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Thread started 10 Feb 2013 (Sunday) 16:55
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Why does FF yield shallower DOF?

 
khwaja
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Feb 11, 2013 19:16 |  #91

sandpiper wrote in post #15599383 (external link)
The actual EV doesn't change, exposure needs to be the same regardless of sensor size. However if you are standing in the same place and wanting the same image (so a given FOV) then you would need to stop down the aperture on the FF camera, relative to the crop camera to give the same DOF. If you were using f/5.6 on the FF you would probably find that around f/3.2 on the crop would give you the same DOF. You would also need to adjust the shutter speed or ISO accordingly of course.

Thanks sandpiper. Forgive me for another newbie question. For given FOV, FL and A value, we are always going to get same DOP irrespective of exposure values (1/4000 vs 1/1000, under exposed vs over exposed) or light conditions right?.


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Feb 11, 2013 19:18 |  #92

JohnB57 wrote in post #15599410 (external link)
Exposure value is the same regardless of format. However, to achieve the same DoF, a 35mm format sensor requires an f number 1.6x that of a 1.6x crop - approximately one and one third stops smaller in aperture.

Thanks JohnB57. For similar EV, crop needs one and one third stops faster shutter speed.


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Feb 11, 2013 19:25 |  #93
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sandpiper wrote in post #15599811 (external link)
What's the point? We all know the DOF won't be different.

If keeping all parameters the same, and only changing sensor size, does NOT change the DOF, why are so many people claiming that sensor size affects DOF?

smorter wrote in post #15599847 (external link)
DOF will be different, you get LESS dof with a APS-C camera in your situation, not more.

Try it - photograph a ruler at a 45 degree angle, same lens, same aperture, same distance to subject, and then output at the same size.

The APS-C camera photo will be more magnified showing LESS dof than the FF image.

I'm not trying to confuse things even more, but in the above situation, FF has MORE DOF than a Crop camera

I contend that YOUR scenario: same lens, same aperture, same subject difference will yields THE SAME DOF. Seems most posters in this thread already know that. You are just plain wrong, here. Oh, and output size has nothing to do with it. Does a 24x36 poster have more or less DOF than a 4x6 drugstore print?


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Feb 11, 2013 19:37 |  #94

TSchrief wrote in post #15599630 (external link)
And this has nothing to do with the fact that you were either shooting from a shorter distance, or using a longer lens? C'mon. Distance and focal length affect DOF. It was not the sensor.

Circle of Confusion is also a factor in the DOF calculation (the hyperfocal distance). Sensor sizes therefore do play a part in DOF calculations due to their part in the CoC value.

A few easy to follow links:
http://www.rags-int-inc.com/PhotoTechStuff​/DoF/ (external link)
http://photo.net/learn​/optics/dofdigital/ (external link)
http://www.lensrentals​.com …ensor-size-matters-part-2 (external link) (scroll down to CoC)


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Feb 11, 2013 19:48 |  #95

TSchrief wrote in post #15599896 (external link)
If keeping all parameters the same, and only changing sensor size, does NOT change the DOF, why are so many people claiming that sensor size affects DOF?

"So many people" ???

NOBODY IS CLAIMING SENSOR SIZE DIRECTLY AFFECTS DOF !!!

I have stated that several times now, we all agree that KEEPING ALL PARAMETERS THE SAME AND ONLY CHANGING SENSOR SIZE WILL NOT CHANGE THE DOF !!!!!

Can you explain how to get the same image (with a FF and a crop) WITHOUT CHANGING THE SODDING PARAMETERS !!!

Sorry for shouting but you seem to be having incredible difficulty understanding the whole "same image" thing.

TSchrief wrote in post #15599896 (external link)
Oh, and output size has nothing to do with it. Does a 24x36 poster have more or less DOF than a 4x6 drugstore print?

It has less DOF of course, if viewed from the same distance.

That's because the circles of confusion are enlarged and you can see the softness sooner. DOF tables are based on viewing a specific sized print at a specific distance (something like a 10x8 at arms length I think, but I really don't worry about knowing the exact parameter as I don't use DOF tables). Change the parameters, print size or distance, and you change the point at which the CoC become noticeable, thus reducing the DOF.

It is the same basic principle as the way pixel peeping your images can make everything look softer, but a shot reduced in size for the web will look pin sharp. The DOF in the web sized image will be much deeper, because it is so much easier to see the pixels becoming softer, as you move further from the focused distance, in the 100% image.




  
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Feb 11, 2013 19:51 |  #96

TSchrief wrote in post #15599896 (external link)
I contend that YOUR scenario: same lens, same aperture, same subject difference will yields THE SAME DOF. Seems most posters in this thread already know that. You are just plain wrong, here. Oh, and output size has nothing to do with it. Does a 24x36 poster have more or less DOF than a 4x6 drugstore print?

Wow. Just wow.


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Feb 11, 2013 19:52 |  #97

TSchrief wrote in post #15599896 (external link)
If keeping all parameters the same, and only changing sensor size, does NOT change the DOF, why are so many people claiming that sensor size affects DOF?

Because we are not so literal minded that we cannot put the question into context of the real world.

In a very technical sense, a larger format will give slightly more DOF when you shoot a scene with the same focal length, focus distance and aperture. This is your point, and no one is disagreeing. So you are correct in a very narrow sense that if you change only the format and not the focal length, aperture or focus distance then the DOF is very nearly the same (larger actually has slightly more DOF). Our point is that this is irrelevant because nobody uses cameras this way.

Your mistake is clinging to a false premise that we cannot change the focal length. But the reality is that when someone asks how format affects DOF, use of longer focal lengths on the larger format is almost a given.

Why do you assume focal length must be the same in this comparison but field of view can be changed?

In reality, focal length is not what photographers interact with when forming a picture, they interact with the field of view.

So what most people in this thread realize is that for the same field of view, focus distance and aperture, a larger format will give less DOF. This is because we have to use a longer focal length on the larger format.

This is very natural to just about everyone and I don't understand why it puzzles you. You have selected focal length arbitrarily as something that cannot be changed in this comparison. That makes no sense in real world photography. Most of the time it is field of view that we will keep the same when we try to take the same picture with two different formats.

Focal length is just where the indicator on my zoom lens happens to be. I don't even know what focal length I'm shooting at a lot of the time when I am using a zoom lens. Field of view is what I see through the viewfinder. It is field of view that I am directly considering when I am framing a photo.


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Feb 11, 2013 19:55 |  #98

Jeffrey the reality is for many applications, e.g. Sports, Birding, Wildlife etc, we are focal length limited

If I have a 600L with me in the field with a 7D and a 5D, I can't magically pull up a 800L when I switch over from the 7D to a 5D

In theory, we would equalise framing to get whatever framing we originally envisaged for the photo - but I don't think this is always practical

Also I don't think TSchrief actually is saying all the stuff you're saying, I don't think he has grasped the concept of DOF if he thinks output size doesn't affect DOF (everything you say is correct - I just disagree with your base assumptions)


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Feb 11, 2013 20:03 |  #99

smorter wrote in post #15600003 (external link)
Jeffrey the reality is for many applications, e.g. Sports, Birding, Wildlife etc, we are focal length limited

If I have a 600L with me in the field with a 7D and a 5D, I can't magically pull up a 800L when I switch over from the 7D to a 5D

In theory, we would equalise framing to get whatever framing we originally envisaged for the photo - but I don't think this is always practical

Also I don't think TSchrief actually is saying all the stuff you're saying, I don't think he has grasped the concept of DOF if he thinks output size doesn't affect DOF (everything you say is correct - I just disagree with your base assumptions)

I know about focal length limited case - but that's pretty much not going to help given the current state of the discussion.

And really - I think most of the time when people are asking about this topic it is hardly the "400mm and wish I had more" case they are really thinking about. It's more within the realm of 17-55 and 24-105 and 35L and 85L kinds of cases.

I shoot with a 100-400L, and sometimes I wind up focal length limited. This discussion seems to have little to do with that case in my mind....I just use the highest pixel density camera I own (which also is the one that can AF with a 100-400 + 1.4X TC) and that's good enough.


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Feb 11, 2013 20:04 |  #100

smorter wrote in post #15600003 (external link)
Jeffrey the reality is for many applications, e.g. Sports, Birding, Wildlife etc, we are focal length limited

If I have a 600L with me in the field with a 7D and a 5D, I can't magically pull up a 800L when I switch over from the 7D to a 5D

In theory, we would equalise framing to get whatever framing we originally envisaged for the photo - but I don't think this is always practical

Yes, but that then means that you are cropping the image to get the framing that you want, and so changing the effective sensor size as you are not using the outer areas of the sensor. In effect you are turning the FF sensor into a crop sensor, if you only use the same area of it.

We are working on the premise of framing the subject as desired. For most people, most of the time, that is possible within the range of focal lengths at their command. When it isn't possible then, of course, we revert to the "if all parameters are the same" scenario, which doesn't give comparable framing (i.e. the bird will look much smaller in the frame of the FF image than the crop image) until you crop the image in post.

That doesn't alter the fact that when you CAN frame an image the way you want, the longer FL required on the FF camera will give shallower DOF.




  
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Feb 11, 2013 20:06 |  #101

OP Here.

Thanks all for your input. It will take me a bit to wade through this. The discussion seems to have shifted a bit from my original Q but I'm OK with that as I'm pretty sure my original question has been answered 80 times over in 80 different ways (Yes, I'm tossing some prop's to Canon_Lover for his 80 coment :-)


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Feb 11, 2013 20:07 |  #102

because you need a larger FL to achieve the same framing.... and focal length is a variable in DOF equations.


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Feb 11, 2013 21:34 as a reply to  @ Charlie's post |  #103

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Feb 11, 2013 22:23 |  #104

n1as wrote in post #15600045 (external link)
OP Here.

Thanks all for your input. It will take me a bit to wade through this. The discussion seems to have shifted a bit from my original Q but I'm OK with that as I'm pretty sure my original question has been answered 80 times over in 80 different ways (Yes, I'm tossing some prop's to Canon_Lover for his 80 coment :-)

:lol::lol:

I have been around long enough to know which topics go in circles page after page.

Your answer was done early, but then you get people arguing semantics, and then there is no hope for you to sift through who is right or wrong even if they are trying to argue the same thing at each other. :lol: :(

Since I am bored with cabin fever myself, I will be generous and give you the final and correct answer to this topic. I suggest you might want to just save yourself a headache and go with what I have posted here. :lol:

PART ONE: What defines DOF

DOF is defined by focal length (100mm for example) + Subject Distance + Aperture

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.

PART TWO: What defines crop factor

Crop sensors are nothing different than taking a full frame image into Photoshop and using the crop tool to take out all you want from the edges. There is ZERO difference in optical parameters when doing this. A Nikon D800 has around a 15MP crop sensor's worth of megapixels crammed into the center, and it can shoot in crop mode, or be cropped for the same image in Photoshop. No difference, at all.

PART THREE: What defines FOV

FOV is defined by focal length, where a 10mm lens is wider than a 200mm lens. Taking an image into Photoshop and cropping it will reduce the FOV or Field of View, or otherwise known as Angle of View. A crop sensor is no different than reducing FOV by the same amount in Photoshop. You can easily crop a 10mm image to the FOV of a 200mm image, but you might be looking at some blurry pixels. :lol:

PART FOUR: Framing and FOV

If you have a full frame sensor and take a picture of a person's head and fill the entire frame with their head, you must get say 10 feet away with a certain lens, right? If you crop it in Photoshop to the FOV of a crop sensor, then there are going to be missing parts of the person's head, right? YES!

If you have a crop sensor, you must BACK up to not cut off the person's ears on the outside of the frame, when taking that same photo, with the same lens, at 10 feet. Right? YES! You must go to, let's say, 16 feet to get their head back in.

PART FIVE: How it all relates!

Now go back to PART ONE. What did we learn there? Increasing your distance to the subject makes the DOF larger and less background blur when you are using the same focal length and aperture, right? YES! In order to not cut off ears and chins and foreheads, you must back up, OR use a shorter focal length (which we learned in PART THREE makes a wider FOV) to get their full head back into the shot. Backing up to increase subject distance AND/OR using a wider focal length increases DOF and reduces background blur, as we learned in PART ONE, right? YES!

So in the end, the only difference from Full frame and Crop, is that for crop, you must either back up or use a shorter focal length lens to get a person's head to fill the shot without cutting off ears. Both of which make the DOF larger AND reduces background blur.

There is really nothing more to it than that.

You can test ALL of this yourself with any camera, crop or full frame, by just using Photoshop to mimic a smaller sensor than the one you are using.


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Feb 11, 2013 22:34 |  #105

Ignoring mathematical equations of course, and the other factors that play into the final result... ;)


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Why does FF yield shallower DOF?
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