Little Confused... 4k 3k 2k Question.

[FreakIndeed]

Newbie here. I'll be placing my deposit soon and I'm trying to absorb as much info as I can from the forum here. After doing several forum searches I'm still remaining unclear on something.

Do you lose shallowness of depth of field when shooting in 2k over 4k or does it simply crop the sensor, not effecting depth of field, but giving the sense of magnification thus requiring wider lenses to achieve the same field of view that a longer lens may give you at 4k? I read somewhere about using S16 lenses when shooting 2k, but can you still use Nikon lenses with the Nikon lens mount?

The reason I ask is because I come from a still photography background and shallow (sometimes very shallow) depth of field is part of my style. Nothing I ever shoot will be played at anymore than 1080p so I can't see why I would need to shoot at 4k when I could get 4X the amount of footage for my disk space at 2k.

Thanks!!!

 

[Simon Valderrama]

hello there!
2K is a sensor crop, anyway there's a nice post by Brook Willard that covers the topic extensively here:

http://www.reduser.net/forum/showthread.php?t=1487&page=2

 

[FreakIndeed]

Valde, thanks for the response. I did read that post and that is where I got the impression that 2k would NOT effect the depth of field but give the sense of magnification. However, later I read in another post (don't know where it is now) where someone said that the advantage of shooting 4k over 2k is increased shallowness of depth of field.

Hence the confusion!

 

[Matthew Rogers]

Here's the deal... You can shoot 4k/2k with a 35mm lens or you can shoot 2k with a 16mm lens. Both can work at 2k but there are optical differences. You really only want to shoot 4k unless you need more than 25fps. 4k is much sharper and you will see less grain. The less disk space that 2k offers isnot worth the quality loss that is there because it is cropping the sensor.

Matthew

 

[Tyler Black]

Freak,

I think to answer your question - you can shoot in 2k with a 100mm lens and get the same dof that you would get in 4k with a 50mm lens (in general terms... I know it's not that neat and tidy).

If you've got the room to back up and get the same shot with a 100... go for it.

 

[Eric Lange]

Funny you should say that as a “circle of confusion” is a technical term used by technical photographers and optical instrument makers to define the smallest resolvable blob or point of focused light onto a focal plane, and these diameters usually of the order of 15-60 micron (in film systems and older optical systems are used to calculate the depth of focus (hence the illusive depth of field) you speak of.

The modern formula for calculating shifts in depth of focus is by Washer 1945,

d= 4 (f-number)/R,

Where d is the distance that the focal plane (of the sensor) can be shifted by in mms (forwards or back)
f-number is the f number (you know that one)
R is the resolution in line pairs per mm.

The relation to focal length is that the “f-number” is the focal length in mms divided by the diameter of the clear aperture. So a 50mm lens with an clear aperture of 25mm gives a f number of 2.

You will notice that the depth of focus is not directly affected by focal length but as you will see the depth of field is.

If for example you shoot a picture with a 50mm lens on a 4” by 5” transparency (diapositive) and have it developed, you will notice if you take a pair of scissors to the transparency and cut it down to 2” by 2.5”, the depth of field in the shot does not magically change. Similarly the same lens on the red one sensor then cropped down to 2k does not change the depth of field of the original subject. However the angle of view (if using the same lens has changed) a 50mm lens on a 24mm sensor has an approximate angle of view of 27 degrees. If you crop the sensor to half its size e.g. 12mm you still have a 50mm lens on there so your angle of view will be approximately half or 13.5 degrees.

Now, if you want to effect the same angle of view on your 24 mm sensor as you do on your 12mm sensor then you will use a lens with half the focal length or 25mm. However a lens with 50mm at f4 on a 24mm sensor will give you the same angle of view and same DEPTH OF FOCUS as a 25mm lens on a 12mm sensor at f4. HOWEVER the depth of focus refers to how much you can shift the image plane in and out of best focus before the image becomes unacceptably blurred or the circle of confusion becomes too large. The same little image shift has different consequences on a long focal length than on a short focal length. A 0.25mm focal plane shift (to a blurrier image) on a 25mm focal length when projected out into the room (object space) yields a larger depth of field approximately twice that of the same 0.25mm image plane shift on a 50mm lens. Therefore if your approach is to maintain the same angle of view and same f stop on a sensor that is half the size, then you will have to use a focal length that is half the distance. The same permissible focal shift on a lens with half the focal length will yield a depth of field that is roughly twice as much. This is because permissible shift of depth of focus is only controlled by desired resolution (or blurr) and the f number, but the depth of field is controlled by the depth focus in relation to the focal length.

{I know this is essentially correct but horribly explained}

In summary for a given constant angle of view and f number, shorter focal lengths have greater depth of field than long focal lengths.

To maintain the same angle of view on a comparatively smaller sensor then a smaller focal length is needed, and therefore it has a larger depth of field at the same f number than the equivalent larger sensor and longer focal length maintaining the same angle of view.

There you go clear as mud!

 

[Ryan Patch]

What he's trying to say is:

You will get the same DOF on a 50mm in 4k as you will on a 50mm in 2k. A 50mm is a 50mm is a 50mm is a 50mm. Always.

However, you won't get the same angle of view between the two. So, if you want a 50mm field of view, if you shoot 2k, you will have to go down to a 24/25mm. Then your DOF will change (or at least your minimum DOF. Obviously they can be made to match with the 50mm on f/4 and the 24mm on f/2.)

And yes, shooting on Nikon lenses is fine with the 2k sensor. That's what I've been doing. Again, you just get half the view.

Also, keep in mind that you can get a great S16mm zoom for a fraction of the cost of 35mm glass.

Ryan

 

[Tom Lowe]

Just to put icing on this, YES shooting at 2K on a cropped sensor will effect your DOF - if you are trying to get the same angle of view, aka the same "frame".

Windowing or cropping a sensor simply makes it a smaller sensor, so all the same rules apply.

 

[FreakIndeed]

Ah, ha. I do believe I've had the revelation. From my still photography experience I know that an 85mm is gonna give more depth of field blur than a 24mm. So to get the field of view of an 85mm in 4k you are only going to have to use something like a 50mm, thus changing the depth of field... or you could leave the 85mm on and scoot back.

Thanks guys for clearing that up!

 

[Tom Lowe]

If you ever see indie guys shooting narrative films on cameras like the HVX200, which has a very small "sensor", you will see them pull the camera back like 30 feet and shoot damn near maxed out telephoto just to get the "DOF" they want.

 

[Gunleik Groven]

My guess is that you'll quickly find that the datarate for 4k is quite low, while the quality of 3k/4k images over 2k is ... huge... (even for 1080 finnish).

I think we all have been through a "2k is surely enough" phase, untill we tasted the 4k drugs...

In other words:
35mm glass is cool for RED
2k is fine for slomo
4k is the best freakin' video you'll see in a while...

 

[FreakIndeed]

If you ever see indie guys shooting narrative films on cameras like the HVX200, which has a very small "sensor", you will see them pull the camera back like 30 feet and shoot damn near maxed out telephoto just to get the "DOF" they want.

I'm currently shooting with a Sony EX1... I'm that guy. :biggrin:

 

[RivaiC]

Altho i'm very much like Super 35mm framing and recorded in 2K resolution. But that is not how the camera works. If you need 2K version from the 4K framing, u have to scale in post. That is how RED works, each version of the res is the crop format.

 

[Raphael Varandas]

Very nice this thread guys...thanks

 

[Daniel Browning]

Ah, ha. I do believe I've had the revelation. From my still photography experience I know that an 85mm is gonna give more depth of field blur than a 24mm. So to get the field of view of an 85mm in 4k you are only going to have to use something like a 50mm, thus changing the depth of field... or you could leave the 85mm on and scoot back.

If you ever see indie guys shooting narrative films on cameras like the HVX200, which has a very small "sensor", you will see them pull the camera back like 30 feet and shoot damn near maxed out telephoto just to get the "DOF" they want.

Shooting telephoto and backing up to compensate does not make the depth of field thinner, because the increase in focus distance cancels out the increase in aperture. However, it does increase magnification, so whatever background blur there is will become more magnified in relation to the subject. It's a different look from the increase in blur caused by a *true* change in the depth of field. As explained in Myth busted: telephoto lenses have thinner DoF

 

[Tom Lowe]

Shooting telephoto and backing up to compensate does not make the depth of field thinner, because the increase in focus distance cancels out the increase in aperture. However, it does increase magnification, so whatever background blur there is will become more magnified in relation to the subject. It's a different look from the increase in blur caused by a *true* change in the depth of field. As explained in Myth busted: telephoto lenses have thinner DoF

That's why I said "DOF" rather than DOF. :tongue: