Crop factor on a camera = zoom factor?

[snivlem]

Hi,

In DSLR's there are crop factors (ie 1.6, 1.3 and full frame DSLRs etc...).

Is my understanding correct that when you enter in the film gate as being 35mm, the physical V-Ray camera becomes a full frame DSLR?

Now if I were to emulate a cropped camera (let's say a 1.6 cropped sensor), does entering 1.6 into the Zoom factor parameter emulate this or is it just a different parameter?

Thanks!

[vlado]

You'd have to set the film gate as the actual sensor size, and use the zoom factor to compensate for it.

Best regards,
Vlado

[snivlem]

Thanks vlado, so for the canon 7d (1.6 crop), I would enter in its film gate size (not 35mm) then 1.6 for the zoom factor?

[jujubee]

Might be worth adding in a sensor drop-down menu for people such as myself that can't remember numbers.

[snivlem]

After some tests I think I should be doing it like this?

Canon 5D. Sensor size of 35.8mm (Full Frame)
Canon 7D. Sensor rize of 22.3mm (1.6 Crop Factor)

So, in Vray if I wanted to emulate the Canon 5D, I'd enter in this?

Film Gate = 35.8mm
Zoom Factor = 1

If I were to emulate the Canon 7D, I'd enter in what?

Film Gate = 22.3mm
Zoom Factor = ???

.6 Gives the best result but I'm not sure what this "Zoom Factor" number should actually be ?

[plastic_]

why should there be a zoom factor?
changing the film gate should be enough, in my understanding. after all it's just a cropped sensor. the lens stays the same.

[joconnell]

Will the film gate take into account its relationship between the size of the sensor and the section of the lens it sees though?

[simmsimaging]

As I understand it you can leave the film gate size as 35mm, and you apply the crop factor to the lens length to find the matching view. To convert for max you would take your DSLR lens and multiply (i.e. 18mm is actually 28.8mm on a full sensor camera or in Max).

This is how pano stitching software seems to deal with it.
B

[KAKTUSDIGITAL]

I think changing the lens length is pretty easy. If you're dealing with APS-C sized sensors. On the Canon's, they are whatever your lens length X 1.6. On the Nikon its X 1.5. Canon 1D series - X 1.3
Not sure, but all the other typical point and shoot type stuff should utilise a small sensor that translates to approx X 4 in 35mm equiv terms. ie: 7mm = 28mm etc.

Canon APS-C:
7D, 50D, 40D, 30D, 20D, 10D, D60, D30, Digital Rebel T2i, Digital Rebel T1i , Digital Rebel XS / EOS-1000D, Digital Rebel XSi / EOS-450D, Digital Rebel XTi / EOS-400D, Digital Rebel XT / EOS-350D, Digital Rebel / EOS-300D

Canon 1D series
Canon 1D, Canon 1D Mk II, 1D Mk II N, 1D Mk III.

Nikon DX sized cams

Nikon D7000, Nikon D3100, Nikon D300s, Nikon D3000, Nikon D5000, Nikon D90, Nikon D60, Nikon D300, Nikon D200, Nikon D80, Nikon D70, D70s, Nikon D50, Nikon D40, D40x, Nikon D2Xs, D2X, D2H, D2Hs, Nikon D100, Nikon D1X, D1H, Nikon D1

[greysheep5]

so why on earth is the standard film gate size of a newly placed vray cam always 36mm? did i get something wrong there and shouldn´t it be 35mm then? where the hell does this "just off full format" 36mm film gate size come from? this would also explain to me why the quickly placed standard cams always slightly deviate from the vray ones i place to finally render my scenes (of course with the same focal length). is this correct vlado? and... could i chance this preset somehow?

ck.

[snivlem]

I believe the FF sensor size is actually 35.8mm, so rounded to 36mm. It's not much of a difference actually.

BTW thanks for the idea everyone. I'm still unsure to how it all works so for the moment, I'll pretend i'm using a FF camera, not a crop (which is what I'm used to in real life). Cheers!

[plastic_]

Well the sensor size of my 5d II is (exactly) 36x24mm. The old 5d is slightly smaller. Nikon is different again.
With cams things are not as nicely mathematical as with rendering.

As I said in another thread, the focal length value is worthless to describe FOV of lenses.
Lens manufacturers don't care about accurate FOV, every lens is different by as much as +-10%. It's more like a rough estimation.
The only way to find the real FOV of a lens, is calculating it with boujou, syntheyes, etc, or guessing it by eye.

[kein_plan]

@greysheep: pay attention! 35mm is the size of the film in the good old analog cameras. the size of the sensor or the film gate was and is for "full format" 36x24mm!
read: http://en.wikipedia.org/wiki/135_film

[greysheep5]

to make it clear for me or everyone else confused... so the film gate value in the vray cam is the wider (36mm) value of a 35mm film full frame which has 36x24mm dimension? hence the standard 36mm value there? makes sense to me this way.

@kein_plan: nice user name by the way, hehe. i was talking about the 35mm value as brett simms suggested using it, was never sure about the whole topic since i once discovered the slight mismatch between max and vray cams... it´s not clear to me if the vray film gate value is meant to represent the size of the film or the size of the full frame of a given film... whatever!?

now why, and i know plastic in real life it´s all a bit different but i am talking about max, is the field of view of a max cam slightly different to the one of a vray cam? or to ask the other way round, which value do i have to feed the vray cam to match the one of the max standard cam? or, with which values does a max standard cam operate internally?

[plastic_]

I think the max standard cams use 35mm and Vray Cam 36mm, that's the difference.
Changing default parameters of max objects is a pain, one way that works, albeit ugly:

put this into a textfile with the file extension ".ms", copy it into /3dsmax/scripts/startup/

with redraw off (
local currentPanel=getCommandPanelTaskMode()
setCommandPanelTaskMode #modify
delete (VRayPhysicalCamera film_width:35 isSelected:on)
setCommandPanelTaskMode currentPanel
resumeEditing()
)

This will permanently change the Vray Physical film gate default value from 36mm to 35mm.