F/Stops and DOF

[out n about]

Hi, this probably is a dumb question but here goes anyway, recently I read that all f/stops or aperture settings let in the same amount of light at each stop regardless of the brand of lens or size of lens ie F8 on a nikor 105 macro lens allows the same light in as f8 on a 70 - 200mm canon lens. So here is my question my brother in law owns a Ziess 100 mm macro lens which goes from f2 to f22 and we own a nikor 105 mm and it starts at f2.8 and finishes at f40, so although at F8 both lens supposedly allow the same amount of light in IS the depth of field the same at f8 on both lens or is F8 on the Ziess equal to f16 or there abouts on the nikor or because the nikor goes to f40 does that give it an overall greater DOF.

[ameerat42]

Hi Gayle and Colin.
I'd like to address some of the points raised in your post, so, in point form:
1. "...all f/stops (sic) ... let... same amount of light...regardless..."
This is not an exact statement. Rather, all f-stops theoretically allow the same intensity of light to pass. Intensity is "amount of light per unit area".
Take f/8 on an f=105mm lens, then f/8 represents ~13mm of aperture. But on an f=200mm lens, f/8 represents ~26mm of aperture. So, more light
gets in through the larger aperture than through the smaller one. What remains the same is how strong that light is when it hits the sensor, ie, its intensity.
Furthermore, the intensity can be attenuated by the amount of glass, how the light is dispersed, and other factors.

2. The question about depth-of-field at the same f-stop.
Given that DOF depends, amongst other things, on the actual aperture of lens you're using, then from the example above,
the DOF will NOT be the same at the same f-stop for two different lenses of different focal lengths.

There are charts and such about this, but that's the nub of it.
Am.

PS: NOT a dumb question.

[bitsnpieces]

Perfect - I understand this stuff, but would have no idea how to phrase it. Thank you for the simple explanation ameerat42 :)

[arthurking83]

Also to add to Am's reply.

the difference in DOF @ f/8 between a 100mm lens and a 105mm lens is minimal, to negligible.

The lens itself may(will) have light transmission differences(to other lenses of similar types) .. ie. T-stops.
So while the Zeiss may be @ f/2.8, it's T-stop could be something like f/4(don't know .. just an example).

Also, as lenses focus closer and closer, you will get focal length shortening. Very few lenses don't do this nowadays.
Some of those lenses can also misreport the real aperture value as you focus closer, others will report the actual aperture value.

I remember that Canon's don't adjust for aperture as you focus closer, Nikon's(and AFAIK) most third party lenses do(not sure about Zeiss)

So for the Canon 100/2.8 if you focus to MFD the camera may still indicate an aperture of f/2.8, where the reality could be something like f/4.8 or so.
The Nikon 105/2.8 will display the physical aperture(ie. I think f/4.8), even tho you haven't adjusted it at all.

if you have the Nikon, it will go down to a small as f/57(at MFD) and will stop at f/32 at infinity.

At f/8 you probably won't notice this, but at wider apertures vignetting also has an impact on the intensity of the light through the iris and onto the sensor.
So two lenses at f/8 may not produce the same (overall) exposure because the resultant image is affected by various degrees of vignetting.

So(to answer part of the question), in theory two different lenses set to the same aperture with the same scene(eg. say a plain white wall) should produce the same exposure no matter the focal length. This assumes no vignetting and the same T-stop from both lenses(which can happen).
of course in real life you don't shoot plain white walls, and it's almost impossible to get the same framing from two disparate focal lengths(ie. 70mm to look the same as 200mm).

One of DxO's strengths as a review site for lenses is that they test for T-stops of the lenses when they test. it gives you some idea of the differences you may get in any exposures if you're using the same camera/lens settings across different lenses.
To answer another part of the question, although not directly ..

Don't use f/40 on the Nikon 105VR!!
I meant to say Don't use f/40 if you value sharply rendered fine detail!
I've used f/32 which is about as far as I'd stop down, but the images require a fair amount of USM/highpass/sharpening of some sort to recover fine details.

(Haven't yet tested the 105VR thoroughly on the D800 yet to see how low I can go with aperture .. did my tests on the D300 early on(ie. years ago).

As to the DOF from the Ziess compared to the Nikon .. it depends.
It depends on how the Ziess is designed. If it reduces focal length as it focuses closer and doesn't report that, then the comparison is hard to judge without physical testing.
We know that the Nikon 105 does both report AND report an accurate aperture, so with that in mind you can easily test with side by side comparison images.

Something to note. The Ziess doesn't focus down to 1:1 as does the Nikon. As I don't have any experience with it, if the Ziess lens extends considerably as you focus closer, then the likelyhood is that it doesn't reduce focal length. But it may still not report an accurate aperture.
Also with the Zeiss lens(es) some of the older version don't have CPU chips on them, the newer ones I think do(well they do for Nikon) .. this could make a difference in the the way that the aperture is reported to the camera.

I think that from about 3 or so meters almost all macro lenses begin to report the physical aperture value(ie. it starts at about f/3 or so).
So to compare the DOF of each from 3 meters at f/8 or f/16 could be a bit hit and miss. You could be reliant on personal opinion as to how each will be rendered considering that the lenses may be mounted onto different camera brands .. ie. different sensor types.

I probably confused you more than helped .. if so ... sorry! :p

[out n about]

Hi Gayle and Colin.
I'd like to address some of the points raised in your post, so, in point form:
1. "...all f/stops (sic) ... let... same amount of light...regardless..."
This is not an exact statement. Rather, all f-stops theoretically allow the same intensity of light to pass. Intensity is "amount of light per unit area".
Take f/8 on an f=105mm lens, then f/8 represents ~13mm of aperture. But on an f=200mm lens, f/8 represents ~26mm of aperture. So, more light
gets in through the larger aperture than through the smaller one. What remains the same is how strong that light is when it hits the sensor, ie, its intensity.
Furthermore, the intensity can be attenuated by the amount of glass, how the light is dispersed, and other factors.

2. The question about depth-of-field at the same f-stop.
Given that DOF depends, amongst other things, on the actual aperture of lens you're using, then from the example above,
the DOF will NOT be the same at the same f-stop for two different lenses of different focal lengths.

There are charts and such about this, but that's the nub of it.
Am.

PS: NOT a dumb question.

Thanks heaps Am think I get the gist of it, :o I was confused as when my brother in law and I shot the same subject, him with D200 and ziess macro and me with D300s and nikor macro at the same f stop he always seemed to have a greater dof :(

- - - Updated - - -


Also to add to Am's reply.

the difference in DOF @ f/8 between a 100mm lens and a 105mm lens is minimal, to negligible.

The lens itself may(will) have light transmission differences(to other lenses of similar types) .. ie. T-stops.
So while the Zeiss may be @ f/2.8, it's T-stop could be something like f/4(don't know .. just an example).

Also, as lenses focus closer and closer, you will get focal length shortening. Very few lenses don't do this nowadays.
Some of those lenses can also misreport the real aperture value as you focus closer, others will report the actual aperture value.

I remember that Canon's don't adjust for aperture as you focus closer, Nikon's(and AFAIK) most third party lenses do(not sure about Zeiss)

So for the Canon 100/2.8 if you focus to MFD the camera may still indicate an aperture of f/2.8, where the reality could be something like f/4.8 or so.
The Nikon 105/2.8 will display the physical aperture(ie. I think f/4.8), even tho you haven't adjusted it at all.

if you have the Nikon, it will go down to a small as f/57(at MFD) and will stop at f/32 at infinity.

At f/8 you probably won't notice this, but at wider apertures vignetting also has an impact on the intensity of the light through the iris and onto the sensor.
So two lenses at f/8 may not produce the same (overall) exposure because the resultant image is affected by various degrees of vignetting.

So(to answer part of the question), in theory two different lenses set to the same aperture with the same scene(eg. say a plain white wall) should produce the same exposure no matter the focal length. This assumes no vignetting and the same T-stop from both lenses(which can happen).
of course in real life you don't shoot plain white walls, and it's almost impossible to get the same framing from two disparate focal lengths(ie. 70mm to look the same as 200mm).

One of DxO's strengths as a review site for lenses is that they test for T-stops of the lenses when they test. it gives you some idea of the differences you may get in any exposures if you're using the same camera/lens settings across different lenses.
To answer another part of the question, although not directly ..

Don't use f/40 on the Nikon 105VR!!
I meant to say Don't use f/40 if you value sharply rendered fine detail!
I've used f/32 which is about as far as I'd stop down, but the images require a fair amount of USM/highpass/sharpening of some sort to recover fine details.

(Haven't yet tested the 105VR thoroughly on the D800 yet to see how low I can go with aperture .. did my tests on the D300 early on(ie. years ago).

As to the DOF from the Ziess compared to the Nikon .. it depends.
It depends on how the Ziess is designed. If it reduces focal length as it focuses closer and doesn't report that, then the comparison is hard to judge without physical testing.
We know that the Nikon 105 does both report AND report an accurate aperture, so with that in mind you can easily test with side by side comparison images.

Something to note. The Ziess doesn't focus down to 1:1 as does the Nikon. As I don't have any experience with it, if the Ziess lens extends considerably as you focus closer, then the likelyhood is that it doesn't reduce focal length. But it may still not report an accurate aperture.
Also with the Zeiss lens(es) some of the older version don't have CPU chips on them, the newer ones I think do(well they do for Nikon) .. this could make a difference in the the way that the aperture is reported to the camera.

I think that from about 3 or so meters almost all macro lenses begin to report the physical aperture value(ie. it starts at about f/3 or so).
So to compare the DOF of each from 3 meters at f/8 or f/16 could be a bit hit and miss. You could be reliant on personal opinion as to how each will be rendered considering that the lenses may be mounted onto different camera brands .. ie. different sensor types.

I probably confused you more than helped .. if so ... sorry! :p

Thanks Arthur, and you are right I have read your reply 4 times and even though I understand in practical terms what you are saying some of the technical stuff is over my head :( As i said to Am when we have compared the pics of the same subject taken from the same tripod both with nikon cameras albeit one a d200 and the other a d300s both with the same aperture the zeiss seemed to have a greater dof.
When you mentioned that the ziess wasn't 1:1 instead being 1:2 I remembered that fact and it was something we talked about at the time as maybe being a reason for the difference.
I haven't been past F32 yet and now I won't , thanks for the advice :th3:
Also at the risk of sounding even dumber :o can you explain what you mean when you use the MFD in your post.
Thanks again for taking the time to answer, appreciate it heaps.

[arthurking83]

MFD = Minimum focus distance.

On the Zeiss it will give you 1:2 magnification ratio. It's MFD may be something like 40cm, or 50cm or whatever it is.
The 105VR has an MFD of about 33cm and gives you a repro ratio of about a smidge above 1:1.

Note that MFD is not to the font of the lens, it is to the plane of the sensor. On the top plate on the LHS, just to the side of the top LCD is a small circle mark with a horizontal line though it.
This is the sensor plane .. ie. where you measure this focus distance to and from.

The Nikon lens is marked for it's reproduction ratio(there are three levels on the distance scale. In meters, in feet and also in magnification(or reproduction ratio).

I'm pretty sure that the Zeiss has similar markings.

Now that you mention A D200 and a D300s .. it should also be noted if you're still a bit curious about light intensity and apertures.

If you set the cameras both to manual settings and shoot with just the one lens(on both cameras) at the same settings, you will probably find to different exposures too.
I say that because part of your question involved light intensity for a given aperture value.

Different sensors and different metering systems give slight variations on exposure.
I'm sure I've seen that the D200 usually gives a slightly lower exposure even at full manual mode on the camera/lens of any given constant settings.
it's both a sensor thing(sensitivity?) and a rated ISO discrepancy.

My D70s is about -2/3Ev darker than my D300 was, and my D800E is slightly brighter than the D300 is(maybe +1/3 to 1/6Ev(it varies, and I haven't checked it extensively).

So if the Zeiss is Nikon mount(ie. ZF) is it a ZF2 model or just ZF?
ZF2 will have the electronic contacts on the rear mount, just like the Nikon lens has, only not as many gold contacts.
A ZF mount doesn't have the electronic contacts, and hence CPU.

I did a quick test (for focus stacking more than aperture vs detail testing) a long time back with my D800 and 105VR and I got very good IQ at f/16, but it's visibly better at say f/8.
Of course the problem with f/8 is the shallow DOF you get.
So I tried to compare it with bigger aperture and focus stacking and so on.

I figured out it was easier to get usable images at say f/16 and f/22 with some PP, than to muck about with focus stacking and f/5.6 or f/8.

If you're seeing a deeper DOF from the Zeiss lens, make sure of a few things:

1. magnification is about the same.

2. don't worry about MFD in this comparison. The reason not to worry about MFD is because I'm sure the Zeiss doesn't suffer from focal length shortening. If it does, it's not as much as the Nikon does.
So while the Nikon says it's a 105mm lens, at macro distances it's more like an 85mm or maybe even shorter lens. MFD and macro magnifications vary according to focal length.
If the Zeiss lens doubles in physical length as you focus closer, then it most likely doesn't have focal length shortening .. MFD will be longer because it could still be a true 100mm lens.

3. check exposures on one camera between the two lenses. That is, don't compare the Zeiss at f/16 on the D200 with the 105VR on the D300 at f/16 .. the exposures WILL be different no matter what. Only on the once camera can you tell if the Zeiss doesn't report working aperture(which I'm 99.99% sure it won't, as it doesn't have focus distance info to give the camera).

- - - Updated - - -

Oh! I forgot to say too.
(I know I can babble on a bit, but you may be interested)

When you start to get into the macro range, I think from about 1:2 or higher(as in 1:1) DOF is about the same irrespective of focal length.

So while a 35mm lens has a deeper DOF at f/2.8 than a 105mm lens will have at f/2.8 ... once you have both lenses at the 1:2 or higher magnification rate, the DOF of both lenses set to the same aperture will give you the same DOF even tho they have massively different focal lengths.

Just a fact of life, and one that confuses folks. It's obviously has to do with the distance to the subject for each focal length.

And one other note.
I've made comments previously that the Nikon is not the greatest macro lens in the world. It's great for chasing bees, or occasional snaps at close up, but it suffers from lots of CA(usually green/magenta fringing).
The Zeiss is (from what I read of it) much much better.
if you read reviews of both lenses(such as those on Photozone), both lenses seem pretty much on par .. the Zeiss maybe a touch better but not by a lot.
yet in real life the Zeiss is a lot more sharp, and renders a scene better simply because CA is so much less.
CA is equivalent to a loss of sharpness.

The test pattern paper prints that most lens review sites use to measure lenses are black and white. It's very rare to photograph a purely black and white subject.
So while the Nikon looks usually as good as the Zeiss in most tests, in real images it's been hailed as much better in overall image rendering.

Remember Zeiss could do 1:1 with extension tubes(about 50mm or so) but I beleive it may cause some vignetting .. maybe not so much on Dx, but more so on Fx.

[out n about]

MFD = Minimum focus distance.

On the Zeiss it will give you 1:2 magnification ratio. It's MFD may be something like 40cm, or 50cm or whatever it is.
The 105VR has an MFD of about 33cm and gives you a repro ratio of about a smidge above 1:1.

Note that MFD is not to the font of the lens, it is to the plane of the sensor. On the top plate on the LHS, just to the side of the top LCD is a small circle mark with a horizontal line though it.
This is the sensor plane .. ie. where you measure this focus distance to and from.

The Nikon lens is marked for it's reproduction ratio(there are three levels on the distance scale. In meters, in feet and also in magnification(or reproduction ratio).

I'm pretty sure that the Zeiss has similar markings.

Now that you mention A D200 and a D300s .. it should also be noted if you're still a bit curious about light intensity and apertures.

If you set the cameras both to manual settings and shoot with just the one lens(on both cameras) at the same settings, you will probably find to different exposures too.
I say that because part of your question involved light intensity for a given aperture value.

Different sensors and different metering systems give slight variations on exposure.
I'm sure I've seen that the D200 usually gives a slightly lower exposure even at full manual mode on the camera/lens of any given constant settings.
it's both a sensor thing(sensitivity?) and a rated ISO discrepancy.

My D70s is about -2/3Ev darker than my D300 was, and my D800E is slightly brighter than the D300 is(maybe +1/3 to 1/6Ev(it varies, and I haven't checked it extensively).

So if the Zeiss is Nikon mount(ie. ZF) is it a ZF2 model or just ZF?
ZF2 will have the electronic contacts on the rear mount, just like the Nikon lens has, only not as many gold contacts.
A ZF mount doesn't have the electronic contacts, and hence CPU.

I did a quick test (for focus stacking more than aperture vs detail testing) a long time back with my D800 and 105VR and I got very good IQ at f/16, but it's visibly better at say f/8.
Of course the problem with f/8 is the shallow DOF you get.
So I tried to compare it with bigger aperture and focus stacking and so on.

I figured out it was easier to get usable images at say f/16 and f/22 with some PP, than to muck about with focus stacking and f/5.6 or f/8.

If you're seeing a deeper DOF from the Zeiss lens, make sure of a few things:

1. magnification is about the same.

2. don't worry about MFD in this comparison. The reason not to worry about MFD is because I'm sure the Zeiss doesn't suffer from focal length shortening. If it does, it's not as much as the Nikon does.
So while the Nikon says it's a 105mm lens, at macro distances it's more like an 85mm or maybe even shorter lens. MFD and macro magnifications vary according to focal length.
If the Zeiss lens doubles in physical length as you focus closer, then it most likely doesn't have focal length shortening .. MFD will be longer because it could still be a true 100mm lens.

3. check exposures on one camera between the two lenses. That is, don't compare the Zeiss at f/16 on the D200 with the 105VR on the D300 at f/16 .. the exposures WILL be different no matter what. Only on the once camera can you tell if the Zeiss doesn't report working aperture(which I'm 99.99% sure it won't, as it doesn't have focus distance info to give the camera).

- - - Updated - - -

Oh! I forgot to say too.
(I know I can babble on a bit, but you may be interested)

When you start to get into the macro range, I think from about 1:2 or higher(as in 1:1) DOF is about the same irrespective of focal length.

So while a 35mm lens has a deeper DOF at f/2.8 than a 105mm lens will have at f/2.8 ... once you have both lenses at the 1:2 or higher magnification rate, the DOF of both lenses set to the same aperture will give you the same DOF even tho they have massively different focal lengths.

Just a fact of life, and one that confuses folks. It's obviously has to do with the distance to the subject for each focal length.

And one other note.
I've made comments previously that the Nikon is not the greatest macro lens in the world. It's great for chasing bees, or occasional snaps at close up, but it suffers from lots of CA(usually green/magenta fringing).
The Zeiss is (from what I read of it) much much better.
if you read reviews of both lenses(such as those on Photozone), both lenses seem pretty much on par .. the Zeiss maybe a touch better but not by a lot.
yet in real life the Zeiss is a lot more sharp, and renders a scene better simply because CA is so much less.
CA is equivalent to a loss of sharpness.

The test pattern paper prints that most lens review sites use to measure lenses are black and white. It's very rare to photograph a purely black and white subject.
So while the Nikon looks usually as good as the Zeiss in most tests, in real images it's been hailed as much better in overall image rendering.

Remember Zeiss could do 1:1 with extension tubes(about 50mm or so) but I beleive it may cause some vignetting .. maybe not so much on Dx, but more so on Fx.

Thanks Arthur you have certainly got me thinking :lol: laying in bed last night it dawned on me MFD = minimum focusing distance:D
Armed with this knowledge the next time I catch up with my brother in law (which won't be for a while as he lives in Vic and we are in Queensland at the present) we will do some more testing, pretty sure his model is the ZF2 model. Can't wait to try both on the D800.
I reckon you are spot on when you stated that the zeiss is sharper and renders the image overall better and maybe this where I am getting mislead in DOF as well :( and I was thinking incorrectly of course that because the nikon went from f2.8 to say f40 and the zeiss from f2 to f22 then the zeiss' dof at f8 should roughly be equal to some where around f16's dof on the nikor. :D
Thanks again for the detailed replies as it helping to alleviate my confusion and spark my interest... will do some more googling now that I understand a bit more. :)

[fillum]

Perhaps the simplest way to understand this is to recall that DoF is basically dependent on the aperture and the magnification - larger aperture and larger magnification give less DoF. In this case the Zeiss gives a max magnification of 1:2 and the Nikkor gives a max of 1:1 (ie more magnification) so the the Nikkor will show less DoF than the Zeiss when both lenses are at their max magnification for the same aperture. If you were to take shots so that the subject is the same size on both camera sensors for the same aperture then the DoF should be the same for both lenses.


... because the nikon went from f2.8 to say f40 and the zeiss from f2 to f22 then the zeiss' dof at f8 should roughly be equal to some where around f16's dof on the nikor...I think this is probably correct when both lenses are at max magnification, but is due to the magnification and is not related to the min and max apertures of each lens.




Cheers.

[arthurking83]

Perhaps the simplest way to understand this is to recall that DoF is basically dependent on the aperture and the magnification - larger aperture and larger magnification give less DoF. In this case the Zeiss gives a max magnification of 1:2 and the Nikkor gives a max of 1:1 (ie more magnification) so the the Nikkor will show less DoF than the Zeiss when both lenses are at their max magnification for the same aperture. If you were to take shots so that the subject is the same size on both camera sensors for the same aperture then the DoF should be the same for both lenses.

I think this is probably correct when both lenses are at max magnification, but is due to the magnification and is not related to the min and max apertures of each lens.

Cheers.

This is all true if the lenses operated equally, which is why I introduced the possibility of the Zeiss misreporting the working aperture(as opposed to the indicated aperture).
As I don't have a Zeiss to test or compare, if some else does, it could help to explain why the DOF could look deeper on the Zeiss at the same apertures.

if you recall, did you control the aperture on the Z lens via the aperture ring, or via the sub command dial(front dial)?
If it was the front dial, then I think it may be easier to explain.

If you use the Nikon 105VR and have your aperture set to f/2.8 with the lens at infinity focus, the aperture indicated will be f/2.8.
But if you now focus the 105VR to a setting of 1:2(the front most markings on the distance scale) the aperture will now read f/4.something(I think f/4.2 or f/4.5).

If you focus the Zeiss at infinity and set it to f/2.8, and then focus to a 1:2 magnification setting, I'm fairly sure it won't report the actual aperture as f4.2 or f/4.5(I don't think it has any coupling to the focus mechanism for it to know what distance is set) .. that is the lens is only half smart in this respect.

It's a similar concept to using extension tubes(or focusing helicoids). as the lens is moved further from the film/sensor plane the effective aperture is reduced(unless the lens somehow manages to become larger in diameter!!)

So, as an example of how this aperture setting strangeness can affect the actual aperture used:

With the Nikon lens if you have the lens already set to 1:2 and want to use an aperture of f/8 you only need to rotate the sub command wheel 6 clicks(if you're using 1/3 stops)
But you can also set the lens to f/8 while it's at infinity focus and f/2.8 with 9 clicks(1/3stop exposure settings) focus to 1:2 and it will still stay at f/8.

With this lens and two different aperture control operations you still achieve f/8. ie. the camera knows the actual aperture set at rest(which is really f/4.2) and will move the aperture lever by a certain amount to achieve an f/8 shooting requirement.

Now(if) with the Zeiss you have the aperture set as described before, and you want to set it to f/8 you need to turn the sub command wheel ... 9 clicks to set it to f/8 no matter what focus distance you have preset.
The question is, are those 9 clicks going to produce an effective, or indicated aperture setting?
Because the Zeiss isn't fully electronically coupled, I think the aperture reported(to the camera) is not the effective aperture, but more likely the indicated aperture.
(ie. as with using extension tubes, the aperture you see indicated is not the working aperture).

So in effect, the Zeiss will be operating at f/11(ie. deeper DOF) than the Nikon will be as it's fully connected message to the camera is to set x amount of travel on the aperture lever.
if the Zeiss isn't fully connected, and is reporting f/2 or f/2.8 at a magnification setting of 1:2 then the camera is fooled into thinking it needs to move the aperture lever by x+? amount to achieve the same setting.
Note that it's more likely to be f/16(as you seem to think it is) as part of the job of the CPU chip is to report maximum aperture value. This of course will be f/2 from the Zeiss lens.

The amount of ? is going to be the difference between the 105VR's reported max aperture at that focus setting(remember f/4.2) and the Zeiss lens's reported max aperture(most likely f/2).

Hope this makes sense. Like I said tho, I have no experience with the Zeiss, and my comments are more theory than anything else .. basically extrapolated observations of what I've seen with some of my stuff.
For it to make sense to you too, you need a basic understanding of how Nikon camera/lenses work too.

[out n about]

This is all true if the lenses operated equally, which is why I introduced the possibility of the Zeiss misreporting the working aperture(as opposed to the indicated aperture).
As I don't have a Zeiss to test or compare, if some else does, it could help to explain why the DOF could look deeper on the Zeiss at the same apertures.

if you recall, did you control the aperture on the Z lens via the aperture ring, or via the sub command dial(front dial)?
If it was the front dial, then I think it may be easier to explain.

If you use the Nikon 105VR and have your aperture set to f/2.8 with the lens at infinity focus, the aperture indicated will be f/2.8.
But if you now focus the 105VR to a setting of 1:2(the front most markings on the distance scale) the aperture will now read f/4.something(I think f/4.2 or f/4.5).

If you focus the Zeiss at infinity and set it to f/2.8, and then focus to a 1:2 magnification setting, I'm fairly sure it won't report the actual aperture as f4.2 or f/4.5(I don't think it has any coupling to the focus mechanism for it to know what distance is set) .. that is the lens is only half smart in this respect.

It's a similar concept to using extension tubes(or focusing helicoids). as the lens is moved further from the film/sensor plane the effective aperture is reduced(unless the lens somehow manages to become larger in diameter!!)

So, as an example of how this aperture setting strangeness can affect the actual aperture used:

With the Nikon lens if you have the lens already set to 1:2 and want to use an aperture of f/8 you only need to rotate the sub command wheel 6 clicks(if you're using 1/3 stops)
But you can also set the lens to f/8 while it's at infinity focus and f/2.8 with 9 clicks(1/3stop exposure settings) focus to 1:2 and it will still stay at f/8.

With this lens and two different aperture control operations you still achieve f/8. ie. the camera knows the actual aperture set at rest(which is really f/4.2) and will move the aperture lever by a certain amount to achieve an f/8 shooting requirement.

Now(if) with the Zeiss you have the aperture set as described before, and you want to set it to f/8 you need to turn the sub command wheel ... 9 clicks to set it to f/8 no matter what focus distance you have preset.
The question is, are those 9 clicks going to produce an effective, or indicated aperture setting?
Because the Zeiss isn't fully electronically coupled, I think the aperture reported(to the camera) is not the effective aperture, but more likely the indicated aperture.
(ie. as with using extension tubes, the aperture you see indicated is not the working aperture).

So in effect, the Zeiss will be operating at f/11(ie. deeper DOF) than the Nikon will be as it's fully connected message to the camera is to set x amount of travel on the aperture lever.
if the Zeiss isn't fully connected, and is reporting f/2 or f/2.8 at a magnification setting of 1:2 then the camera is fooled into thinking it needs to move the aperture lever by x+? amount to achieve the same setting.
Note that it's more likely to be f/16(as you seem to think it is) as part of the job of the CPU chip is to report maximum aperture value. This of course will be f/2 from the Zeiss lens.

The amount of ? is going to be the difference between the 105VR's reported max aperture at that focus setting(remember f/4.2) and the Zeiss lens's reported max aperture(most likely f/2).

Hope this makes sense. Like I said tho, I have no experience with the Zeiss, and my comments are more theory than anything else .. basically extrapolated observations of what I've seen with some of my stuff.
For it to make sense to you too, you need a basic understanding of how Nikon camera/lenses work too.

Hi Arthur, thanks again for the response, you have made me look at my 105 in a new light :o and when I catch up with my brother in law later in the year will do so testing and will post the results.