Back Focus - Different for all lenses?

[Rick Jones]

I've had the Elite for a couple of weeks now. My first outing I used the 135mm and 85mm and all looked ok. But when I put on a 24mm I couldn't get any medium to far focus. So I adjusted the back focus. But then the 85mm and 135mm focus was out of whack. So I played around later and I've found that I had to adjust the back focus for each lens. None of them have the same plane for infinite focus.

I have the Sony EX3, Letus Elite with EX3 optimized kit. I use Canon FD lenses, 24mm 2.0, 55mm 1.2, 85mm 1.8 and 135mm 2.0. I've made a mark for each lens on the Letus body so I can reset when I change lens but it just doesn't seem like I should have to do this.

Is this the way it's supposed to be? Anyone else have to do this?

[David W. Jones]

Backfocus should be set using your widest lens.
By the way, what was your procedure for setting the backfocus?

[Mitchell Lewis]

You should only have to change back-focus when you change the brand of 35mm lens (like from Canon to Nikon). Obviously you'd have to change the mount as well.

I followed the Letus instructions on how to set back focus and didn't have much luck with it. What worked for me was to just set the lens to infinity and adjust the back-focus so that you get sharp focus on far away objects. In addition, I have a couple of Nikon zoom lenses. I double check that when I zoom all the way in and then back out, the image stays in focus.

I haven't set the back focus in quite a while now.

Nikon Lenses:
17-35mm f/2.8
50mm f/1.4
80-200mm f/2.8

[Bob Hart]

Rick.


Mitchell's wordstuff is pretty much on the mark. Using an infinity target is the safest bet. There is sometimes to be found variation between lenses in build quality, or fair wear and accident will have shifted the backfocus (collimation) of the lenses.

There is also sometimes interference between the ridge on back of earlier Nikon lenses and the mount because of a mismatch in the clearance groove around the outside of the Nikon mount.

In extreme cases the fit will very tight and the aperture ring will be found baulky or jammed altogether.

The outcome in this situation is in combination with a worn lens, the lens focal plane will be moved slight forward of the adaptor focal plane. The lens becomes held forward in the mount against the front face of the lugs by the ridge riding in the groove and there will be a slight clearance between the true flange faces of lens and mount.

Do all of your lenses fit snugly with some slight resistance to rotating the full distance to the pin locking. Are some a loose fit in the mount after locking home on the pin. Are some binding tight before the pin clicks into place.

When you adjust the Elite's backfocus for such a lens to bring the infinity focus back into place, the other lenses which sit normally may then have insufficient focus movement left to bring infinity in sharply and the witness marks on the barrel will be incorrect.

[Rick Jones]

Thanks for the responses guys. All my lenses fit snugly and click into place correctly. I did have to grind down a pin on a couple of the lenses to make them fit but this worked well and doesn't hurt the lens at all. Before grinding down the pin a couple of the lenses wouldn't fit at all or would grind and or bind up when trying to mount on the adapter.

My procedure for setting back focus is the following: I set the lens at infinity and dial the back focus ring until I get an infinite focus. The 135mm and 85mm are pretty close to the same setting so you could possibly not change the setting. The 55mm is a little further off but the 24mm is has spun the dial WAY around.

I had all the lenses cleaned inside and out by a local camera repair shop that's been in business for over 20 years so they know what they're doing.

Anyway, I went out shooting this weekend and whenever I changed lenses I just moved the ring around to line up with my markings for that particular lens. This worked well but it's just another step to slow you down.

[Bob Hart]

With some lenses, the barrel around the outside of the focus ring is adjustable by looseninbg off some tiny grubscrews, then moving the barrel with its metric or imperial witness marks until they co-incide with the sharp focus of the lens at those measured points from the focal plane.

Your lens man may be able to check this for you.

[Francois Xavier]

We have had a few screw loose on our Letus Ultimate, and received replacement screws from Letus and now the Letus is no longer wobbly. However, suddendly we find ourselves with a different backfocus setting for each of our Nikon lens, and one (17-35 f2.8 Nikon) for which we cannot find any backfocus that will make a sharp image. The lens is brand new and works fine on a camera, but on the Letus it just makes a soft image which ever position you select on the back focus ring. This is like impossible, we lost already 3 shoots to this, the image looks sharp in the EX3, but on the broadcast monitor it is soft. We use the broadcast monitor to set the back focus, but in the field you just can't do it accurately on the EX3 monitor.
Until now we had only one backfocus setting for all our Nikon lenses and it worked fine.

[Bob Hart]

Francois.


It may be that your zoom lens is soft. Zoom lenses are more complex and many people discover them to be less sharp at the same focal lengths than prime lenses.

You may have the backfocus correctly adjusted but if the lens is soft it may not be any better. You may find it improves at f4. Groundglass image relay seems to amplify any softness in a lens image.

If your lens focus does not drift when moving through the zoom range, then backfocus is probably correct. It may be what you are getting is as good as that lens well ever be.

I have a 17mm-35mm f2.8 Tokina-for-Nikon zoom and I stopped using it for the same reason. Compared to the prime lenses it was much softer wide-open.

[Francois Xavier]

Thanks Bob, no I tested the lens with a D300 camera and it works just fine and is sharp at all apertures. I also stopped down the lens on the Letus, all to no avail.

The bloody Letus has become a liability, this morning with filmed 2 interviews and could only use one lens out of the 5 we tried. If you are going to buy a DOF adapter I really recommend you either by a cheaper one or one that is worth the steep price. The Letus Ultimate is definitely not worth its price - to me. How could I need to readjust the backfocus for each Nikon lens we are using?

[Bob Hart]

Have you blown your D300 images up to TV screen size. If you shoot a resolution chart, you may be in for a surprise when you compare lenses.

As I previously mentioned, a groundglass has the effect of amplifying quite strongly, any softness in a lens image, be it the lens itself at fault or operator error.

Still-camera zoom lenses are pretty good these days but may not hold sharp focus through their entire zoom range. They certainly won't if the backfocus is also off.

However, chances are, if you are adjusting backfocus for the zoom to operate correctly you may be actually compensating for a build issue for that particular lens and the Letus backfocus for the less complex primes, which are more likely to be internally correct will then be off as a consequence.


There are four possible causes for the backfocus problem :-


The spring pressure arrangement in the mount has been damaged - usually a diaphragm spring or ring shaped spring in genuine Nikon, or a slotted and bent arrangement of the parent metal of the mount of most else.

This arrangement exerts a rearwards pressure on the front face of the lugs on the lens, forcing the flange faces into direct contact. One overtight lens, forced on, is enough to flatten the pressure arrangements in non-genuine designs.


The backfocus assembly is not locking tight after adjustment. - If one particular lens holds sharpness for the same focus setting after being fitted on and off a few times, then this eliminates anything wrong with the Ultimate unless the mount has already been damaged.


Collimation variation beween individual lenses due to build quality, fair wear and tear or damage.


Interference with the correct flange contact between the lens and mount faces due to something holding the lens forward. This is commonly a problem with older Nikon F-mount lenses which have a rearwards facing protrusion related to a mechanical linkage between lens and camera.

Some have a sort of lever extending rearwards off this ring. Sawing this lever off may have enabled the lens to be fitted but it still may be held forward against spring pressure by the remaining surrounding protrusion which will remains.

The iris adjustment may be tight or may have been until the lens went on and off a few times and the surfaces became polished. This has been an issue with several types of adaptors, non-genuine mounts on macro tubes and non-genuine mounts to enable fitment of Nikon lenses to camera like the Canon XL and JVC GY HD--- camera families.

There is a channel cut around the perimeter of the front of a genuine Nikon mount to accommodate this ring on older lenses. The ones with the "lever" may still hang on a modern Nikon mount, notably some Micro-Nikkors. The non-genuine mounts either do not have this channel at all or have a channel of incorrect dimensions and the ring or lever rides on it.

If several lenses have this rearwards facing protrusion, there will likely be variation between lenses as the thickness of this protrusion varies a little and how high the lens rides forward against spring pressure will also vary.


Given that one of you has had to file down some bits on the back of the lens to achieve a fit, this suggests to me that the last problem may be what is going on, maybe in combination with the first, if the armstrong method was employed to make a lens fit on.


If any of the lenses are a loose fit or rattle in the mount, then this has to be fixed before anything else. For long lenses it won't matter a scrap but with lenses wider than 28mm focal length you are going to have a problem.

You may need to use later model lenses or take your Letus - Nikon mount to a machine shop and have the correct relief turned into the mount face to ensure a clearance fit of the rearwards protrusion of the older lenses.

This issue is not unique to the Letus Ultimate. Some other adaptors have this problem as well when attempts are made to fit older Nikons.


If you have time, maybe post photos of the back of your lenses so we can take a closer look.

For a quick check, use a black parcel marker to paint over the flange face of your Letus-Nikon mount, then fit the tightest lens to it enough times to bruise the marker ink off the flange face.

The removed marker ink should be an even pattern all around. If it is not removed on one side, the lens is riding on something or one of the three pressure spring sections in the mount has been crushed or both factors are at work in combination.

[Francois Xavier]

Dear Bob,
Thank you so much for your extensive answer and detailed insight as to how lenses attach themselves and can cause problems.

Well, I took the Letus off tonight after spending yet another hour trying to find an impossible backfocus setting with my sharpest prime lenses. And guess what? A huge piece of glass just fell out of the Letus. Like this. The big lens that sits at the back of the Letus Ultimate just next to the front glass of the Fujinon lens it screws on, well this bit just fell off. No surprise I can't get anything to be really sharp, right?

Did I already mention that the screws had fallen off the Letus before this happened? I think we Letus Ultimate owners paid 5000$ for something that would be too expensive at 500$. Just another Internet rip off I guess.
I ask those of you who really like this accessory to think how much they'll like it after the screws fall off and then the actual glass lens follows suit.

[Bob Hart]

Francois.


I can sympathise with you about the glass falling out. I too would be quite vexed.

However, this raises another possibility for your backfocus problem within the dark recesses of my ageing mind.

There is a known and reported issue regarding video cameras including the RED and SI2K, even though the last two are single-sensor cameras.

Posts relating to the SI2K on the Silicon Imaging forum advise of a backfocus issue with lenses. This is related to the combined thickness of infrared filters and an extra anti-aliasing filter in the SI2K between the lens and the sensor. These also double as a dust excluder which is what the piece of glass is for in the Letus Ultimate and most other adaptors.

In the SI2K, the combination is about 2mm thick. This is a refracting layer. The effect is to alter the optical path. Least affected are the long lenses. Most affected are the wides and of course the zooms. The wides have to be moved closer to the sensor to maintain infinity focus.

In the case of the SI2K, the solution is apparently to use extra individual IMS intermediate mounts with each intermediate mount shimmed for correct backfocus of an individual lens it remains matched to.

P+S Technik are apparently working on a more elegant solution, which I expect will be a backfocus device similar to the Letus Ultimate. This solution would also carry over to the P+S Technik MINI35 and PRO35 adaptors as they also use the IMS intermediate mount system and both adaptors have a glass panel behind the lens.

There is a little bit of wriggle room to a mount for the very widest lens. It might also be used for the next less wide lens, say 9mm and 12.5mm. When matching up a Kinoptik 9.8mm to the SI2K, I found I had to skim a fair bit off an ARRI B to PL adaptor ring to restore sharpness.

That glass in the Ultimate is likely to be close to 1.5mm to 2mm thick. If so, then the same issue may be occurring which puts much but not all of my suggested causal possibilities aside.

If you have not already put that glass back in, it would be interesting if you run another backfocus test with your lenses with the glass removed to see if there is an improvement. If there is, I would be inclined to leave the glass out but to be sure to cap the mount when no lens is fitted. Dust on a spinning groundglass is rarely a problem but dust on the condenser front face will be an increased risk.

If it is any consolation, about every other adaptor out there except a few home-mades may have this issue to varying degrees. It may only the more developed or professional operators who decide to use wide and ultra-wide lenses on adaptors, also those who will be always pushing for perfection who will pick up on this issue.

My guess is that if the relatively thick dust glass has to remain installed in adaptors, then a quick release system for the existing intermediate mounts might have to be evolved. A glass panel thin enough to eliminate this issue would introduce an injury risk to anyone cleaning it so we are not likely to see one for liability reasons. A thin clear gel will not stay put when being cleaned.

Like the P+S Technik IMS intermediate mount system, individual mounts then would be shimmed for correct backfocus and matched to each lens or groups of lenses - like one for wides and one for normal lens and beyond.

The alternative is for the existing backfocus systems on adaptors to be refined and made much more precise to ensure repeatability of marked settings for individual lenses. A strong pre-load spring between the travelling mount and the front of the groundglass enclosure would in most cases suffice. It is a long used and proven method in zoom lenses and a few primes to take up lost movement due to necessary tolerated clearances and wear.

If you re-install that glass panel, use three small spots of water-cleanup white bathroom sealer on the faces with no spillover into the edge. Once the glass is in place, put three more small spots in the front corner. This stuff sets firm but not brittle hard and will retain the glass through impact events heavy enough to wreck the rest of the adaptor. You don't want any in the edge otherwise you might not be able to remove the glass in future.

[Francois Xavier]

Bob, do you recommend I try to glue the fallen lens back on the Letus myself rather than sending it back for repair? It really sounds like a recipe for me leaving fingerprints, smudges, dust and glue flecks all over the lens - if only because I've never done something like this. I will see with my local camera shop if they could do it and rediscover life without DOF control until they do!

[Bob Hart]

Francois.


That's what I would do. However I recommend you to contact the vendor who sold you the Ultimate to be sure you won't void any warranty by doing it yourself.

If you hold the glass up to a strong light it will have fingermarks on it already from being picked up.

If you fix it yourself, first check the glass closely to make sure no pieces have flaked out of the contact face where the glue spots are.

To clean this glass, first go to your camera shop and buy a microfibre lens cleaning cloth. The purple coloured ones seem to be best.

Wash your piece of glass in water and bathing soap, the natural soap cake kind. It won't hurt to wet the cake of soap and peel a bit of wet soap off and touch it onto the glass spread it around and then wash it off. Dry the surfaces with a tissue then polish off the dry surfaces with the lens cloth. Hold it up against strong light to check for marks.

If marks are stubborn or oily, use some kitchen detergent to dissolve them, wash off, then repeat that natural soap wash, then dry, then polish off the dry surfaces with a lens cloth. If you get a bit of grease or dandruff from your hair on the cloth, you may get an oily mark coming back. Flip the cloth over to find a clean area.

The bathroom sealer is a water soluble material in a caulking gun tube. Don't use the sealer which is silicone and smells strongly with an acid odour like squashed ants. That stuff is impossible to clean off from glass.

When you put the drops of sealer on, use a long roasting skewer or satay stick. Squeeze a worm of sealer onto a clean surface. Pick up a small piece with the sharp end of the stick. Dob three or four spots about 1mm to 2mm wide on the face of the ledge inside the adaptor where the original glue spots are.

I would simply rest the glass on my thumb, turn the Ultimate face down and raise the glass into the hole until it beds onto the internal shoulder, then turn it over and pull my thumb out, let the sealer set and clean up the front face of the glass later. As long as your rear face is clean, you can always get at the front face afterwards.

Just push the glass on hard enough to spread the glue spots wider but not too hard because they will become too thin and some sealer may squeeze out and drop onto the groundglass. If the spots grow to about 5mm wide, that is okay. It does not matter if the glass is not touching the shoulder all the way around. As long as it is on the three or four spots of sealer, that will be okay.

If you want to be more thorough, you could perhaps get hold of a small suction cup like you find on an old fashioned child's bow and arrow toy, or a suction cup on a stick used for hand-lapping valves in automotive engines.

Use this to hold the glass as you place it into the Ultimate from above with the front facing upwards.

You could use a piece of Blu-Tac on a piece of wooden dowel or a pencil to support the glass as you put it in but it is likely to come off and go sideways in the hole.

Do not borrow a valve grinding cup from a mechanic because it will be contaminated with grinding paste which will scratch your glass when you clean it off. Buy a new one from an automotive accessories shop.

Cleaning glass properly takes a bit of practice but it can be done.

Look at it this way. If you return your Ultimate to the vendor, it goes via courier. It risks getting a good beating on the journey and may come back to you damaged in some other way.

[Francois Xavier]

Wow, Bob you could have your own TV show about advanced DIY optics maintenance and creation! Thanks for those detailed and precise instructions!

Now I just spoke to Letus and they offered to send me a replacement part. You see, the glass is damaged where it kept banging on the my Fujinon lens, and you can just unscrew the back end of the Letus Ultimate and replace that part with a new one - my preferred option.

But I'll give a try to the bare EX3 for a while, I shot Letus only for months now and it will be much lighter in every sense of the word without the adapter on.

Thanks again for your support!