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Thread started 10 Feb 2013 (Sunday) 16:55

# Why does FF yield shallower DOF?

Feb 11, 2013 22:38 as a reply to  @ post 15600513 |  #106

What happens to DOF if you stitch two(or more) shots together?

Mark

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Feb 11, 2013 22:47 |  #107

Canon_Lover wrote in post #15600476
PART ONE: What defines DOF

DOF is defined by focal length (100mm for example) + Subject Distance + Aperture+Final image enlargement to display size.

The wider the aperture (f2 is wide, f22 is narrow), closer the subject, and longer the focal length, THE thinner DOF and more background blur. Go to the opposite of any of those factors and you increase the DOF to be less thin and less background blur. The more you enlarge the final image, the less depth of field because you are enlarging the blur, including that which appeared sharp at lesser enlargement.

The bold is mine.

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Feb 11, 2013 22:49 |  #108

TeamSpeed wrote in post #15600513
Ignoring mathematical equations of course, and the other factors that play into the final result...

There are no other factors into understanding the question posed by the OP. None, at all.

Everything the OP needs to understand the relationships of factors is contained in my single post. This is not an exact science, it is an art that takes massaging for desired results.

There is nothing more to add or debate on this exact topic posed by OP. Sorry.

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Feb 11, 2013 22:53 |  #109

RDKirk wrote in post #15600554
The bold is mine.

You are posing something that is completely irrelevant to this thread. That is a debate for more MP vs. less MP, NOT full frame vs. crop DOF. Sorry. Even then, you are also trying to split really small hairs there, and the difference is irrelevant to any of us.

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Feb 11, 2013 22:56 |  #110

Canon_Lover wrote in post #15600572
You are posing something that is completely irrelevant to this thread. That is a debate for more MP vs. less MP, NOT full frame vs. crop DOF. Sorry. Even then, you are also trying to split really small hairs there, and the difference is irrelevant to any of us.

RDKirk is absolutely correct and his statement has absolutely NOTHING to do with the resolution (pixel count) of the camera(s) involved.

Skip Douglas
A few cameras and over 50 years behind them .....
..... but still learning all the time.

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Feb 11, 2013 22:56 |  #111

rrblint wrote in post #15600529

What happens to DOF if you stitch two(or more) shots together?

Nothing, unless you change a parameter I listed in PART ONE.

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Feb 11, 2013 23:07 |  #112

SkipD wrote in post #15600581
RDKirk is absolutely correct and his statement has absolutely NOTHING to do with the resolution (pixel count) of the camera(s) involved.

I see no point in telling you that you guys are wrong and are just making it harder for the OP to understand this issue.

On top of that, the issue you are trying to bring up, makes such a insignificant difference, that it's rather pointless to make any mention of it, even in the proper discussion of MP density. That is an issue of MP density and nothing more. A d800 with 15MP of crop density offers the exact same values across the board (when cropped and enlarged in PS) as a crop camera with 15MP.

My first post is completely correct in what increases DOF and what decreases DOF. What you guys are arguing is how much of that thin part of DOF is magnified in MP. Pixel density does not change the optical formulas of what create more or less DOF.

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Feb 11, 2013 23:17 |  #113

To the OP. You should be set with just reading my post for understanding your question.

This thread has gone off topic and is becoming of no use to you in other regards.

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Feb 11, 2013 23:50 as a reply to  @ Canon_Lover's post |  #114

Thread on a completely different subject:

But IMO the photos included contain an interesting illustration of the affect of cropping on perceived DOF.

Mark

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Feb 11, 2013 23:58 |  #115

Canon_Lover wrote in post #15600611
I see no point in telling you that you guys are wrong and are just making it harder for the OP to understand this issue.

On top of that, the issue you are trying to bring up, makes such a insignificant difference, that it's rather pointless to make any mention of it, even in the proper discussion of MP density. That is an issue of MP density and nothing more. A d800 with 15MP of crop density offers the exact same values across the board (when cropped and enlarged in PS) as a crop camera with 15MP.

My first post is completely correct in what increases DOF and what decreases DOF. What you guys are arguing is how much of that thin part of DOF is magnified in MP. Pixel density does not change the optical formulas of what create more or less DOF.

No, you've missed the key issue of output size/image enlargement. It's nothing to do with the megapixels or pixel density as they've been trying to tell you.

Re-read what RDKirk and SkipD wrote, and then consider this illustration if it still doesn't make sense:

Consider a 6X4 print versus a 60 X 40 print

Obviously when you view it at the same distance, on the 60 X 40 print it will be so much larger that you're able to see the focus transition much more clearly than you can on the 6X4 print

The amount of the "in focus" part will look like it's bigger on the 6X4 print as the smaller size of the print "hides" more of the out of focus area

By definition, DOF is the amount that is perceived to be in focus. Therefore, surely you will perceive more to be in focus on the 6X4 print (since the smaller size is hiding more OOF areas) and therefore, logically, the output size affects DOF

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Feb 12, 2013 01:58 |  #116

smorter wrote in post #15600728
No, you've missed the key issue of output size/image enlargement. It's nothing to do with the megapixels or pixel density as they've been trying to tell you.

Re-read what RDKirk and SkipD wrote, and then consider this illustration if it still doesn't make sense:

Consider a 6X4 print versus a 60 X 40 print

Obviously when you view it at the same distance, on the 60 X 40 print it will be so much larger that you're able to see the focus transition much more clearly than you can on the 6X4 print

The amount of the "in focus" part will look like it's bigger on the 6X4 print as the smaller size of the print "hides" more of the out of focus area

By definition, DOF is the amount that is perceived to be in focus. Therefore, surely you will perceive more to be in focus on the 6X4 print (since the smaller size is hiding more OOF areas) and therefore, logically, the output size affects DOF

Isn't DOF measured in actual distance which covers acceptably sharp plane away from the lense? How do you measure DOF on a print? With a ruler? What should be measured on a 2D image to represent DOF which exists purely in 3D world?

P.S. i find this thread to be quite hilarious to be honest. It seems 99% of the posters understand how sensor size relates to images they take but nobody agrees on how to put it to words (which is not relevant in any way in real world practical shooting).

P.S.S. best idea yet - "Size matters"

6D and glass

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Feb 12, 2013 02:05 |  #117

Canon lover, Ive made it simple,

I drew a picture, and have framed it with with the same lens, focal length, aperture etc, on a a CROP (left) and FF (right).

Notice, the different framing

I then print these out at the same size.
CROP

FF

Yes they are framed differently, but thats what happens when you use the same focal length and aperture on a different sized sensor.

The FF image has more DOF! Because it has been enlarged LESS!

If i were to crop the FF image to match the framing of the crop factor then print it i would get.
(DUPLICATE IMAGE)
THIS IS BECAUSE WE HAVE NOW MADE THE ENLARGEMENT EQUAL

Normally this does not matter because we would either walk closer or use a longer focal length and that will overpower the effect of enlargement in DOF!

EDIT:
Just want to add, pixels just confuse this, the image is 36*24mm on FF and something smaller (22*15mm? or thereabouts?) on a crop. If you are so stuck in the digital age that this confuses you, think of it as film. Its not how many MP or such that matters, its the physical size of the image on the sensor.

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Feb 12, 2013 03:43 |  #118

I hope the attached shot clearly explains why, when a shot is framed the same, FF has less DoF than crop.

In the first example the subject fills the frame of the FF sensor but the head and feet are cropped on the 1.6x sensor.

In the second example the subject is 1.6x further away so the crop sensor now has the same framing as the FF has in the first example.

In the final example the subject is back in his original position but the lens has been changed to give a wider field of view. Once again the framing on the crop is now the same as it was for the FF in the first example.

DoF is the result of magnification and aperture - changing the distance and/or focal length just change the magnification.

In the 2nd and 3rd examples the magnification is lower which means more DoF but in the first the greater magnification on the FF sensor means less DoF. In this example the crop sensor will have exactly the same amount of DoF but the shot isn't usable. You need to change something to get the same shot on the smaller sensor and that something is magnification and, yes, changing that affects the DoF....

HOSTED PHOTO

-- PXL8
1DmkIV, 5DmkIII + 135mm f/2L, 24-70mm f/2.8L, Sigma 35mm f/1.4

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Feb 12, 2013 04:17 |  #119

marsaz wrote in post #15600909
Isn't DOF measured in actual distance which covers acceptably sharp plane away from the lense? How do you measure DOF on a print? With a ruler? What should be measured on a 2D image to represent DOF which exists purely in 3D world?

P.S. i find this thread to be quite hilarious to be honest. It seems 99% of the posters understand how sensor size relates to images they take but nobody agrees on how to put it to words (which is not relevant in any way in real world practical shooting).

P.S.S. best idea yet - "Size matters"

This total misunderstanding of depth of field is probably typical for many (if not most) folks.

Depth of field is NOT (and can not be) "measured". It is calculated with a rather complex formula which uses several assumed variables. The assumed variables include:

• assignment of a standard size for a print that would be viewed by people
• assignment of the standard distance from which they would view the standard sized print
• an assumed standard visual acuity of the people viewing the print
• how large a blur the viewers of the print would be likely to interpret as a point vs interpret as a blur
Then, the assumed minimum blur size is reduced to the size that it would be on the film or the in-camera image on a digital sensor to get the "circle of confusion" variable in the formula. Then they have to deal with the optics that involves focal length, distance to the "subject", and more.

I don't pretend to fully understand the mathematics involved in the formulae, but I understand enough to know why the camera format (size of the film frame or digital sensor in a camera) is one very important factor in calculating depth of field.

Skip Douglas
A few cameras and over 50 years behind them .....
..... but still learning all the time.

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Feb 12, 2013 05:38 |  #120

Canon_Lover wrote in post #15600572
You are posing something that is completely irrelevant to this thread. That is a debate for more MP vs. less MP, NOT full frame vs. crop DOF. Sorry. Even then, you are also trying to split really small hairs there, and the difference is irrelevant to any of us.

As far as the question the OP asked, you are pretty much correct. As you and I and Skip and Smorter and RDKirk and most others have pointed out, larger formats require longer focal lengths for equal field of view, and the longer focal lengths make for shallower depth of field.

Even the OP understood this (in his first paragraph) and so that is clear.

But one other thing (and which RDKirk is pointing out) is that the DOF itself is a funny thing in that it is a really fuzzy, subjective thing that we can calculate with great precision. This precision makes people forget just what kinds of assumptions we are making.

DOF is the very subjective appearance of what is and what is not in focus when we view the output image. If this is a print, then it serves to know that the bigger the print we make or the more closely we view the print, the easier it is to detect fuzziness in the areas that are near the plane of focus. So for any one given image, the bigger we print or the more closely we scrutize the print then the smaller the DOF. I can print off an image at 4x6 and again at 20x30 and the latter print of the exact same image will appear to have much less DOF to anyone who views it.

So in a DOF calculator, the DOF appears to be a strict mathematical formula that is a function of the focal length, aperture and subject distance. But you forget that the CoC is also a term in this calculation.

The CoC itself is a not so strict function of how big the sensor or film is, how big the final print is, how closely we are holding the final print from our face, and how good the eyesight of the person judging the DOF is.

All this means is that larger formats actually do slightly increase DOF because of the effect on the CoC, but that the opposite effect of longer focal lengths used with larger formats swamps the effect.

Finally - DOF existed as a concept before the invention of digital cameras. The magnification to print size we are discussing here for the CoC is completely unrelated to the pixels. It's a physical thing, like where I magnify an image that was 24mm x 36mm on the sensor (or film) to create a print that is 200mm x 300mm in my hand.

My personal stuff:http://www.flickr.com/​photos/jngirbach/sets/
I use a Canon 5DIII and a Sony A7rIII

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Why does FF yield shallower DOF?
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