A good question I often get about camera lenses and focusing on the stars is why can't we just
turn the manual focus ring to the infinity symbol and all the stars should be in focus because in
terms of a lens everything in the night sky is at infinity. Of course it's not literally infinity
but to the lens it is. For a lens anything further away than what's called the hyperfocal distance
is at infinity focus and the actual position for perfect focus is really a very small range
of focus positions called the critical focus zone or CFZ and that critical focus zone gets smaller
and smaller as you go to brighter and brighter f ratios so if I'm at f/2 like with this Rokinon 135
wide open i have to be very exacting to achieve perfect infinity focus the problem with hard
stops for infinity is that that entire CFZ where you're at best focus actually moves with changes
in temperature the reason is because when glass gets cold it contracts and this contraction
actually changes where the focal plane is and this is why astrophotographers often end up with crazy
systems like this where a computer can change the focus point for you throughout the night
as the temperature changes to recap infinity focus is not one thing but a small range of focus points
called the critical focus zone and that zone is very small with fast focal ratios and that zone
can also move with temperature change and this is the problem with just saying just use the infinity
hard stop that's not precise enough at least not for astrophotography and i know i'll get
the question of why infinity hard stops seem to work in the film days and my guess is just that
it comes down to where the lenses and cameras were then versus now we're really pushing what you can
do with sharp lenses and digital sensors today and the lenses are faster and sharper the sensors
can resolve fault smaller and smaller details and so the focusing ability has to go up so why does
any of this matter well when you've tried as many cameras and lens combinations as i have eventually
you'll get into the frustrating experience of finding a camera and lens that fit together
physically but the lens won't focus to infinity and the stars it's just short and usually it
happens with a manual film lens with a hard stop that's keeping it from actually reaching infinity
focus you'll also find that this is more common when you try with modified cameras of all kinds
because whenever we modify a camera it's very difficult to have the exact same back focus since
we're removing elements changing elements and with some lenses even a fraction of a millimeter
could make the difference between achieving focus or not now i want to be clear here this
can't reach infinity focus problem is mostly an issue with manual lenses autofocus lenses already
have a bigger range at the infinity end because i believe they need it for the way that autofocus
systems work by sort of hunting for best focus so with manual lenses like these when you have
that issue what do you do well my solution is to fix the lens with a little bit of lens surgery
and i know that sounds scary it's not for everyone but to wrap up this five minute friday i thought
i'd show you how to fix a very popular lens for astrophotography this Rokinon 135 f/2 and so if
you're running into this problem of not being able to reach infinity focus with it especially
if you're using it with a modified camera or a dedicated astronomy camera then hopefully this
will help and our goal here with this surgery is to change the position of infinity focus from
being a hard stop right at the end of the focus travel at the right side of the infinity symbol
to just before that on the left side of the infinity symbol which is just going to give
us a little bit of play so as the temperature changes and that critical focus zone shifts we
can adjust the infinity focus and not run out of focus travel to get us there and it's actually a
pretty simple modification but i recommend before you start it's nice to have a nice clean space
i like a magnetic project mat so that because we're going to be working with very small screws
and you'll also need a small flat head screwdriver and a size 0 0 0
Phillips screwdriver and i'll put the link to these things in the description and lastly before
i show this i'll just say that i don't think this is a very risky procedure but you're doing it at
your own risk so only proceed if comfortable after watching this video step one pry the rubber focus
grip up with a flat head screwdriver and push that rubber grip out of the way with your fingers
step two make sure the lens is set to infinity focus and then use a number zero zero zero
Phillips head screwdriver to remove the three small screws and set these aside somewhere safe
step three push the now loose central plastic piece down
to reveal the silver piece that controls the hard stops and remove it
step four adjust the positioning of the central plastic piece so that the left side of the
infinity marker lines up with the focus line and then re-screw in the three small screws
step five push the rubber focus grip back over the lens and test infinity focus on something
far away adjust if necessary okay and now for the moment of truth i live on the third floor of an
apartment so it's pretty easy for me to test with things that are far away during the daytime so
here's the before shot and i'm gonna have to zoom way in here for you to see the difference
so back here this may be hard to see but this is actually out of focus
and i think it'll be clear once i show you the after so this is before when i was having trouble
reaching infinity focus and then there's after and it's really easy to see in this tree
right here but it looks sort of blurry and then after there's so much more detail it's also easy
to see on the right side of this building over here you can see there it's very sort of blurry
and then after the adjustment we have all that detail back and this will also be really clear
on the stars as well especially if you're using a bahtinov mask or something like that if you
have this problem you'll be able to tell and then you'll also know if you fixed it by the ability to
actually reach infinity focus and have a bit of room to spare on the far end of the focus travel
so i hope this was helpful this has been Nico Carver nebulaphotos.com Clear Skies!