35mm stills converision formual

[sasha2222]

I have a question that does not exactly relate to red, but I think the guys on this forum might have some more information on this then the guys in DVX, nothing against those guys buy I already posted this question there and I did not get an answer. what is the math on how to calculate focal lengths still cameras when compared to Video cameras. IE. If you are zoomed in on the HVX at lets say Z50, what would be the 35mm Still equal. I have a job where I must match the same 35mm lens with my HVX, they will be using a 50mm lens.

Also how to Video Lens size markers relate to 35mm size markers. I noticed that on all video cameras the markers are usually something like 4.5-9.5 or something like that, how does that relate to 35mm. I hope I have gotten my questions across. Thanks for the help guys.

[Andrew M.]

Use horizontal dimension of 35mm still and horizontal dimension of your format on HVX, then divide the FL by the ratio of lengths.
Here you can find the dimensions of both formats.
http://en.wikipedia.org/wiki/Super_35
http://en.wikipedia.org/wiki/135_film

[Dominic Jones]

Hi Sasha!

It should be noted that the focal length of a lens does not change when you move it between different formats - a 50mm lens is a 50mm lens on any size of gate. What changes is the field (or angle) of view, so that whilst a 20mm lens is a wideangle on 135, it's a telephoto on a 1/3" system.

135 (SLR 35mm) FoV has a relative FoV of 7.2x the FoV of 1/3" systems - so a 50mm lens on an SLR would have the same FoV as a 7mm lens on your HVX.

That also answers your question about video lens lengths - they are the actual length of the lens, often something like 4.5-54mm or so (I think that's what the Z1's lens is, for instance) - giving a 35mm relative FoV of about 32-390mm, for that example...

Hope that helps!

[sasha2222]

Thanks for the answers that definitely explains a lot for me, But how does one calculate what the FOV is in order to come up with the right number for multiplication how did you come up with 7.2, or is there a chart that gives you that number so that you don't have to think about it. One last thing I just wanted to say that this forum and DVXUSER are really awesome place I have learned much being a member and a browser its great that we all have a place to share and teach each other about are crazy business. I currently do not have a reservation for red due to finances but i will get in the game when they hit the market I have been following this camera from when it was rumor. I even got a friend of mine to reserve one. I can't wait to have the opportunity to have a camera that can compete with my still photography friend’s image quality.

[chuck colburn]

The ratio is the difference in the diagonals of the two formats your comparing.

[Illya Friedman]

Both Dominic and Chuck got it partially correct- You'll want to divide the hypotenuses to determine your LMF. This will give you FL with equivalent AOV, but it doesn't take into account AOV H & V when mismatching aspect ratios. If you want to get that specific I recommend a little light reading/research. All the info you need is in this book:

http://www.amazon.com/American-Cinem...3345675&sr=1-1

The quick cheat for 2/3"- LMF for 35mm (1.85) is 2.54. The 2/3" LMF (4:3) is 2.8.

I.
Dalsa

[Brian Ferguson]

Hi Sasha!

It should be noted that the focal length of a lens does not change when you move it between different formats - a 50mm lens is a 50mm lens on any size of gate. What changes is the field (or angle) of view, so that whilst a 20mm lens is a wideangle on 135, it's a telephoto on a 1/3" system.

Yes thank you! That was very well worded.
I have often tried to explain that motion picture film goes through the camera vertical and stills horizontal. That Leica cameras were made to test motion picture films. That made them about twice as wide.

If you worked in a still studio a generation ago you dealt with 35 mm, 2 1/4 sq (0r 6 x 6 cm) 4x5" format and 8x10" format and you learned the FOV for each format. But the laws that govern depth of field were the same - focal length and aperture. So as you went larger your depth of field decreased as your format got larger. You learned the format and what the wide and telephoto parameters are. It is easier to learn the focal lengths for a given format and it is much more organic that having a formula. The depth of field charts will always be the same.

This is a good topic.

BriFerg

[Stephen Williams]

But the laws that govern depth of field were the same - focal length and aperture.
BriFerg

Hi,

Remember the DOF at a given f stop will be identical regardless of lens if the image size is the same on the sensor/ film!

A 100mm @ 20 feet equals a 50mm @ 10 feet equals a 25mm @ 5 feet. Assumptions that the lenses are of 'simple' design, physically small and & F stop is identical.

Stephen

[James T Mather]

Apparently 7.2 seems to be a conversion number but differences in the aspect ratio make it only an approximation. (this was for a 1/3 chip 4x3 camera so 16x9 aspect ratio might need further calculations)

So according to the links below - divide the focal length of your stills cam by 7.2.
50mm/7.2=6.944.

On these threads:

http://www.dvinfo.net/conf/showthread.php?t=3726

http://www.dvinfo.net/conf/showthread.php?t=15881

[Dominic Jones]

Both Dominic and Chuck got it partially correct- You'll want to divide the hypotenuses to determine your LMF. This will give you FL with equivalent AOV, but it doesn't take into account AOV H & V when mismatching aspect ratios.

Ah yes, I decided not to get into the aspect ratio mire!! But you're quite correct, of course!

Since I've just noticed jtm's brought this up as well, it's worth mentioning, I guess, that the hypotenuse conversion method only really works between identical ratio systems - if you're converting FoV between differing ratios you really need to do a separate calculation for both the H and V angles - so you might end up with a conversion factor of 7.2 for the horizontal AoV and 6.4 (this is purely an example, btw, and bears no relation to any reality as far as I'm aware!) for the vertical. Working out the conversion factors gets a little more complex though...

Oh, and thanks BriFerg, glad to have helped!!