RPO's quest to build a f/0.7 lens.

[Ryan Patrick O'Hara]

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.

Interesting. I'm still not certain on this.

To play devil's advocate, if I were to go along with this notion and say that the lens has changed qualities and now inherent the DoF of the faster lens it has appeared to become, wouldn't the focal length change be directly proportional to the gain in sensitivity? For instance, because of the inverse square law, a 2x teleconverter loses 2-stops of light, because the surface area is four times as large. So playing with this reasoning, theoretically, this lens were to have shallower depth of field, but at the same time inversly be a wider focal length of proportional means, and we know that wider focal lengths have deeper stops.

Lol. Something to think about.

I assumed the lens would keep it's same properties, the only difference is that we see more of what used to fall off the sensor, thus it's more of a FoV change than it is a DoF change. It's the CoC part I've got to figure out. I love this talk though. Good way to start my brain in the morning. Thanks to everyone who is participating. Can't wait to figure this all out.

[Ryan Patrick O'Hara]

The depth of field depends on the circle of confusion, the f-number and the distance to the object that is supposed to be
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.

I'm on my way to LAX, but I'll be plugging away at the DoF formula later to see. It's been awhile since I've actually used the formula and not some DoF calculator online. Going to get back to the ground floor. And yes, the focal reducers are made for F/10 -> F/3.3 telescopes. Things are going to get messy. I'll be trying some achromats out to see if I can negate some of the effects. I said it was going to be funky!

[Dustin Moore]

To play devil's advocate, if I were to go along with this notion and say that the lens has changed qualities and now inherent the DoF of the faster lens it has appeared to become,

If you put a bunch of glass together that makes an f/0.7 beam hitting the image plane, you got a f/0.7 lens. The
only caveat is that if you can't get a picture out of the lens because of other aberration problems, depth of field
might not be that important.

wouldn't the focal length change be directly proportional to the gain in sensitivity? For instance, because of the inverse square law, a 2x teleconverter loses 2-stops of light, because the surface area is four times as large.

So it is true that a lens with a 2x teleconverter will have focal length that is twice that of the lens alone and that
therefore the f number will be the original f number divided by two.

This does not mean that you have four times as much light hitting a particular pixel. The doubling of light
goes by f/2.8->f/2.0->f/1.4->f/0.98->f/0.7. An f/0.7 lens is four times as bright as an f/1.4. That's also a
good idea of how much harder it is to make an f/0.7 lens than an f/1.4.

The brightness goes like (f#)^(-2). The inverse square law doesn't work on images and the actual math is a great
way to ruin a perfectly good weekend. If you want to see the proof it's equation 2-77 in
http://books.google.com/books?id=Jg1...n disk&f=false

So the size of the image doesn't really determine the brightness. It's dependent on magnification which
is dependent on focal length which is dependent on f/number which controls brightness.

[Ryan Patrick O'Hara]

Before I upload photos and information, just wanted to say that so far, my initial tests have been interesting.

I've been using a f/2.5 105mm lens to test, and so far have gained a stop and a half - a stop and a quarter range on it. At an f/4, I got it to match my Cooke Panchro/i at T/2.8.

The Nikon was definitely the way to go. Because of flange distance, the Nikon has one the longest/deepest, while the camera I'm using (NEX-5 right now, later the GH2) is short. This allows me room to add elements in the middle. Sadly, because of where I have to place the focal reducer, it does not reduce to the amount it does in telescopes. It was advertised to gain at least two+ stops in telescopes, of course placed differently in the assembly. I think a f/10 - f3.3. Instead, I'm seeing a stop and change.

I will be keeping at it, but it seems if the f/1.2 50mm follows suit, we may be looking at something below a f/1 but not a .7. Maybe a .8 or so.

I've taken apart about $2,000 worth of lenses I no longer use. They are really quite fun to take apart. It's a learning experience and I'm having fun.

Not giving up yet.

-ryan

[BlackWolf]

Interesting project. AFAIK, Zeiss simply used a focal reducer for the Planar f0.7 to reach its fast aperature.

I've been playing around with some fast lenses myself recently, including this 60lb 130mm f/0.7 monster. Covers 80x80mm sensors!

[TimurCivan]

I've seen this in person... its awesome.

[Jordan Scott Price]

interesting project. Afaik, zeiss simply used a focal reducer for the planar f0.7 to reach its fast aperature.

I've been playing around with some fast lenses myself recently, including this 60lb 130mm f/0.7 monster. Covers 80x80mm sensors!

Awesome!