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View Full Version : film back - wtf is 1.0512 inches?



jin choung
03-01-2010, 03:16 AM
howdy,

so i'm looking at the film back or "frame" setting in lw camera properties and that's supposed to be the vertical height of the film back (1.0512 inches being the one for still photo 35mm (135 film)) and i have no idea what that number is supposed to be... it doesn't correspond to an actual 135 film frame (36mm x 24mm or 1.417" x .945") so what the heck is 1.0512" (26mm)?

and the other dimensions in the pull down look pretty unrecognizable too....

anyone got any insight?

jin

MrWyatt
03-01-2010, 03:29 AM
very easy to explain. this would be the film back of 35mm film cameras actually I think it is S35. Super 35 is a bit broader than normal 35 as it also uses the space usually reserved to the soundtrack. 35 mm filmcameras use the exact same filmstock that SLR's use. the only difference being that film cameras use it vertically, thus a 35 mm film frame has a width of 24 mm (26 mm respectively for S 35).
:D

Edit: now that I reread your post. I see I misunderstood you. when it is the vertical measure. this really makes no sense at all.

jin choung
03-01-2010, 03:38 AM
Edit: now that I reread your post. I see I misunderstood you. when it is the vertical measure. this really makes no sense at all.

yup

there are separate entries for super35 and it's much smaller (vertically) cuz of crop. but i think it's still wrong!

36mmx24mm is 35mm still photo and like... vistavision!

but 26mm vertical... i have no idea wtf that corresponds too... is it measuring to the extents to the actual strip of film past the sprocket holes?!

jin

jin choung
03-02-2010, 01:10 AM
bump

meshpig
03-02-2010, 03:33 AM
Lens properties as in the diagonal not the vertical height.

jin choung
03-02-2010, 03:41 AM
What do you mean "lens properties"?

Especially since the pulldown menu lists other film backs. Not lenses.

Also I tried diagonal dimension. Doesn't work. Still wrong number. So still a mystery.

Jin

meshpig
03-02-2010, 04:10 AM
Bull at a gate... the way it's laid out is confusing. The point being lenses not film backs.

jin choung
03-03-2010, 03:12 AM
Bull at a gate... the way it's laid out is confusing. The point being lenses not film backs.

what's bull at a gate?

and what do you mean "lens properties"? what EXACTLY is that number for the "frame" setting denoting?

hmmmmm..... needing to match film backs is pretty important so it would be distressing if lw didn't have a way to do that.

jin

meshpig
03-03-2010, 06:59 AM
The expression is "like a bull at a gate"...

Choose from the small drop menu otherwise it appears to me to be a matter of lens diameter.

jin choung
03-04-2010, 01:55 AM
right...

but that's the problem, when you select from the pulldown, you get a number that is completely unexplicable.

the answer to the question "what EXACTLY does that number in the 'frame' field represent"? still seems to be "nobody has a fing clue".

jin

meshpig
03-04-2010, 11:56 PM
right...

but that's the problem, when you select from the pulldown, you get a number that is completely unexplicable.

the answer to the question "what EXACTLY does that number in the 'frame' field represent"? still seems to be "nobody has a fing clue".

jin

Damn, I was hoping someone would by now because it isn't in any manual and I've asked myself the same question many times.

If you set your own values there it seems to make no difference to anything either so seems arbitrary... if you look up the various lens specs you see nothing like it.

- 6.9 mm is to 16 mm film as 26.7 mm is to 35 mm? They read more like preset focal lengths and apertures????

meshpig
03-05-2010, 12:48 AM
Ha!

Was just looking through v8 manual and there it's called aperture height...:)

meshpig
03-05-2010, 01:18 AM
http://en.wikipedia.org/wiki/Aperture

... So it's the height of the cone in inches:D

Lightwolf
03-05-2010, 02:22 AM
If you set your own values there it seems to make no difference to anything either so seems arbitrary...
It affects DoF. But that's also the only case where it makes a difference. Without DoF the camera rays are computed like a pin-hole camera (which makes sense).

Cheers,
Mike

meshpig
03-05-2010, 02:45 AM
... Without DoF the camera rays are computed like a pin-hole camera (which makes sense).

Cheers,
Mike

Would be awkward otherwise.

jin choung
03-05-2010, 04:38 PM
http://en.wikipedia.org/wiki/Aperture

... So it's the height of the cone in inches:D

height of WHAT cone?

it's CERTAINLY not the height of the aperture itself.

jin

jin choung
03-05-2010, 04:41 PM
i'm pretty sure newtek meant this to be the measure of the filmback... especially from the choices in the pulldown, nothing else makes sense.

but then numbers supplied by newtek don't make sense either... so it's still a mystery.

jin

Jarno
03-05-2010, 05:13 PM
It is the height of the filmback. They have nothing to do with the lens properties. Where those actual numbers came from I don't know. They were like that when I got here.

---JvdL---

jin choung
03-05-2010, 05:14 PM
It is the height of the filmback. They have nothing to do with the lens properties. Where those actual numbers came from I don't know. They were like that when I got here.

---JvdL---

right but if it's the height of the filmback, they're WAAaaaaaaaay wrong.

jin

Lightwolf
03-05-2010, 05:47 PM
Where those actual numbers came from I don't know. They were like that when I got here.
It was probably a feature request ;)

*ducks_for_cover*

Cheers,
Mike

meshpig
03-05-2010, 07:15 PM
It is the height of the filmback. They have nothing to do with the lens properties. Where those actual numbers came from I don't know. They were like that when I got here.

---JvdL---

Well, how so since it only affects DoF. Logically it's to do with aperture. They are most likely an old fashioned way of saying f2, f5.6 etc.

Besides, why would a virtual camera need a film back and why in v8 is it called "aperture height"?

jin choung
03-05-2010, 08:53 PM
Well, how so since it only affects DoF. Logically it's to do with aperture. They are most likely an old fashioned way of saying f2, f5.6 etc.

Besides, why would a virtual camera need a film back and why in v8 is it called "aperture height"?

aperture simply means "opening"... so maybe they looked at the film back frame and thought of it as an opening.. who knows... and also, if it's fstop, then the drop down presets make no sense whatsoever.

---------------------------------------------------

film back determines dof TOGETHER WITH iris aperture. it's part of the formula.

that's why in small sensor consumer level cameras, almost everything is in focus.

the larger the film back, the smaller the dof per t-stop.

that's why everyone likes the canon 5d mark2. it's sensor is the size of a 35mm film frame and that gives it a very nice "bokeh" that peeps associate with film (both still and motion picture).

film back also changes the relationship of the lens size to the final image... a 50mm for a 35mm film camera would yield a VERY TREMENDOUSLY DIFFERENT RESULT for a 1/3 in ccd camera!

also, film back is important because it allows you to determine measurements that pertain to film back like pixels per inch or mm or whatever.

in maya right now, my stereoscopic rig gives me parallax readouts in pixels of shift and that wouldn't be possible unless i knew the film back.

and finally, film back is important if you need to comp something with something shot in some physical media with a given lens and given film back size.

a really important setting!

jin

meshpig
03-05-2010, 10:57 PM
Ah! OK, thanks for that. Aperture, I thought in photographic optics also implied/included the distance to the plate as in aperture height.

Admittedly I don't understand digital cameras because they're so incredibly perfunctory.

Why then is the film back height in inches? :)

jin choung
03-05-2010, 10:59 PM
Why then is the film back height in inches? :)

arbitrary.

it could be in mm. in photo discussions, it comes up as both...

jin

jin choung
03-05-2010, 11:03 PM
oh, and if you've ever looked at digital camera or camcorder manuals and they say "20mm lens (35mm equivalent)", it's this difference in film back that is necessitating this language.

the lens for the digicam or camcorder is NOT a 20mm lens for a 35mm slr. but they replicated the final image size difference and regardless of the actual size of the lens, they say "35mm equivalent" to give peeps an idea of how wide or how zoomed in because they rightly assume that most people for whom lens numbers mean anything to would know them in terms of 35mm cameras.

jin

druitre
02-11-2011, 09:25 AM
I'm gonna BUMP-O-RAMA this thread -

as it's the only one on filmback I can find on these forums and although it has succeeded in establishing what filmback IS (and I already knew that) it still hasn't come up with an answer as to HOW Lightwave measures it. The numbers on the dropdownlist still don't make sense and I still don't know what to set when I want to use my 5D footage. Or any other footage, for that matter.

As Jin has pointed out, filmback is a very important thing for almost any job where live action footage has to be combined with CG.

Anybody have a clue?

jrandom
02-11-2011, 12:06 PM
I'd love to know the answer to this as well.

About the only thing I can think of is set up a simple scene to photograph with the 5D, recreate that scene in lightwave (including camera position), and then mess with the filmback until it matches.

I might have to give this a go this weekend if I have time.

druitre
02-11-2011, 01:08 PM
:) that's exactly what I'm doing right now!

Question I've ran into: what actually IS the position of the camera?

Is it the position of the filmback (= the sensor) or
is it the position of the front of the lens?

In my case, I have a EF 24-70 L zoom on the camera. When I try and match a photo taken at 24mm I have to position my LW camera closer to the subject than when matching one taken at 70mm (this lens reverse-extends when zooming in; it physically gets longer at shorter angles). That suggests the 'position of the camera' is actually at the front of the lens. Or somewhere about 15mm behind it, inside the lens.

It makes quite a difference in my setup. I'm taking MFD pics (minimal focus distance), which is around 38 cm. acoording to specs. The physical length of the lens is 13 cm @ 70mm and 16 cm @ 24mm - in other words, the front end of the lens is 3 cm closer to my subject when fully zoomed out. Which is 8% of distance to subject...

Hmm... complicated... perhaps it's better to set up a much larger scale test. And then see if results translate well to my miniature setup.

By all means JRandom, go ahead and do your test too! ;) Two can find out more than one.

JMarc
02-11-2011, 01:27 PM
Shoot a still with the same lens settings on your 5D (best while on location for the footage being shot) then in LightWave change the camera to the "Real Lens Camera" and from its properties panel click the "(all)" drop-down menu on the left side, just under the "Zoom Factor" readout. From the drop-down list select (from image) and load in the still you shot with your 5D. This will read the EXIF data in the JPEG and replicate the DSLR settings in the LightWave Real Lens Camera.

Try it out. It's pretty cool. I haven't tested a matching render composited into footage from my DSLR this way yet but this is the workflow as it I believe it was intended to be used. It should at least provide a better starting point than guessing. :)

You might have to match the resolution of the test still frame you shoot to the footage size, if possible, for this to match properly. I don't know for sure. Good luck!

jrandom
02-11-2011, 02:44 PM
From the drop-down list select (from image) and load in the still you shot with your 5D. This will read the EXIF data in the JPEG and replicate the DSLR settings in the LightWave Real Lens Camera.

!!

I didn't know Lightwave could do that! Cool!

druitre
02-11-2011, 03:46 PM
That's quite brilliant, thank you Jean Marc for pointing it out.

For my 5D mkII, it gives me frame size 0.958" (that is for a photo, not a movie) and it seems to be quite accurate. Will do more tests tmrw to find out about the cameraposition-thing.

(Unless someone with better knowledge than me can tell me, ofcourse)

jrandom
02-11-2011, 03:51 PM
For my 5D mkII, it gives me frame size 0.958" (that is for a photo, not a movie) and it seems to be quite accurate.

Unfortunately, the video files do not have EXIF information, although it might be possible to calculate the new filmback by comparing frame ratios between the still-image aspect ratio and the 16:9 video.

rsfd
02-11-2011, 03:57 PM
the film back is meant to be the height of the "filmgate" (the area through which the film is exposed - on a 35mm SLR this would be 24mm or 0,9448818897648 inches in landscape mode).

The position of the camera is from a technical point of view the position of the nodal point of a lens (where the rays meet in one single point).
Using a zoom lens makes it a bit harder as the nodal point shifts depending on the focal length used.
In general, the nodal point is near the diaphragm but it can also be in front of the lens on certain types (e.g. real tele-lenses).
Btw. the data from tech spec sheets is usually not very precise, a 24-70mm zoom can in fact be a 25-68mm or 23,6- 71,2mm and the like. Even LW reads the EXIF data I wouldn't expect a perfect match. (too much variables)

druitre
02-11-2011, 04:20 PM
Thanks, very useful information. Maybe I can find specs for my lens that tell me where it's nodal point is.

Is MFD usually measured from the nodal point (by manufacturers in their spec sheets)?

So the deviations in the lens account for the film back being given as 0.958" instead of 0.94488"?

colkai
02-12-2011, 03:14 AM
!!

I didn't know Lightwave could do that! Cool!

Likewise, what a little gem of information. :D

alexos
02-12-2011, 04:48 AM
That's quite brilliant, thank you Jean Marc for pointing it out.

For my 5D mkII, it gives me frame size 0.958" (that is for a photo, not a movie) and it seems to be quite accurate. Will do more tests tmrw to find out about the cameraposition-thing.

(Unless someone with better knowledge than me can tell me, ofcourse)

Yes, Lightwave's "frame size" is pretty bizarre to work with - even using the EXIF data from real lens camera doesn't always work for some reason. Interesting fact I've discovered a few weeks ago though: it accepts sizes in millimeters and converts them to inches. So if you have a classic 24*36 camera (like the 5D) and input "24 mm" in the "frame" field it will magically become 0.9449 and give you a rather good match. Why Real Lens Camera gives you a 0.958 I have no idea, I guess the 5D sensor is in fact a bit bigger... but it's close enough anyway.

ADP.

rsfd
02-12-2011, 07:00 AM
…Maybe I can find specs for my lens that tell me where it's nodal point is.

I bet you won't find it in any spec sheet. I never saw a spec sheet from Canon, Nikon, Zeiss, Leitz, Rodenstock, Schneider-Kreutznach aso with a note for the nodal point's position.
Determining the nodal point location(s) of lenses is vital e.g. for cylindrical and spherical panoramic photography. There are lots of tutorials inside the www about how to do that.
These are two of them:
http://www.vrphotography.com/data/pages/techtutorials/technotes/nodalptalign-tn.html
http://dgrin.smugmug.com/gallery/2114189


Is MFD usually measured from the nodal point (by manufacturers in their spec sheets)?

Manufacturers measure from the film plane (or the chip plane on digital cameras).
Many cameras have this plane marked somewhere on the outside of the camera body (Nikon e.g. usually draws a line on the bottom of the camera body near the tripod mount).


So the deviations in the lens account for the film back being given as 0.958" instead of 0.94488"?

Don't have an answer for that one.
I don't know, from where NT gathers the information for the various film backs.
Some are standards, some are special sizes from individual cameras (with little variations).
Wikipedia lists a bunch of those here:
http://en.wikipedia.org/wiki/List_of_film_formats
http://en.wikipedia.org/wiki/Film_formats

Boris Goreta
02-12-2011, 07:33 AM
This is what I do to calculate correct horizontal field of view when matching 3D elements with pthorograps. I hope it helps someone because I was so lost before using this technique. Now I can have perfect matches.

http://www.borisgoreta.com/forum/CCD-doodle.jpg

rsfd
02-12-2011, 08:05 AM
you can do that to calculate the horizontal FOV for any type of camera.
It will lead to a good approximation. You need to be aware, that the lens focal length read out from the EXIF data is rarely exact. It's programmed by the Manufacturer, but the real focal length is mostly slightly different. (No Manufacturer wants to write in "19,8 mm", "20,2 mm" or even "21,4 mm". They just write in "20 mm". There is a certain standard range of lens focal lenghts and Manufacturers are just rounding the real data to the closest "standard". Most users aren't interested in exact data either).

druitre
02-12-2011, 11:53 AM
Thanks for all that information! Essential stuff.

archijam
02-12-2011, 01:24 PM
Nice Boris! :)

druitre
02-13-2011, 07:33 AM
I did some tests today and here's what I found. Thought I'd share it, maybe it'll help someone. Or someone can correct me if I'm making a mistake somewhere.

When matching photos taken with my 5D mkII the closest match I get is by using framesize 0.9448" (=24mm). Works better than what the EXIF data provides real lens camera with (0.956"). The focal length of my lens (a 24-70mm L) seems to be slightly different in real life than what it says on the ring: at 70mm it is exactly what it says, but at 24mm I have to set LW's camera to 25.3mm to get a close match.

So my settings are: Width 2784, Height 1856, Aspect 1.0, Frame 0.9448"

(Actually, EXIF provides for Aspect 1.0158. Why is that?)

Then matching that to a movie of the exact same scene, I have to set my frame to 0.8" to get identical results.

Settings: Width 1920, Height 1080, Aspect 1.0, Frame 0.8"

If I'd calculate it and assume that the width of the film back doesn't change (remains at 36mm/1.4173") the film height would have to be 20.25mm/0.8" (at 16:9). So the theory fits actual life. Woohoo.

Remains the mystery why none of the 35mm Frame presets in LW give you that.

rsfd
02-13-2011, 08:56 AM

(Actually, EXIF provides for Aspect 1.0158. Why is that?)


Aspect is the Pixel Aspect Ratio.
Not all pixels are square as for print or computer monitors. NTSC pixels are taller then wide, resulting in values below 1.0 (0.86-0.9), where PAL pixels are wider then tall, resulting in a ratio upper 1.0 (1.01-1.06).
Widescreen pixels are even wider both for NTSC and PAL.

For still images I would expect a ratio of 1.0, while 1.0158 could be the aspect ratio for HD video output from your camera (PAL).


Remains the mystery why none of the 35mm Frame presets in LW give you that.

That's a good one to NT. :D
I never found LW to update the frame value in camera properties window when changing e.g. a Real Lens Camera from a (known) full-frame body to a camera with a (known) smaller chip.
It seems, it's just not working and one has to enter that value manually.
(Even changing from a 24x36 SLR to a medium-format body doesn't change the frame value, which should be 2.2047" for 6x6 - quite a difference)

druitre
02-13-2011, 10:59 AM
I've found that in LW you have to close then reopen the camera properties panel to see Frame value updated, or to see focal length updated after manually changing Frame value.

Regarding (pixel) Aspect (ratio), yes I know what it means (no offense though at your explanation), my question was about it being 1.0158 when taken from EXIF data in a 5D photo - that should be 1.0, I'd think. HD video doesn't store EXIF, and as far as I know 1920x1080 is supposed to be pixel aspect ratio 1.0 too. I could try taking a photo (and movie) of a perfect sphere as subject matter and see how it pans out in LW viewport when lining up with a LW sphere.

[EDIT] - I just did; and photos & movies both have a pixel aspect ratio of exactly 1. The EXIF value 1.0153 doesn't make sense.

rsfd
02-14-2011, 03:33 AM
I've found that in LW you have to close then reopen the camera properties panel to see Frame value updated, or to see focal length updated after manually changing Frame value.
I've never found the values updated, but that's probably another LW-Mac issue…


Regarding (pixel) Aspect (ratio)…

(Sorry, explanation wasn't meant to teach you, I just added it to create a common base.)
You are right that Aspect for a still camera should be 1.0.
Why EXIF data stores 1.0153 would be a question to Canon, I guess.