2/3" lens on Sony F3 via homebrew adapter

[Alister Chapman]

Yes, it is basically an extender, only to get up to super 35 from 2/3" you need a magnification factor of around 2.5x. A few 2/3" lenses with built in extenders will just cover a s35 sensor without any modifications, just a simple B4 to PL adapter. Mike Tapa has one for the Panasonic AF101 but the micro 4/3 sensor is a little smaller than s35 so most 2/3" lenses with extenders will cover the AF101 sensor.

I'm investigating two approaches. One is to use the lenses built in extender with a simple doublet reduction lens with a 1.25x power the other is to use the lens without the built in extender and then an external 2.5x extender.

I'm getting similar results with both methods. Using the integral extender requires a simpler more compact design, but you must have a lens with an extender. The external extender is more complex and longer, but you can use the Canon cinestyle lenses which often don't have integral extenders. Either way compared to the non extended lens you get a 2.5 stop light loss. Consider that you are taking the same amount of light but now spreading it out over a larger area, so the amount of light falling on any given point is reduced.

[Tim Polster]

Thanks for your reply. Makes sense. When I engage the extender on a Canon broadcast zoom there is a softening of the image to some extent. With the light loss and the loss of sharpness I would wonder if the final product would be as good as the 2/3" lens on a proper sensor/mount setup.

[Alister Chapman]

All lenses have a finite resolution. The reason why the lens softens when you engage the extender is because if you take the lenses projected image which if we say for arguments sake has 1000 lines resolution falling on the 2/3" sensor and then enlarge it by 2x with the extender the resolution falling on the sensor is halved to 500 lines (the other 500 lines falling off outside the sensor area) and any aberrations are increased in size by two times.
So if you take that larger image, which with the extender engaged now almost covers the F3's sensor. That entire original 1000 lines now falls on the larger pixels and sensor and you get back to a sharp, 1000 line resolution image with the same size aberrations. Assuming the extender itself is well designed the difference in image quality will be minimal between 2/3" no extender and 35mm with extender.

[Dustin Moore]

Originally Posted by Tim Polster

I agree! One question - Is there a way to make an adapter with less than two stops of light loss? Since the F3 has power it would be quite a tool to have ENG & EFP in one camera but having your ENG end at F4 is kind of a drawback. F2.8 would be fine.

The light loss is exactly coupled to the amount of extension. You are changing the focal length
through the extender but the pupil size is set by the pupil installed in the main lens.

The formula for f stop is basically focal length divided by pupil size. If you double the focal length
the f number has to get bigger. Only way to keep it constant is to violate conservation of energy.
If use an image intensifier tube to pump energy into the system, you can increase the effective
f number but that's cheating.

Originally Posted by Alister Chapman

The corner softness looked like astigmatism. The adapter contains a pair of doublets.

It's probably astigmatism plus Petzval curvature since the two are closely related. Two
doublets in a row is a lot of positive power to be adding on to the end of a well corrected
system and you are going to have field curvature problems. Field curvature is the classic
limiting problem in lens design and you usually have to engineer the entire lens around
it from the start. On the plus side, you say you don't have coma so you don't have to fix that.

If I saw this problem on the test I would say add a negative element somewhere,
look the problem up in a book and call an expert. I have Sidney Ray's "Applied Photographic
Optics" book in front of me and chapter 53 has a few words on teleconverters.

What you want to do take a small image plane and convert it into a big image plane
like a barlow lens or a door peephole. Usually you do this with a negative overall power
so I'm not quite sure how your are doing this with two positive elements (are you
inverting the image?).

The two examples that Ray gives for teleconverters both involve a doublet and a singlet
and one of them is major negative power while the other is somewhat positive and
balances abberations. The major negative element keeps the field curvature down as
well.

Another book to see is Herbert Gross's Handbook of Optical Systems volume 4 but
only a major university might will have that sitting around.

Originally Posted by Alister Chapman

I'm investigating two approaches. One is to use the lenses built in extender with a simple doublet reduction lens with a 1.25x power the other is to use the lens without the built in extender and then an external 2.5x extender.

I think the trick is that if you have a built in extender, you don't want the tubing/mechanics after
the extender to vignette (much). That's probably gonna limit you to 1.25x internally. There might be
a lens with a 2x extender internally that you could hack everyone off past the extender and
get a larger image "for free". Probably not as likely to produce a perfect image since the folks
designing the extender will not have optimized the picture out to a super-35 sized frame.

[Tim Polster]

Thanks for your thoughtful responses guys. Very informative!

[Alister Chapman]

Status update:

I have two working adapters. Both use optical extenders made up from 4 lens elements, One pair are reduction lenses followed by a second pair that provide correction for field curvature and coma.

Both work acceptably well but in different ways.

Option one is a short adapter that I have fitted with a Canon DSLR mount. So it will work on a Canon DSLR or F3/FS100 with a Canon mount adapter. It only works with broadcast lenses that have a 2x extender and makes use of the in built doubler (so yes, I am using an extender, followed by a further extender). The adapter provides an additional 1.25x image magnification.

Option two is a much longer adapter, specifically for the F3. It has a 2.5x magnification factor so works without the need to use a lens with an extender.

At the moment I am favouring option one. Image quality wise there is very little to choose between them. Option one with a top end HD lens is extremely sharp with excellent contrast. It appears to perform slightly better than option 2 due to the way the integrated lens extender is very carefully matched to the individual lens, so there is less variation in performance from lens to lens.

Option two's corner performance is more variable due to differences in field curvature from lens to lens which is then magnified 2.5x. But it still performs well and I would be happy to use it given that options for 20x zooms for the F3 are near zero. It does have the advantage that if you really want to go crazy, you can use the lenses extender! Performance is better when the lens is stopped down more than f2.8 due to flare at f1.8

The cost for either as a completed product would be around $1,300 USD. Anyone else want to express a preference: Higher IQ, Canon mount, but must use lens with integral doubler, or Dedicated F3 mount, no need to have extender, but lens should be stopped down to f2.8 or slower.


Either way you loose approx 2.5 stops.

[Michael Palmer]

I'm a picture guy, it would be great to see these items Alister.

[Alister Chapman]

I'm going with the 1.25x adapter design. I get much more consistent results across differing lenses with this design. I also have my first cable design up and running so you can use the lens to trigger recording in the F3 and the zoom servo works too. I think it might be possible to get the auto iris working as well. The nice thing is that by adding an external EVF the length of the lens and integral lens handgrip makes for a nice shoulder mount system.

No pictures yet, this is still a rough prototype. One a get the first fully machined version I'll post pictures.

[Alister Chapman]

OK, here's a sneak preview.

[Cees van Kempen]

Originally Posted by Alister Chapman

My old Canon J16x8 f1.8 becomes a 24 to 320mm f4(ish) par-focal lens which is actually quite handy. Next step is to make up a power cable for the lens so I can use the zoom servo.

Dear Alister,

I believe your canon lens is a SD lens, isn't it. How well do lenses like this perform on HD camera's like the F3 and EX3? Do you loose a lot of quality compared to the 'real' HD lenses. Is lens quality (picture) as good as using dslr lenses on these camera's?

[Cees van Kempen]

Okay, just found a thread called "HD 2/3" Lens Tests" where amongst others Alister and steve Phillips discuse a lot of ins and outs of SD lenses on HD cameras. So found alot of my answers already, except for how it compares to prime dslr lenses.

[Alister Chapman]

That depends on the DSLR lens. The Canon lens in the picture becomes roughly a 20 to 320mm lens with perfect focus tracking and constant aperture. Such a DSLR lens afaik does not exist. The pictures are sharp and with good contrast. An HD lens would possibly a perform a little better (higher contrast) I would expect.

A good DSLR prime lens should outperform a zoom in terms of contrast, flare and resolution, but the differences may be small. The question is will the versatility of a 20 to 320mm zoom (admittedly now only f4) outweigh the possibly very small and not necessarily noticeable difference in image quality. Only you can decide that for yourself based on your specific needs.

Certainly for many of the shoots that I do a decent zoom will be a godsend. For other projects primes will be preferable.

[Cees van Kempen]

Thanks Alister,

The case is that I am using a Nikon 80-200/2.8 zoom, a 300/4 and very occasionally a 400/3.5 for wildlife filming. But i miss a decent zoom, not for making nice zooms, but mainly for finding my subject in wide and then focus in. That's why I look for an affordable broadcast zoom, but i don't want to (significantly) compromise on picture quality compared to the dslr lenses i mentioned. The 80-200 is my main lens. I see that you can buy an ocasion Canon J16x8 for less than 2000 euros. If it is as good as the 80-200 it could be a great benefit to have the 16x zoom. Would like to have a bit more tele however.
One day I might also look further for a real big zooms like the A36x14,5 you metion in the other thread. Realise that would be in another price league, but investing good money in a lens that will serve me for the rest of my life feels better than investing in cameras that will last for 5 years or so. espicailly now you prove that these lenses also do their job on large sensor camera's. That is very, very valuable information.

[Alister Chapman]

Image wise I would say that the Canon J16 is on a par with my Sigma 70-300 f4 APO. I'm much happier using the Canon J16 as the focus ring is so much smoother and has far greater travel, so getting spot on focus is much easier and of course servo zoom makes life much simpler.

There are plenty of low cost 20x SD zooms second hand. Most of these perform reasonably well for HD, but best if you try before you buy.

[Sander Vreuls]

Very interesting :) I looked into doing similar for the EOS 550D, but couldn't find any reasonably priced solution.. It almost made me want to trade in the 550 for a GH1 since people have been using 2/3th inches on those for a while..