VERY technical question about depth of field (you've been warned)

[Alex_B]

ok, maybe I made sloppy use of the word sharpness here. my apologies.

DOF in a sense refers to what appears sharp, not blurry, it refers to the detail on objects being resolved or not. I agree that way of phrasing it is not very precise.

So my basic point is, in order to define DOF via the circle of confusion on the recording medium (be it film or a sensor), the size of the recording medium (format) has to be taken into account. For a smaller format, the circle of confusion will be smaller since it will be enlarged more than a larger format.

On 35mm film this circle might be .03mm, then it will be .019mm on a 1.6 crop sensor. This means it gives the same sized circle in the final image on screen or print.

At least this is my understanding of this.

You never view the image at its original size, you always enlarge.

 

[Sark]

Originally Posted by Senor Hound
I'm not talking about field of view, I'm talking about depth of field. Why does a digital camera have a shorter depth of field when using the EXACT same lens and aperture as a film camera?

Because to get the same final print size (or size on your computer screen or whatever medium we talk of), you have to enlarge more if you record on a smaller sensor than on a large one (or film).

The above is why I had some confusion (and disagreement) with your reply. You seem to have suggested print size and image sharpness effect Depth of Field. I dispute this. DOF relates to sharpness I agree, but image sharpness is not the same as DOF.
The original post was discussing the relevence of sensor size on DOF. I would still argue that sensor size has no effect on DOF.

Sark

 

[Alex_B]

The original post was discussing the relevence of sensor size on DOF. I would still argue that sensor size has no effect on DOF.

And I would say it has.

And that effect of sensor size alone points just the opposite way than the effect of focal length:
So when you want the same FOV, you will need a longer lens for the larger sensor. The longer lens will reduce your DOF.

And this effect is larger than the sensor size effect. So overall, if you try to take the same image, same composition, you need the same FOV, meaning a longer lens for the larger sensor, meaning a net reduction in DOF.

All assuming the same f-stops being used of course.

 

[Alex_B]

He, I like this discussion, but I have the feeling we are confusing those watching us. Where is Helen when you need her??

 

[Sark]

Quote:
Originally Posted by Sark
The original post was discussing the relevence of sensor size on DOF. I would still argue that sensor size has no effect on DOF.

And I would say it has.

And that effect of sensor size alone points just the opposite way than the effect of focal length:
So when you want the same FOV, you will need a longer lens for the larger sensor. The longer lens will reduce your DOF.

And this effect is larger than the sensor size effect. So overall, if you try to take the same image, same composition, you need the same FOV, meaning a longer lens for the larger sensor, meaning a net reduction in DOF.

All assuming the same f-stops being used of course.

I guess it depends on whever you are discussing DOF from an optical point of view, or from an image point of view. I think it can get confusing if you are refering to DOF in relation to the final image. There are so many factors that determine the quality of the detail in both the intended out of focus part of the image and the intended in focus part of the image.

I do agree it's relevent and your argument may be right from a practical point of view, but technically speaking, DOF is still a factor of the focal length rather than the sensor size. The fact that sensor size can effect focal length choice doesn't directly infer sensor size effects DOF. Two identical lenses at the same f-stop will always produce the same DOF.

Sark

 

[Alex_B]

OK, I just did some googleing and want to cite some things:

"An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot."
[http://www.cambridgeincolour.com/tutorials/depth-of-field.htm]

And if you use their DOF calculator and just vary the format size while all else is fixed, and you will see the effect of format size.

 

[Sark]

Will check this out, but must go for now, but will hopefully get back to this. Would be nice to read some input from others. I guess they're all on the Nikon v's Canon thread.

Sark

 

[Alex_B]

I guess they're all on the Nikon v's Canon thread.

:lmao:

 

[RyanLilly]

I guess all that has been said above makes pretty good sense with the clarification. Good topic, I enjoyed it.

Holy crap Alex, I replied to this topic yesterday, went to sleep, woke up today, and your still online talking about it! I guess thats all in the same day for you, but man, you a trooper, and its not even time for diner yet! :mrgreen:


I'm sure Helen and dispel all of our deep thinking and ramblings with a swift easy to understand reply.
Its still early in New York, I guess we can giver her some time to finish her coffee.

 

[Alex_B]

I guess all that has been said above makes pretty good sense with the clarification. Good topic, I enjoyed it.

Holy crap Alex, I replied to this topic yesterday, went to sleep, woke up today, and your still online talking about it! I guess thats all in the same day for you, but man, you a trooper, and its not even time for diner yet! :mrgreen:

Right you are

I'm sure Helen and dispel all of our deep thinking and ramblings with a swift easy to understand reply.
Its still early in New York, I guess we can giver her some time to finish her coffee.

That is my fear .. errm, hope I mean!

 

[Senor Hound]

OK, I just did some googleing and want to cite some things:

"An acceptably sharp circle of confusion is loosely defined as one which would go unnoticed when enlarged to a standard 8x10 inch print, and observed from a standard viewing distance of about 1 foot."
[http://www.cambridgeincolour.com/tutorials/depth-of-field.htm]

And if you use their DOF calculator and just vary the format size while all else is fixed, and you will see the effect of format size.

Maybe you are arguing the same point. Perhaps he's saying the depth of field would be the same if there was the same percentage of enlargement for each medium. For example, if you made the film photo a 12x9 and the digital 8x6, they would have the same depth of field.

In other words, what I think he's saying is its not the film or sensor that's changing the depth of field, its the resulting extra enlargement necessary to compensate that's the difference.

Then again, maybe he's not. I will say as the OP of this thread I am very thankful to Alex, because he taught me something (that isn't easy to teach, also), and because he's just a cool cat from what I know of him

BTW, I don't know if Sark is wrong, but I do know Alex is right, now that he explained it to me and I've looked at other sources to help clarify (and consequently confirm).

 

[Sark]

Last night I read the Depth of Field Tutorial Alex linked and began to doubt some of my own preconceptions. This article suggests Focal length has no effect on DOF. There is a link to further reading matter that claims to prove this fact. However, the DOF calculator clearly contradicts this by changing the DOF when altering the focal length. DOF is clearly shown to decrease with an increase in focal length. I then re-read the article and it appears that this statement is based on the following: “If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length”. I don’t disagree with this, but you can’t make a statement like “focal length has no effect on DOF” and then base that statement on more than one variable.
Photographer don’t, or rarely at least, change focal length to retain the same FOV.

If you accept Depth of Field relates to how much of the foreground and background of an image appears in focus (or out of focus), then how much of the image circle your sensor uses doesn’t change the foreground and background DOF. The DOF is the same across the entire image circle. It is therefore the same across the entire sensor area, regardless of sensor size. Only the Field of View changes with changes in sensor size. The sensor does no more than crop the image circle.

If you trim a piece of 35mm film so that it is the same size as an APS sized sensor.it won’t change the DOF of the remaining smaller piece of film, and trimming/cropping the film area is all you’re basically doing to the image circle when using a smaller sensor.

If you want to introduce image sharpness into the equation, then print size becomes relevant, but this is just another variable. Ultimately, sensor size cannot change the information on the image circle, and therefore has no effect on DOF within that image circle. Unless I'm wrong of course?:hail:

Sark

 

[Alex_B]

If you accept Depth of Field relates to how much of the foreground and background of an image appears in focus (or out of focus),

But "appear in focus" always relates to print size and viewing distance.

 

[Alex_B]

“If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length”. I don’t disagree with this, but you can’t make a statement like “focal length has no effect on DOF”

That statement has to be seen in the context it was given. And it in fact has little to no effect if you make the subject occupy the same fraction of the viewfinder and given the same format of the recording medium. Because then, if you change the focal length, you will also change the distance to the subject, and the effect of using a shorter lens is compensated by the effect of getting closer to the subject by the same fraction.

If, however, you chose a different focal length in order to maintain the same FOV on a smaller or larger format of the recording medium, then this implies that you remain at the same distance to the subject. Hence DOF will vary.

 

[Alex_B]

Photographer don’t, or rarely at least, change focal length to retain the same FOV.

This is exactly what you do. This is why with your medium format camera you have a totally different set of lenses than with your 35mm small format. And this is also why zoom lenses made in particular for crop sensors, are often shorter than their 35mm counterparts.

Assuming that you as a photographer compose the shot, then you will control two things composition wise. And those are perspective and framing.
Now, the perspective depends on the distance to the subject and on the direction your camera points at, and the framing depends on FOV and again the direction.

So in order to get the same composition on two different formats, you will need two different focal lengths respectively so you can keep your FOV identical.

Of course I am assuming that the photographer controls his composition, and does not just let it happen randomly