The TC Factor and Telephotos-the f3.3 Inflection Threshold

[chambeshi]

I have finally finalized and published a series of Blog posts. These 6 pull together some ideas I've developed over the past couple of years on what's good, missing, and achievable in telephotos in performance on the best Nikon DSLRs. Although purely Nikon-centric, the principles are much broader. I ended up scoping out a summary of how I see the specs of the Optimal Telephoto len(ses). The TCF concept is key - the Teleconverter Factor. It's just a simple calculation of 2 key variables - T Score (Telephoto-Gain) and C (Converter-Scope).

Obviously, this is a personal view but I find it useful to explain my thinking on the design space and functionality of telephotos for FX Nikon DSLRs. I also see a streamlined Internal TC System holding great promise in future lenses. All the better as a bespoke design dedicated for high optical performance with 1 or 2 models of a light-weight compact telephoto. It's clear a growing market sector (birders etc) will welcome the lens with the best optimal merging of ergonomics, function into a package of a 400 f3.3 and/or 500 f3.3.

Why f3.3? It is intriguing to discover a threshold in telephoto lens design of how TCF [ie T3C2.75] relates to maximum lens aperture. This f3.3 Rule reflects how lens diameter (i.e. f3.3 widest lens aperture) is the optimum where flexible AF Performance meets Teleconverter Design. This allometric constraint also reflects the importance of ergonomics and costs of production and market realities.  Smaller f-stop? - lens too heavy! Increase f-stop? Lens gets dinky (yes, great) but it kills off AF flexibility with TCs (bad).

Here's the link - all the essays are cross-linked.

https://chambeshiphotographyblog.wordpress.com/2018/02/17/the-tc-factor-with-telephoto-lenses/

this Post is the focus  https://chambeshiphotographyblog.wordpress.com/2018/02/20/optimal-telephotos-the-optical-space-race/

kind regards

[Les Olson]

I think it weakens the analysis to base it on focal length, rather than angle of view, or, even better, pixels on the subject. 

One problem with focal length is that angle of view is inversely proportional to focal length, so that the 100mm difference between 200mm and 300mm is a lot (10.3 degrees horizontal AoV vs 6.9) but the 100mm difference between 400mm and 500mm is much less (5.2 degrees vs 4.1).  A 1.4 TC gives you the same reduction in AoV on a 200mm as on a 400mm lens, although the increase in focal length on the 400mm is twice as great.

Where focal length is seriously misleading - as is AoV - is when cameras have different pixel density, which is why you need pixels on the subject. A 40cm duck at 10m subtends 2.8 degrees, so on a D850 a 200mm lens has 1122 ppd (pixels per duck), a 300mm has 1675, a 400mm has 2222 and a 500mm has 2819.  A D5 does much less well: for the same focal lengths it has 756, 1129, 1499 and 1901 ppd. The D500 does slightly better than the D850 at each focal length: at 200mm it has 1163 ppd, at 300mm 1732, at 400mm 2292 and at 500mm 2887.  To make a best quality 8 x 10 print you need 3000 x 2400 pixels (if your output is screen you need, as of now, fewer pixels - but soon you will need more).  The utility of a TC is completely different on the D850, the D5 and the D500 and is not constant at different focal lengths: putting a 1.4TC on a 200mm lens on the D5 to photograph the duck is pretty much useless but you might just get away with it on the D850 and the D500; conversely, at 500mm the 1.4TC makes the D5 useful but gives minimal advantage on the D850 or the D500.  And if you want to print larger - or smaller - the relative merits of the TC at varying focal lengths are different again. So whether it is a good thing if a lens can be used with all three TCs depends on both lens and camera.

The DX advantage is not illusory for subjects that occupy only part of the viewfinder at the focal length you have available. But if the subject fills the FX viewfinder at (say) 300mm - ie, a 2.4m object 10m away - and so the DX viewfinder at 200mm there is no DX advantage in pixels on the subject.  Of course, that is a lion-sized animal 10m away, so it will be not very common in wild mammal photography and rare in bird photography.

[MILLIREHM]

Thanks for the contributions to this discussion. It is always good to have several Superteles available, then there is less need for the use of Teleconverters ;-)

[JKoerner007]

... pixels per duck ...

Thanks for making my morning

[JKoerner007]

In your blog post, you said,

This nonsense of a DX camera “magnifying” the “reach” of a lens has become widely believed. It’s fanned in forums by the self-anointed ‘egg-spurts’. Moreover, even paid photography gurus and photographic companies perpetuate the myth. Indeed, these myth-makers trumpet this “DX benefit” in their official marketing – that a 300mm prime becomes a real 450mm, or a 600mm becomes the definitive 900mm! Plain wrong!

First of all, why all the excitement and exclamation points?

Second, as Les Olson points out, a Nikon D5 and D500 have virtually identical MP sensor sizes (albeit one full frame, the other crop).

Whatever you want to call it (be it magnification, or "reach," or whatever), the D500 puts more pixels onto the subject with the same lens.

Sure, if you can get close enough to 'fill the frame' with the D5, it will take better photos than the D500. However, if you can't, or (worse), if you're cropping-in a little with the D500 (which I do all the time), then you're going to be really cropping-in with the D5, which at the end of the day means the D500 will be putting more pixels on the subject than the D5. Hence the D500 image will be better than the D5 image.

As Les boils down into pixels, the D5 only puts 1901 "pixels per duck" (lol, still makes me grin) where the D500 puts 2887 pixels on the same subject.

Now, if you don't feel comfortable calling this "magnification," or "reach," that is your idiosyncrasy.

But, whatever you want to call it, the end result is an advantage.

[chambeshi]

Interesting critique :-) Introducing PpD [Pixels/ per / Duck  ] as the additional variable is a useful way to consider the dilemma of choice of the optimal telephoto to invest in. If we introduce choice of camera - recording device - in comparing telephotos. Then ISO - wrt sensor-noise - becomes yet another factor to consider for the best combination of camera and lens.

My comparisons of DX vs FX assumes the same sensor resolution, which obviously is not always the case. Provided one is able to afford 2 cameras and can carry them 'Out There'

Then we have to consider ISO performance wrt to the camera.  I find too often ISO has to increase in poorer lighting conditions to maintain minimum shutter speed-speed where one is handholding the rig and/or moving subjects. Where choice of camera is possible, the D500 does have the edge in higher resolution. Based on your example this is 3.7 to 2.5% more pixels with a 200, 300 ,400, 500 lens and the subject of the exemplar Duck.

many thanks of all the feedback.

I think it weakens the analysis to base it on focal length, rather than angle of view, or, even better, pixels on the subject. 

One problem with focal length is that angle of view is inversely proportional to focal length, so that the 100mm difference between 200mm and 300mm is a lot (10.3 degrees horizontal AoV vs 6.9) but the 100mm difference between 400mm and 500mm is much less (5.2 degrees vs 4.1).  A 1.4 TC gives you the same reduction in AoV on a 200mm as on a 400mm lens, although the increase in focal length on the 400mm is twice as great.

Where focal length is seriously misleading - as is AoV - is when cameras have different pixel density, which is why you need pixels on the subject. A 40cm duck at 10m subtends 2.8 degrees, so on a D850 a 200mm lens has 1122 ppd (pixels per duck), a 300mm has 1675, a 400mm has 2222 and a 500mm has 2819.  A D5 does much less well: for the same focal lengths it has 756, 1129, 1499 and 1901 ppd. The D500 does slightly better than the D850 at each focal length: at 200mm it has 1163 ppd, at 300mm 1732, at 400mm 2292 and at 500mm 2887.  To make a best quality 8 x 10 print you need 3000 x 2400 pixels (if your output is screen you need, as of now, fewer pixels - but soon you will need more).  The utility of a TC is completely different on the D850, the D5 and the D500 and is not constant at different focal lengths: putting a 1.4TC on a 200mm lens on the D5 to photograph the duck is pretty much useless but you might just get away with it on the D850 and the D500; conversely, at 500mm the 1.4TC makes the D5 useful but gives minimal advantage on the D850 or the D500.  And if you want to print larger - or smaller - the relative merits of the TC at varying focal lengths are different again. So whether it is a good thing if a lens can be used with all three TCs depends on both lens and camera.

The DX advantage is not illusory for subjects that occupy only part of the viewfinder at the focal length you have available. But if the subject fills the FX viewfinder at (say) 300mm - ie, a 2.4m object 10m away - and so the DX viewfinder at 200mm there is no DX advantage in pixels on the subject.  Of course, that is a lion-sized animal 10m away, so it will be not very common in wild mammal photography and rare in bird photography.

[chambeshi]

In your blog post, you said,
First of all, why all the excitement and exclamation points?

Second, as Les Olson points out, a Nikon D5 and D500 have virtually identical MP sensor sizes (albeit one full frame, the other crop).

Whatever you want to call it (be it magnification, or "reach," or whatever), the D500 puts more pixels onto the subject with the same lens.

Sure, if you can get close enough to 'fill the frame' with the D5, it will take better photos than the D500. However, if you can't, or (worse), if you're cropping-in a little with the D500 (which I do all the time), then you're going to be really cropping-in with the D5, which at the end of the day means the D500 will be putting more pixels on the subject than the D5. Hence the D500 image will be better than the D5 image.

As Les boils down into pixels, the D5 only puts 1901 "pixels per duck" (lol, still makes me grin) where the D500 puts 2887 pixels on the same subject.

Now, if you don't feel comfortable calling this "magnification," or "reach," that is your idiosyncrasy.

But, whatever you want to call it, the end result is an advantage.

"the D5 and D500 can have virtually identical MP sensor sizes (albeit one full frame, the other crop)." Nearly similar numbers of pixels yes, but of distinctly different sizes - hence the significant differences in ISO and sensor-noise. Again, it is obvious that choice of camera changes the resolution obtained with a lens, but the questions as to choices of telephotos and TCF remain.

Resolution and Magnification are two different properties of the image projected on the DSLR Sensor of a camera. Keeping the camera constant, you can project the same sensor through a 400 or 600 lens but it's the same respective magnification regardless of how it is cropped. As Les Olsen points out the numbers of pixels of the image focused through a 400 or 600 lens can have a different Resolution BUT this is contingent on what Sensor.

We can try and avoid using Focal Length as the variable here, but this is the universally used proxy for lens magnification. Yes, with a 400 or 600 lens, you can crop the respective Image on to the 20mp in a D500 or the DX crop mode on a 45 mp sensor in a D850. And we can crop the same projection using the in-camera controls in the D850 to 1:1 etc.  So a 1:1 frame would complement the options as I tried to summarized in the graphic.

Each of the 3 Teleconverters on the lens enlarges the image by its set factor of Magnification, and the image can be cropped to the dimensions of DX or 1:1 on a FX. Or the same TCs and lens can magnify the same image on to a DX sensor - cropped - at its respective Resolution. I do not see any idiosyncrasies.

The distinction between Magnification and Resolution - and more - was debated at quite some length a while back in this Thread http://nikongear.net/revival/index.php/topic,3972.0.html.

As we have been discussing constructively on other threads, it is not always possible to hike with the biggest of lenses on a DSLR on a tripod or monopod, plus an alternative rig of 2nd DSLR and smaller lens. My interests are toward a better understanding of the optimal telephoto lens, which permits the most flexibility of use in outdoor photography. Teleconverters are vital and invaluable accessories in such situations (besides reducing costs). It's a  trade off. But the new 180-400 f4 TC14 Nikkor points to what the potential holds to integrate TCs into future telephotos....this will arguably minimize the penalty of image quality. And the concept of the TeleConverter-Factor - imho - shows up how well (and not) the extant telephoto design-space serves us - especially where you cannot take it with you - http://www.vividlight.com/articles/403.htm

thanks

[JKoerner007]

"the D5 and D500 can have virtually identical MP sensor sizes (albeit one full frame, the other crop)." Nearly similar numbers of pixels yes, but of distinctly different sizes - hence the significant differences in ISO and sensor-noise. Again, it is obvious that choice of camera changes the resolution obtained with a lens, but the questions as to choices of telephotos and TCF remain.

Translation: In the end there is no "one," magical, silver bullet

There are times where the reach is going to favor a D500, times where the high-ISO is going to favor the D5, and times when the high-res, great base ISO will favor the D850.

Which brings us back to what Wolfgang said, on my 400/600mm thread, to compensate for this ... he forsees himself buying all 3 bodies

Unfortunately, I do too

Resolution and Magnification are two different properties of the image projected on the DSLR Sensor of a camera. Keeping the camera constant, you can project the same sensor through a 400 or 600 lens but it's the same respective magnification regardless of how it is cropped. As Les Olsen points out the numbers of pixels of the image focused through a 400 or 600 lens can have a different Resolution BUT this is contingent on what Sensor.

In the end, pixels on the subject may be the best way to look at it.

If the pixel density is equivalent, the FF sensor advantage becomes relevant.

If the pixel advantage of the DX is significantly greater, however, this becomes the relevant determiner.

Further, nowadays, the better our sensors become, the more and more birders are migrating toward the D500, especially with small birds.

(The D500 rates better than the Canon 1Dx of a few years back.)

The D500 has optimal reach, superior AF coverage across the frame, and the second-best tracking of any camera on the planet (behind only the D5).

We can try and avoid using Focal Length as the variable here, but this is the universally used proxy for lens magnification. Yes, with a 400 or 600 lens, you can crop the respective Image on to the 20mp in a D500 or the DX crop mode on a 45 mp sensor in a D850. And we can crop the same projection using the in-camera controls in the D850 to 1:1 etc.  So a 1:1 frame would complement the options as I tried to summarized in the graphic.

Not sure I follow as none of these are 1:1 (macro) in magnification/reproduction ratio.

Each of the 3 Teleconverters on the lens enlarges the image by its set factor of Magnification, and the image can be cropped to the dimensions of DX or 1:1 on a FX. Or the same TCs and lens can magnify the same image on to a DX sensor - cropped - at its respective Resolution. I do not see any idiosyncrasies.

You're losing me on the 1:1 statement ...

The distinction between Magnification and Resolution - and more - was debated at quite some length a while back in this Thread http://nikongear.net/revival/index.php/topic,3972.0.html.

Thanks for the link.

Interesting (albeit painful) reading

I get that focal length = focal length. (A tautology.)

Notwithstanding the dissenting opinion of certain curmudgeons (), the attempt to describe "equivalent" focal length merely has to do with framing ... not with any attempt to 'change the focal length' of the lens.

The real subject of concern is FRAMING: we are trying to frame our subjects adequately when we purchase (or fantasize about) super-telephoto lenses

So, how do we get the framing ... without losing quality?

That said, with our current choices, the optimal image possible would be via the D850, perfectly-framed, with no cropping, at base ISO.
(None of the others, neither the D5 nor the D500, could compare.)

The trouble is, unlike with landscapes, when we encounter wildlife, we are faced with different distances, different lighting conditions, and (often) with various degrees of movement.

And, at the end of the day, there is no 'one' way to deal with all of these variables optimally with 'one' setup (camera + lens)

There are arguments to be made for all 3 cameras, but Nikon prices the D5 at the summit simply because it handles two of these items (movement, low-light) better than the other two.

The D850 is second in price, simply because (properly-framed, in good light) it renders the best images.

The D500 isn't really the best at anything (neither base ISO nor movement/low-light) ...
However, the D500 is freaking close to handling movement like the D5, and it does give the most 'reach' (or magnification, or 'pbd'—pixels-per duck—lol) at the long-end of any lens.

The D500 is, in essence, "a built-in teleconverter" with any lens, in a small, dexterous, very capable body.

Where reach is paramount, the quality of the D500's images is "good enough" for most applications. Indeed, the quality from the D500 is indistinguishable from the other two cameras in the majority of instances, especially where it has the reach advantage. (It can very often be superior to the other two in these instances.)

As we have been discussing constructively on other threads, it is not always possible to hike with the biggest of lenses on a DSLR on a tripod or monopod, plus an alternative rig of 2nd DSLR and smaller lens. My interests are toward a better understanding of the optimal telephoto lens, which permits the most flexibility of use in outdoor photography. Teleconverters are vital and invaluable accessories in such situations (besides reducing costs). It's a  trade off. But the new 180-400 f4 TC14 Nikkor points to what the potential holds to integrate TCs into future telephotos....this will arguably minimize the penalty of image quality. And the concept of the TeleConverter-Factor - imho - shows up how well (and not) the extant telephoto design-space serves us - especially where you cannot take it with you - http://www.vividlight.com/articles/403.htm
thanks

I appreciate your passion.

IMO the D500 + 400mm f/2.8 + TCs will provide the most flexibility in a single choice (or, possibly now, the 180-400 f/4 + 1.4 TC).
I can't imagine any other single choice handling more options than the described.

But there will always be specific instances were other choices are more optimal (600mm, 800mm ... D850/D5) ... and that will always be the case, regardless of any choice we make.

Even with these other choices (an D850/800mm combo, e.g.,), there will be times where these are not the right tools for the job.

The takeaway here is no single choice we can make will ever be 'the' optimal choice for every possible encounter.

Cheers 

[Les Olson]

As we have been discussing constructively on other threads, it is not always possible to hike with the biggest of lenses on a DSLR on a tripod or monopod, plus an alternative rig of 2nd DSLR and smaller lens. My interests are toward a better understanding of the optimal telephoto lens, which permits the most flexibility of use in outdoor photography.

No: it is always possible to carry two cameras and half a dozen of the biggest lenses made.  All you need is a well-designed backpack and a good breakfast.  You don't choose to, and neither do I, but saying it is not "possible" is giving a false appearance of objectivity to a mere preference. In the same spirit, you write about "the optimal telephoto lens, which permits the most flexibility of use". If flexibility is the highest priority, of course the number of TCs a lens can use is important: but why is flexibility the highest priority?  It is not that you are making an unreasonable choice, but that presenting a choice as a given creates a circular argument.

[MILLIREHM]

Which brings us back to what Wolfgang said, on my 400/600mm thread, to compensate for this ... he forsees himself buying all 3 bodies

Unfortunately, I do too

Well actually I already use all three types of systems in all sort of combinations (currently the D4S which i still see highly competitive, the D500 and now the D850)
It started with the D700/D300 combo, D800E added FX highres and took the role of the DX body from D300 until D500 took it again. I dont want to pursue every upgrade so  I left the D810 until the D850 came and I bought the D4S a year before the D5 entered the stage, so sooner or later there will be a better version of the topspeed, high iso FX body available and make it into my bak.
I am sure any FL series Supertele that is currently on the list will still be top-line, when all the camera bodies we are talking about now are already "outdated"

[MILLIREHM]

No: it is always possible to carry two cameras and half a dozen of the biggest lenses made.  All you need is a well-designed backpack and a good breakfast.  You don't choose to, and neither do I, but saying it is not "possible" is giving a false appearance of objectivity to a mere preference. In the same spirit, you write about "the optimal telephoto lens, which permits the most flexibility of use". If flexibility is the highest priority, of course the number of TCs a lens can use is important: but why is flexibility the highest priority?  It is not that you are making an unreasonable choice, but that presenting a choice as a given creates a circular argument.

Some contradiction: The posibility to lug around or handle Supertele Equipment is depending on physical consitution and fitness - that can be trained but it remains that some people have better preconditions for that than others. There are people that can use them freehand, even for a longer period of time, others get tired soon or even fail,
climatic factors, nutrition water supply and lung capacity also play a role and a good backpack can help. I have already carried a Kiboko with a 600/4 (or 800/5,6) plus 200-400/4 plus 2-3 heavy bodies accessories and a Sachtler tripod and moved the thing over distances further than three paces from a car (that i dont own), (need to regain fitness for repeating an undertaking like that) - there were even people climbing high mountains with a 600/4 in the Backpack which seems to be beyond my range for the rest of my life.

Short answer: Yes i agree it is possible but not for everybody and not at any given point of time

[JKoerner007]

No: it is always possible to carry two cameras and half a dozen of the biggest lenses made.  All you need is a well-designed backpack and a good breakfast.  You don't choose to, and neither do I, but saying it is not "possible" is giving a false appearance of objectivity to a mere preference.

If you're traveling by boat, or by vehicle (or are operating from a blind), I can see the point.

However, on a nature hike?

No, it is not possible to carry "half a dozen of the biggest lenses made," on a long, challenging hike. Period.

You could not physically do that, I don't care what backpack you purchased ... or what you had for breakfast

I'd like to see any photographer tote an 800mm lens, a 600mm lens, a 400mm lens, and a 200mm lens on a mountain hike (and that's just 4 lenses .

Further, a person's budget might render purchasing "half a dozen of the biggest lenses made" impossible as a target as well.

Either reality creates the desire to blog about 'getting the most' from a single lens investment.

In the same spirit, you write about "the optimal telephoto lens, which permits the most flexibility of use". If flexibility is the highest priority, of course the number of TCs a lens can use is important: but why is flexibility the highest priority?

It's quite obvious why considering flexibility (getting the most from one lens) is so important: cost and weight, not to mention having the most options with one lens, so you don't have to carry/switch lenses if a moment presents itself.

For example, if you're shooting an antelope with a 400mm lens, and suddenly need 800mm for a bird, it's a lot easier to leave the 400mm lens mounted on the tripod, remove the camera, and quickly add an 11 oz (330g) 2x teleconverter that you pull out of a front belt-pouch ... to achieve a very nice 800mm (1200mm equiv. framing on a D500) ... than it is to take a remove the the entire 400mm lens + camera from the tripod, then take your horrifically-heavy backpack off, place it on the ground, then carefully pull out your $16,000, 10-lb (4.6 kg) super-tele out ... mount this to your tripod ... then carefully place the 400mm in the pack, remove the camera, and then re-attach the camera to the 800mm. Your moment is likely gone, your back is likely broke ... all so you can do what? Possibly get a slightly-nicer image?

Doesn't make any sense.

The only way two+ super-telephoto lenses make sense is to operate from a blind, or a safari vehicle (so you're not walking for miles burdened this way), and with each having a camera already on it, both also mounted on a tripod at the-ready.

It is not that you are making an unreasonable choice, but that presenting a choice as a given creates a circular argument.

I'd say you're the one who made an unreasonable counter point.

No one on earth is going to hike with "half a dozen of the biggest lenses made," and not many can afford them either.

And even among those fortunate few who can afford several super-telephotos, they don't bring them 'all' with them on a nature hike

[Ilkka Nissilä]

The 400/2.8 FL with D810 and TC-20EIII has center MTF50 of 2000 (lw/ih) at f/5.6, whereas the 800 FL gives 3441 lw/ih (photographylife reviews). So you get a lot more detail on the duck with the 800mm prime. Even at f/8 the TC provides 26% lower score than the 400mm prime without TC. So at most you get detail comparable with a 600mm prime, when mounting the 2X on the 400. So based on those results the 400+2X is not that good an 800mm.

I would think that if flexibility in framing while retaining the highest level of detail on the subject  is the goal, the D850 with 600/4 would work better than the D500 with 400/2.8. The full recorded image in each case has the same framing but the former setup provides more detail by virtue of the longer focal length and high pixel count. You don’t have to take off the TC or put it on, and still should get similar detail even on a deep crop than the 2X+400, or better wide open. Which probably explains a part of why the D850 is in high demand.

Personally I find TCs quite time consuming to put in (remove TC front cap, store the cap, take off the camera, mount TC, take off the rear cap of the TC, store the cap, mount the camera body all the while making sure that no dust, sand, rain, snow, or other dirt gets inside the camera, the lens, or on the TC, periodically clean the caps and TC etc.). And then you lose so much image quality unless you can stop down to f/8... I think it may be just more pragmatic to use a high pixel count camera to gain the flexibility and speed of framing (just shoot and crop in post) and much better quality when using the whole image or cropping just a bit. If flexibility and ease of use of use in the field (and avoiding loss of subject while switching focal length) are the goals.

Now I do get it that the D500+2X+400 shows a larger subject in the viewfinder than the D850 + 600. But it may be a legitimate question to ask which setup will give better subject detail. My guess is that the longer primary lens wins at least when shooting wide open.

[MILLIREHM]

My two cents again:

it is possible to afford several superteles without being rich - buying them second-hand or doing without a car can help.

It hardly makes sense to bring more than two for a shooting (except you fill up your car and stay in the vicinity)

I can carry the big package for a significant while (for instance to get to a location in the woods) but doing it only when I have got an idea where my shooting opportunity is, not for random events. A full day hike would be too much by far. Dont forget you not only need to carry it but keep the agility to shoot it. Being exhausted and tired (also in your mind) does not help.

I once bought the 200-400 because I realized changing Teleconverters is just not fast enough to react to birds of different size on a location, Switching form FX to DX and back on a camera is much faster if the setup is properly done. I frequently have a longer supertele on a tripod and a shorter one lying on the backpack below to quickly adapt.

It is fine to do careful technical quality comparisons for all sorts of options but IMHO that does not give you the best options, for me its depending on the situation, my mood how I feel in shape to make the choice. Sometimes it turns out that a 200-500 or 80-400 is the better choice though not providing equal technical quality but I might get images i cant have take with the big glass on that day (and vice versa)

[JKoerner007]

The 400/2.8 FL with D810 and TC-20EIII has center MTF50 of 2000 (lw/ih) at f/5.6, whereas the 800 FL gives 3441 lw/ih (photographylife reviews). So you get a lot more detail on the duck with the 800mm prime. Even at f/8 the TC provides 26% lower score than the 400mm prime without TC. So at most you get detail comparable with a 600mm prime, when mounting the 2X on the 400. So based on those results the 400+2X is not that good an 800mm.

There is no question a bare 800mm will produce a cleaner image than a 400mm with a 2x TC.

The trouble is "flexibility" ... which the 800 isn't and the 400 is.

I can shoot both 400 and 800 with the latter + TC, but in no event can I shoot 400mm with an 800.

I would think that if flexibility in framing while retaining the highest level of detail on the subject  is the goal, the D850 with 600/4 would work better than the D500 with 400/2.8. The full recorded image in each case has the same framing but the former setup provides more detail by virtue of the longer focal length and high pixel count. You don’t have to take off the TC or put it on, and still should get similar detail even on a deep crop than the 2X+400, or better wide open. Which probably explains a part of why the D850 is in high demand.

Interesting point

I happen to take 2 bodies with me, typically (D500 / D810), and so a 400 would be a 400 on the D810 and the (dare I say it?) "equivalent" to a 600 on the D500.

Do I really need to print anything bigger than what I can get with a D500? No, I don't (and I doubt most people need to print to the full capacity of D850's ability).

You're also forgetting weight + Af acquisition + buffer, where the D500 has the edge and the D850 not as much.

Further, downloading 1000-2000 rapid-fire D850 files is a long, arduous hassle ... compared to D500 files.

But, hey different strokes.

I might wind up with a 300 f/4 E PF on my bare D500 + 1.4 TC (600mm equiv), holstered in a Cotton Carrier, and a second D500 with a 600 mm + 1.4 TC (~1200mm equiv) on a tripod over my shoulder.

Lots of options, and the weight of the PF on a chest holster is negligible.

Personally I find TCs quite time consuming to put in (remove TC front cap, store the cap, take off the camera, mount TC, take off the rear cap of the TC, store the cap, mount the camera body all the while making sure that no dust, sand, rain, snow, or other dirt gets inside the camera, the lens, or on the TC, periodically clean the caps and TC etc.). And then you lose so much image quality unless you can stop down to f/8... I think it may be just more pragmatic to use a high pixel count camera to gain the flexibility and speed of framing (just shoot and crop in post) and much better quality when using the whole image or cropping just a bit. If flexibility and ease of use of use in the field (and avoiding loss of subject while switching focal length) are the goals.

I don't.

TCs are a thousand times easier to carry (plus take-off, put-on) than super-telephoto lenses.

You also will not get the same benefit cropping as with a TC. Steve Perry proves that in his videos.

Lenses with TCs built-in would be the best solution IMO ... would be ecstatic if Nikon began implementing this technology in their primes ...

Now I do get it that the D500+2X+400 shows a larger subject in the viewfinder than the D850 + 600. But it may be a legitimate question to ask which setup will give better subject detail. My guess is that the longer primary lens wins at least when shooting wide open.

I would be curious to learn this as well, but I would bet the D500 combo is better.

The D500 + 2x + 400mm is the equivalent of 1200mm.

The D850 + 600mm is 600mm.

If you had to crop-in to half the total image size with the D850 ... to achieve what the D500/400mm/2x TC would achieve "filling the frame" ... no way is the D850 image going to be comparable.

The D850/600mm + 2x extender would be a better comparison. Same "equivalent focal length," but here the D850 would have more pixels on the subject ... and, both have the same "hit" with a 2x TC.

I would imagine the D850 wins, hands-down, here ...

All that said, at some point, one has to stop "measurebating," and just shoot

Whether you have a D500 + 400 + 2x ... or a D850 + 600 + 2x ... you're going to get some good shots, and miss some others.

If you don't mind missing some AF moments, and lots of huge, time-consuming files being downloaded at the end of the day ... to be rewarded with an occasional 'perfect' shot taken in a great file size ... the D850 is for you

However, if you want to have better AF, and smaller (less space-wasteful) files streaming onto your hard drive ... and don't really need to make 30" prints with your best images ... the D500 would be the way to go.