An email sent on Aug 28, 2014 in response to a question about the value of Arri Open Gate on Alexa versus normal gate for a film that will likely use anamorphic lenses but might use regular flat lenses or possibly the new Panavision 70mm flat lenses:
Okay, thanks, man.
The very short answer to your question is that Open Gate probably won’t be worth it in this particular case. But let me give you the tools to figure it out for yourself instead of just blindly taking my advice.
First off, without actually making a judgment on any of them, here are some of the parameters you need to think about in order to decide about your film-back or target size or whatever you want to call it (let’s just settle on the word “target size” or “target area” to describe the area in millimeters that you’re framing for — not the full sensor, just the part your framing for):
-None of the sensor sizes are going to have the same aspect ratio as your theatrical display ratio, so the target area is ALWAYS going to be hovering within the full aperture area with extra pixels either on the sides or the top&bottom… no matter which sensor size you choose. So, part of the decision isn’t just which sensor is bigger, but how your aspect ratio fits in the sensor.
-If you use Arri’s Open Gate, most (maybe all) normal spherical or anamorphic lenses won’t cover the full gate from left to right. Of course, the 70mm lenses should (as far as I know), but normal flat lenses won’t.
-Open Gate has on-set logistical issues (even if it has none in post). I’m not sure how the new version of the Alexa firmware is, but with the version we used on San Andreas, when you reboot from normal to Open Gate, you lose you framelines, your shooting format, your ArriLooks, etc. All kinds of stuff that’s dangerous and could actually mess up footage. This means that if you have to reboot into normal mode to over crank (since max speed in Open Gate is low), you get into dangerous territory of messing up footage when you reboot to change sensor mode.
-With open gate, there is a more data to copy, transcode, store, etc. How much of the data is picture area versus the excess padding beyond the aspect ratio of the film or beyond the coverage of the lenses?
Okay, those are some of the issues at hand. Now let’s apply them:
IF you're shooting 2x anamorphic, it seems to me totally unnecessary to shoot Open Gate. Here’s why:
To shoot 2.39 using 2x squeeze anamorphic lenses, you are shooting with a 1.195:1 ratio on the sensor (2.39/2=1.195). That is fixed; you can’t change it. Even if you change the SIZE, you can’t change the ratio. It’s fixed at 1.195:1. Now, regular gate and open gate are both WIDER than 1.195:1. So, with regular gate, your excess padding, the unused area, is all on the sides, not on the top and bottom.
But the main difference between open gate and normal gate is that open gate is WIDER. So, all you’re doing by going to Open Gate is adding more excess/unused area out there on the already-excessive sides.
Here it is in precise numerical values:
Arri regular gate: 23.760 x 17.82 mm
Arri open gate: 28.17 x 18.13 mm
Standard film-camera gate anamorphic area: 20.96 x 17.53mm
So, as you can see, if you’re shooting regular 2x anamorphic, the most you’d get out of the Open Gate is an increase of 1.7%. That’s not a typo. It’s 1.7% (that’s linear measurement, not area measurement). Because 18.13/17.82=1.017. You have to leave your ratio fixed, and the vertical is the limiter, not the horizontal. So your only gain is in the difference in the vertical sizes, not the difference in horizontal sizes.
Now, couple that with the fact that we know there is the “anamorphic egg” where no anamorphic lens can focus on the edges. Look at the vertical sizes of the three target areas above and notice that Arri’s regular gate is already including MORE of the out-of-foucs part of the optics than traditional film is, and Open Gate is even worse.
In fact, because the supposed resolution advantage of Open Gate over normal gate is all out past the good part of the lens, you could almost look at it as a degradation instead of an advantage. Every single one of the 1.7% more pixels you add in open gate is recording information about the bad part of the lens. So, instead of saying “there are 1.7% more pixels making up the image” you could equally say “1.7% MORE of the image is now made up of the bad part of the optics.”
I guess that could be an aesthetic choice: to make the blurry part of the lens push farther into the frame, but it doesn’t seem like a great aesthetic choice. Anyway, whether it’s an advantage (more pixels) or a disadvantage (more of the frame is blurry)… either way (advantage or disadvantage), it’s only 1.7%, which seems pretty negligible.
Now, if you’re shooting spherical, it’s different. You could conceivably get some actual resolution advantage from Open Gate, since spherical’s target area is a horizontal strip in the middle of the sensor and Open Gate does indeed have a wider sensor. However, if you use regular spherical lenses (not the 70mm lenses), you’d have to do tests and make sure that you select some custom target size (presumably greater than 23.760mm across and less than 28.17mm across) that ALL of your lenses cover, otherwise you’ll get vignetting. Now, you could get around that by using the 70mm lenses, but then that gets into questions of if you want a larger, more expensive, less versatile lens system to gain 18.5% of nominal resolution increase (that’s nominal because it’s in area measurement only and doesn’t take into account the reality of the optics).
Also, if you shoot with 70mm and open gate, you have to remember that when you over crank beyond what the camera can do in Open Gate mode, you’re lenses are going to jump to a narrower field of view.
So, to sum up, my personal feeling is:
-No reason at all to use open gate with 2x anamorphic.
-We haven’t discussed 1.3x anamorphic
-Not a whole lot of argument for using regular spherical with Open Gate, although it could be of some value with a custom target size (not the full width of the Open Gate), depending on how much coverage your lenses get.
-70mm is the only optical format where there seems to be some sort of very clear advantage to Open Gate, but you still have to weigh the cost, the lack of versatility in the lenses (like, they don’t have anything like the PCZ 19-90 for 70mm), potential issues with the camera losing settings when rebooting, and the change in field-of-view when over cranking. Also, the 70mm lenses just seem philosophically like overkill for such a small increase: I mean, 65mm film has a 48.56mm horizontal and Arri Open Gate only has a 28.17mm horizontal, so you need all this extra glass just to cover your sensor, but you’re stepping up into way more glass than you actually need.
Okay, hope this helps.
Cheers!
-Steve