learning anything; it’s just for fun. As best I can determine, this post has absolutely no practical use whatsoever. It’s just something to amuse and entertain those of you who are amused and entertained by such things.
A couple of years ago I gave a talk on the history of lens design at the Carnegie-Mellon Robotics Institute. The faculty members were kind enough to spend the day showing me some of their research on computer-enhanced imaging. I’m a fairly bright guy with a doctorate of my own, but I don’t mind telling you by the end of that I was thoroughly intimidated and completely aware of my own limitations. Continue reading →
We’re growing, which means, we’re hiring. Lenrentals is a fantastic place to work. We believe in working hard, having fun, and giving our customers the best possible experience. Our team members get great benefits, including health, dental, paid vacation, and 401(k), not to mention – FREE RENTALS! Continue reading →
When you run a rental house, you basically function as a torture-test lab for equipment. For many years I’ve put out a Repair Data list annually, showing which photography equipment is more likely to fail than others. I get asked to do the same thing for video equipment, but I usually just shrug and say it’s not necessary. I can sum it up simply.
If the product is new and exciting it will probably fail.
If the company is new and cutting-edge, the product will probably fail.
This sounds like a generalization, and it is. It also sounds like an exaggeration, and it’s not. New, cutting-edge video equipment from new, cutting-edge companies has an extremely high failure rate. For a lot of these products, nearly 100% fail within a few months. Continue reading →
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 →