QUESTION ABOUT DOF

DreDaze wrote in post #15922023
but if you took a picture at 50mm, f2 and 6 ft away on a crop, and on a FF camera...would the larger sensor size really give you shallower DOF...

no...it'd actually give you more DOF...

^
OK folks, here goes...

• Shooting 50mm FL on APS-C from 20' away a yardstick target, our frame captures an area of 6' x 9' and a 3' tall yardstick is 50% of the height of the frame. We magnify the 14.9x22.3mm frame by 13.6x to make the 8x10" print (the standard by which DOF calculators assume). The yardstick is 4" high on the 8" print. At f/2 our DOF zone is from 18.6' - 21.6', or 3.0' deep.
  
• Shooting 50mm FL on FF from 20' away, our frame captures an area of 9.5' x 14.3' and a 3' tall yardstick is 32% of the height of the frame. We magnify the 24x36frame by 8.5x to make the 8x10" print (the standard by which DOF calculators assume). The yardstick is 2.56" high on the 8" print. At f/2 our DOF zone is from 17.8' - 22.7', or 4.9' deep.

Why do people keep bringing FF versus crop into this thread?

This thread is about the same camera with two different lenses.

Same Camera!!!

Heath wrote in post #15922189
Why do people keep bringing FF versus crop into this thread?

This thread is about the same camera with two different lenses.

Same Camera!!!

I was responding to the point raised...which was addressing the point 'format size does matter'.

Yet, from the standpoint of two lenses on APS-C body 17mm f/4 is 17mm f/4, regardless if the lens has the EF-S Canon tag or the D-II Tamron tag vs. EF Canon tag or D-I Tamron tag.
EF-S or D-II only have meaning in the context of FF, simply because they cannot fill the larger FF frame area.

gasrocks wrote in post #15921400
I was always taught (and I teach) that the 4 factors are: 1 - Aperture (the one everyone figures out.) 2 - working distance. Closer = less DOF. Reason macro is such a difficult thing for many. And astrophotography does not care about DOF. 3 - MM of the lens. Teles have less DOF, wides more. 4 - Size of sensor (negative). Larger = less DOF, smaller more. Pocket P&S cameras typically have a whole lot of DOF. The reason Ansel Adams shot at f/90. Gene

We've gnawed over this bone before, and doubtless will again.

What I have put in bold is quite false. It is backwards. The larger the sensor, the greater the DoF, because less enlargement is needed to reach the same print/display size, so any blur present is less pronounced. This is readily verified using any DoF calculator.

The ONLY way your statement can be considered at all valid is if, quietly, behind the scenes, you secretly change the focal length to match compositions, and don't mention this to any one, on the basis that it is obvious to everyone and naturally assumed by even the greenest novice. If you do that, the DoF indeed is shallower, but it is not because of the change in sensor size, as you state, but because of the unmentioned change in focal length.

Argue pragmatics all you like, but there is no reason not to present the physical, optical facts, and it is a positive disservice to provide erroneous information.

xarqi wrote in post #15922357
We've gnawed over this bone before, and doubtless will again.

What I have put in bold is quite false. It is backwards. The larger the sensor, the greater the DoF, because less enlargement is needed to reach the same print/display size, so any blur present is less pronounced. This is readily verified using any DoF calculator.

The ONLY way your statement can be considered at all valid is if, quietly, behind the scenes, you secretly change the focal length to match compositions, and don't mention this to any one, on the basis that it is obvious to everyone and naturally assumed by even the greenest novice. If you do that, the DoF indeed is shallower, but it is not because of the change in sensor size, as you state, but because of the unmentioned change in focal length.

Argue pragmatics all you like, but there is no reason not to present the physical, optical facts, and it is a positive disservice to provide erroneous information.

^
see my examples in post 16
But, before digital cameras, few people tried to shoot with a 100mm large format lens mounted on 135 SLR simply because you couldn't mount the lens, like you can with FF lenses on APS-C camera!

Yet 'Larger format = less DOF' is indeed true, assuming you use a lens FL for each format which obtains the same FOV from the same camera position.

• APS-C with 45mm lens (3 * short dimension of frame), captures area of 6.6' x 9.9' at a distance of 20', with DOF zone of 17.4' at f/8
  
• 4x5 format with 288mm lens (3* short dimension of frame), captures area of 6.7' x 8.4' (different aspect ratio frame) at distance of 20', with DOF zone of 2.1' at f/8
  

for a gien size of output, DOF, depends only on 2 factors
1: F stop (focal length/aperture diameter). the large, the more dof
2: Magnification factor (size of image on sensor to size of objecte) the less, the more DOF
let go in stops of controversy
1 is not controvertial.
2: dof is in fact not a function of focal lenght or distance to subject, although both of those can change dof if you keep the other constant. However, for the same framing of the subject, unless you are in macro range, dof is independent of those two factors. so you can shoot with a 50mm, then double your distance and shoot with a 100mm lens, and you will achieve same dof on your subject, and same magnificaiton (but different perspective of course).
Now sensor size is a funny thing. sensor size is no more of a factor in dof than is cropping an image, except that interestingly, a larger sensor will actually require less maginification in its final output which I discuss later, Hence, it has a more favorable circle of confusion for giving you more dof. but I digress. I think it is easier for people to think in terms of sensor size and thats fines, since with a smaller sensor, to get the same framing, you need less magnification (so it would fit on the smaller sensor) so you end up with more dof.
the final Factor, is output size, and thats where circle of confusion comes in. the DOF calculators assume an 8x10 print size. for a larger output (say 10 feet by 8 feet) at the same viewing distance (impractical) dof would be less. but given we usually view objects at same magnificaitons (I.E, you wont be pressing your eye against a 20x30 print), the general circle of confusion used for 8x10 print is good enough.
anyways, hope I did not add to the confusion. If you want to use focal lenght, and distance to subject, and all that stuff, thats cool, as long as it works for you.