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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

understanding my true focal length

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MiguelOrtega

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im a bit confused, if anyone can be so kind to answer.

if im shooting with a 50mm prime,
if i understand correctly its only a true 50mm if i shoot full 4k?
so if im shooting 3k or 2 k how do i know what that translates to in terms of my focal length?

if this has been asked before i apologize but i cant find it. i did search.
 
Focal length doesn't change. 50mm is always 50mm, doesn't matter what sensor size or crop. The field of view (or angle, if you like) changes :biggrin:
 
A 50mm at 4K has a set field of view. If you shoot 3K (cropped inside 4K), you are getting less field of view so there is a magnification factor associated with decreased sensor size (that you are using).

A 50mm is a 50mm. But a 50mm means something different with every different sensor size. Hence... a 50mm in still full frame (36x24) is normal. It is a fairly long telephoto when used with a 2/3" sensor.

There are tables that give you field of view for a given focal length and sensor size. Also tables that give you focal lengths of equivalent field of view for a given sensor size.

Hope that makes sense and helps. Someone can step in and say it better. Won't hurt my feelings...

Jim
 
In simple terms shooting with a 100mm in 4k is the same as shooting with a 50mm in 2k field of view. The resolution you record at wont change the parameters in the equation to calculate the F point of the glass concave in the lens other wise changing your focal length.
 
so am i correct to say a 50mm lens on a red shooting 3k is equivalent to a 109mm lens in terms of focal length?

i apologize if this is trivial but im learning all this as u can see.
if u guys ever have questions about making monsters or blowing up people i can help.... :) :ninja:
 
Quickest way to understand is to use a zoom lens. Pick a focal length somewhere in the middle to long range. Select 4K and mark lines on your wall (big red crayons work really well here...) where the field of view ends. Then switch to 3K and see how much wider you have to zoom out until you see those marks. Or you can do the math. Both ways work. Crayons are probably better to get a graphic look at what is going on. Make sure to leave the markings on the wall to remind you not to forget. :-)

Jim
 
4K is 4 times as big as 2K... go figure. :-)

(and 5x bigger than 1080P)

Jim
 
Jannard said:
Quickest way to understand is to use a zoom lens. Pick a focal length somewhere in the middle to long range. Select 4K and mark lines on your wall (big red crayons work really well here...) where the field of view ends. Then switch to 3K and see how much wider you have to zoom out until you see those marks. Or you can do the math. Both ways work. Crayons are probably better to get a graphic look at what is going on. Make sure to leave the markings on the wall to remind you not to forget. :-)

Jim
Click to expand...

But...but, i just put on new wallpaper!!!! :turned:

yean
 
MiguelOrtega said:
so am i correct to say a 50mm lens on a red shooting 3k is equivalent to a 109mm lens in terms of focal length?
:ninja:
Click to expand...

Yes if shooting the same resolution with both lenses the focal length of a lens won't change no matter what you do. You must differentiate between focal length and field of view. Field of view is how much you are seeing or how big the object is, focal length is basically how you are seeing it. The millimeter number on the lens is the distance of the F point from the center of the the lens a set value. The smaller the focal length (millimeter number) of the lens the wider the field of view. So when you change the resolution from 4k to 2k that has no bearing on the focal length of the lens yet it does mean you are cropping out a part of the image the lens is focusing on the chip. There for your field of view will change when lessening the resolution.
 
MiguelOrtega said:
so am i correct to say a 50mm lens on a red shooting 3k is equivalent to a 109mm lens in terms of focal length?

i apologize if this is trivial but im learning all this as u can see.
if u guys ever have questions about making monsters or blowing up people i can help.... :) :ninja:
Click to expand...

Hi Miguel,

If you've got an iPhone there's a great little application for it called "iSee4K" (I keep banging on about it on these forums & I don't even have any shares in it...honest!!). Anyway it's got a great Field of view calculator for 2K, 3K & 4K (you just type in your lens and subject distance and it gives you all the dimensions - you might find it handy for your line of work?)

All the best,
 
thanks guys, im trying to track something in 3D and i have a suspicion my settings are off hence my bad tracks with red footage. i would get much better results if i dont let the computer guess my focal length but i was just unsure if this 2-3-4k change affected it.
 
Jannard said:
Quickest way to understand is to use a zoom lens. Pick a focal length somewhere in the middle to long range. Select 4K and mark lines on your wall (big red crayons work really well here...) where the field of view ends. Then switch to 3K and see how much wider you have to zoom out until you see those marks. Or you can do the math. Both ways work. Crayons are probably better to get a graphic look at what is going on. Make sure to leave the markings on the wall to remind you not to forget. :-)

Jim
Click to expand...

you realize you are going to get someones kids in trouble right? heheh, classic.
 
This is the most common question I've gotten over the years. I can't count how many times I've been asked this. This thread explains it well. One day we should have a RedWiki that could contain all the knowledge that this forum has gathered over the years in an easy to search format.

BTW, I have noticed that different focal lengths have different "perspective signatures" that are consistent even when the field of view changes due to different sized image capture planes.

An 85mm looks like an 85mm in both a S16mm sized aperture and and S35mm sized aperture. One is a bigger field of view than the other, but they both have that signature.
 
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