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Alexa latitude and EI, how to best take advantage of the latitude shift?


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#1 Markus A Ljungberg

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Posted 17 May 2012 - 06:29 PM

Let's discuss the latitude of the Alexa, and how it's shifting depending on EI/ASA. How do we best take advantage of this?

At first I thought it was pretty straight forward, but the more I work with the camera I find myself puzzled with this.

If you'r not familiar with the Alexa, the latitude for highlights contra shadows is dramatically different depending on set EI. On EI160 you have 5 stops latitude above 18% grey and 9 stops below, whilst on EI3200 you have 9,4 stops latitude above and 4,6 stops below
Posted Image.

From what I understand this shift is mainly because of engineering reasons, can anyone explain this or refer to a more detailed explanation of this? It also seems to fit many situations nicely though so it also feels like it's deliberately designed to perform this way? If the latitude was locked around 18% grey independent of other factor it would be more limiting, but it would at the same time be easier to predict the results.

The only reason I've ever bumped up my IE or ASA is to get exposure in less light, or occasionally to get more grain, now suddenly another factor comes into play; where do I want to shift my latitude? Highlights or shadows?

My first thought was that when shooting a dark set, most of the scene is likely to be in shadows, so I would want more details in the shadows. Bumping up the EI would give me the opposite. Strange? Then I think again, and realize that with a dark set, I will expose for a dark set, and any highlights appearing in that set will be way off the exposure and clip. Say a street light appearing on a dark street. So it makes total sense to have more details in the highlights.

Ok so when shooting a bright scene, with more details in the shadows, would I want more details in the highlight? Say if I have a portrait in sunlight, the bright side would be very bright, and if I expose close to this then it would be vital to keep the details in the shadow side of the face. But then in a situation like that I suppose I would prefer the sun to be on the verge of clipping and just lifting the shadows to get the exposure I want. I don't really see when I would want to be shooting at EI160 given how much it reduces my latitude in the highlights, unless I needed absolute minimal grain/SNR in the image.

Like David Mullen pointed out in a previous thread, when you choose a low EI obviously you need to carefully watch your highlight details, and on high EI you need to be extra careful with your shadow details. The recommendation overall seem to be shooting at IE800 (Which is the native IE right? With the Alexa are there any special advantages of shooting on the native IE other then the balanced latitude?) and perhaps dropping to IE500 in sunlight. Mullen advises to avoid EI200 or lower because your more likely to clip highlights and get a more videoish look, makes total sense.

What are you thoughts on this? How do you take advantage of the latitude shift in the Alexa to achieve the results you want? Would there be situations where you want a dark scene but because of the design you would want to shoot it at EI160? Or what would a situation look like where you choose to use IE1600 and stack ND:s even on a bright scene, just to get the latitude you want? Ideas? Thoughts?

Thanks,

/Markus
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#2 David Mullen ASC

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Posted 17 May 2012 - 08:23 PM

The Alexa has such a wide dynamic range in Log mode, 14-stops, that the look is not that different between 400 to 1600 ASA, it's not like it looks dramatically clippy at 400 ASA or dramatically crushed at 1600 ASA. It's more a matter of just compensating a bit. If you are shooting at 1600 ASA and want very dark shadows, then you don't really have to do anything, just make the blacks really black in post. But if you want more "open" shadow detail while still color-correcting for deep blacks, you may want to add a touch more fill light, that's all. But as I said, the dynamic range is so wide in general that most people are not going to worry about some loss of shadow detail at 1600 ASA because there is already quite a bit anyway, most of the time people are trying to get more contrast into the Alexa image, not get less. It's not like you are using a camera with only 9-stops of dynamic range and really have to worry where you place your highlights or shadows.

I do find that in low-light at high ASA ratings and using a fast lens, there is a tendency for the Alexa image to get a bit foggy, which is made worse by the fact that an LCD monitor in low light levels tend to glow in the dark, also lifting the blacks. The end result is that you think the image is flatter than it really will be later in post when you set your blacks at "0". So I try to remember to crush the blacks a bit on the monitor in a dark room to force myself to add a bit more fill light to compensate.
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#3 Markus A Ljungberg

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Posted 19 May 2012 - 05:51 PM

Thanks David. Indeed the latitude doesn't look that dramatically different on high/low ASA. I suppose it's only in extreme situations it would make a difference, just curios on what those situations could be. I suppose you would know when you'r looking at it, and at that point you'd make the correct adjustments.

Maybe I'm making too big of a deal of this shifting latitude range, it's just a new factor for me and I want to really understand it.

Is ASA800 somehow a better choice of EI in general, being the native ASA of the camera? I think the grain and performance looks nice throughout, but the graininess of course being very apparent on higher ASA.
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