Busting the FF vs Crop DoF Bokeh Myth

Started Jul 27, 2009 | Discussions
therickman Senior Member • Posts: 1,305
Busting the FF vs Crop DoF Bokeh Myth
2

There's been so much discussion, argument, and name-calling because of the disagreements (misunderstandings) over the depth-of-field and bokeh of full frame versus 1.6x crop cameras, I decided to finally do a controlled test with my 50D and 5D.

My conclusion:

At the same distance from subject, using the same exact lens, there is absolutely no difference in DoF and bokeh between images produced by a full frame and crop camera.

Test setup consists of tripod with camera exactly 5 feet from the Ansel Adams book with center focus on the "L". John Freeman's "Photography" book is exactly 1 foot behind, "Lighthouses" book 2 feet behind. Images are JPG straight out of camera with no PP applied. Sharpening in-camera at 7. Contrast in-camera at +1.

Here is the setup shot at 5 feet with the 5D with 50mm f/1.4 lens

Here is the setup shot with the 50D with 50mm f/1.4 lens

And here are my findings

Here is the setup shot with the 5D with 85mm f/1.8 lens

Here is the setup shot with the 50D with 85mm f/1.8 lens

And here are my findings

Here's a link to my full-size test images:
http://www.flickr.com/photos/picsurephoto/sets/72157621866502388/

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rman18 Regular Member • Posts: 205
Re: Busting the FF vs Crop DoF Bokeh Myth
1

Nice comparison - and thank you for clearing that up... you know you still will have those that will disagree

Rob

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Kuivaamo Senior Member • Posts: 2,248
Re: Busting the FF vs Crop DoF Bokeh Myth

therickman wrote:

There's been so much discussion, argument, and name-calling because of the disagreements (misunderstandings) over the depth-of-field and bokeh of full frame versus 1.6x crop cameras, I decided to finally do a controlled test with my 50D and 5D.

What, precisely, is the "myth" you're attempting to bust?

Without knowing more, I can make one comment on your test. You can't resize images to different final output sizes and claim that DoF is the same (you have artificially made the same features the same size in your crop comparisons). After all, DoF calculations assume a constant print size and viewing distance.

OP therickman Senior Member • Posts: 1,305
Re: Busting the FF vs Crop DoF Bokeh Myth
1

Kuivaamo wrote:

What, precisely, is the "myth" you're attempting to bust?

The "myth" is that the DoF is different between a full frame and crop sensor camera. I have clearly shown that there is no difference. Bokeh is exactly the same for each camera at the same distance with the same lens at the same aperture. Making the Field of View exactly the same for each camera will affect DoF and bokeh ... but that's not what I'm testing here. I'm simply testing DoF between two camera formats at the same distance and aperture.

Without knowing more, I can make one comment on your test. You can't resize images to different final output sizes and claim that DoF is the same (you have artificially made the same features the same size in your crop comparisons). After all, DoF calculations assume a constant print size and viewing distance.

Resizing an image doesn't change the depth-of-field. That's preposterous.

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R Yenko Regular Member • Posts: 139
Re: Busting the FF vs Crop DoF Bokeh Myth
2

I think what people are saying is that FF BG blur is better because you have to move closer to the subject to get a similar composition in the image, thus creating a greater distance to subject/distance to background ratio which increases background blur.

Another simlilar principle applies to super zooms and aperture size. The BG blur appears greater in longer focal lengths even when a 500mm is stopped down to 5.6. The DOF is the same as a 24mm lens, you're just magnifying the BG blur.

Maybe someone could fill me in on the physics of macro photography...

R Yenko Regular Member • Posts: 139
Re: Busting the FF vs Crop DoF Bokeh Myth
1

therickman - you just explained it while I was busy typing.

Gene L. Veteran Member • Posts: 3,788
Experiment ignores FOV (Field Of View)
2

What you have demonstrated is the fact that when the things that control DOF are the same, you get the same DOF. Seems kind of intuitive. For your shots you have the same focal length, aperture, and distance, so the DOF will be identical. If you crop the image from the 5D to match the 50D, they should look virtually the identical in terms of DOF.

What you are ignoring is that to take the same shot from the same perspective, one must necessarily use a shorter focal length with the 50D. Using a shorter focal length increases the DOF. Therefore, for a given field of view, the 50D has greater DOF at a given aperture. This is the source of the "myth" to which you refer.

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Hugowolf Forum Pro • Posts: 12,676
You are adding to the myth

You are not busting the myth, just expounding on it. Your test shows why the myth exists.

You either need different focal lengths that create the same field of view, or better still, just move the camera so you have the same field of view. If you keep the same distance and same focal length, you will always have the same approximate DoF.

And by the way, bokeh refers to the quality of the out of focus area, not the out of focus area itself. Bokeh can be good, bad, dreamy, creamy, and awful, amongst other things, but not deep or shallow.

Brian A.

OP therickman Senior Member • Posts: 1,305
Re: Experiment ignores FOV (Field Of View)

Gene L. wrote:

What you are ignoring is that to take the same shot from the same perspective, one must necessarily use a shorter focal length with the 50D. Using a shorter focal length increases the DOF. Therefore, for a given field of view, the 50D has greater DOF at a given aperture. This is the source of the "myth" to which you refer.

I'm not ignoring anything. I understand that for the same shot (identical field of view, composition, whatever...) from both cameras, you must either use a lens 1.6x longer on the 5D, or increase the distance by 1.6 times. Of course this will change DoF, bokeh, and even cause compositional inconsistencies. Duh! That's not what I'm trying to prove here.

Many people have argued that the DoF and bokeh using the same lens on a full frame and 1.6x crop camera at the same aperture and the same distance from the subject will yield different DoF and different bokeh... And they're flat out wrong! Even DOFMaster's calculator shows a difference between the two at the same FL, distance and aperture. That makes no sense, and I've proven it. There's absolutely no difference. If you took a 4" X 6" printed photo from a full frame 5D, and with scissors cut the edges off to make the photo 2.5" X 3.75", that doesn't change the depth of field. This is the same thing as using a 1.6x crop sensor camera. All the crop sensor is doing is producing the central 63% of a full frame sensor.

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Steve H Senior Member • Posts: 2,648
Re: Experiment ignores FOV (Field Of View)

therickman wrote:

Many people have argued that the DoF and bokeh using the same lens on a full frame and 1.6x crop camera at the same aperture and the same distance from the subject will yield different DoF and different bokeh... And they're flat out wrong

Except for the 60% of the shot that the crop body doesn't even capture

Steve H
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bronxbombers Forum Pro • Posts: 18,226
your samples are not the point

you need to stand in the same spot, shoot the same subject, focused on the same spot using a DIFFERENT lens (or focal length on a zoom) to produce the SAME image on both

Yaamon Senior Member • Posts: 1,751
How about adjusting the distance

Thanks for your test but how about adjusting the distance with the crop body so that it matches the 5D to compare with your original photos?

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pixseal
pixseal Veteran Member • Posts: 3,840
Re: You are adding to the myth

I don't think that the OP understands DOF.
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Cheburashka Regular Member • Posts: 297
I've never seen anyone make that claim

therickman wrote:

Many people have argued that the DoF and bokeh using the same lens on a full frame and 1.6x crop camera at the same aperture and the same distance from the subject will yield different DoF and different bokeh... And they're flat out wrong!

When people say that the DOF is different on FF and 1.6x using the same focal length and the same f-ratio, they mean for the whole of the frame and displayed at the same dimensions. They don't mean to crop the FF image to the same framing as the 1.6x image. Why would they do that?

I mean, who, for example, uses a 50/1.4 at f/2 on FF and crops to the same framing as they would get with a 50/1.4 at f/2 on 1.6x? Now, in some cases, such as extreme telephoto, where the FF camera is focal length limited (due to availablity, expense, size, or weight) or extreme macro where more than 1:1 is needed, this may be common. But other than that, it's unheard of.

Jason Hutchinson Forum Pro • Posts: 12,177
you've changed the enlargement ratio
1

You've changed the enlargement ratio between the two, changing the CoC. The reason you think the DOF master page is wrong is because you don't understand CoC. If you kept the enlargement ratio the same, you'd find DOF master is right, assuming you don't exceed the CoC possible with a particular sensor/lens in which case you are just putting garbage in to the DOF calculator and getting garbage out.

Jason

Daniel Browning Senior Member • Posts: 1,058
What about when you print them at the *same* size?
1

therickman wrote:

... depth-of-field and bokeh of full frame versus 1.6x crop cameras, I decided to finally do a controlled test with my 50D and 5D.

Thank you very much for taking the time to execute a test. It correctly demonstrates the fact that when you print APS-C 1.6X smaller than FF, DOF is the same. It also shows that if FF is cropped 1.6X digitally, DOF is the same. However, you failed to address the case where both are printed at the same size and neither is digitally cropped. If you had, you would have found the reason why DoFMaster shows thinner DOF on the 50D: because it has a higher reproduction magnification (enlargement ratio).

In any case, I find that any DOF discussion based on varying angle of view to be unhelpful, since it has nothing to do with the way a photograph is composed in the real world. Photographers do not restrict themselves from shooting wide angle just because they happen to use a crop camera: instead, they use a shorter focal length. Neither do they force themselves to stop shooting telephoto just because they went from from 12mm on a digicam (short tele) to full frame (ultrawide). Instead, a photographer first chooses the angle of view, perspective, and composition they desire to shoot, then chooses what focal length to use (or buy) for the format. Some might even sacrifice the perspective (subject distance) just to get the angle of view they desire.

The comparison of a single focal length among multiple sensor sizes therefore does not correspond to reality. It is far more informative to consider the situation where focal length is varied among sensor sizes to retain the same angle of view.

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Jason Hutchinson Forum Pro • Posts: 12,177
when you try to defy physics...

Cheburashka wrote:

I mean, who, for example, uses a 50/1.4 at f/2 on FF and crops to the same framing as they would get with a 50/1.4 at f/2 on 1.6x? Now, in some cases, such as extreme telephoto, where the FF camera is focal length limited (due to availablity, expense, size, or weight) or extreme macro where more than 1:1 is needed, this may be common. But other than that, it's unheard of.

When you try to explain away physics you have to go strange places.

Jason

Cheburashka Regular Member • Posts: 297
On the 'preposterous' effect of resizing on DOF

Kuivaamo wrote:

Without knowing more, I can make one comment on your test. You can't resize images to different final output sizes and claim that DoF is the same (you have artificially made the same features the same size in your crop comparisons). After all, DoF calculations assume a constant print size and viewing distance.

therickman wrote:
Resizing an image doesn't change the depth-of-field. That's preposterous.

It's not "preposterous", it's a fact. The "plane of perfect focus" has zero depth (well, it's not a plane, actually, due to imperfections in the lens, but that's another story). As we move away from this "plane" in the image, the focus gets less sharp.

So, the DOF is not the thickness of this "plane" that is in focus, but the depth that appears to be in focus. The larger you display the image, the more obvious it will become that deviations from the "plane of perfect focus" are out of focus, and the smaller you display the image, the less obvious it will be.

This is accounted for with the CoC (circle of confusion) in the DOF forumulas. A good discussion of this can be found here:

http://www.josephjamesphotography.com/equivalence/#dof

and here's an excellent visual demonstration of that fact:

http://forums.dpreview.com/forums/read.asp?forum=1029&message=23910313

chinch Senior Member • Posts: 1,731
Re: disproved a myth noone ever heard of LOL

you missed the point, explained by others.

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Cal Dawson Veteran Member • Posts: 4,810
Re: Busting the FF vs Crop DoF Bokeh Myth

I did this years ago and all it brought was grief.... Nobody wants to see the truth they all want to change something distance, FOV etc then they say the test will be fair or unfair dependent upon whether your results are what they want....
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