DOF Question, non-mathematically

[Ulophot]

Friendly note: Optical and related formulae offered in response to this question may serve others, but will be out of reach for me.

Question: Doesn’t depth-of field follow a widening curve, rather than straight lines, with increasing distance? Is there some region of change in this, perhaps around half the hyperfocal distance (just guessing?)

Reason for question: When I first read Stroebel’s View Camera Technique long ago, I was struck by illustrations that showed the DOF widening out like a trumpet bell as distance increased. As I thought about it, that made sense, since the increase of DOF as distance increases does this, as shown with an online DOF calculator; for instance, doubling the distance each time (with focal length and aperture constant). I later purchased Merklinger’s Ins and Outs of Focus, in which his lines are always straight. Given his elaborate calculations (which I had to skip over), I hesitate to imagine that his illustrations contradict the data. But, leaving aside the special instance I know of, that DOF is reportedly equal before and beyond the plane of focus at very small distances, one of these gents must be more correct than the other, no?

Example: DOFMaster.com shows DOF for a 50mm lens at f/5.6 progressing thus (distance in ft/total DOF in ft): 2.5/0.24; 5/1.01; 10/4.25; 20/19.7 (DOF increases slowly beyond a multiple of 4); 40/205.6; 80/INF (very rapid increase from 10 ft). Hyperfocal distance is 48.5 ft.

[MAubrey]

Sounds to me like Merklinger's illustrations contradict his data. The "trumpet bell" is indeed a more accurate image.

[Jac@stafford.net]

Sounds to me like Merklinger's illustrations contradict his data. The "trumpet bell" is indeed a more accurate image.

And consider the odd DOF of a lens with profound spherical aberration.

[Ulophot]

Sounds to me like Merklinger's illustrations contradict his data. The "trumpet bell" is indeed a more accurate image.

Thank you, MAubrey. I write this belatedly, after the conversation has taken off into an entirely different direction from the specifics of my question -- which is fine; our forum is open an generous in this way. Perhaps your very concise reply is sufficient. I will keep it in mind as I continue my work.

[jp]

DOF varies from lens to lens; some lenses are easily calculated hyperfocal distance. Lenses like Kodak Portrait, all the DOF is behind the focus and none in front. Lenses like the Nikon DC variety you can change the nature of the not-quite-depth of field variably affecting the DOF some. And with a normal lens, not only does depth of field decrease with distance, the focal length increases as bellows are racked out for closeup work, again changing any calculations.

Bokeh is a big buzzword, but it's essentially describing how not quite DOF is imaged.
http://jtra.cz/stuff/essays/bokeh/

If you want a non-math way to understand how depth of field affects your photography, shoot a whole year with one lens. You'll learn subject matter can also affect your image's depth of field as much as any math.

[Pere Casals]

Friendly note: Optical and related formulae offered in response to this question may serve others, but will be out of reach for me.

Question: Doesn’t depth-of field follow a widening curve, rather than straight lines, with increasing distance? Is there some region of change in this, perhaps around half the hyperfocal distance (just guessing?)

Reason for question: When I first read Stroebel’s View Camera Technique long ago, I was struck by illustrations that showed the DOF widening out like a trumpet bell as distance increased. As I thought about it, that made sense, since the increase of DOF as distance increases does this, as shown with an online DOF calculator; for instance, doubling the distance each time (with focal length and aperture constant). I later purchased Merklinger’s Ins and Outs of Focus, in which his lines are always straight. Given his elaborate calculations (which I had to skip over), I hesitate to imagine that his illustrations contradict the data. But, leaving aside the special instance I know of, that DOF is reportedly equal before and beyond the plane of focus at very small distances, one of these gents must be more correct than the other, no?

Example: DOFMaster.com shows DOF for a 50mm lens at f/5.6 progressing thus (distance in ft/total DOF in ft): 2.5/0.24; 5/1.01; 10/4.25; 20/19.7 (DOF increases slowly beyond a multiple of 4); 40/205.6; 80/INF (very rapid increase from 10 ft). Hyperfocal distance is 48.5 ft.

For the DOF math you can see https://en.wikipedia.org/wiki/Depth_...e_DOF_formulae, sections 10 and 11. Also this article shows the limitations that calculation has.

The in focus range is also related to the Circle Of Confusion (CoC) you would tolerate to say an spot is in focus, in general you tolerate larger CoC for larger formats, because an array of reasons we could debate.


DOFMaster.com is a nice tool that's a very good guide for general usage, also you have Apps for the phone, but if wanting refined calculations we need to go to the Through Focus MTF charts, basicly this is plots of actual "sharpness" of a particular lenses as you go outside of the perfect "plane" (not completely flat!) of focus.

Here you will find some TF MTF charts, this would show how actual out of focus roll-off may behave in reality: http://cinematechnic.com/optics/super-baltar

...just to know why that different lenses have different personality for the OOF roll off, but anyway you will find that DOFMaster and other tools are a good guide as an initial guess about what you will obtain, always thinking in that you will have to refine the way you use DOF from the results you are obtaining, comparing to what you want to get.

Bokeh is another thing...

http://jtra.cz/stuff/essays/bokeh/

This is a really nice article !

[mdarnton]

Look at the DOF scale of a 35mm camera lens, and that might simplify it. It shows based on those scales that what you are seeing is that equal movement of the lens in either direction affects near vs far in a way that's very easy to see on those scales, and that the controlling factor in calculating DOF is lens movement, not distance. Distance is an effect, not a cause. This makes sense if you regard it as a circle of confusion issue, inasmuch as the diameter of the cone of light forming a point will be equal as it is equal distances ahead and behind the film, affected by film or lens movement, not subject distance.
https://www.google.com/search?q=dept...ms&tbm=isch&sa

[Dan Fromm]

Stupid question of the day: Why do we use lenses instead of pinholes? Pinholes are generally believed to give infinite depth of field. Fuzzy from near to far. Why isn't that good enough for everyone?

I was raised to believe that depth of field given how much the negative is to be enlarged -- that's what the widely feared circle of confusion defines -- is controlled by magnification and relative aperture and nothing else. It is the same for all lenses given CoC, magnification and relative aperture. Why am I mistaken?

[Pere Casals]

...that depth of field given how much the negative is to be enlarged...

I'd add that it also depends on the CoC we want to tolerate in the print itself !!!

We may want 6 lp/mm on a 2m print to view it with the nose on it, or perhaps we can tolerate 2 lp/mm because the print anyway has to be viewed from at least 1m far...

Why am I mistaken?

I guess this is a rethoric question

It would be hard to find mistakes from you, Dan, possibly some are there, but they are well hidden !

[Jac@stafford.net]

How many discussions regarding DOF do we need? The topic has been beaten to death for at least 100 years.