Call me crazy, but I plan on experimenting to make a lens that will hopefully achieve an exposure equivalent of what a f/0.7 lens would provide or perhaps even a little under.
Let me say that this will be for fun, and if it works, which it very well might not... the lens will be very funky in regards to optical quality. I just want to see if I can do it.
Right now, the plan is to use a Nikon f/1.2 55mm lens, which is full frame. That means the projection circle is at least 11x the needed surface area of 16mm film.
What I plan to do, which has been briefly attempted before, is to take the full frame image circle and condense it to an image circle suited for a 16mm chip size. Thus we get more of the image circle falling upon a smaller sensor, so mathematically, I'm anticipating a two stop gain in light with an increase in the field of view; meaning the 55mm lens will look wider than it normally would on that particular camera.
The F/1.2 lens will still behave like a F/1.2 lens. The depth of field will be the same, however, the exposure will have increased by 4x the amount of light.
As mentioned, this will likely end up as a very 'vintage' looking lens, with aberration and sharpness issues, but I hope to make something that is fun to play with. I'd imagine getting any image at f/0.7 should be a fun time.
Depending on light loss, there is a small chance I could break the f/.07 barrier, by perhaps a sixth or a third of a stop. I'm just wanting to hit f/0.7.
Mind you I'm using F-stop to measure the amount of light, but the lens will always really be a f/1.2 and have all the DoF qualities of a 55mm f/1.2. Only my exposure will change because of the focal reducer assembly.
So my lens will technically be aiming to be a 55mm f/1.2 lens that exposes as if it's letting in light that a f/0.7 lens would.
See what I mean? It won't actually have an aperture opening of F/0.7
I'll update this thread as it comes together over the next month or so. Depending on how much I work.
I'll be starting with a 50mm f/1.4 and will buy the 55 f/1.2 as necessary.
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05-15-2012 11:56 AM
05-15-2012 11:56 AM
* most def not to scale... at all
So here is the first basic game plan.
The obstacles are immediately recognizable.
First, thank goodness for Nikon flange distance! At 46.5mm, it is one of the longer FFD's out there. This gives me some working room. However, if I'm not mistaken, the biggest reason this won't turn out pretty is because I believe the focal reducer should actually come before the focal point which occurs inside the lens. So I am immediately fighting an uphill battle trying to wrangle the light after it's past that point. I could be wrong, and perhaps it is bad to try to design this lens after how it's done in the telescopes, but it is a concern for sure.
Second, to utilize the space as much as possible and to test the focal reducer at different positions, I will remove the flange and any necessary back end parts of the lens. I imagine this won't be much, as I just took apart another Nikon lens of mine to see what I was getting into.
As of right now, the plan is to keep it simple and go from there. I will be suspending the 55mm lens, focal reducer, and camera (now a GH2 in 1:1 crop mode) as my test subject. The GH2 is a wonderful choice as it has one of the shallowest FFD's of the bunch thus giving me more (if there was anything to begin with) room to work. This is under the assumption the focal reducer does not change the back focus distance of the lens. I hope it does by means of making it longer. Of course this runs the risk of letting the image circle get bigger over distance and cancel out the condensing effect. Having all of these parts on a bellows system track will allow me to move them very precisely and by careful amounts to find the optimum distances between the lens, reducer and camera. I will have an assortment of lens elements and achromats on hand to experiment with and attempt to improve the results.
I originally desired a PL version of my creation, but unless everything can fit within the RedEpic PL port, and do so safely, I am afraid the deep flange depth is not helpful. Only helpful when the lens has a deep flange depth. The camera is better to have a shallow depth so I have more room to work in the open, and by open I mean the two inches and change.
Here is a goodexample of how this works. It was attempted on GH1's awhile back, but in my opinion, failed in many attempts by many people because of improper mounting, which I plan to explore the best I can. However, my deepest fear is that this won't work considering the reducer might be optimal in front of the point of focus which happens in the lens. I don't know if that is the case, but it seems to be how it works in telescops:
I believe the reducer typically goes right before the mirror, which is before the point of focus. Above, I can only put it right after.
If this were easy, it wouldn't be as fun, would it?
Last edited by Ryan Patrick O'Hara; 05-15-2012 at 11:53 PM.
05-15-2012 01:12 PM
Tagged for the epic mad-scientist glass whoreage.Matt Gottshalk - Dp/Editor
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05-15-2012 02:22 PM
- Join Date
- Feb 2012
nice idea my 2 pence... Maybe purchase a achromatic doublet (Tokina +0.4 closeup). these are known to improve drastically any chromatic aberration problems from less than optimal lenses. eg. my olympus 50mm f1.4 is a lot better with the achromat when wide open. little difference in brightness too.
05-15-2012 02:51 PM
- Join Date
- Apr 2011
It is really hard to make the opposite of a telecompressor (tele-decompressor?) It is also really hard to make
things that are f/0.7 and are not narrow field microscope objectives.
You might want to grab some lens design software like OSLO or Zemax, pull in one of the basic 50mm lens
models and see if you can squeek out an f/4.0 teledecompressor that does what you want in terms of
the image size. Then think about f/2.0 and let us know if you figured out how to make it all work.
Then you will need to find someone to grind the lenses and a machinists to put them in a housing. Hopefully you
are already a machinist or a lens designer. That will save some headaches.
Actually, if you just design a f/1.0 lens for a 16mm camera image circle and build that from scratch you will be
much happier. You can make all of the image quality versus lens complexity decisions when it's easy to
fix the problem. Once you are using a lens someone has already designed your hands are tied when it comes
to speeding the system up more or less.
05-15-2012 02:59 PM
- Join Date
- Apr 2011
f/0.7 lens acts like an f/0.7 lens and has the depth of field you would expect from an f/0.7 lens. (Assuming the f/0.7 lens has decent
imaging performance and one can talk about depth of field and mean something.)
05-15-2012 03:24 PM
What I want to do is the opposite, and those seem to be called focal reducers. It takes the image circle and condenses it into a smaller circle. Therefore the same amount of light is being pushed into a smaller space, which means more photons per photosite.
The goal is to compress full frame image circle down to what is required of an image circle for 16mm sized sensors.
I'm really doing this for fun. I'll be experimenting with different lens elements from broken lenses, focal reducers, condensers and other fun stuff in concert with the already made f/1.2 55mm lens, which I may tamper with, if I find I need to. My goal as of now, was to allow the lens to operate as normal, and simply do my best to wrangle the light coming out of the lens and reducing the image circle down to as small as 16mm sized sensor. I know what you are saying and I'm not out to create a f/0.7 lens from scratch and have it properly housed. There will be no lens designer. I'm just playing with focal reducers from telescopes and various optical elements, using those elements to teach myself more about lenses, and to see if I find anything cool along the way. Most likely the contraption will be temporarily housed by using a bellows system track modified to hold lens elements, and the final design will be made from spacer tubes and adapter rings.
I'm more focused on using a telescope focal reducer instead of the condenser. The condenser is going to be for experimentation, as it seems completely inappropriate for my goals.
I'm doing this to learn and explore, so I really do appreciate you mentioning this. Can you please go more in depth because I'm trying to wrap my head around what will happen when I condense an image circle.
Last edited by Ryan Patrick O'Hara; 05-15-2012 at 03:35 PM.
05-16-2012 04:45 AM
- Join Date
- Apr 2011
perfectly in focus. Over on http://en.wikipedia.org/wiki/Depth_o...e_DOF_formulas they define the depth of
field as the difference in distance between the farthest thing that is mostly in focus and the nearest thing that is mostly in focus. Then they
rewrite the distance to the object that is supposed to be perfectly in focus in terms of the magnification m. If c (CoC) and m in constant,
for close objects they find
DOF ~ 2 N c (m+1)/m*m
where N is the F number (for close focus). If you half the F number, you will half the depth of field. For far focus, the formulas are messy but
if you plot them relative to F number you'll find that smaller F number always pulls down the depth of field.
Side note on the telescope telecompressor, remember that most of those are designed for an f/10 or f/6.3 input beam and go to around f/3.0
at best. Going from f/1.2 to f/0.7 is a big deal in terms of lens complexity.
Last edited by Dustin Moore; 05-16-2012 at 04:52 AM.
05-16-2012 05:24 AM
- Join Date
- Mar 2011
The focal reducer will shorten the effective focal length of the lens and consequently (assuming the aperture remains constant) change the f-stop. It should have DOF similar to the faster lens it becomes, or perhaps I'm missing something.
Last edited by nothing; 05-16-2012 at 05:30 AM.