# dof calculator ff vs crop



## candc (Mar 3, 2014)

Here is a link to an online dof calculator

http://www.dofmaster.com/dofjs.html

I understand that as focal length increases, dof decreases and as subject distance decreases then dof decreases. If you use the online calculator it will show less dof at a given distance and fl using a crop sensor opposed to a ff one? I don't see why dof would be any different. The crop sensor is just looking at a smaller part of the image circle that the lens is projecting so it should be the same as cropping a ff image. Why would the dof change if the fl and distance are constant?


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## Mt Spokane Photography (Mar 3, 2014)

Lots of people get confused by this, but its true. A smaller sensor will have more depth of field given the same lens focal length, aperture, and distance to subject. 
The web site tries to explain this, its based on the circle of confusion, which is different for small or large sensors. They use the term DSLR to mean a crop sensor.
http://www.dofmaster.com/dof_dslr.html

If you frame the subjects the same, the depth of field will be the same, The depth of field gets larger as you backup with a crop camera in order to frame the subject the same.

With the point and shoot, if you frame the subject the same, the DOF will be the same.


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## sagittariansrock (Mar 3, 2014)

Mt Spokane Photography said:


> Lots of people get confused by this, but its true. * A smaller sensor will have more depth of field *given the same lens focal length, aperture, and distance to subject.
> The web site tries to explain this, its based on the circle of confusion, which is different for small or large sensors. They use the term DSLR to mean a crop sensor.
> http://www.dofmaster.com/dof_dslr.html



I think you mean *shallower* depth of field.
This has been explained in depth (pun not intended) by Neuro and PBD on these pages.
Please see below (sorry, couldn't resist it  )


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## neuroanatomist (Mar 3, 2014)

Mt Spokane Photography said:


> Lots of people get confused by this, but its true. A smaller sensor will have more depth of field given the same lens focal length, aperture, and distance to subject.
> The web site tries to explain this, its based on the circle of confusion, which is different for small or large sensors. They use the term DSLR to mean a crop sensor.
> http://www.dofmaster.com/dof_dslr.html
> 
> ...


Sorry, but only your very first sentence and your reason (circle of confusion, CoC) were correct - an accurate response to the OP's question (does a smaller sensor have shallower DoF when FL/aperture/distance are constant?). That's true, and the circle of confusion is the reason, and many people are confused by the whole concept. 

DoF is determined by subject magnification and aperture (the physical diameter of the iris diaphragm), and the CoC enables practical comparisons of DoF. Focal length and subject distance affect subject magnification, therefore they affect DoF. 

If you match subject framing, the smaller the sensor, the deeper the DoF. That's generally the most useful type of comparison, because we are usually interested in comparing hope the same picture."




candc said:


> The crop sensor is just looking at a smaller part of the image circle that the lens is projecting so it should be the same as cropping a ff image. Why would the dof change if the fl and distance are constant?


Cropping an image _does_ change the DoF (pauses until the sound of heads exploding fades  ). Well, sort of. 

PBD summed it up nicely - fundamentally, DoF is very subjective. Since DoF is really about differential sharpness in an image, it's affected by more than just the parameters of how the image is captured. Cropping and enlargement, viewing distance (for the monitor or print), and even the viewer's visual acuity all affect DoF. CoC attempts to control for those post-capture factors by assuming a specific, fixed print size, viewing distance, and viewer acuity. 

The fixed print size means that an image from a smaller sensor must be enlarged more, i.e., the original magnification was relatively lower. Similarly, cropping an image in post (where aperture, focal length, and distance cannot be changed) results in a shallower DoF given the assumptions of CoC (i.e., the cropped image would be enlarged to the same final output size).


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## candc (Mar 3, 2014)

alright, i thought this was a pretty straightforward concept and i knew all there was to know about it but i guess not?

here is an excerpt from the wiki page about this. its what i knew before, at least i think so?

If pictures taken from the same subject distance using the same focal length, are given the same enlargement, both final images will have the same DOF. The pictures from the two formats will differ because of the different angles of view. If the larger format is cropped to the captured area of the smaller format, the final images will have the same angle of view, have been given the same enlargement, and have the same DOF

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

i read the luminous landscapes article and that is very interesting. it says that all lenses at a given aperture and subject framing have the same dof, that sounds wrong but when you read the article it makes sense, its a good rule to know
http://www.luminous-landscape.com/tutorials/dof2.shtml

but this is what throws me off, the picture is of 2 dof calcs, the only difference is the camera. dof for the 7d is shallower than that of the 5d when fl and distance are equal?

i guess that may be true in perceived relative terms but in absolute terms i think the wiki page sums it up correctly.


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## philmoz (Mar 3, 2014)

candc said:


> alright, i thought this was a pretty straightforward concept and i knew all there was to know about it but i guess not?
> 
> here is an excerpt from the wiki page about this. its what i knew before, at least i think so?
> 
> ...



The key here is the images are given the *same enlargement*. In other words if you print the FF image at 4x6 inches then you would only print the APS-C image at 2.5x3.75 inches. If you then view the prints from the same distance the perceived DOF is the same.



> i read the luminous landscapes article and that is very interesting. it says that all lenses at a given aperture and subject framing have the same dof, that sounds wrong but when you read the article it makes sense, its a good rule to know
> http://www.luminous-landscape.com/tutorials/dof2.shtml
> 
> but this is what throws me off, the picture is of 2 dof calcs, the only difference is the camera. dof for the 7d is shallower than that of the 5d when fl and distance are equal?
> ...



In this case the DOF calculation is assuming you are viewing the resulting images at the same physical size (from the same viewing distance) - in order to do this you must enlarge the APS-C image more (so the images no longer have the same enlargement).

Phil.


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## neuroanatomist (Mar 3, 2014)

A great and succinct explanation, philmoz. 

@candc - there's a difference between "same enlargement" and "enlarged to the same output size". The latter is the basis for CoC (and factors into the DoF calculators).


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## candc (Mar 4, 2014)

I think I have it all straight now, thank you all for taking the time to respond.


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