Depth of field question.

A

Adamsenoj

Well-known member
Optics can be quite confusing. When it comes to depth of field we normally hear things like you have to open up the lens by so many stops to match the DOF of super 35mm film or full frame.
But do I understand it right that for example a 50mm lens on a super 35 camera and on an AF100 would have the same exact DOF, and the whole opening up the lens to match DOF only applies if you are trying to match the FOV as well?
If that's right then a 50mm lens has the same DOF properties regardless of sensor size but it would naturally have different FOV with the different sensors. So 50mm on a super 35 camera and on an AF100 have the same DOF but different FOV. But if I try to match the FOV of a 50mm lens on a super 35 camera with the AF100 I would have to use a 35mm focal length lens, and then to match the DOF too I would have top open up the lens? Is this it or I have it totally wrong?


How many stops I have to open the lens to match super 35? I know to match academy is only 1/4 of a stop. But what about super 35?
 
you got it, a bigger sensor size decreases DOF and a longer lens also does... a 100mm FF lens at 4.0 will give you the same DOF as a 50mm 4/3" lens at 2.0
we have to know if you have 4/3" lenses or FF ones to answer you question correctly.

regards Steve
 
I think you didn't understand my question. Besides a 50mm lens is a 50mm lens.
But let's say I have the same 50mm lens, same model, same lens. I put it on a F3 and then take it off and put it on an AF100. Is the DOF the same with the F3 and the AF100 with this 50mm at say F2.8? I know the FOV is different. I'm just speaking of DOF here.
 
Yes, your depth of field would be the same, only your angle of view would change.
Think of it as a photo - you can zoom in on it, what you see changes but not the characteristics.

To be a 100% accurate - there are miniscule differences from sensor to sensor (or film) but not that you'd notice.
 
Adamsenoj said:
How many stops I have to open the lens to match super 35? I know to match academy is only 1/4 of a stop. But what about super 35?
Click to expand...

To match super 35 DoF to 4/3 there is a bit less than 1/2 a stop difference - 5/12
 
That's what I thought. So basically if you have space to back the camera up to compensate for FOV instead of using a wider lens, as when shooting outside, DOF is no longer an issue as you can potentially even get 5D shallow.
 
Wait, so you guys are saying a 50mm on a F3 with a Super 35mm Sensor will have the exact same DOF as an AF100 with a 4/3 sensor and the exact same 50mm lens?
 
Adamsenoj said:
But do I understand it right that for example a 50mm lens on a super 35 camera and on an AF100 would have the same exact DOF
Click to expand...
At the same f-stop, yes. They would have a different fov, but they would have identical DOF.

and the whole opening up the lens to match DOF only applies if you are trying to match the FOV as well?
Click to expand...
Yes, because in order to match FOV you'd have to change the focal length, and changing the focal length changes the DOF, and so you'd have to open up the f-stop to equalize the DOF.

If that's right then a 50mm lens has the same DOF properties regardless of sensor size but it would naturally have different FOV with the different sensors.
Click to expand...
EXACTLY!
So 50mm on a super 35 camera and on an AF100 have the same DOF but different FOV. But if I try to match the FOV of a 50mm lens on a super 35 camera with the AF100 I would have to use a 35mm focal length lens, and then to match the DOF too I would have top open up the lens? Is this it or I have it totally wrong?
Click to expand...
You have it exactly right.

How many stops I have to open the lens to match super 35? I know to match academy is only 1/4 of a stop. But what about super 35?
Click to expand...
About 1/2.
 
Adamsenoj said:
That's what I thought. So basically if you have space to back the camera up to compensate for FOV instead of using a wider lens, as when shooting outside, DOF is no longer an issue as you can potentially even get 5D shallow.
Click to expand...
Yes, but you don't typically want to go backing the camera up, because that changes your perspective. Change the lens to a wider lens instead.
 
LiamR said:
Wait, so you guys are saying a 50mm on a F3 with a Super 35mm Sensor will have the exact same DOF as an AF100 with a 4/3 sensor and the exact same 50mm lens?
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Yes, of course. But the field of view will be tighter on the AF100.

The image from the lens is exactly the same, with the exact same DOF, but on the smaller sensor it's like you went in and trimmed off the edges of the bigger picture.
 
Barry_Green said:
Yes, but you don't typically want to go backing the camera up, because that changes your perspective. Change the lens to a wider lens instead.
Click to expand...

Sure. I just wanted to make sure I got it right. But when it comes to the 4/3 chip and super 35mm the frame is so close. We are talking about using a 35mm to match the 50mm FOV in super 35 with the AF100. Do you really think moving the camera back in this case would affect perspective significantly? Perspective is very subjective and an artistic decision anyway in most cases. I feel the difference here is so small that it wouldn't play much of a role.
 
I'm just relaying that moving the camera will be introducing a difference in perspective. It's up to you to determine whether the difference makes a significant enough impact on the picture... the difference from 35 to 50mm is about 1.3, so it's not a massively huge difference (not like 2.0) but it's still significant. I would rather use the 35mm lens and open up the iris half a stop to equalize the DOF, than I would want to move the camera back 30%. But hey, the great thing about video formats is you can look through the viewfinder and try it and see what you like.
 
Barry_Green said:
Yes, but you don't typically want to go backing the camera up, because that changes your perspective. Change the lens to a wider lens instead.
Click to expand...

Exactly. And this is the problem. A given lens, at a given aperture, and a given focus, gives you the same DOF no matter what camera you use it on. But when you back a camera up to get the same FOV, you change perspective, and you also have to change focus. And when you change focus, you change the DOF along with it.

The workflow that works best for me is simple. First, find the correct perspective, and place the camera at that point (if you're going to truck/dolly the camera, find the start and end points, etc.). Second, pick a lens that will give you your desired FOV, at that point. Third, decide what aperture to use with that lens, to give your desired DOF. Forth, using that aperture, figure out how to get the exposure you need (lighting, iso, NDs, etc.).

The trick, if it is in fact a trick, is to find the correct perspective without using a camera. Use your feet, and use your eyes. Learn to see what's actually there. All of it, especially what is not the primary subject of interest. How much of that do you want to include, and how much do you want to exclude?

"The camera is an instrument that teaches people how to see without a camera." -- Dorothea Lange
 
I think, that it would be more simple to have in mind, that for the same image magnification, dof depends only from the f-number.

Or, for any given f-number setting, the greater the image magnification, the less the dof; the less the image magnification, the more the dof.
Applied Depth of Field, A. Blaker
 
Adamsenoj said:
That's what I thought. So basically if you have space to back the camera up to compensate for FOV instead of using a wider lens, as when shooting outside, DOF is no longer an issue as you can potentially even get 5D shallow.
Click to expand...

It's not quite that simple unfortunately - because you have moved your camera further away from your subject your focus point is further away so your DoF will change.

If you double the distance from your subject and open up two stops (1 & 3/4 to exact) you have a similar look to a full frame 35 mm still - but you would see less background.
That's fine if the 5D is shooting at f4 or f2.8 but if it's shooting at f1.4 you need a lens that can open to f0.75.
 
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