Equivalent Depth of Field Parameters?
Depth of fields is affected by image magnification and f/stop. For example, if I come close to an object and photograph it, depth of field will be shallower than if I back away. Likewise, switching to a longer lens to enlarge the object again restores the depth of field to the previous (shallower) range.
There is a calculator here:
http://www.cambridgeincolour.com/tutorials/depthoffield.htm
Now, I want to figure out the exact paramters needed to maintain the visual appearance of a consistent constant depth of field, but with while systematically alterting three out of the following three variables between each frame: lens focal length, f/stop, and shooting distance.
According to the calculator:
For a 50mm lens, aperture of f/1.2, and distance of 2m (assuming 1.5x sensor factor), the DOF range will be: 0.079m, with a near limit of 1.961 and a far limit of 2.04m.
The above parameters describe a recent shooting situation I encountered.
I would like to now use the calculator to extrapolate to a longer shooting distance, a smaller f/stop, and a longer lens focal length (while maintaining roughly the same DOF range). Reason is, I'm curious whether or not I could have made a similar image (speaking strictly in terms of DOF) with two other lenses: an 85/1.8 and a 135/2.8.
I will now use guessandcheck while inputting differing parameters and attempt to maintain the 0.079m DOF:
135mm @ f/2.8: 3.56m shooting dist.
85mm @ f/1.8: 2.245m shooting dist.
Now here is a question: would the distribution and degree of blur in the background actually look the same for all of these combinations, or do laws of optics predict subtle differences even though the DOF supposedly remains the same?
Also: the calculator has no way to lock image magnification. Therefore, it's not clear whether or not these combinations would yield equivent image sizes.
I'm still struggling. If someone could help by providing equations and guidance, this would be helpful. Again, I want to keep image size and DOF range constant and vary the shooting distance, f/stop, and focal length to achieve the same DOF range. I've had math up to basic calculus.
Thanks.
Nikon user since 2000

#1. "RE: Equivalent Depth of Field Parameters?"  In response to Reply # 0
Thu 15Sep11 05:12 PM  edited Thu 15Sep11 05:19 PM by benvenisteNow here is a question: would the distribution and degree of blur in the background actually look the same for all of these combinations, or do laws of optics predict subtle differences even though the DOF supposedly remains the same?
A longer lens will have more of the overall DOF in front of the focal point, and less in back of the focal point, but until you start approaching the hyperfocal distance of the shorter lens the difference may not be noticeable.
Differences in Bokeh and Spherical aberration among lenses will also have an impact on perceived DOF.
Also: the calculator has no way to lock image magnification. Therefore, it's not clear whether or not these combinations would yield equivalent image sizes.
Unless you can measure the exact focal lengths at your planned shooting distances, you would have a hard time matching image size exactly. The current 85mm f/1.8 is a rear focus design, which means that focal length changes with focus distance. I don't remember if the old nonAI version was as well.
I'm still struggling. If someone could help by providing equations and guidance, this would be helpful. Again, I want to keep image size and DOF range constant and vary the shooting distance, f/stop, and focal length to achieve the same DOF range. I've had math up to basic calculus.
The formula for magnification at the focal plane is fairly simple:
m = f/(sf), where
m = magnification
f = focal length
s = subject distance from the optical center of the lens.
I use an old copy of f/Calc, which computes magnifications, DOF, and fields of view. The online user manual contains several of the formulae you are looking for. As with most "real world" optical formulae, these are somewhat simplified, but will yield valid results for the shooting distances you are looking at. After all, trying to measure the distance from your subject to the optical center of a lens with millimeter accuracy quite an undertaking in itself.
If you want to photograph a man spinning, give some thought to why he spins. Understanding for a photographer is as important as the equipment he uses.  Margaret BourkeWhite 
#2. "RE: Equivalent Depth of Field Parameters?"  In response to Reply # 0
Thu 15Sep11 06:55 PMAdding to reply 1 changing magnification has a greater effect on depth of field than changing aperture.
Unfortunately dof sources generally ignore two common situations where they are not as accurate as they could be.
In close up many lens do not retain there infinity focal length and aperture  and the amount of change varies with the lens design. In close magnification and dof can be more complex than close up dof tables suggest.
This is an issue where the aperture varies in apparent size when viewed first through the back of the lens and then through the front. If the 2 apparent aperture sizes are measured and the ratio of size difference is applied to m (known in the close up world as the T factor) it is possible to make dof adjustments.
With extreme wide angles (as a guide wider than about 20mm on FX) circles are stretched into ovals in the corner of the frame  reducing resolution along the long dimension of the oval.
It is wise to stop down 1 stop more than "just enough" for sufficient dof to bring the corners up to acceptable sharpness shooting wider than about 20mm on FX.Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.
Len Shepherd 
#3. "RE: Equivalent Depth of Field Parameters?"  In response to Reply # 0
Sun 18Sep11 02:49 PMIn addition to what has already been mentioned by Michael & Len I'd like to add two more things for you to consider.
1. Advertised focal lengths (at infinity) are not always accurate  you'll have to be mindful about that when using calculators.
2. Shape of the aperture determines the shape of the out of focus 'circles'  therefore the number of blades in the iris diaphragm plays a major part in rendering the oof background.
>>would the distribution and degree of blur in the background actually look the same for all of these combinations
The simple answer I can think of is NO!Regards
Dinil
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