Calculating DOF for macro work

[DanK]

Apologies, in that I think someone may have answered this question during the past year, but I haven't been able to find the thread.

I have never used a rail in macro work, but a comment by Grahame in a recent thread got me thinking about it again. People who use them often say that one can get more precise control of focusing for stacking using a rail than changing focus manually. However, to take advantage of that, one would need to know how much to change focus in a given shot, which in turn requires accurate calculation of DOF. For example, the Kirk rail moves 0.042" per revolution. That's not useful information unless you know how far the camera should move. In adjusting focus manually, I have a sense of how much I have to rotate the focusing ring to make sure I have overlapping focus, but I am probably shooting more shots than I need, and I have no corresponding sense for a rail.

So, my problem is calculating DOF given the change in focal length and effective f-stop at close to minimum focusing distance. The tutorial on this site indicates that for a 50mm macro lens, effective f-stop increases by two stops at minimum focusing distance. It's slightly more for longer lenses. The tutorial notes that this increase in effective f-stop has all the effects of a normal change in aperture, including increasing depth of field. Here is the explanation:

*Technical Notes: The reason that the f-stop changes is because this actually depends on the lens's focal length. An f-stop is defined as the ratio of the focal length to aperture diameter. A 100 mm lens with an aperture diameter of 25 mm will have an f-stop value of f/4, for example. In the case of a macro lens, the f-stop increases because the effective focal length increases

This implies that if one uses a standard tool like DOFmaster to estimate DOF, one would need to make two adjustments: increase the aperture by two stops, hence increasing DOF, AND double the focal length, hence decreasing DOF. The latter effect is huge at these distances. Here are some calculations of total DOF from DOF master, starting with a nominal f/8 with a 100mm macro lens at 32 cm (roughly the minimum focusing distance for the Canon 100mm L lens I usually use):

uncorrected: .34 cm
f-stop corrected: .68 cm
f-stop and focal length corrected: .09 cm

Anyone know if there is a flaw in my logic? It would be nice if there were, since the final estimate--the one I think is correct--is only 13% as large as the second.

Many thanks

Dan

[Mark von Kanel]

This might help Dan

DOF calculator

My gut feeling is that you will find that some experimentation is required depending on your camera/lens combination. if you use nikon your in luck in that a program called control my nikon will automate this for you!

[ashcroft]

I have a rail, but rarely use it, due mostly to the fact I'm probably too lazy. One thing that I do find very helpful is that I have very good eyesight for my age (62) with no need to wear glasses. I can therefore manually focus with some accuracy, and I find that very useful for macro and close-up work. I also use Helicon stacking software which has a remote shooting feature that automatically adjusts the DOF intervals for you. You can even fire flash with it and set it to wait for the flash to recycle. See this video.

[FrankMi]

I would test by shooting a series of images and noting the distance between shots, then look at the results on the screen. The calculations can give you a starting point, but no matter what the calculations tell you, it's what you get for results that will determine what works for your situation.

[DanK]

Thanks, all. Mark, that may have been the thread I vaguely remembered. I tried the second of the links John provided. it appears to implement neither of the corrections in my note; it gives very close to the uncorrected estimates from DOFmaster. I couldn't get the first of them to work. My browser refused to let Java run it, even after I did what answered the dialogs in the affirmative.

Rob--Thanks for the link to the video. I have been thinking about using Helicon remote. It too seems to require some trial and error at first to set the "custom" parameter for a given lens. Still, over time, it would save me a lot of effort. I find that vibration from the wood floor where I do my work can create enough motion to ruin the long-exposure shots I use for stacking, so I end up spending quite some time standing stock-still, waiting for everything to settle and each exposure to be taken. With Helicon remote, that would be less of an issue.

Dan

[george013]

The DoF-calculator is based on the lens-formule. There is no "effective f-stop" involved.
More important is diffraction. The used calculator is for the ease using the f-stop, but it should be the effective f-stop. In macro that makes a big difference.
George

[DanK]

The DoF-calculator is based on the lens-formule. There is no "effective f-stop" involved.
More important is diffraction. The used calculator is for the ease using the f-stop, but it should be the effective f-stop. In macro that makes a big difference.
George

George,

thanks. This takes us back to my original question. If one has only DOF formulae that do not take into account the change in focal length in 1:1 macro, then one would need to correct the results. The question is whether I am right that one would need to correct them both for effective f-stop and for the change in focal length that causes the change in effective f-stop. The latter correction is in the opposite direction, and in the several examples I tried, it was far larger than the former.

If I am right, diffraction is a somewhat different question. Effective aperture will not equal nominal aperture even if the aperture is large enough that the lens is not diffraction-limited. However, it is the case that diffraction is determined by effective aperture, and therefore, it ought to be more of a problem in macro work than nominal aperture suggests.

However, my experience (consistent with what Colin has often posted here) is that the impact of diffraction is quite minor unless the aperture is far smaller than the point at which the lens becomes diffraction-limited. Here is an example, which I took years ago, when I was starting on macro. It was taken with a Canon XTi with a 60mm lens at f/20. That means that the effective aperture was approximately f/40. I hung this on my wall, printed at 8 x 10 (approximately A4). It looked just fine.



I routinely shoot bugs at nominal f/13, with a 100mm lens and a 36mm extension tube, which means an effective aperture greater than f/26. Here is one example:



Dan

[george013]

George,

thanks. This takes us back to my original question. If one has only DOF formulae that do not take into account the change in focal length in 1:1 macro, then one would need to correct the results. The question is whether I am right that one would need to correct them both for effective f-stop and for the change in focal length that causes the change in effective f-stop. The latter correction is in the opposite direction, and in the several examples I tried, it was far larger than the former.

Only change the focal length, if you know. If you use the "effectieve f-stop" you will get wrong answers. The formule is based on calculating the diameter aperture from f-number and focal length.
Somewhere halfway, http://www.bythom.com/105AFSlens.htm
There is somewhere a list with macrolenses giving the changed focal length at 1:1 magnification. I don't know where anymore.

George

[george013]

If I am right, diffraction is a somewhat different question. Effective aperture will not equal nominal aperture even if the aperture is large enough that the lens is not diffraction-limited. However, it is the case that diffraction is determined by effective aperture, and therefore, it ought to be more of a problem in macro work than nominal aperture suggests.

I think for most macro photographers it's easier to accept some diffraction and gain more DOF. I'm not a macro photographer. It's up to you. But you can always try it out, one setup and two photo's with different diafragma.

The diffractiontabel on this site is based on the f-number. So if you are very nearby or using extension-tubes, you should correct that.

George

[DanK]

Here's an interesting bit of information (interesting to me, at least). Following Rob's suggestion, I just downloaded a trial of Helicon Remote. It has a DOF calculator. Its internal DOF calculator must be making the appropriate adjustments, since it will control focusing to take the required number of images for any given stack, given the aperture and FL of the lens. However, the user-accessible DOF calculator does not apply any adjustments; if you enter the nominal f-stop, it returns the same number as does DOFmaster.

[xpatUSA]

Hi Dan,

That calculator I sent you is still there:

http://kronometric.org/phot/xfer/4DanK/DOF 2.2.ods

It is intended as a macro calculator and calculates both DOF and distance using the magnification, f-number and and marked focal length as inputs. It also gives the "effective" focal length from f ' = f.(1+m).

Where f = marked focal length and m = (an object's image dimension)/(an object's real dimension)

It should help in your quest.

[TonyW]

How do you define the focal length of a lens that is not focused at infinity?

[george013]

Here's an interesting bit of information (interesting to me, at least). Following Rob's suggestion, I just downloaded a trial of Helicon Remote. It has a DOF calculator. Its internal DOF calculator must be making the appropriate adjustments, since it will control focusing to take the required number of images for any given stack, given the aperture and FL of the lens. However, the user-accessible DOF calculator does not apply any adjustments; if you enter the nominal f-stop, it returns the same number as does DOFmaster.

You tell the program the limits of the image you want to be sharp. The program calculates the dof and then calculates the number of pictures you need for the total distance. Lens and camera must be able to produce some variables the program can use.
But you have still to be sure about the right focal length. It's a property of your lens.
Good luck with it.

George

[chauncey]

I almost always shoot tethered using Canon Utility Software which allows computer controls of
camera settings, including focusing. It uses "arrow buttons" to control the focus adjustments
(<<<, <<, <, >, >>, >>>).

I spent an afternoon using a ruler as a subject determining the focus movement of those arrows at
various f/stops and distances from subject, was a real PITA.

Came to the conclusion that using f/13 gave me the best image IQ and allowed me to use the two
arrow button for all focus adjustments...regardless of subject distance. It should be noted that getting
close to the subject requires more slices as DOF is lessened. The number of slices is determined by
eye based on that live view image on my monitor.

We all have different techniques...I'm very satisfied using PS CC for all my stacking and merging.

[DanK]

Hi Ted,

Thanks. Sorry, I had forgotten about that.

I just tried Helicon Remote. I set up a clip that is approximately 2.6 cm deep, with the near end roughly 32 or 33 cm from the focal plane, so not much beyond MFD for my 100mm lens. I then set the camera to a nominal aperture of f/8 and manually set the nearest and farthest focal points in the software and let it calculate how many images to take to create a stack. It took 14. Thus, it took one image per .19 cm. Going back to the top of the thread, this is about twice the DOF I calculated with both corrections (effective aperture and focal length), but far smaller than the DOF with no correction or with correction only for effective aperture.

The software allows you to adjust the algorithm for calculating intervals to best fit your specific lens. I haven't yet taken a series with enough detail to allow me to do that. However, given that all of these calculations are approximations, this result is broadly consistent with the notion that you have to correct for effective focal length, if you can find out what the actual effective FL is for your lens at 1:1 magnification. The software is clearly not using the unadjusted DOF estimate that its own dialog box gives.

George--the software reads the (nominal) focal length of the lens and sets it on its own. It appears that the way you would adjust for variations in the final focal length is by means of the "correction factor" adjustment, which does not use that scale.

[Stagecoach]

Dan,

From what I recall the FL for my 105mm at MFD of 1:1 magnification ratio is 70mm.

Another variable here is what 'overlap' factor has the software used in deciding upon the divisions? Out of interest are you able to adjust this?

[DanK]

Dan,

From what I recall the FL for my 105mm at MFD of 1:1 magnification ratio is 70mm.

Another variable here is what 'overlap' factor has the software used in deciding upon the divisions? Out of interest are you able to adjust this?

Grahame,

Yes, you can adjust the overlap. The correction factor simply shrinks or enlarges the depth between images. They have a help page that explains how to use it: http://www.heliconsoft.com/HeliconRe...RECTION_FACTOR. I haven't tried it yet.

Dan

[Stagecoach]

Thanks Dan,

I have just been watching the Helicon tutorial. I presently have the 'ControlMyNikon' programme but will download the Helicon Remote to see if there's any advantages.

[george013]

Thanks Dan,

I have just been watching the Helicon tutorial. I presently have the 'ControlMyNikon' programme but will download the Helicon Remote to see if there's any advantages.

ControlMyNikon doesn't do the stacking, only the photo's. I understood that Helicon does the stacking also.

George

[DanK]

Same company, different software. Helicon Remote controls focus to photograph the stack. Helicon Focus does stacking. I'll be using Helicon Remote with Zerene Stacker.