 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 F2, 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, 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 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 on 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 wanna 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 F2. 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 gonna 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'd recommend before you start it's nice to have a nice clean space. I like a magnetic project mat so that because we're gonna be working with very small screws and you'll also need a small flathead screwdriver and a size 0000 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 flathead 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 00 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. That 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 Bodinov 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 at NebulaFotos.com. Clear skies.