The F-number of a photographic lens is a measure of its light-gathering ability, and is expressed as its aperture diameter divided by its focal length. Lenses with low F-numbers are prized by photographers for their properties, but are usually expensive because making a good one can be something of a challenge. Nevertheless [Rulof] is giving it a go, making an 80mm F0.5 lens with a Sony E-mount. The video below the break has all the details, and also serves as a fascinating primer on lens design if you are interested.
Rather than taking individual lenses, he’s starting with the second-hand lens from an old projector. It’s got the required huge aperture, but it’s by no means a photographic lens. An interesting component is his choice of diaphragm for the variable aperture, it’s a drafting aid for drawing circles which closely resembles a photographic part. This is coupled with the triplet from an old SLR lens in a 3D-printed enclosure, and the result is a lens that works even if it may not be the best. We know from experiences playing with lens systems that adjusting the various components of a compound lens like this one can be very difficult; we can see it has the much sought-after bokeh or blurred background, but it lacks sharpness.
Perhaps because a camera is an expensive purchase, we don’t see as much of this kind of hacking as we’d like. That’s not to say that lenses don’t sometimes make their way here.
Small correcrion, f-number is measure by focal length divided by the aperture diameter. That why it is written f/8. The f stands for focal length. Great read! Thanks for the article.
vaguely the right concept, but wrong arithmetic. think it through.
Indeed. As stated in the article, photographers would appear to desire lenses with the smallest value of the aperture diameter divided by the longest focal length, which would be a pinhole.
Yes, that’s it : a 50 mm lens with a F:2 aprture will have a diaphragm hole of exactly 25 mm as 50:2 is 25, a 100 mm at the same aperture will have a diaphragm hole of 50 mm, and a 600:4 will have a minimum size hole of 150 mm, that’s why long focal bright (wide open) lenses are so big !
Almost. The aperture is defined as the size of the entrance pupil. That is determined by the diaphragm size (in a correctly designed lens), but generally is not exactly the actual size of the hole in the diaphragm.
The entrance pupil is the size that the aperture stop (diaphragm, usually) appears, as seen from the entrance of the lens, through the lens components.
Go play with an unmounted SLR zoom lens, looking in from the front. It’s fun to see how the entrance pupil changes size with focal length, even if the diaphragm itself isn’t changing size.
If the F-number is the aperture diameter divided by the focal length, wouldn’t you want a very small lens if you wanted to achieve a low F-number?
It depends. If the goal is just to lower the f number then yes you could make a tiny lens with nearly any focal length. But that’s usually not the only goal.
Let’s use the other extreme so it’s more obvious. What if they used a microscope slide? Kind of large but the focal length is about infinite so it would be a nearly tiny f stop.
So f stop is important when we talk about a lense relative to other lenses in an optical system. Less so on its own. So what the author is trying to do probably is to find a nice depth of field or good image resolution for their system. In doing so they want a low f stop. The challenge with making a low f stop lens that is large is getting accurate and small geometry on glass.
I hope this makes sense… Basically it’s not incidental that they need a low f number but it’s only part of the puzzle.
More optics please, but;
If 1/2 is a lower number than 1/16 then yes, this is a low f number lens.
Otherwise this is a high f number lens.
Or, you could confuse some people by referring to is as a f divided by low number lens.
In other old news https://en.wikipedia.org/wiki/Third-pound_burger
Are there any F/0.0625 lenses?
I don’t think so but there was a video for something around 0.3 – but not a simple mount, I believe it was some multi-stage digital hybrid.
There’s a 50mm f/0.7, so pretty close!
https://en.wikipedia.org/wiki/Carl_Zeiss_Planar_50mm_f/0.7
The notation for F number is f/n when written correctly, which accurately conveys the relationship. So what is typically said as “to follow the sunny 16 rule, shoot at F-16 on a sunny day”, would actually be written correctly as “shoot at f/16”.
So, hopefully this help demonstrate how in fact f/.5 is in fact much larger than f/16. The other thing to know is that the major “stops” on a camera represent increments of 2, so each major stop represents twice as much light entering as the stop below it.
The importance of f-stops is understanding their relationship in the exposure triangle. A higher number let’s in less light, but captures a greater depth of the picture as being in focus. This is ideal for landscapes or images where you want alot of layers to be captured clearly. A lower number f-stop captures more light and renders a shallower depth of field. This is preferred for capturing people and single subjects, as it can blur out other things in frame, allowing the subject to be the only thing in focus.
One thing the article does not address is the depth of field of a f/.5 lens. My fastest lens is f/1.2, and it has an absurdly shallow depth of field when wide open. I can’t imagine you could even capture a face fully in focus at f/.5, which is probably why they aren’t made.
Yup, my f/1.2 will get your eyes in focus but not your ears. And not your nose if it’s too big!
The DoF of this will be wafer thin. If you focus this on the iris, the eyelashes will be blurred.
The aperture of that lens isn’t 0.5. The original video, and the person aggregating it here, don’t understand how focal length is calculated, so their extrapolating the f/# from a lens they assigned a focal length using nothing but vibes, even the aperture diameter is vibes based haha. If they set the camera to live view (or video recording) and slowly stopped down the lens, they wouldn’t see any change in the image recorded for 3 or 4 “clicks”. The image circle looks to be about 10mm across at the sensor, and as it’s showing the video he recorded, the right side up, that means he did manage to get the light to cross over and project the image circle upside down (the correct way, it might be the one thing he actually got right in his video haha). So draw two straight lines from the outer edge of the front element, that cross over a centimeter or two before the sensor, ending 10mm apart. Whatever their distance apart as they pass through the location of the adjustable aperture, is the required diameter of the aperture… And what I would consider the maximum entrance/aperture diameter for that lens. If he actually calculated the correct focal length (it’s not less than 150mm, I’m certain of that), and measured the aperture diameter properly, he made the worst 180mm f/2.8 I’ve ever seen, or something thereabouts. There may not be a single interchangable lens for sale on eBay worse than the lens that guy made.
So in Europe this would be a Royale lens?
A royale with cheese
Focal length has nothing to do with the size of the lens. The f# uses physical measurements in its calculation (aperture diameter), but focal length is calculated using a functioning image with an image circle large enough for a 36x24mm sensor/film to have proper coverage. You can make a pretty good guess at focal length using only image magnification, or field of view (degrees from center lens the image circle utilizes to cover the 36×24 sensor), if the image circle is the correct size. And while it’s not technically required for the image the lens creates be sharp, or even functional, it’s fair to dismiss this design as not a camera lens, as moving the lens further from the sensor, to gain correct image circle, will lower sharpness so far it wouldn’t have any use-case, a bit like holding a magnifying glass infront of a camera body with no lens attached and claiming it’s actually a zoom lens.
Focal length has nothing to do with the size of the image circle.
Yes… It does. Pick any lens designed for a crop sensor, MFT sensor, etc. And in every piece of documentation you will see the “effective focal length” noted. For the Nikon 35mm AF-S DX f/1.8, it’s 56mm. Want to know how the focal length of 56mm is calculated? It’s the 35mm focal length the lens would have if a full frame camera was used to calculate the focal length, multiplied by the 1.6x the Nikon APS-C sensors were shrunk by. The smaller sensor creates a smaller field of view angle, which is literally in the equation used to find focal length.
That bit of “effective focal length” marketing fluff also is independent of the actual focal length.
The “effective focal length” really means “the field of view is equivalent to this focal length on the standard full frame sensor”. Field of view is a result of the focal length and the sensor size (and shape though that’s usually a 3:2 rectangle). Photographers use “focal length” to mean the field of view most of the time, as that’s the practical implication it has on the act of photography, but focal length is an optical property entirely independent of what you’re projecting the image onto.
NO !
Nikon APS sensors have a « crop factor »* of 1,5.
Only Canon has the 1,6 ratio, so the Nikon DX 35:1,8 gives the same field of view of a 50 mm on a 24 x 36 sensor on an APS sensor.
And focal length does not exactly have a relation with the size of the sensor or the film, it’s only the minimum distance between the diaphragma and the film or sensor to have an image (as closer will be only blurr).
If the lens is a 50 mm and the distance between film or sensor and diaphragma is 50 mm, the picture will be sharp at the infinite, if the distance is longer than that, it will be sharp at a closer distance, and if that distance is fx2 (100 mm in that case),
you get 1:1 magnification ratio, witch means 1 cm on the subject will be 1 cm on the film or sensor, and loose one stop of light (it’s called macro photography, and after that, bigger magnifications can be obtained by adding more lenght with rings or a below to make microphotography).
Lens design give an image circle that can be quite different for the same focal length, and then give a different field of view, and THAT makes the difference.
On large format cameras, it’s possible to use the same lens on a 8 x 10 inches (20 x 25 cm) camera and on a 4 x 5 inches (10 x 12,5 cm) one, for instance. The « standard » (the closest to human field of view) lens for the 8 x 10 is a 300 mm (it’s simply the diagonal of the film format !), it can be used on a 4 x 5 inches camera, and there will be a « crop factor »* of 2, and the shifting possibilities will be enormous (if the lens is made for 8 x 10 of course).
It’s not possible to do the contrary : if the image circle is smaller than the size of the frame, there will be vigneting (althrough I saw once someone shoot with a Nikon 40:2,8 DX on a Nikon F 100 film camera, and the lens has in incredibly big image circle and the negative had only a milimeter rounded corners !, it’s also possible to cut the very little sunshade on the Nikon 10,5/2,8 DX Fisheye to have a round image on a 24 x 36 film or sensor).
about «crop factor» and « full frame » : those are marketing garbitch, 24 x 36 is in fact a very poor reference for image quality : a 4 x 5 inches film is nearly 15 times bigger, and 8 x 10 close to 60 times… And I love my Olympus that’s MFT (Micro Four Third, what a weird name) and has a « crop factor » of 2 : I can use any of my Nikon and Leica lenses on it with pleasure.
Here is Canon’s focal length calculator. It’s a nice handy one, since it has every mainstream consumer sensor and film size (or the varied image circle sizes,, if you want to get technical). Go enter that 35mm Nikon DX lens into a few of those spots, and see how much the rest of the lens attributes change.
https://www.usa.canon.com/pro/electronic-range-calculators/object-dimension-object-distance-focal-length?srsltid=AfmBOoqsYY3wwTEjGaSXP6zFfPU4ZwCR1V37gyu8oTIYkvjHQY56QYQh
If your effective image circle is smaller due to a smaller sensor or film only recording the central portion of the image, the field of view angle is smaller. And I’m sure you’re a strong independent proud young girl, but you can’t just remove entire variables from extremely well known and understood equations/calculations, because you think lenses having different focal lengths when the sensor size changes, is some marketing slop cooked up to personally torment you.
The true (not “effective”) focal length of a lens is not dependent on the film or image sensor behind it. Focal length is purely a function of the design of the lens.
The focal length of a lens influences, but is not dependent on, the field of view (“image circle”). The field of view is determined by the size of the field stop (NOT to be confused with aperture stop or diaphragm) — again, a function of the lens design.
Those assertions are supported by that Canon calculator.
People do a terrible job explaining aperture’s f-number, and how it relates to depth of field in a way that’s easily understood.
Aperture manages depth of field in a photograph. Depth of field refers to how much of the overall plane of the photo is in focus. Generally, it can be sorted into shallow, medium, and great depth of field, while also allowing for zero – no focus anywhere – and infinite – focus everywhere.
Who, except an engineer, is going to be dividing the aperture diameter by the focal length when taking a photo? The reason the smaller f- numbers are the larger lens openings – responsible for the smallest, shallowest, most bokeh depth of field – is because the f- number is a fraction of the diameter of the lens. So f2 means that the opening of the aperture will fit across the diameter of the lens twice, pretty wide open, letting in a lot of light for the duration of the exposure. Focus is more important when a lens is wide open, from f1.2 – 5.6, because of the way the light crosses the lens. There’s a wider light scatter when the lens is open, so where you’ve focused becomes the deciding factor on what’s in focus.
The smaller openings are the larger f numbers – f16, f22, f36 – and are responsible for the greatest depth of field. F22 means that the aperture opening fits across the lens 22 times, which is a tiny opening. The light scatter crosses the opening (aperture) in a narrower pattern, which allows for increase in the area of focus and maximizes depth of field. Ansel Adams famously used f64 in many of his photos, so everything in his gorgeous landscapes is sharply focused. You don’t have to worry as much about where to focus when you use the larger f numbers (smaller apertures), because that will give you access to greatest depth of field. Because they are the smaller openings, your shutter speed may have to slow down to get an adequate exposure. If you are shooting the photo with less than 1/60, or a sixtieth of a second, if you don’t want motion blur to affect the sharpness of the photo, you may want to use a tripod or a stable surface to balance your tripod on. That’s a whole other lecture on how shorter speed and aperture have to coordinate with iso to get the best exposure, but hopefully someone gets a glimmer of understanding about why the larger f numbers are actually the smaller openings. When someone says to ‘stop down’ they mean pick the larger f numbers, because it’s the smaller openings. There’s an endless amount of math in photography, which can make it confusing if you don’t know about it. If you decide to learn about it, it gives you much more control over the outcome of the photo. Your intentions will show in the outcome of the photo, rather than having no idea how to replicate the gorgeous photo you just took. You’ll be able to develop your photographic style and vision, which will set you apart from other photographers.
So many words. So many terrible explanations and incorrect interpretations. It looks like maybe a non-native language translation issue for some of it. But in any case, please take the above as an ‘interpretive dance’ of the concepts: Go find one of the many, many good books on optics and photography to get a better handle on the concepts and terminology.
You’re wrong about almost everything you said. But with several other people misunderstanding the fundamental basics of optics in the comments, I’m gonna keep this short…
The f/#, when you get to individual lenses, does not provide any discreet data on how shallow or deep the depth of field is. Even when two lenses have the same focal length. There are 55mm lenses that provide very impressive bokeh, and quite shallow depth of field. Many of the f/1.4 manual focus lenses from the 70s come to mind. Then there’s Nikons 55mm f/2.8 micro (macro lens design in most other brands lingo) from the 70s, that does have incredible sharpness wide open at f/2.8, but makes a very very different image, due to its wildly impressive sharpness. Even as the distance increases from the sharpest distance, it remains well within what is considered “in focus” for lenses designed for portraiture, where the designers engineer as shallow an in-focus depth as possible.
The 55mm example is one of my favorites, but the difference in depth of field between two lenses of the same focal length, but different design languages or just difference in quality, is so obvious, im wondering if you even take photos.
It was really after you mentioned Adams and his f/64 that I was sure you just watched a movie about photography once… Ever seen the camera he used to take those photos? They were full plate cameras, 4″x6″ or 5×7. His f/64 would be a setting around f/16, possibly even more open than that, using a 35mm camera.
Forgot to add that I meant using a dealers choice 55mm f1.4 stopped down to f2.8, and the Nikon 55mm f/2.8 micro, wide open. Everything else being exactly the same, camera, subject, lighting, etc. etc. You would never suspect two lenses with the same focal length, set to the same aperture, took the pictures.
The video from Applied Science is extremely relevant and got a better result too :/
https://www.youtube.com/watch?v=DQv0nlGsW-s
Bonus: Ben Krasnow actually mostly understands what he’s doing.
The point of the video was that he mostly didn’t, but wanted to, so he did research and experimentation and now he does know. It’s extremely informative how the full calculations work and why the simplification that often confuses people was done. I find it very similar to how wing lift descriptions are wrong but also functional.
i just think the phrase ‘from scratch’ doesn’t mean what you think it does
You’re right, step 1 should be creating the atoms needed for the lense
Doesn’t Carl Sagan have a related recipe for cake?
Ackshewally, it was apple pie, but yeah… first you need to invent the universe.
More like I’d assume that “making an 80mm F0.5 lens” involves, you know, making a lens. Not assembling a pre-made lens into some plastic. Words mean things.
If i ordered a 50mm f/2.0 lens from Canon, and they delivered a few pieces of glass without the structure around them, I’d be pretty disappointed…
Maybe they should call them “lens assemblies” instead, then.
That’s your misunderstanding, because the full title describes not a glass lens but an assembly.
If you want DIY pens making look up DIY telescopes. There’s a guy that goes through the entire process of refining a blank at home.
We are too spoiled from Huygens Optics. I’d love to see him make a very fast lens one day.
But for now, here’s making a solid telescope from scratch quite literally.
https://youtube.com/watch?v=A0bysBIj0FA
Adapting projector lenses to cameras is a regular thing among YouTubers. See https://hackaday.com/2022/08/02/ultimate-bokeh-with-a-projector-lens/ in which a 35mm f/0.4 lens was made.
The great irony is that, because of the high refractive index of silicon, the image sensor used simply can’t accept light coming in from a cone much wider than a plain f/2.8 lens will produce: All that additional light outside that cone from a faster (but simple) lens is simply wasted.
That’s why you see modern lenses labeled as “digital”: They are a much more complex design (like the phone camera lens he uses as an example), and specifically designed to accommodate the restricted acceptance angle of silicon sensors.
It’s worth having a look at the Zeiss f 0.7 lens that Stanley Kubrick had adapted for movie cameras to use in the candle-lit shots in “Barry Lyndon”, that were filmed without artificial lighting.
https://www.dpreview.com/news/8390711699/stanley-kubrick-s-rare-zeiss-planar-50mm-f0-7-barry-lyndon-lens-now-in-zeiss-museum-of-optics
That class of lens is nowhere near as rare as that article implies. Many, many thousands of very fast lenses were made for medical imaging devices (X ray image intensifier tubes), that suddenly became surplus when the industry transitioned to flat-panel detectors. Depending on the system they were liberated from, they were 42-75 mm, and f/0.75 to f/1.1. Rodenstock was a big supplier (“Heligon” model) , but there were others. For a while they were dirt cheap, literal garbage, until the eBay bottom feeders found their value.
If you do score one, you’ll find that: a) they were intended to be in contact (or nearly so) with the image sensor, usually with optical oil or grease coupling it, usually to a vidicon (or plumbicon) television camera tube. It’s hard coupling that to a CMOS sensor (requires cover window replacement and optical oil or epoxy to fill the void). And b), the output light cone is well over 100 degrees wide, again not useful for most sensors, as a silicon imaging sensor can’t accept input light that wide anyway: most light reflects or scatters right back out.
Fast indeed (and pricey – yikes), but as the article describes the Zeiss units were built for “satellite imaging” (other sources say this was for three-letter agencies). The real magic was getting them to work at all with a movie camera with its shutter assembly and moving media.
I did a bit of lens hacking back in the early 90’s, though modern fab tools didn’t exist and the state of my art was more sticking a surplus lens at the end of a pair of sliding PVC pipes with a T-mount to the camera body. That said, some amazing glass was being surplused in those days as photocopiers and lithography machines switched from analog (requiring clear sharp page images) to digital (scanning with simpler optical) techniques. They had to be color-corrected because lasers for illumination were far too exotic and projector lamps were used. I got a 300mm F/5.6 Zeiss copy lens for $25 which must have cost over $1000 new, and several Bausch & Lomb 240mm x F4 with actual built-in diaphragms to stop down to F/16 for $20 each. Any of those lenses would take a full-35mm image of a butterfly that could be blown up to full page with razor sharpness. They were awkward as hell to use but nothing like them was ever made for general use. Any of those lenses would also probably have worked with a 4×5 or 8×10 view camera negative too. For what I was doing they were overkill to the nth degree.
I also tried the superfast projector lens, snagged in my case from a home projection TV system. Like the OP I found it fuzzy, because projector lenses were designed for light wrangling rather than sharpness.
I found that with fast homemade lenses even flat black paint in the tubes doesn’t suppress internal reflections well enough; you need baffles. Tight enclosures like the OP made are therefore not ideal.
Sadly, in a world of 10mm digital sensors those massive lenses simply don’t provide much added value. If film was still in general use I’d probably have some creative uses for them though; I still have the glass if I think of a use for it. From the OP it sounds like there’s another wave of good glass coming through channels as old broadcast equipment is retired; those won’t be as wide-field or as sharp, but stupidly telescopic zoom is a standard thing in that world. Snag stuff like that while you can, because once it’s gone they won’t be making any more of them.
Maybe you don’t care about the topic, but some of us do. If you’ve outgrown hackaday, I get it, but to act like the problem is anything other than you?
Hmm, the message I was replying to disappeared. Sorry, you can delete these.
I was always confused about f-stop, the weird f/n notation (so F16 is same as F/16?), depth of field, and what or why, exactly lenses were called “fast” or “slow.” It comes up because I like amateur astronomy and have a simple 8″ Newtonian telescope. Darn thing just works, I don’t even have to think about it.
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And the amount of “explanations” above that were promptly called totally wrong and promptly “corrected” really doesn’t help. Hahaha. See, photographers. This. This is the problem.
Wow, some folks here in the comment section might have a deep understanding of optics and photography, but they’re still outnumbered by the ones who THINK they do…
The ending was kind of funny to me. So much work and the result looks like if you put a plastic lens from a toy camera up to a webcam sensor. Womp womp! Still worth it for the process and the optics lesson, so I’m not really criticizing I just thought it was kind of a good unintentional punchline