35mm stills converision formual [Archive]

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sasha2222

03-07-2007, 09:11 AM

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.

03-07-2007, 09:54 AM

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

03-07-2007, 01:46 PM

sasha2222

03-07-2007, 06:22 PM

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

03-07-2007, 06:49 PM

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

Illya Friedman

03-08-2007, 01:37 AM

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-Cinematographer-Manual-Ninth-Stephen/dp/0935578242/ref=pd_bbs_sr_1/104-4688842-9439911?ie=UTF8&s=books&qid=1173345675&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

03-08-2007, 02:27 AM

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

03-08-2007, 03:05 AM

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

03-08-2007, 03:25 AM

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

03-08-2007, 04:19 AM

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!!

James T Mather

03-08-2007, 06:47 AM

Look what happened to the last guy who tried to figure it out.
http://img138.imageshack.us/img138/6261/hippo0ot.jpg

Martin Drew

03-08-2007, 09:32 AM

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

I don't think this is correct (assuming I understand you correctly Stephen). You position a 100mm lens at 20 feet from an object it is focused on (object A) and there is another object (object B) 2 feet further on, so the second object is 22 feet from the lens. Then you select an aperture so the second object is just outside the DOF. Take a picture.

Now change to a 10mm lens and position it 2 feet from object A. Object B will therefore be 4 feet from the lens. Focus on object A and set the same aperture on this lens as you set on the 100mm lens. take a picture.

Object A will be in focus and the same size on both pictures, object B will have the same amount of blurryness on both pictures but it will be smaller on the second picture.

DOF is the depth of acceptable focus. Object B is just outside the range of acceptable focus on the first picture because the blur is just perceivable. On the second picture however object B is significantly smaller so the blur cannot be perceived.

So the 10mm lens will have a greater absolute DOF than the 100mm lens at a given aperture.

M

Stephen Williams

03-08-2007, 10:18 AM

I don't think this is correct (assuming I understand you correctly Stephen). You position a 100mm lens at 20 feet from an object it is focused on (object A) and there is another object (object B) 2 feet further on, so the second object is 22 feet from the lens. Then you select an aperture so the second object is just outside the DOF. Take a picture.

Now change to a 10mm lens and position it 2 feet from object A. Object B will therefore be 4 feet from the lens. Focus on object A and set the same aperture on this lens as you set on the 100mm lens. take a picture.

Object A will be in focus and the same size on both pictures, object B will have the same amount of blurryness on both pictures but it will be smaller on the second picture.

DOF is the depth of acceptable focus. Object B is just outside the range of acceptable focus on the first picture because the blur is just perceivable. On the second picture however object B is significantly smaller so the blur cannot be perceived.

So the 10mm lens will have a greater absolute DOF than the 100mm lens at a given aperture.

M

Hi Martin,

If the image size remains the same on the sensor the DOF is identical at the same F stop regardless of lens focal length.

If you shoot a packshot with a 100mm lens @ f5.6 and the label is not fully in focus, somebody always says 'easy use a wider lens'. The 25mm comes out the camera is moved in to 1/4 of the distance and the label is still soft at f5.6. Only stopping down the lens will increase the DOF. (Assuming a thin lens that does not change focal length as it focuses!)

I am not talking about changing anything! A am not talking about the rendering of the out of focus background just what IS in focus!

Take a quick look at standard DOF tables and they will agree totally!
DOF tables for thick lenses that are corrected for where the front nodel point actually is will vary slightly. (The front nodel point or enterance pupil is where DOF is actually calculated from, that is not the film plane. That is why a zoom lens will have a different DOF at a given distance, however the magnification at the sensor is also different. Match up the image size and the DOF will also match)

Stephen

chuck colburn

03-08-2007, 10:28 AM

Stephen Williams

03-08-2007, 10:43 AM

Martin,

A 10mm lens is probably a retrofocus or inverse telephoto lens. It will therefore PROBABLY have less DOF than a 50mm thin lens at the same magnification. A 500mm telephoto lens will probably have slightly greater DOF than the 50mm thin lens.

I do realise this goes against what seems obvious!

Stephen

Martin Drew

03-08-2007, 10:52 AM

Hi Martin,

If the image size remains the same on the sensor the DOF is identical at the same F stop regardless of lens focal length.

If you shoot a packshot with a 100mm lens @ f5.6 and the label is not fully in focus, somebody always says 'easy use a wider lens'. The 25mm comes out the camera is moved in to 1/4 of the distance and the label is still soft at f5.6. Only stopping down the lens will increase the DOF. (Assuming a thin lens that does not change focal length as it focuses!)

I am not talking about changing anything! A am not talking about the rendering of the out of focus background just what IS in focus!

Take a quick look at standard DOF tables and they will agree totally!
DOF tables for thick lenses that are corrected for where the front nodel point actually is will vary slightly. (The front nodel point or enterance pupil is where DOF is actually calculated from, that is not the film plane. That is why a zoom lens will have a different DOF at a given distance, however the magnification at the sensor is also different. Match up the image size and the DOF will also match)

Stephen

What you are describing is a particular case where the DOF in both situations was insufficient. It is possibly more helpful to think of a more extreme case, then the logical point becomes more obvious. If you had a very large pack (possibly a RedOne camera box) and the pack label was at an angle to the lens so that one end of the label was 20 feet from the 100mm lens and the other end was 22 feet from the lens and then you moved 2 feet from the label and used a 10mm lens... You see where I am going. I am not talking about rendering an out of focus background either. The issue is that there is only one point of focus, in front of that point and behind that point will be out of focus but if the bluriness is less than the system can resolve it is within the depth of field.

My example may be extreme but it illustrates the logical point.

M

Stephen Williams

03-08-2007, 11:02 AM

What you are describing is a particular case where the DOF in both situations was insufficient.

M

Martin,

In both cases the DOF was identical! Thats the point!

Try making the background soft when you shoot people head to toe in 16x9 and you're on a zoom (25-250) that won't open more than say F2.8! That 1000mm lens won't help either as it's a F4 or 5.6. The 24mm or 35mm f1.2 (I think that's the F no of a Super Speed) will win the game!

Stephen

Martin Drew

03-08-2007, 12:24 PM

Hello Martin,

Mr. williams is correct. Open up this site listed below and scroll down about a third of the way until you see Grommit and read that section. It gives visual proof with an easy to understand explanation.

http://www.vanwalree.com/optics/dof.html

Chuck

I think this article illustrates my point. Look further down to the section headed: "DOF at constant magnification II"

Steven Parker

03-08-2007, 01:08 PM

Hey sasha2222--

Panavision has a handy calculator on their website:

http://www.panavision.com/tools.php

and then check Table of 'Equivalent Focal Lens and AOV'

I haven't used it for 1/3" chip conversions, but it bears out the rule of thumb 2.5x for 2/3" chips...

Hope that helps

Stephen Williams

03-08-2007, 01:09 PM

I think this article illustrates my point. Look further down to the section headed: "DOF at constant magnification II"

Hi Martin,

The car is equally soft, just smaller so appears sharper. Look at the ground the Focus clearly ends before the car in both pictures.

There is a bright 'piece of paper'? half way between the car and where the focus ends, it seems to be the same in both pictures!

I do however understand what you are saying.

Stephen

Martin Drew

03-08-2007, 03:09 PM

Hi Martin,

The car is equally soft, just smaller so appears sharper. Look at the ground the Focus clearly ends before the car in both pictures.

Stephen

Yup. The car is blurred to the same amount in each picture but because it is smaller in picture B the relative blurring between the car and Grommit is all but lost to the screen resolution. DOF is dependent upon magnification and relative magnification of elements in a frame at different distances changes as viewing distance changes.

Bottom line is your pack shot example is sound, in order for the focal length to be sufficiently short that the whole pack is acceptably sharp the geometric distortion would be unacceptable. But if you are trying to isolate the pack from the background use a long focal length and go further back, the first Grommit set of pics (http://www.vanwalree.com/optics/dof.html) is the perfect illustration of this.

M