Quick DOF question.

[Alex Zustra]

I've known forever now that larger image formats characteristically have shallower depth of fields. After reading a ton about crop factors/FOV, I recently asked myself if a full frame "x" millimeter lens has the same field of view as for example a 4/3 "x" mm lens. I found the answer to be yes after a quick bit of research, which made sense because I remembered using something like a 14mm when shooting S16 for a wide. But what got me was that depth of field would be the same. So it has nothing to do with the intended lens format or the image circle it projects. Now I just don't understand why/how the larger format has less DOF. I understand how F-stops affect it, is it kind of the same principle?

 

Also, I'm having trouble visualizing how a closed aperture can soften the image even in the focal range.

 

Thanks in advance ladies and gents.

 

 

 

Edit: I think I confused myself before even getting to the question. A 50mm is a 50mm, yes, I know. But, cropped or not, a 50mm on 4/3 is still comparatively telephoto, FOV is not the same, which is why you would use a 14mm or something for a wide on 4/3 rather than 28ish on 35mm. Am I getting this right? It's been a long night of staring at numbers and charts. The other questions still stand.

[Adrian Sierkowski]

DoF in terms of lenses being lenses... Ok, so as you know a 50MM lens is slightly telephoto/normal, which gives you let's say, on 35mm, a nice head and shoulder's shot from about 4 ft away. Now, that combo of iris and distance from subject gives you some depth of field (I think about 5" or so, but that's immaterial.) So, now you want to compare this to say, S16mm, well, that 50mm lens won't give you the same shot from the same distance, it's much more telephoto as the format is using less of the image circle projected by the lens. So you change you to something, let's say a 25mm for simplicity's sake, and this lens at the same distance gives the same shot on S16mm. If we swapped this 25mm back to the 35mm camera and tried to get that same head and shoulders shot, we'd have to move the camera closer in, and focus closer, this reducing the DoF to about what it would be with a 50mm from further away!

 

I hope that makes some sense.

[Mitch Gross]

It's because you would need to position the camera differently dependent on the size of the frame used, which would then effect the appearance of depth. Perhaps this video I did might help:

 

http://blog.abelcine.com/2010/05/14/a-lens-is-a-lens-is-a-lens/

[John Sprung]

Also, I'm having trouble visualizing how a closed aperture can soften the image even in the focal range.

 

That's a difficult concept for everybody, and not really related to DOF. It's because of diffraction, which has to do with the aperture getting small enough to run afoul of the wavelength of the light. That oversimplification will probably suffice for most of cinematography, but if you want to thrash about down in the weeds on this, here's a place to start:

 

http://en.wikipedia.org/wiki/Airy_disk

 

 

 

-- J.S.

[Alex Zustra]

Thanks guys. So is it correct to say the difference is both apparent due to the need to use different focal lengths between formats to achieve a similar frame and actual due the focal distance (shorter focal distance to the subject = shallower depth of field, correct?)

[Alex Zustra]

Lost again. At the end of Mitch's video, we have the 2k/25mm on the left, 4k/50mm on the right. Same distance to subject, same size of subject and background, much different DOF. If focal length doesn't actually change depth of field

(because it is really just apparent as a telephoto lens simply compresses space, making the background object appear larger in the frame and magnifies the same blur that was there with a wide), then I still don't understand why the 2k/25mm with the same relative sizes and magnification at the same distance has more depth of field. Logic tells me that difference here is that the magnification from taking a smaller section of the image circle is different than magnification by focal length. Yet this contradicts that focal length does not actually change DOF.

 

I wish I could just live with knowing that there is a difference, but I'm too curious I guess.

[David Mullen ASC]

Lost again. At the end of Mitch's video, we have the 2k/25mm on the left, 4k/50mm on the right. Same distance to subject, same size of subject and background, much different DOF. If focal length doesn't actually change depth of field

(because it is really just apparent as a telephoto lens simply compresses space, making the background object appear larger in the frame and magnifies the same blur that was there with a wide), then I still don't understand why the 2k/25mm with the same relative sizes and magnification at the same distance has more depth of field. Logic tells me that difference here is that the magnification from taking a smaller section of the image circle is different than magnification by focal length. Yet this contradicts that focal length does not actually change DOF.

 

I wish I could just live with knowing that there is a difference, but I'm too curious I guess.

 

For all practical purposes, focal length affects DOF but as modified by distance focused and f-stop (of course.)

 

So yes, at the same distance focused, the longer focal length has less depth of field at the same f-stop, assuming that the same Circle of Confusion figure is used. The distance focused is critical because some people think that with the same format / sensor size, you get more depth of field when you shoot a close-up with a wide-angle instead of a longer lens, but they don't take into account that you have to move closer with the wide-angle lens to maintain the same screen size, and DOF drops off as you focus closer.

 

But in this case, the difference in format / sensor size affects the FOV, so you don't have to change the distance to maintain the same view when comparing the 2K/25mm shot to the 4K/50mm shot.

 

Actually, the Circle of Confusion figure is a factor here too... if you actually look on a DOF chart, a 50mm lens focused at the same distance and at the same f-stop has FOUR stops less depth of field than the 25mm, not TWO stops like you'd think.

 

But by switching from 4K to 2K, but comparing the two images at the same display size, now the 2K image needs to use a CoC figure that is twice as critical because it has to be enlarged twice as much to match the viewed 4K image. And thus you lose 2-stops of DOF with the format that is twice as small, so the four-stop DOF difference between the 50mm and the 25mm becomes only a 2-stop difference.