Why backfocus on DOF adapter
- Thread starter booggerg2
- Start date
Chris Santucci
Veteran
Backfocus isn't something that concerns me with the adapter. It's a nice idea to be able to depend upon the lens indexing for distance, but if you're using still camera lenses (which I am) I don't sweat it. Especially if your using an auto focus lens which has a ridiculously short focus throw.
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Lenilenapi
Carbonite Member
Chris,
if your back focus is off, you may never hit infinity at all, your wide angle lenses may never focus properly at open apertures and your zoom lenses won't track focus at all. Its a lot to give up.
Lenny Levy
if your back focus is off, you may never hit infinity at all, your wide angle lenses may never focus properly at open apertures and your zoom lenses won't track focus at all. Its a lot to give up.
Lenny Levy
Chris Santucci
Veteran
Yeah, I get the infinity focus thing, but if I have a 20mm on the adapter or even a 28mm and I'm anywhere close to infinity focus on the lens, the depth of field should carry the focus.
Right?
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Right?
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DO any of your zooms actually track? I haven't tried it yet, but do SLR zooms track properly?
I'm looking at replacing a heap of my primes with the Nikon 28-70, so that'd be nice if it tracks.
It's not just the infinity, but your near focus as well. Having the backfocus off is just never a good thing at all.
Lenilenapi
Carbonite Member
I never thought my zooms tracked until I took my adapters to a collimator at a still lens shop. They were all off and after I collimated them properly the zooms tracked perfectly.
Nikon 80-200 f2.8 and Tokina 28-70 f2.8. I believe it was the same for my Tokina f2.8 80-200 but i can't recall.,
i thought my back focus was correct before I did the collimation. Quite enlightening. I don't think you can do it by yourself accurately except with a zoom that you know tracks correctly.
- Lenny
Nikon 80-200 f2.8 and Tokina 28-70 f2.8. I believe it was the same for my Tokina f2.8 80-200 but i can't recall.,
i thought my back focus was correct before I did the collimation. Quite enlightening. I don't think you can do it by yourself accurately except with a zoom that you know tracks correctly.
- Lenny
CinemaElectronika
Well-known member
Lenny,
Would you please explain what "the collimation" is... and how someone that have an adapter like an M2, can perform it?
By the way, I know how to calibrate the back focus on the RedRock.
Thank you.
Leni, how bad does this lens breathe?
Backfocus refers to the lens focusing it's image properly on the sensor which is in this case the ground glass in the adapter. Different lenses are designed to be spaced at different distances from the sensor. This distance is called the flange-focal length or FFL. If a lens has a flange focal length of 40mm then it needs to be 40mm from the gg in order for it to focus properly. If the lens is to far away you won't be able to focus to infinity (but you will get better close up focus). The opposite occurs if the lens is to close: You won't be able to focus as close up. In either case having the FFL be off makes the barrel marking on the lens useless because they are calibrated for when the lens is properly distanced from the sensor.
Collimiation refers to adjusting a lens so it is focusing properly. It more or less means the same thing as calibrating back focus.
Thanks for the explanaition, could back-focus affect overall sharpness/image quality???
Chris Santucci
Veteran
I can tell when I have focus with my primes, and I have yet to even use a zoom on an adapter.
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Lenilenapi
Carbonite Member
A collimator is an optical device that precisely determines proper backfocus of a lens. It is used in all lens repair houses and in all motion picture rental houses. It projects a pattern through the lens that is optically designed to mimic light from infinity. When the lens is at infinity you can see the pattern become perfectly sharp on the film surface.
It takes some experience to use properly because some lenses are meant to end their movement exactly at infinity and some go a hair past it. Motion Picture zoom lenses are designed to keep their focus from wide angle to telephoto and if their backfocus is off even a hair they will not track properly.
You can get away with sloppy backfocus more with a prime than a zoom so many don't notice it or like Chris don't even care. If you're not far off you can get away with it in many circumstances.
When will it bite you? Well if your infinity is off you won't reach it at all or you may tend to go past it. A wide angle at open fstops will tend to be compromised more and it is harder to see the focus clearly on a wide. A zoom lens is compromised by not tracking. I thought I had adjusted my adapters with my primes carefully until I actually brought them to a shop and tested on a collimator. all 3 of my adapters were off and my personal experience after 5 adapters is that every single one was wrong from the shop.
I don't personally trust the method of setting backfocus by matching the focus marks on the lens. Still lenses aren't designed to be that perfect. Getting close to the infinity mark is probably the best method outside of a collimator and getting a zoom to track (if it is a zoom that is supposed to track) is probably the most precise.
Davide's explanation of back-focus was good except that "Different lenses are designed to be spaced at different distances from the sensor" is not quite accurate. I think what he probably meant was that different lens mounts require different backfocus distances so that say, all Nikon F lenses should have the same back focus. All canon would have a different backfocus length.
Hopefully your still lenses have their own backfocus correct but in fact that is another parameter in all this and a lens could be off. Generally Nikons and Canons are probably pretty reliable.
Lenny Levy
Now that my backfocus is precise my lens focus just at infinity , my zooms track and my footage marks are pretty accurate.
It takes some experience to use properly because some lenses are meant to end their movement exactly at infinity and some go a hair past it. Motion Picture zoom lenses are designed to keep their focus from wide angle to telephoto and if their backfocus is off even a hair they will not track properly.
You can get away with sloppy backfocus more with a prime than a zoom so many don't notice it or like Chris don't even care. If you're not far off you can get away with it in many circumstances.
When will it bite you? Well if your infinity is off you won't reach it at all or you may tend to go past it. A wide angle at open fstops will tend to be compromised more and it is harder to see the focus clearly on a wide. A zoom lens is compromised by not tracking. I thought I had adjusted my adapters with my primes carefully until I actually brought them to a shop and tested on a collimator. all 3 of my adapters were off and my personal experience after 5 adapters is that every single one was wrong from the shop.
I don't personally trust the method of setting backfocus by matching the focus marks on the lens. Still lenses aren't designed to be that perfect. Getting close to the infinity mark is probably the best method outside of a collimator and getting a zoom to track (if it is a zoom that is supposed to track) is probably the most precise.
Davide's explanation of back-focus was good except that "Different lenses are designed to be spaced at different distances from the sensor" is not quite accurate. I think what he probably meant was that different lens mounts require different backfocus distances so that say, all Nikon F lenses should have the same back focus. All canon would have a different backfocus length.
Hopefully your still lenses have their own backfocus correct but in fact that is another parameter in all this and a lens could be off. Generally Nikons and Canons are probably pretty reliable.
Lenny Levy
Now that my backfocus is precise my lens focus just at infinity , my zooms track and my footage marks are pretty accurate.