Thanks useful information: 
It's no secret that I enjoy using the odd non CPU lens from time to time, but most of the time these lenses are hard(er) to use than a modern equivalent with all the modern features and functions.
Adding a CPU to the lens eliminates some of the difficulty in metering.Using the Dandelion CPU partly assists in the most difficult part .. the focusing.
On my D300, I took the time, effort and expense of installing a Katzeye focusing screen, which greatly helps in determining focus more accurately than the standard screen does when using manual focus lenses.
I've never found the lenses focusing system, ie. manual focusing mechanics and quality to be all that much of help or hindrance to achieving accurate focusing. It is the focusing matte screen that help more so than a lens with 'beautiful focusing quality'.
But! .. one lens has bugged me from day one, and that one is the Nikon 500mm f/8 mirror lens, and getting a reasonably sharp shot from it has always been a challenge and subsequently rewarding.
Although the reward in getting a nice sharp shot from it is more about luck and persistence, I'll take any usable shot I get from it, anyway I can!
Luck, skill, persistence... really who cares, I got the shot and that's that
Although hopefully this will now change.
With the D800, now comes the need for a new focusing matte screen, but before I do that, I also want to try out another system I've been meaning to do for a while, and lucky for me that I held off for a bit(a couple of years) as this Dandelion CPU chip has been improved upon since the first incarnation I read about years back.
For those that don't know, a quick run down on what a CPU chip is .. very simple, it's an added set of CPU and contact blocks added to the rear of the lens(usually permanently) that basically turns your non electronically coupled Nikon mount lens into one with electronic information now passed onto the camera.
There are many benefits in doing this, even for cameras with the ability to add Non CPU lens info into the camera .. ie. D7000/300/700 and higher.
While your old non cpu lens will work well with metering and collect some lens data and save it into the exif data, there are still many advantages of using a CPU chip instead.
That I can quickly think of ..
1. you don't need to continually alter the menu item when attaching different lenses.
2. spot metering works better!!
3. (specific to the Dandelion) focus is greatly assisted.
First of all lets see this CPU chip:
1.
Of course I didn't take the time to take a photo of it on it's own, I only thought to take some photos of the install AFTER I'd installed it.
Also, using the term installed is a bit misleading in this particular instance, as can be seen!.. it was simply glued to the filter ring.
The lens here is the 500 mirror, which is usually a right ol PITA to focus accurately
So my priority was to see how this device works on this specific lens, even tho it's my least used lens due to it's extremely slow nature, dark vf view, non adjustable aperture range, and not the greatest of image quality rendering .... it's merely 'acceptable'.
But what it has in it's favour is ease of transportability, that is it's about the size of the average large 27-70/2.8 sized lens, and if you find you need a super telephoto lens in an instant, it's easy to keep in your bag at all times. Even the slow xx-500 telephoto zoom lenses are massive by comparison, and two of these mirror lenses will fit into the space of a normal sized consumer 500mm zoom.
So this is one lens that has found it's spot in my bag since day one, simply due to this fact but it's rarely used because of the difficulties in actually using it effectively and consistently.
So a couple of pics of how the lens looks with and without the CPU.
2.
lens without CPU(ie. filter removed)
3.
lens with filter attached and hence now it's CPU'ed.
The CPU is programmable, so if I wanted to fool people into thinking it's an 5mm super super wide angle lens, then this is possible. But it has been programmed with the correct info and now when the lens is attached to any Nikon camera it announces itself as a 500mm f/8 G lens .. and the G naming is technically correct in this instance, as the mirror lenses have no aperture ring!
There are other versions of CPU blocks available for purchase and as far I know about these other versions, they're all screw mounted to the lens.
The only worriesome aspect of the Dandelion chip is that it's a glue on solution only. The one aspect that is of concern is long term durability of the mounting procedure.
The 500 mirror lens installation was a doddle! a quick 2 min glue on process, as the CPU has to be glued onto the rear 39mm filter on the back of the mirror lens.
This filter must be fitted to the mirror lenses for two reasons. One is to stop ingress of foreign matter into the optics of the lens(as far as I can tell), but more importantly it completes the optical formula of the lens.
That is, without the rear filter attached to the lens, you won't get proper focusing .. especially through the vf.
I have a few other lenses that will be modded, but I won't be modding the 50/1.2 as the rear element is a major concern for this particular CPU chip type.
But some, if not all, of the other lenses may need to be chopped up a bit with a dremel to allow fitment of the CPU.
Hence the initial installation of this CPU onto the 500 mirror .. speed of fitment and will be used as a test for durability of the attachment process.
If I have any problems with this chip coming off the lens, I may look to another CPU product for other lenses.
So back as to the one single attraction I had to this particular CPU.
It has a feature I've yet to see listed on other CPU products, and that is the ability to use a focus trap feature. That is, this CPU has a feature that allows the user to set the camera to expose only if focus is confirmed first.
I have my cameras set up where the release is the priority, not focus, so that way I can take a photo of a blurry scene if I wish, but with all other manual focus lenses, this is obviously not available.
I'm assuming that other CPU products will also allow this focus trap system of set up, but the difference that I'm reading into this particular feature is that it's separate to the way exposure priority is setup in camera.
(this may be hard to follow, but it eventually makes sense!!)
I have the camera still set to release priority, so that I can take a shot even if focus is not confirmed .. nothing has changed here and I've used this method for years now(my particular preference).
If you set the camera up the normal manner(ie. as default) it may probably be set so that the camera will only expose if focus has been confirmed(I hate that!!)
With manual lenses this is moot, and the only way to get a good image with the 500 mirror was to focus bracket .. usually a series of 3 or 4 shots.
The Dandelion CPU has this feature(that can be turned off if need be!!), where the camera will not fire the shutter unless focus is confirmed.
Initially this may sound annoying to a person that wants the freedom to shoot at any time, but then in actual use it makes perfectly good sense, and is the feature that now allows the operator to achieve accurate focusing with a single exposure.
It seems weird at first, but the way it works is via the AF system. Even tho the lens is manual focus if I press the AF-On button on the camera, the camera will not fire(initially) unless focus is confirmed.
If I don't press the AF-On button, it works as per normal, and I can happily shoot away taking all the blurry shots I'm accustomed too with this lens
To say it's the best of both worlds, is a massive understatement .. it feels in practical every day usage to be the natural way to do it.
So the way to use focus trap is to press and hold the AF-On button, and the shutter release at the same time. Nothing happens .. no image is exposed if the image is not focus confirmed(green dot).
Then you focus towards the subject, and as long as pressure is set on the release, as soon as the focus is confirmed, the camera makes the exposure.
This is manual auto focus. That is it works as AF does, the only difference is that you manually do the focusing yourself(and it's much slower than even the slowest AF system
The real question is: does it work? and the best answer is my oath it does.
If the camera is set to single shot exposure mode, it will take only a single shot and then you do it all over again.
if the camera is set to continuous exposure mode, the first image only will be the focus confirmed image .. all others will be whatever. That is, these other images if shot in continuous mode can also be used to account for subject movement, camera shake, or even micro movements in the fore/aft plane where focus may move in and out of the correct plane.
a couple of images taken to confirm that not only it works as intended, but that it works well too:
D800 is actually 'quite easy!' to get a higher volume of usable images with the mirror lens.
the only processing step was to bump ISO way up as high as possible to maintain a higher shutter speed. Massive NR was used on the image.
D70s image 500 mirror, in the 'dark' and what can only be described as amazing!
Whilst it may not look like it from your point of view, this single image is what stunned me to be honest.
Having toiled with the D70s and mirror for so long, prior to the D300 coming and even then only when I finally got and installed the improved focusing screen, it was close to impossible to get a single workable image from the D70s and 500 mirror.
Especially in such darkness, at such low shutter speeds and basically spot on in terms of focusing and exposure .. well ok exposure is a bit dark here but the poor old D70s is already maxed out at ISO1600 and shutter is already pushing the friendship at the lowly rate of 1/8s .. this is 500mmm and only at 4m focusing distance.
The fact that this image could be called usable, even tho the content is questionable, is something I've never seen in the preceding 6/7 years of use.
I would estimate that in the time I've had the D70s and 500mm, I may have got only 5 shots in at least a thousand attempts that I could call sharp .. and even then I'd say the total number of shots may have been in the multiple thousands .. all I know is that when I deleted all irrelevant files from the archives, the hard drive space contracted by an alarming amount!
Almost all files in the 500 mirror archive were deleted, and as far as I can tell, only 1 or 2 remain.
Once again very heavy NR was applied on the D70s file an that there is any detail remaining is a testament not only to the ability of this focus trap system, but also to Nikon's improved NR routine in CaptureNX2.
(for those that want the guts of the NR step:
Another image using the focus trap method(excuse the subject matter!)
Simple summary: if you are looking to modify an older manual lens, consider the Dandelion CPUs as an option, but beware the caveat that it is a glue on procedure, not a screw in procedure.
So far in the few dozen times I've mounted and unmounted the lens to various camera bodies, it doesn't feel as thou it's going to be an issue, but until a long term test is done, durability of the mounting procedure is unknown. So far that I've researched, this isn't a huge concern, and the quality of glue used will have an impact on this anyhow.(I used Selleys Super Glue to fix mine).
Also note that these CPUs can also be fitted to adapters and other accessories such as extension tubes and suchlike. The beauty of these types of in camera programmable chips is that you can change the programming(tedious but feasible!) to whatever value is required .. an important consideration for adapters and extension tubes where the actual lens may vary.
Because of this ability, I'm also considering fitting one of these chips to my Tamron Adaptall mount too which then allows the ability to program the lens details as required .. ie. from 300/2.8 to 860/5.6.
The cost of this CPU was a reasonable $30 incl postage.
Hope this helps anyone else looking to similar solutions.
- - - Updated - - -
I should have added a 100% crop of an image, but didn't think it to be important, as the lens is not really the type where you'd do 100% crops anyhow.
But for anyone interested here's one, strictly for the purpose of assessing focus accuracy(which BTW looks to be spot on).
some details of the image tho ... no sharpening was applied to the image at all. Not in camera nor in PP. I have setup my own Picture Controls via the Utility in ViewNX2 where I have the contrast controls set the same way as they are by Nikon, but with all sharpening turned off. I prefer to use USM via CaptureNX2 anyhow.
One thing I know is that in the rare instances that I have managed a sharp shot, the 500 mirror can accept a lot more sharpening than other lenses I've seen.
The lack of 'image sharpness' must be due to the lower contrast images rendered by this, or these types of lenses.
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