Epic-S: Shooting Format Affecting FOV/DOF?

[Mathew Joki]

Haven't really seen anything about this posted yet, and sorry if I missed a topic discussion on this already. I've been browsing the REDUSER forums religiously for a year now but finally joined the community to answer this question I was having about the upcoming Epic-S.

One of the most intriguing things to me about the Epic-S (also, I apologize if this should be in EPIC section instead) is the 35FF censor size, and the DOF that it will produce. However, many of the applications that I need to apply this shallow DOF look too do not not require that the footage be recorded in a sizes greater than 2K (ie. professional looking advertisments for web, TV, etc.).

So the question is: Will recording in resolutions less than 5K (ie. 2K) affect the FOV and DOF for the same lens? Or will it be similar to the differences you would see if you decided to use that same lens from 35mm camera on a 16mm? Will there be some sort of firmware option to change how resolution is recorded, either at a pixel-per-pixel censor crop, or by "turning off" select pixels in the censor to maintain 35FF (or other such censor area options)? Example, if I wanted to record 2K with a 35FF censor size for a wider FOV and shallower DOF, and then the same 2K resolution at a 2K censor size for a narrower FOV and deeper DOF. Would that be possible? Or is one or the other?

Hopefully my terminology isn't TOO far off and you all understand what I'm traing to ask. :P

[Justin O'Neill]

Hi Mathew, everything is subject to change but last we heard all of the lower resolution RAW options on Epic would be windowed (meaning FOV change). There was a 1080p RGB option in the original specs that would be scaled (same FOV as 5k). As far as I know that is still in the plans.

[David Mullen ASC]

The 5K Epic M-X sensor is not FF35 (36mm x 24mm) -- it's around 30mm x 15mm, closer to what would be called APS-H in still photography, and the recorded 5K area is more like 27mm wide, so not a huge leap over the 24mm width of Super-35 photography. I mean, if you want the FOV of a 50mm lens in Super-35 photography, you'd have to use something like a 57mm lens on the Epic in 5K mode, so it's not a big change in depth of field or field of view characteristics over Super-35 photography.

Lower resolutions in RAW would involve cropping the sensor, so you can't record 2K RAW but use the full 5K sensor area to keep the same DOF / FOV of 5K photography.

[Mathew Joki]

Thanks for the quick responses and for clarify things for me!

[Keith Putnam]

Lower resolutions in RAW would involve cropping the sensor, so you can't record 2K RAW but use the full 5K sensor area to keep the same DOF / FOV of 5K photography.

Cropping the image circle affects depth of field? In my mind, the lens is still throwing an image over the same area as it was in 5K when you crop to 2K, so it seems counterintuitive that merely choosing to look at the center of the image circle rather than the whole of it would change the depth of field within the cropped image. I imagine that cropping to 2K hasn't changed anything about the geometry that physically produces depth of field; you're just choosing to look at a different part of the image in 2K.
Isn't the fact that the RED system allows one to achieve an approximate 35mm format DOF even at a 2K cropped FOV part of the appeal?

[Dan Kanes]

The best way to produce a 2k image (or 4k,3k,1k,etc) is to shoot at the highest allowable resolution for your desired framerate, then downscale to your deliverable. Delivering 1080 from a 5k master gives you the flexibility of a rerelase at higher res later, as well as the ability to pull high res frames from the stream.

DSMC takes the notion of digital camera raw imaging and does it at motion picture speeds.

[D Fuller]

Cropping the image circle affects depth of field? In my mind, the lens is still throwing an image over the same area as it was in 5K when you crop to 2K,



so it seems counterintuitive that merely choosing to look at the center of the image circle rather than the whole of it would change the depth of field within the cropped image. I imagine that cropping to 2K hasn't changed anything about the geometry that physically produces depth of field; you're just choosing to look at a different part of the image in 2K.

The lens is still throwing the image over the same area, but when in 2K mode, the RED camera is not seeing that whole area. It's only seeing the 2K area in the center of the sensor. So, while you are correct that DOF characteristics of the lens haven't changed, the field of view has changed dramatically.

That changes our perception of DOF. With a 50mm lens in an S35mm frame, you get a certain field of view. When you crop to 2K, you get a narrower field of view - similar to the FOV of a 100mm lens in the S35 frame. But as you say, the DOF characteristics haven't changed, they are still the DOF characteristics of a 50mm lens.

It's when you compare the same field of view between two different sensor formats that you see differences in DOF. This is because you need to use a 100mm lens on the 4K sensor to get the same FOV as a 50mm lens on the 2K crop. So when you stand in the same place and make the same shot, you have to use different lenses - with different DOF characteristics.

[Sam Eilertsen]

Cropping the image circle affects depth of field? In my mind, the lens is still throwing an image over the same area as it was in 5K when you crop to 2K, so it seems counterintuitive that merely choosing to look at the center of the image circle rather than the whole of it would change the depth of field within the cropped image. I imagine that cropping to 2K hasn't changed anything about the geometry that physically produces depth of field; you're just choosing to look at a different part of the image in 2K.
Isn't the fact that the RED system allows one to achieve an approximate 35mm format DOF even at a 2K cropped FOV part of the appeal?

As I understand it, DOF has nothing to do with the image circle of the lens. If that were true, you could get shallower DOF on 16mm just by throwing a 35mm lens on the camera, which you can't.

The reason DOF is higher for smaller formats is that, to achieve the same FOV and f-number, you need a smaller focal length and aperture. To get the FOV of a 35mm lens on a full-frame camera, you would use a 24mm lens on super-35, and thus you have more DOF.

[David Mullen ASC]

Sure, cropping in itself doesn't change DOF -- if anything, technically it decreases DOF because once you enlarge the cropped image to match the size of the uncropped image, you have to use a more critical Circle of Confusion figure because the enlarging makes focus more critical.

But you don't get the same image once you crop it, the field of view gets more telephoto... so you compensate by switching to a more wide-angle lens, which has more depth of field.

In fact, cropping by half in both directions, vertically and horizontally, requires using half the focal length to compensate and maintain the same field of view. If you look on a DOF chart, for example, you'd see that at the same f-stop & distance, a 25mm lens has 4 stops more DOF than a 50mm lens. However, because you are enlarging the cropped image by 2X to match the same viewing size as the uncropped image, you lose 2-stops of DOF, so the difference in DOF between a 25mm lens on a sensor area half as large as a 50mm lens on the uncropped sensor is 2-stops.

So, no Keith, you don't get 35mm DOF when shooting in 2K mode, you get something more like 16mm DOF because you have to use focal lengths that are twice as short to maintain the same FOV.

[Keith Putnam]

...because you have to use focal lengths that are twice as short to maintain the same FOV.

Ah, see, I wasn't considering matching FOV at all, but with that in mind what you're saying is totally sensible.

[Mathew Joki]

In fact, cropping by half in both directions, vertically and horizontally, requires using half the focal length to compensate and maintain the same field of view. If you look on a DOF chart, for example, you'd see that at the same f-stop & distance, a 25mm lens has 4 stops more DOF than a 50mm lens. However, because you are enlarging the cropped image by 2X to match the same viewing size as the uncropped image, you lose 2-stops of DOF, so the difference in DOF between a 25mm lens on a sensor area half as large as a 50mm lens on the uncropped sensor is 2-stops.


Thanks David for helping to clarify my short formed layman's terms. :P