# FF vs APS-C DOF question



## kalgra (May 25, 2016)

So I am very familiar withe concept of crop factor in terms of focal length and how a crop sensor affects that. However I am curious about DOF. So I know if I have a 100mm f/2.8 EF lens that on my 70D that will equate to ~150mm but how if the aperture affected?

I think I read somewhere that you also have to apply the crop factor to the aperture as well but does that only really effect how much light is allowed in but not affect DOF or does it affect both?

I know this is a noob question so I hope I have explained what I am asking clearly enough.

Basically this is what I want to know. I have a 24-70mm 2.8 I use with my 6D. If use that same lens on my APS-C camera to extend the 70mm to 105mm for portraits am I going to lose some of the nice bokeh I get at 2.8 or will it improve because I have extended my focal length but lose a stop or two of light?

I suppose i can just experiment with it when  I get home tonight but this question has been driving me nutz all day and didn't want to wait anymore.


----------



## Ysarex (May 25, 2016)

You apply the crop factor to the f/stop to get a DOF equivalency. This does not apply to exposure in any way. F/2.8 is f/2.8 for all lenses no matter what the format of the camera the lenses are used on.

Smaller sensor cameras are going to give you more DOF all things else being equal (taking the same photo). So if you want to equalize the DOF say between a FF and 1.5 crop sensor camera you would take the f/stop value of the FF lens and multiply by .66 to get an f/stop value for the crop camera lens that would produce the equivalent DOF.

Joe


----------



## kalgra (May 25, 2016)

Ok so basically my ability to blur the background with 2.8 on a an APS-c will be reduced? The lens would actually need to go down to f/1.8 to effectively have the same DOF of 2.8 on a FF correct?


----------



## Ysarex (May 25, 2016)

kalgra said:


> Ok so basically my ability to blur the background with 2.8 on a an APS-c will be reduced? The lens would actually need to go down to f/1.8 to effectively have the same DOF of 2.8 on a FF correct?



That is correct (assuming a 1.5 crop factor for the APS sensor).

Joe


----------



## kalgra (May 25, 2016)

got it!! thanks so much!


----------



## astroNikon (May 25, 2016)

you can use a DOF calculator to play around with settings and compare FF to a crop sensor.  Such as ==> A Flexible Depth of Field Calculator


----------



## KmH (May 25, 2016)

Blurring the background (attaining a shallow depth of field) is about more than just the lens aperture.
In fact, the point of focus distance has a bigger effect than does the lens aperture, but lens focal length and how far behind the far limit of the DoF also affects how blurred a background will be.

In this photo the lens aperture was f/6.3. The total DoF was 1.5 feet. The near DoF limit was 0.73 feet in front of the point of focus. The far DoF limit was 0.75 feet  behind the point of focus.






The lens had a focal length of 500 mm and the point of focus distance was approximately 70 feet.
The camera was a Nikon D90 with an APS-C size image sensor.


----------



## kalgra (May 25, 2016)

KmH said:


> Blurring the background (attaining a shallow depth of field) is about more than just the lens aperture.
> In fact, the point of focus distance has a bigger effect than does the lens aperture, but lens focal length and how far behind the far limit of the DoF also affects how blurred a background will be.
> 
> In this photo the lens aperture was f/6.3. The total DoF was 1.5 feet. The near DoF limit was 0.73 feet in front of the point of focus. The far DoF limit was 0.75 feet  behind the point of focus.
> ...



Sure I understand that it's a lot more than just the aperature but all things being equal I was trying to understand the effect on DOF for Crop vs FF. Thanks!


----------



## fmw (May 30, 2016)

kalgra said:


> Ok so basically my ability to blur the background with 2.8 on a an APS-c will be reduced? The lens would actually need to go down to f/1.8 to effectively have the same DOF of 2.8 on a FF correct?



Understand that the reason for the change in DOF is not because of the format of the sensor but rather the focal length of the lens.  In other words to get the same framing in an APS-C as opposed to 35mm format with the same subject distance, you will use a shorter focal length lens.  That is why the DOF is greater.  If you use a 35mm format camera and lens and move yourself forward or backward to achieve the same framing for each format, you will not only change the perspective but the depth of field as well because depth of field is greater at distance and less when closer to the subject.  The rules of optics don't change because of sensor size.

I think rather than to say that DOF is greater with APS-C it is more accurate to say that DOF is greater with APS-C because of the shorter focal lengths used.  As an example, if I put my 35mm 80-200 zoom lens on my APS-C camera it will have the same DOF at every focal length with the same subject distance as it would on a 35mm format camera.  The only difference is that the camera will capture less of the lens's field of view.

Another strange concept I read around here is that of increasing "reach" with a smaller sensor.  Again, the rules of optics do not change because of sensor size.  The subject image size will be identical between the different sized sensors with the same lens.  The difference is that the smaller sensor only records a portion of the field of view.  Assuming the two sensors have the same pixel density, the same thing can be accomplished by cropping the 35mm format image in post production.


----------



## Ysarex (May 30, 2016)

fmw said:


> kalgra said:
> 
> 
> > Ok so basically my ability to blur the background with 2.8 on a an APS-c will be reduced? The lens would actually need to go down to f/1.8 to effectively have the same DOF of 2.8 on a FF correct?
> ...



This is incorrect. The DOF *does* change because of the sensor size. The size of the recording medium is one of the essential factors in a DOF equation and it is correct to say that the size of a digital sensor is a DOF determinant factor with smaller sensors producing more DOF all other things being equal.



 

That's the equation for calculating hyperfocal distance. "f" is the lens focal length, "N" is the f/stop and "c" is the circle of confusion. The circle of confusion is derived from the size of the recording media.

Joe



fmw said:


> I think rather than to say that DOF is greater with APS-C it is more accurate to say that DOF is greater with APS-C because of the shorter focal lengths used.  As an example, if I put my 35mm 80-200 zoom lens on my APS-C camera it will have the same DOF at every focal length with the same subject distance as it would on a 35mm format camera.  The only difference is that the camera will capture less of the lens's field of view.
> 
> Another strange concept I read around here is that of increasing "reach" with a smaller sensor.  Again, the rules of optics do not change because of sensor size.  The subject image size will be identical between the different sized sensors with the same lens.  The difference is that the smaller sensor only records a portion of the field of view.  Assuming the two sensors have the same pixel density, the same thing can be accomplished by cropping the 35mm format image in post production.


----------



## fmw (May 31, 2016)

Ysarex said:


> fmw said:
> 
> 
> > kalgra said:
> ...





Ysarex said:


> fmw said:
> 
> 
> > kalgra said:
> ...



OK, I accept that.  But it seems to me that a difference in DOF because of sensor size would be fairly trivial compared to the difference from lens focal length or subject distance.  Whenever you deal with circles of confusion, you also deal with the issue of the eye's ability to distinguish the differences and that can vary from person to person and subject to subject.


----------



## Ysarex (May 31, 2016)

fmw said:


> Ysarex said:
> 
> 
> > fmw said:
> ...



Just making sure the facts are accurate. The whole point of your post was to say the size of the sensor isn't a factor in determining DOF. It's a common misconception. You said this, "As an example, if I put my 35mm 80-200 zoom lens on my APS-C camera it will have the same DOF at every focal length with the same subject distance as it would on a 35mm format camera." That's incorrect.

The eye's ability to distinguish differences in sharp focus is the basis of all DOF calculations. Each of the factors involved alter DOF at different but significant rates and are all determinant and essential for a correct understanding. Sensor size is one of them.

Joe


----------



## Dave442 (May 31, 2016)

Don't know if the OP was able to experiment at home. As it is for a portrait, the difference when experimenting will be noticeable in that with the crop sensor camera you will be farther from the subject to have the same amount of the subject in the image. So the greater distance to subject vs. the full frame body will give a difference in the background blur.

What I usually do to compensate is to try and increase the distance between the subject and the background when using a crop sensor body. So for example if I was at 70mm and f/2.8 and 10 feet away with the full frame I would have about 1 foot of DOF and then with the crop sensor would go to around 15 feet for the same subject size in the frame and that would give around 1.5 feet for the DOF.


----------

