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Do larger formats create a unique optical perspective?


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#1 David Mullen ASC

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Posted 05 March 2018 - 03:35 PM

This debate has been raging now for several months, picking up steam again with the announcement of the Alexa LF.

 

I've tended to fall into agreement with Steve Yedlin, that you can match the "perspective", the field of view and depth of field, between formats of different sizes if you adjust the focal length and f-stop.  But some people's tests have shown a shift in optical perspective (compression/expansion) once you switch to the correct focal length that in theory compensates for the difference in format size.

 

To my eyes, that difference seems to be more due to differences in lens designs plus some adjustment not being made for where the optical center of the lens is, the nodal point, etc.

 

I tried to reduce the variables here by shooting this quick test on the same camera with the same zoom lens, once framing for the whole sensor (APS-C in this case) and then zooming out by 2X and framing for a sensor area that would be half the size -- so roughly shooting at 50mm and then at 25mm, and shooting the 25mm version with the f-stop opened up by 2-stops to match depth of field more closely. 

 

I cropped the 25mm version by eye so there is some minor image shift, and I think zooming from 50mm to 25mm does change the "shape" of the lens distortions so there is some shifting of elements in the frame, but my general impression is that the 50mm and the 25mm version (on a format that would be half the size) match.

 

comparison2.jpg

 

When I've tried this test before by switching from a 50mm on a FF35 camera to a 35mm on an APS-C camera, there was some perspective shift as if I would need to move the camera an inch or two forward or back to correct (and zoom in or out a little). Plus I was physically switching from one camera body to another so wasn't in exactly the same place when the two images were shot.


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#2 Phil Rhodes

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Posted 05 March 2018 - 03:46 PM

To my eyes, that difference seems to be more due to differences in lens designs plus some adjustment not being made for where the optical center of the lens is, the nodal point, etc.

 

Quite.

 

If you overlay them and A/B them rapidly, the only real difference is a bit of corner sharpness and the barrel distortion. This is an objective difference, but it isn't one that needs discussing with the sophistry and overcomplication that the subject has attracted.

 

bigchip.gif

 

Different lenses look different. That's all I see.


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#3 Macks Fiiod

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Posted 05 March 2018 - 04:17 PM

That GIF creates a lot of despair..


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#4 Samuel Berger

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Posted 05 March 2018 - 04:41 PM

That GIF creates a lot of despair..

 

Needs a transparent green hand.


Edited by Samuel Berger, 05 March 2018 - 04:41 PM.

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#5 Neal Norton

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Posted 05 March 2018 - 05:14 PM

Thanks, David, this is a very interesting subject considering the somewhat contrary ideas of manufacturers pushing a larger negative (sensor) size at the same time we see smart phones becoming the cinema screen of choice for many.

 

At the very least we are discussing the way light is being manipulated by lenses and not the strange fetish for file sizes produced by Bayer imagers.

 

The depth of field for any given shot can be a great tool in our kit to help tell a story the way we want.  Early in my career I was working for Vilmos Zsigmond as a B camera operator and he asked me what stop I wanted for that set-up.  I thought he was joking with me so I looked to my AC and smirked "sixteen".  He pushed me away from the camera and looked through and dialed in a stop and then adjusted his ND to suit.  From that point on I paid more attention to how the aperture worked for each and every shot. 

 

The idea that larger sensors will always provide one with limited depth of field implies that we would ignore using this effect for the benefit of the story we are telling.

 

Neal Norton

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#6 Stuart Brereton

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Posted 05 March 2018 - 06:03 PM

Here's a post with some tests that I shot a while back. I tend to agree that any differences are more likely down to lens design than format.

 

http://www.cinematog...=76028&p=489075


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#7 Mark Kenfield

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Posted 05 March 2018 - 10:47 PM

I mean to dig a little further into this 'scene geometry' tack that Hrvoje is talking about over on my Reduser thread. Like you David, I'm also still not convinced that it isn't simply an optical issue (I assume slight wide-angle distortions from wider-angle lenses on S35mm for an equivalent FoV), but there may be something to it.

Whatever the case, it seems pretty clear (from every test posted thus far) that it's an issue that only presents itself on the edges of the frame, with foreground objects that are particularly close to the camera. 

And processing all of it through the objective measure of how we actually compose a shot, it seems pretty clear to me that these slight differences (if they do exist) simply aren't significant enough to offer any perceptual difference in image rendering that would affect the way we go about composing a shot between one format or the next.


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#8 David Hessel

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Posted 05 March 2018 - 11:42 PM

Physics, geometry and math basically prove that sensor size doesn't have a look. The images should be identical, the only reason they are not comes down to lens design. For instance it is possible for a 60 degree fov full frame image to have more barrel distortion that a 60 degree fov lens on 16mm, it depends on the lenses used. Compression in an image is defined by perspective, which is dependent only on the position of the camera not sensor size.
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#9 Stuart Brereton

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Posted 06 March 2018 - 12:19 AM

I have a few test frames here from different 35mm lenses shot on APS-C Format. It's very noticeable how different they are in terms of FoV, Bokeh, and apparent depth, despite being 'identical' focal lengths. In my (very crude) tests, the difference between lenses was far greater than the difference between formats.


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#10 David Hessel

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Posted 06 March 2018 - 12:08 PM

Here is another example that may illustrate this better. There are cameras out there, Sony mirrorless for example, that allow the user to switch between full frame or aps-c. To keep it simple for this example we have a camera that can switch between full frame and m4/3 with a crop factor of 2.

 

Put a 50mm lens set at f4 and shoot in both formats, full frame and m4/3. Clearly the full frame image will have a wider field of view but the parts of the image are captured in both will be identical, same lens distortion, aberations, compression, bokeh, etc... Both sizes are receiving the exact same image, just the full frame image is capturing more of it.

 

Now do the same thing again with a 25mm lens at f2, same result as before common parts of the images are identical.

 

Since changing the sensor size doesn't change the captured image in anyway and with the 50mm full frame at f4 being equivalent to 25mm at f2 those 2 images should be identical. Chances are they won't be, not because of sensor size but because different lenses were used. 

 

What we are left with are generalities, full frame might be more likely to have less barrel distortion due to the longer focal length. Since optimal lens performance tends to be f4-5.6 one could say that full frame performs optimally with shallower depth of field while m4/3 performs optimally with deeper depth of field. but, in the end it comes down to the lenses used. It is entirely possible to have a full frame 50mm f4 image that has more barrel distortion and a softer image than a m4/3 25mm f2 image, so which one has the full frame look?


Edited by David Hessel, 06 March 2018 - 12:13 PM.

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