DSLR Depth of Field (5D vs. 7D/T2i)

[Jonathan Bowerbank]

So, being a student of film, lenses, optics, etc. Logic tells me "A 50mm, is a 50mm, is a 50mm." Right? But what's all this talk about the 5D having a much more shallow DoF than the 7D or T2i?

 

They use the same lenses, are set at the same flange focal distance, the only difference being the size of the sensor...which has nothing to do with DoF (despite what some misinformed people may believe). I'm guessing this comes up because to achieve a 50mm's field of view, one would have to throw on a focal length close to 30mm, thus increasing your depth of field...which would make sense to me that people would mistakenly make these claims.

 

I'm mostly curious what people here think. Please chime in :)

[Hal Smith]

There are three variables in a given shot's DOF; Focal length, aperture (f-stop), and sensor (film gage) size. Angle of view is a function of focal length and sensor size. For a given shot from a given distance at a given stop you might choose a 30mm lens for a 7D. For the exact same shot from the same distance you'd put a 50mm on a 5D (31.25mm on the 7D for an exact match). Therefore the 5D shot is going to have a shallower DOF.

[David Mullen ASC]

So, being a student of film, lenses, optics, etc. Logic tells me "A 50mm, is a 50mm, is a 50mm." Right? But what's all this talk about the 5D having a much more shallow DoF than the 7D or T2i?

 

They use the same lenses, are set at the same flange focal distance, the only difference being the size of the sensor...which has nothing to do with DoF (despite what some misinformed people may believe). I'm guessing this comes up because to achieve a 50mm's field of view, one would have to throw on a focal length close to 30mm, thus increasing your depth of field...which would make sense to me that people would mistakenly make these claims.

 

I'm mostly curious what people here think. Please chime in :)

 

Yes and no -- a 50mm is a 50mm, but on smaller formats you have to use a more critical Circle of Confusion figure to calculate Depth of Field is comparing to a larger format where the end product will be compared at the same size. And while this would mean that Depth of Field is actually more shallow for smaller formats, because you have to use shorter focal lengths to obtain the same field of view, there the Depth of Field is deeper.

 

Or to put it more bluntly, if you compare a 50mm to a 25mm lens on a Depth of Field chart, you'd see that the 25mm lens has 4-stops more depth of field. However, if you are putting that 25mm lens on a format that is half as small to match the field of view of the 50mm lens on the format twice as large, then the CoC figure has to be twice as critical for the 25mm lens, losing 2-stops of Depth of Field, so a 4-stop difference between the two focal lengths becomes a 2-stop difference.

 

Anyway, it's not a mistaken belief that the 5D has less depth of field than the 7D.

[Jonathan Bowerbank]

There are three variables in a given shot's DOF; Focal length, aperture (f-stop), and sensor (film gage) size. Angle of view is a function of focal length and sensor size. For a given shot from a given distance at a given stop you might choose a 30mm lens for a 7D. For the exact same shot from the same distance you'd put a 50mm on a 5D (31.25mm on the 7D for an exact match). Therefore the 5D shot is going to have a shallower DOF.

 

 

Right, that's what I'm saying. Just from the way others are talking about the 5D vs. 7D, it's as if they're saying a 50mm on the 5D has less depth of field than the exact same lens on a 7D, which can't be the case. I was just making sure I'm not going crazy when I tell an incredulous soul that the depth of field calculations are the same, it's the FoV that's the only obvious difference.

 

It's like shooting with the Red, when you go from 4K to 2K while using the same lenses. Your field of view changes, but DoF remains the same. However, yes, the circle of confusion really has become more critical due to what can be perceived as "in focus" at a lower resolution.

 

Thanks guys, it just all seemed weird. I'd read it mostly on those HD prosumer magazines and websites, which are rarely accurate or trusted, IMO

[Phil Rhodes]

a 50mm on the 5D has less depth of field than a 50mm on a T2i

But it does, to all practical purposes. You only have to make some very basic assumptions - like, the screen is the same size in both places - before it becomes true.

Of course there's a higher level of understanding to attain here, which is that stop, focal length, and the real-world spatial resolution of both capture and reproduction area all interact in complex ways.

[John Sprung]

.... stop, focal length, and the real-world spatial resolution of both capture and reproduction area all interact in complex ways.

 

Right, and the way to deal with that complexity is to shoot and look at tests. Light and lenses are analog. Sharpness doesn't suddenly fall off a cliff where the DOF tables say. How sharp is sharp enough is a subjective decision which belongs to the DP. That's why I recommend picking your own circle of confusion based on testing.

 

 

 

 

-- J.S.

[Andrei Goaga]

A 50mm lens will have a shalower DOF on a 5D than on a crop senzor camera because:

 

Firstly, DOF is endeed determined by 3 things, and these things are: Focal Lenght, Aperture and Distance, not senzor size, even though senzor size affects the distance from where you take your shot.

 

So a 50mm lens will have shalower dof ONLY when you compare it with the same shot size taken with the same lens and a crop senzor. Thats is because you will get closer to get the same shot size with a 50mm lens on 5D than with the same lens on a crop senzor. And being closer will affect the distance where you set your focus and that affects how shallow your DOF is.

 

Hope I'm right

[David Mullen ASC]

Think of it this way, the 50mm lens image on the smaller sensor is actually the SAME image as on the larger sensor but cropped to a narrower view -- in other words, take your shot with the 50mm on the 5D and then crop it in half in both directions, it's no different than putting the 50mm lens on a sensor that is half the size in both directions. So the depth of field characteristics haven't really changed unless you use a different Circle of Confusion figure for the smaller sensor / cropped & enlarged image... but in that case, since the smaller image actually uses a more critical CoC figure, the 50mm would actually have less depth of field on the smaller sensor, not more.

 

But you would normally switch to a 25mm lens on a sensor that is half as big to get the same view as the 50mm lens on a sensor that is twice as large -- and the 25mm lens focused at the same distance and at the same f-stop will have more depth of field than a 50mm on the larger sensor. And this increase in depth of field is much more than the loss in depth of field from using twice as critical a Circle of Confusion figure.