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Tito Ferradans

Rectilux Extreme Low Light Test (f/1.2)

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So I took my Halloween night to shoot some f/1.2 Rectilux footage with Canon's 50 and 85mm. I really liked how it turned out, even though I fucked up alignment in some several shots. Also pictures of the rig there. (:

http://www.tferradans.com/blog/?p=8292

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So I took my Halloween night to shoot some f/1.2 Rectilux footage with Canon's 50 and 85mm. I really liked how it turned out, even though I fucked up alignment in some several shots. Also pictures of the rig there. (:

http://www.tferradans.com/blog/?p=8292

Is it possible that you are underfilling the entrance pupil of the 85/1.2, and hence not actually shooting at f/1.2?  For example, if you were shooting with an iscorama-36 the limiting aperture would be the 36mm diameter at the rear of the anamorphic section, thus giving you a maximum aperture of f/(85/36) ~ f/2.4

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Is it possible that you are underfilling the entrance pupil of the 85/1.2, and hence not actually shooting at f/1.2?  For example, if you were shooting with an iscorama-36 the limiting aperture would be the 36mm diameter at the rear of the anamorphic section, thus giving you a maximum aperture of f/(85/36) ~ f/2.4

It's not a possibility. It's a certainty. The back of the anamorphic is ~43mm, but I don't know exactly how this math works out. Is that a simple division? Why did I always think this was much more complex? Plus, how would the math work out if I wasn't actually shooting wide open?

Sorry for the questions, it's just that I always wanted to understand this and never quite got around it. hahahaah

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Well Tito, regardless of whether you were shooting at f1.2 or 2.4 (did expect it to be brighter, so that explains it), it still looked nice.

Would be interesting to hear from Brian about how you calculate the math to work out if you're doing yourself out of a stop or two of light.

So are you settled with the Rectilux as the best option for the Dual Focus anamorphic attachments?

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It's not a possibility. It's a certainty. The back of the anamorphic is ~43mm, but I don't know exactly how this math works out. Is that a simple division? Why did I always think this was much more complex? Plus, how would the math work out if I wasn't actually shooting wide open?

Sorry for the questions, it's just that I always wanted to understand this and never quite got around it. hahahaah

"In optics, the f-number (sometimes called focal ratiof-ratiof-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil."

So when you change aperture, you are changing the size of the entrance pupil. The best way to do this is usually inbetween elements, but you can also do it in front of the lens. When you put the iscorama in front of the lens, you are making a new maximum aperture. Taking Brian example, 85/36 = 2.4. Choosing values under 2.4, like 1.2,1.8,2 won't give you a larger aperture, but going 2.8,4,5.6 will have an affects obviously.

But there is another problem, in modern lenses the front element is often very big, bigger than the entrance pupil, so although you aren't limiting the aperture you are still interfering with the way the lens is working, this will create vignette. In front of the lens is not the best place to put the aperture iris. I guess Brian can explain this last point a lot better.

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"In optics, the f-number (sometimes called focal ratiof-ratiof-stop, or relative aperture[1]) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil."

So when you change aperture, you are changing the size of the entrance pupil. The best way to do this is usually inbetween elements, but you can also do it in front of the lens. When you put the iscorama in front of the lens, you are making a new maximum aperture. Taking Brian example, 85/36 = 2.4. Choosing values under 2.4, like 1.2,1.8,2 won't give you a larger aperture, but going 2.8,4,5.6 will have an affects obviously.

But there is another problem, in modern lenses the front element is often very big, bigger than the entrance pupil, so although you aren't limiting the aperture you are still interfering with the way the lens is working, this will create vignette. In front of the lens is not the best place to put the aperture iris. I guess Brian can explain this last point a lot better.

Lovely, now I understand.

So is this why they say these attachments cut a 1/3 f stop of light (or there abouts) or is this more to do with the light passing through more glass?

And I suppose this also might explain a little about why the 36 appears to be sharper than the larger Iscoramas, but in reality it is forcing a higher F Stop which is sharper by default.

So a 85mm f1.5 lens with an Iscorama 54 is spot on as a match.

And in turn, does this explain why the 54 takes on more of the original characteristics of its taking lens than say the 36?

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I guess so, but don't say it out loud, I want to buy an iscorama 54 for cheap sometime.

 

The second point is that basically the further away the aperture is from the optical center, the more problems you get with vignetting, even if the aperture is large enough.

I also found that when I place my isco ultra star back element touching the taking lens front element, the fieldcurvature blur you get at the top and bottom of the frame suddenly vanishes and the image gets very sharp.

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I guess so, but don't say it out loud, I want to buy an iscorama 54 for cheap sometime.

I've been singing the 54s praises for ages now & I got the impression people thought I was nuts - I'll be having the last laugh now!

Brand new, they cost £1602.79 (incl. VAT) in 2006 - which is what you pay nowadays.

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So are you settled with the Rectilux as the best option for the Dual Focus anamorphic attachments?

I'm not set yet since I haven't tested the others yet, but the Rectilux didn't let me down on anything I tried so far.

Talking to John about the f-stop value, as long as the lens is wide open, the math is simply dividing the focal length by the anamorphic's rear element diameter, so the 50mm is still at f/1.2 (50/43 = 1.16), and the 85 is actually (85/43 = 1.97) around f/2. Once you start closing the iris, the math is different and I didn't get to that yet.

I don't think that affects the sharpness of the resulting image, though, but definitely affects vignetting. I had some issues with the Isco Pre36 paired with Contax Zeiss 85/1.4, which has a huge front element. These issues aren't noticeable when I pair it to the Jupiter 9, which is f/2 and has a much much smaller front element (65mm vs 45mm). There's also a way to determine the fastest f-stop of any given focal length based on the front element diameter - which is why these super fast lenses tend to have much larger front elements.

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It's not a possibility. It's a certainty. The back of the anamorphic is ~43mm, but I don't know exactly how this math works out. Is that a simple division? Why did I always think this was much more complex? Plus, how would the math work out if I wasn't actually shooting wide open?

Sorry for the questions, it's just that I always wanted to understand this and never quite got around it. hahahaah

Optics is mostly simple, with some occasional complexity thrown in to make it interesting.  In fact, I've almost completely forgotten all the fancy math I learned after high school, since by and large all the math you need to know to be a successful lens designer is geometry, trigonometry, and a bit of algebra.

In your case, assuming that the anamorphic portion is working at infinity (i.e., parallel light in and parallel light out) the aperture of the optical system is determined by the diameter of the iris diaphragm in the taking lens *unless* there is some other limiting aperture in the system.  Imagine that you take a pin and poke a tiny hole in a large piece of aluminum foil.  Next, open the f/1.2 taking lens wide open and place the aluminum foil in front of your Rectilux so that the pinhole is centered on the optical axis.  Clearly, in this case the f/# of your lens system is determined by the diameter of the pinhole and not by the diameter of the taking lens' iris diaphragm.  So we would say that the pinhole is the limiting aperture in your lens system.

In your case the clear aperture of the back of the anamorphic section is 43mm in diameter (assuming it is round and not rectangular).  This means that the collimated on-axis beam of light exiting your anamorphic section cannot exceed 43mm in diameter, and may be less if any of the other optical surfaces in the Rectilux or your anamorphic group are limiting apertures.  The entrance pupil diameter of your 85mm f/1.2 taking lens (most likely 1/3 stop faster than f/1.4, or f/1.2599 in reality) is 85/1.2599 = 67.5mm.  Since 67.5mm is bigger than 43mm you are underfilling the entrance pupil of the taking lens.  As a consequence, you could stop down the taking lens until its entrance pupil is reduced to 43mm, and have no impact on the actual f/# of the system.  In your case, the actual maximum f/# would be f/(85/43) = f/1.98 ~ f/2, and not f/1.2.  Again, this assumes that the limiting aperture of the system is the rear aperture of the anamorph, and not some other surface in the Rectilux or the anamorph.  If either of the latter is true then your true f/# would be slower than f/2.

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My understanding is, the 43mm rear glass of Ana lens could be treated as the front aperture of the 77mm prime lens.  Then looks like it would reduce more than 2 steps.

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