Calculating minimum focus distances

Is there a formula to calculate the change in minimum focusing distances of a lens when using a 1.4x extender or if using extension tubes?
Any help would be appreciated.
Thank you.

The min focus distance should be the same with a teleconverter. But extension tubes...that can get complicated. Here's a link to a site where you can enter the data for extension tubes.

http://www.cambridgein​colour.com …tension-tubes-closeup.htm

Quack Me Up wrote in post #17734396
Is there a formula to calculate the change in minimum focusing distances of a lens when using a 1.4x extender or if using extension tubes?
Any help would be appreciated.
Thank you.

No change with the extender, but big changes with extension tubes.

There is a useful formula, which is

1/subj + 1/sensor = 1/F

where
subj is the distance from the front nodal point to the subject
sensor is the distance from the rear nodal point to the sensor
F is the focal length.

(Edited) The minimum focus distance is approximately the value of subj. With modern lenses, at the minimum focus distance, the value of F is normally not at all what the marked F is. That's because the focal length changes while focusing. The actual value of F is usually unknown. The location of the rear nodal point also probably changes. So unfortunately it is not practical to calculate the minimum focus distance using this formula. But usually the values you are interested in are given in the lens manual.

http://www.dofmaster.c​om/dofjs.html

Just manually calculate your selected FL based on your lens FL and the extender used.

InfiniteDivide wrote in post #17734447
http://www.dofmaster.c​om/dofjs.html

Just manually calculate your selected FL based on your lens FL and the extender used.

It looks to me like that link you provided is dealing with depth of field, not minimum focus distance.

f1.4bthere wrote in post #17734402
The min focus distance should be the same with a teleconverter. But extension tubes...that can get complicated. Here's a link to a site where you can enter the data for extension tubes.

http://www.cambridgein​colour.com …tension-tubes-closeup.htm

Nothing personal, but that page is a fail IMHO.

First fail: "Note how extension tubes provide minimal magnification when used with telephoto lenses — which is unfortunately their main weakness."
That is true for quite a few Canon super teles. Not true for 300 F4 L IS which has high magnification. Also not true for other Canon Telephoto zooms. Eg the original EF100-400L had high magnification, the updated 100-400II even higher.

Second fail: Entering correct data for the 100-400 II with 25mm tube. it calculated "newest closest focusing distance" as 2022.8 mm.
The 100-400 II focuses at 980mm bare. According to Canon with a 25mm tube that reduces to 814mm. Regardless if that is true, it definitely reduces with a tube rather than increasing to more than double.

You will need to research particular lens and if no data found experiment yourself.
Working distance is usually a more useful term than MFD as working distance is from front element of lens.

The max magnification of a lens is easy to find and that will be the best way to get a ballpark idea.
Once you start getting into the wide angle range the working distance will often be so small (if not inside the lens) to be too difficult to be useful.

Choderboy wrote in post #17734788
Nothing personal, but that page is a fail IMHO.

First fail: "Note how extension tubes provide minimal magnification when used with telephoto lenses — which is unfortunately their main weakness."
That is true for quite a few Canon super teles. Not true for 300 F4 L IS which has high magnification. Also not true for other Canon Telephoto zooms. Eg the original EF100-400L had high magnification, the updated 100-400II even higher.

Second fail: Entering correct data for the 100-400 II with 25mm tube. it calculated "newest closest focusing distance" as 2022.8 mm.
The 100-400 II focuses at 980mm bare. According to Canon with a 25mm tube that reduces to 814mm. Regardless if that is true, it definitely reduces with a tube rather than increasing to more than double.

I would suggest that the first point is correct if read to understand that it is talking about increase of magnification (particularly in comparison to the same extension for shorter focal lengths). This is borne out by Canon's specs for the 100-400II showing that 12mm of extension increases max magnification from 0.31x to 0.38x and 25mm of extension to 0.46x. In comparison to the dramatic changes you can get with shorter focal lengths this is a change that could correctly be argued to be minimal.

Secondly I would suggest that you are incorrectly assuming that the 100-400II is actually a 400mm lens at MFD. This is quite likely not the case. It could be quite a bit less. The calculator on that site suggests it's around 180mm which to me sounds pretty short but plugging 400mm into the calculator and expecting correct results is an exercise in futility

smythie wrote in post #17734806
I would suggest that the first point is correct if read to understand that it is talking about increase of magnification (particularly in comparison to the same extension for shorter focal lengths). This is borne out by Canon's specs for the 100-400II showing that 12mm of extension increases max magnification from 0.31x to 0.38x and 25mm of extension to 0.46x. In comparison to the dramatic changes you can get with shorter focal lengths this is a change that could correctly be argued to be minimal.

Secondly I would suggest that you are incorrectly assuming that the 100-400II is actually a 400mm lens at MFD. This is quite likely not the case. It could be quite a bit less. The calculator on that site suggests it's around 180mm which to me sounds pretty short but plugging 400mm into the calculator and expecting correct results is an exercise in futility

I disagree with your first point. "which is unfortunately their main weakness" is clearly not true for many telephotos. Particularly for 300 F4 IS L and 100-400II it is considered a strong point for many buyers and reviewers.

Your second point - I agree. Using 180mm results in correct MFD and magnification but I also doubt that is the true focal length at MFD.

I just tried to get an idea how much focus breathing there is with the 100-4500II and there seems to be little.

Choderboy wrote in post #17734815
I disagree with your first point. "which is unfortunately their main weakness" is clearly not true for many telephotos. Particularly for 300 F4 IS L and 100-400II it is considered a strong point for many buyers and reviewers.

Your second point - I agree. Using 180mm results in correct MFD and magnification but I also doubt that is the true focal length at MFD.

I just tried to get an idea how much focus breathing there is with the 100-4500II and there seems to be little.

"their main weakness" would not be in reference to telephoto lenses, but rather extension tubes

smythie wrote in post #17734818
"their main weakness" would not be in reference to telephoto lenses, but rather extension tubes

Well I will continue to use tubes with my 100-400II. 25mm tube for near 50% increase in max magnification is not a weakness to me, it's a useful result.
(I usually use a single 36mm tube for more increase)

Choderboy wrote in post #17734824
Well I will continue to use tubes with my 100-400II. 25mm tube for near 50% increase in max magnification is not a weakness to me, it's a useful result.
(I usually use a single 36mm tube for more increase)

I agree with smythie that the actual focal length of the 100-400mm II is close to 175-180mm at its minimum focus distance. It seems extreme, but these lenses are known to change F by quite a bit when focusing.

As for using extension tubes to get more magnification, it is well known that this works better with shorter focal length lenses. Of course it also works at longer focal lengths, it's just less effective.

As you say, tubes are useful with the 100-400mm II, and one of the reasons it works as well as it does is because the actual focal length is way shorter than 400mm.