Busting the FF vs Crop DoF Bokeh Myth

Started Jul 27, 2009 | Discussions
climbng_vine Forum Member • Posts: 86
Re: You are adding to the myth

Cheburashka wrote:

therickman wrote:

I do. DoF is much different than FoV - which everybody here seems to be confusing.

DOF is a function of the FoV, among other factors -- something which you seem to be ignoring.

Absolutely, unconditionally false. Depth of field is a function of focal length and actual aperture diamater--nothing else. Period.

Perceived depth of field also depends, somewhat, on enlargement size.

Field of view is itself a function of focal length, negative or sensor size, and crop. It has nothing to do with depth of field. I can take a shot with a 120-degree field of view and cut some off each size with a scissor to make it a 50-degree field of view. This changes the depth of field not one iota.

Some of your other posts here have good info, but this is just dead wrong.

Roger Krueger Senior Member • Posts: 2,785
A crop very obviously changes DoF

chinch wrote:

the 1.6x crop is just that... A CROP so obviously it won't change DOF all it does is crop the outer portion of the image.

LOL not a revelation!

DoF is very specifically about equal print sizes. Getting to said equal print sizes (not equal composition, just equal print size) absolutely changes DoF.

If you want to play silly games with enlargement ratios you can "prove" just about anything. Only when you have enlargement ratios that yield equal print sizes do you have anything meaningful.

William DIllard Senior Member • Posts: 2,492
Re: your samples are not the point

I'm curious if you found the bokeh to have more clarity with the 5D as compared to the 50D? I had a 350D before the 5D and what I noticed was a sharper bokeh with the 5D and more punchy images in general with the 5D. The degree of bokeh probably the same but a difference in the quality was obvious to me. Now your crop I think has 15 megapixels while my 350 only had 8 so perhaps that is the difference but I am more inclined to think that the full frame handles light a little differently. I also noticed that the 5D could deal with awkward color tempertures better such as when shooting at night on the street in the city. Maybe the differences would not be as noticible on the the 5D compared to the 50D?

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OP therickman Senior Member • Posts: 1,305
Here it goes again.

Since many of you think I'm an idiot, yet didn't feel the need to read through my previous posts, I'm reposting this post from page 2.

I shot the same setup with the 5D with 85mm at f/2 at 5 feet, and then with the 50D at f/2 at 8 feet. Obviously the DoF is visibly different in the two pictures. Duh!! Hey, what do ya konw... I'm not an idiot. Wow, I was worried there for a minute. Oh, wait, no. Most of you decided not to read this thread all the way through. Hmm... who's the moron?

Here's the follow-up post from page 2 (if you care to read it):

therickman wrote:

Now, I'm going to explain this like I'm talking to a bunch of 5-year olds so you all understand. I'm incredibly tired of everyone here contorting my original post into something entirely different than what it was, and making me out to be some kind of imbicile. So here it goes...

The 50D has a 1.6x crop sensor, correct? Yayy!! Good job. Now time for some math. To get roughly the same field-of-view (or similar composition) as a full frame 5D at 5 feet from the subject, I moved the 50D 8 feet away from the subject.

5 X 1.6 = 8

Yayy!! You all get a smiley sticker for getting that correct. Below are two snapshots. One of the 5D with 85mm at f/2 at a distance of 5 feet, and one of the 50D with 85mm at f/2 at a distance of 8 feet. I attempted to frame the shots as identical as possible.

Obviously, the DoF of the 5D will be thinner than that of the 50D because of the closer distance, and this is evident in the photos.

(If I read someone post that these samples disprove my own observation, I'm seriously going to blow a stack. Please read my original post before you comment further. I never, I repeat NEVER claimed similar images from a 5D and 50D will have the same depth-of-field. I disproved the myth that depth-of-field will still be different using the same lens at the same distance from subject at the same aperture.)

Why do I get the feeling people are still going to completely ignore the evidence - and my original post - and still try to make me out to be a moron?

Oh yeah, because people refuse to read through things before they jump to conclusions. Such a shame...

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OP therickman Senior Member • Posts: 1,305
Re: your samples are not the point

William DIllard wrote:

I'm curious if you found the bokeh to have more clarity with the 5D as compared to the 50D? I had a 350D before the 5D and what I noticed was a sharper bokeh with the 5D and more punchy images in general with the 5D. The degree of bokeh probably the same but a difference in the quality was obvious to me. Now your crop I think has 15 megapixels while my 350 only had 8 so perhaps that is the difference but I am more inclined to think that the full frame handles light a little differently. I also noticed that the 5D could deal with awkward color tempertures better such as when shooting at night on the street in the city. Maybe the differences would not be as noticible on the the 5D compared to the 50D?

In short, the 5D is an amazing camera. I love it. I don't see a difference in bokeh, as that is associated with the lens. However, (as many have stated as if I didn't know), I have to stand closer to the subject with the 5D than the 50D, so the subjects "pop" more because of greater background blur. Also, the 5D handles colors much better. They're more accurate.

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Roger Krueger Senior Member • Posts: 2,785
Re: You are adding to the myth

climbng_vine wrote:

Cheburashka wrote:

therickman wrote:

I do. DoF is much different than FoV - which everybody here seems to be confusing.

DOF is a function of the FoV, among other factors -- something which you seem to be ignoring.

Absolutely, unconditionally false. Depth of field is a function of focal length and actual aperture diamater--nothing else. Period.

That's simply not correct. Given identical composition, a wider FoV lens will start to have deeper DoF as it gets anywhere near hyperfocal.

It's all about magnification--DoF is really about magnification, a synonym for focal length/distance. The problem is that for different FsoV, you only get identical magnification at the plane of focus. Start considering points very far behind the plane of focus and the difference in magnification begins to make a difference in DoF.

Perceived depth of field also depends, somewhat, on enlargement size.

Perceived depth of field is all there is. DoF at the sensor is an uttterly useless number, save for its ability to get you to "Perceived" DoF.

Field of view is itself a function of focal length, negative or sensor size, and crop. It has nothing to do with depth of field. I can take a shot with a 120-degree field of view and cut some off each size with a scissor to make it a 50-degree field of view. This changes the depth of field not one iota.

Once you blow it up to the original print size, sure it does. Not controlling for print size makes everything else nonsense.

coldbivy Forum Member • Posts: 71
Re: Preposterous? Oh dear.

The depth of field is something that is perceived by the viewer. The actual focal plane is a 2 dimensional plane. Everything not on that plane is out of focus. DOF is simply a definition of how OOF something is before the viewer notices. That point depends on the print size as well as the distance of the viewer from the print. That is why the definition is based on COC and not some other, easily understood metric.

To prove this to yourself, create a large print of a slightly OOF letter or similar simple black on white background. Then walk away from the large print and notice that at some point, you are no longer able to discern the fuzzy edges of the letter, and the letter appears more sharp, but much smaller. This is the reason that small prints (or images on your monitor) are much more forgiving of soft focus.

Your original statement is correct, but only for a final image displayed at the same size (or at the same display size to viewing distance ratio). So the crop camera image can display the crop of the FF image at the same magnification with the same DOF.

Roger Krueger Senior Member • Posts: 2,785
We got yer myth right here...

wittgenstein wrote:

If you take a FF shot and make a crop of it, you'll get a crop camera shot.

A crop camera is only a FF camera taking only central pixels from the lens, so really DOF from it is the same.

NO!

DoF is about enlarging to equal sizes--8x10 print, 640 x480 jpg, whatever.

DoF at the sensor is certainly the same before/after cropping, but you can't just ignore the enlargement factor.

dopravopat
dopravopat Senior Member • Posts: 1,180
Exactly!

I belive such a test would be very interesting. FF vs 1.6 crop. Sigma 50 vs Sigma 30 (same manufacturer). Same scene, same FOV. Aperture calculated as in the link in my post. Also at different aperture values.
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OP therickman Senior Member • Posts: 1,305
Re: Preposterous? Oh dear.

coldbivy wrote:

To prove this to yourself, create a large print of a slightly OOF letter or similar simple black on white background. Then walk away from the large print and notice that at some point, you are no longer able to discern the fuzzy edges of the letter, and the letter appears more sharp, but much smaller. This is the reason that small prints (or images on your monitor) are much more forgiving of soft focus.

Yes, that makes sense to a point. However, no matter how large or small you print this image, you're still not going to be able to read the subtitle of the "Photography" book, nor the price tag on the "Lighthouse" book. The DoF or "perceived" DoF is going to be the same no matter how small/large it's printed, or how close/far you are when viewing it.

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Roger Krueger Senior Member • Posts: 2,785
Almost

GoldenSpark wrote:

DOFMaster's calculator does't show the difference you claim.

It varies the circle of confusion between different cameras to account for different pixel pitch and hence differences in the acceptable sharpness figure.

No, it varies CoC to account for different degrees of enlargement to get to the same physical image size.

Don't take my word for it, go change between cameras with same same sensor format but different MP counts. No change in DoF.

Kuivaamo Senior Member • Posts: 2,248
Re: Preposterous? Oh dear.

therickman wrote:

Kuivaamo wrote:

therickman wrote:

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

You seriously need to read up on DoF and CoC before you engage in these tests and debates. Resizing an image absolutely does change the depth of field.

How so? Most people print 4X6. If I take this image...

and print it at 4X6, then enlarge it to 20X30, that's not going to make the "Photography" and "Lighthouse" books any clearer or blurrier in the picture. The books are staggered 1 foot and 2 feet behind the subject, and that's how it's been captured at that moment in time.

In the picture above, let's say the DoF is about 5 inches - 2 inches in front and 3 inches behind the Ansel Adams book. The DoF is the same whether it's printed as a 4X6, an 8X10, or 20X30 enlargement. The picture isn't changing, just getting bigger.

That's like saying if I take a picture of two men - one standing 10 feet behind the other, and I then enlarge that printed image, he's no longer 10 feet behind the other guy. That's preposterous. DoF is captured by the camera at that moment in that shot, and doesn't change.

You still haven't read up on acceptable CoC and DoF, have you? Until then, I doubt any conversation with you will be very fruitful. But I'll try once more anyway.

DoF is not an absolute property "captured by the camera". DoF is a property at the print (or final output on a monitor) and it's all about your perception when you examine that print. As an exercise, click on the "zoom out" button above your example photo three times. Can you still tell that the "Photography" text on the second book is very blurred? No, because things are too small to tell whether it's blurred or not in a larger image. In other words, the letters are now within a field of acceptable sharpness, or DoF. Once you zoom in, you realise that the letters are blurred. They are no longer within a field of acceptable sharpness, i.e. the text is now outside the DoF.

DoF is absolutely linked with final output size. The larger the print and the closer you examine it, the more rigorous your limit for acceptable circle of confusion and thus shallower your DoF will be. It's basic photographic theory, and you'd do well to get to grips with it.

As for 4x6" vs. 20x30", yes, the text on the OOF books will appear blurred in both. But if you look at other details such as bits of dust on the wood, some of them will appear sharp in a 4x6" but not in a 20x30". The exact same principle applies.

Roger Krueger Senior Member • Posts: 2,785
Re: Preposterous! Oh elk.

therickman wrote:

coldbivy wrote:

To prove this to yourself, create a large print of a slightly OOF letter or similar simple black on white background. Then walk away from the large print and notice that at some point, you are no longer able to discern the fuzzy edges of the letter, and the letter appears more sharp, but much smaller. This is the reason that small prints (or images on your monitor) are much more forgiving of soft focus.

Yes, that makes sense to a point. However, no matter how large or small you print this image, you're still not going to be able to read the subtitle of the "Photography" book, nor the price tag on the "Lighthouse" book. The DoF or "perceived" DoF is going to be the same no matter how small/large it's printed, or how close/far you are when viewing it.

DoF is NOT subject space resolution. (which is what you appear to be arguing for here.) You don't get to start redefining terms to fit your arguments.

OP therickman Senior Member • Posts: 1,305
I think I found out where the DoF confusion is...

I thought about it and I think I know where the DoF confusion is (maybe). Let me know if this is where my detractors are coming from...

Here's a scenario:

  • I take a picture of a 6-foot tall man with my 5D and 50mm lens, standing 10 feet away from him at f/2. The tip of his head is at the top of the frame (landscape) and his feet are at the bottom, perfectly filling the image frame.

  • 1 foot behind the man to the left is a fence post, still relatively in focus.

  • 5 feet behind the man to the right is a boy standing. He is obviously OOF.

  • I print the same image at 4X6 and 20X30

Are you saying that in the 4X6 picture the man is 4 inches tall, yet in the 20X30 the man in 20 inches tall, therefore the DoF is 5 times larger? Is that how DoF changes relative to print size? Because no matter how large or small you print, the boy will still have stood 15 feet from the camera and be OOF, you'll never see the pimple on his cheek, or read the message on his shirt.

I consider DoF through the camera at the time of capture, in real life conditions, hence the DoF button.

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gava Senior Member • Posts: 1,032
Re: you've changed the enlargement ratio

Alas it is an example of circular (sic) reasoning.

I find it best in any discussion of DOF to insist that the subject FIRST understand what CoC is and what it means.

After that one can move on to the DOF equations, but if they don't know what CoC is then it's a rather pointless exercise.

Roger Krueger Senior Member • Posts: 2,785
Very nicely explained

Although thick enough concrete can be impervious to a very large hammer.

Kuivaamo wrote:

You still haven't read up on acceptable CoC and DoF, have you? Until then, I doubt any conversation with you will be very fruitful. But I'll try once more anyway.

DoF is not an absolute property "captured by the camera". DoF is a property at the print (or final output on a monitor) and it's all about your perception when you examine that print. As an exercise, click on the "zoom out" button above your example photo three times. Can you still tell that the "Photography" text on the second book is very blurred? No, because things are too small to tell whether it's blurred or not in a larger image. In other words, the letters are now within a field of acceptable sharpness, or DoF. Once you zoom in, you realise that the letters are blurred. They are no longer within a field of acceptable sharpness, i.e. the text is now outside the DoF.

DoF is absolutely linked with final output size. The larger the print and the closer you examine it, the more rigorous your limit for acceptable circle of confusion and thus shallower your DoF will be. It's basic photographic theory, and you'd do well to get to grips with it.

As for 4x6" vs. 20x30", yes, the text on the OOF books will appear blurred in both. But if you look at other details such as bits of dust on the wood, some of them will appear sharp in a 4x6" but not in a 20x30". The exact same principle applies.

OP therickman Senior Member • Posts: 1,305
Re: Preposterous! Oh elk.

Roger Krueger wrote:

DoF is NOT subject space resolution. (which is what you appear to be arguing for here.) You don't get to start redefining terms to fit your arguments.

Are you saying that DoF refers only to a final printed image and not the actual shooting situation?

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coldbivy Forum Member • Posts: 71
Re: Preposterous! Oh elk.

Yes, I think you may be confusing resolving detail with DOF.

DOF is based on the perception of the viewer. As the OOF background is made smaller, either through a smaller print or the viewer moving away from the print, the Lighthouse book will appear to be in better focus. The viewer will no longer have any expectation that they will be able to read the title since it will be so small and the fact that it is OOF will not be visible to them either.

In other words, making the image smaller will in itself reduce the detail that the viewer can percieve and at some point they will not be able to observe the OOF detail.

RicksAstro
RicksAstro Veteran Member • Posts: 3,879
Re: Here it goes again.

You really need to understand the concept of circle of confusion.

Of course you're correct that the laws of physics don't change if you put a different camera behind the lens....you are proving an obvious point rather than disproving a myth.

You say the the DOF calculators are wrong without first understanding the factors they take into account. They assume the entire image is displayed at a given size and viewed from a given distance. When you ignore the rules, you don't prove the calculators are wrong, you prove you ignored the rules.

You are frustrated that other don't understand your point, but I'm afraid they do very well...they just don't agree that it's a myth. You are proving the the laws of physics and getting wrapped around semantics.

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Roger Krueger Senior Member • Posts: 2,785
Re: I think I found out where the DoF confusion is...

therickman wrote:

I thought about it and I think I know where the DoF confusion is (maybe). Let me know if this is where my detractors are coming from...

Here's a scenario:

  • I take a picture of a 6-foot tall man with my 5D and 50mm lens, standing 10 feet away from him at f/2. The tip of his head is at the top of the frame (landscape) and his feet are at the bottom, perfectly filling the image frame.

  • 1 foot behind the man to the left is a fence post, still relatively in focus.

  • 5 feet behind the man to the right is a boy standing. He is obviously OOF.

  • I print the same image at 4X6 and 20X30

Are you saying that in the 4X6 picture the man is 4 inches tall, yet in the 20X30 the man in 20 inches tall, therefore the DoF is 5 times larger? Is that how DoF changes relative to print size? Because no matter how large or small you print, the boy will still have stood 15 feet from the camera and be OOF, you'll never see the pimple on his cheek, or read the message on his shirt.

He's saying if you print small enough the boy will not be obviously OOF. For this example it might well be thumbnail size, but such a size exists for anything not OOF to the point of obliteration.

Conversely, if you print big enough, the fence post will appear OOF. Although this will be limited by the lens+sensor resolving power–at some degree of enlargement even an object in perfect focus will fail the CoC test due to non-focus-related imperfections.

Again, subject space resolution has zero direct relation to DoF. DoF is about resolution on the print (or jpg).

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