[pixie]

Hi, I will be going to some remote places with very little light pollution in the next two months, and have thought of trying some Milky Way and star trails shots. I will be taking most of my arsenal with me: DA15/4, A20/2.8, FA31/1.8 and K50/1.2. I'd like to ask you about the focus ring infinity hard stop. Is the physical limit of the barrel travel at infinity focus? What adjustments do you recommend per particular lens? What are your experiences with these lenses when it comes to focus to infinity? All and any help is very much appreciated, and I thank you all.

[manntax]

Most of my lenses - even manual focusing ones - are reaching the true infinity slightly earlier than the hard stop. I think only my Super Takumar 50mm f1.4 8-elements and SMC 24mm are having perfect infinity at hard stop - all other lenses from my tokina 17mm to SMC 300 are reaching infinity more or less earlier than infinity. The worse example is my Pentacon 30mm f3.5 which I like a lot - it has the true infinity something like at 3m mark - something about 1 cm earlier than the infinity mark.

[DSims]

Nearly all lenses have infinity before the hard stop. I understand this is to allow for temperature changes, aging, and other factors.

I've had all the same lenses you mentioned (except the FA20 instead of the A20). As expected, it's especially important to get infinity right with the faster FA31/1.8 and K50/1.2 lenses - particularly since you may be using them at dusk or later. Typically I'll put the camera in MF mode and use Liveview long enough to set infinity correctly - then I'll go back to the viewfinder for most of the shots.


BTW, those who shoot the stars are often concerned about lens characteristics such a "coma." I don't (shoot the stars), but I know it might be worth looking at the Samyang 14mm lens, even if you wouldn't normally consider one (like I wouldn't). Also, the O-GPS1 may be of use to you Review: Pentax O-GPS1 GPS Unit - Sample Astrophotography | PentaxForums.com Reviews.

[Na Horuk]

Most modern lenses go past infinity, here is why:
a) Temperature - materials change size depending on pressure and temperature. If the lens has infinity at the end, a little change in temperature can prevent you from reaching infinity. Better to have some extra space
b) AF. AF is a fast motor. You don't want it literally bumping at the end of the lens' focus range. Give it some space, so it finds infinity before slamming into the end of the rails.
c) Cheaper. It is expensive to calibrate hundreds, thousands of lenses very precisely. Especially with modern digital cameras, where in-camera AF adjustment is the norm. This means the lenses you buy cost less than they would if focus scales were perfectly calibrated. If you want that, think about buying Leica or something. And even they are following the rest of the camera/DSLR market.

Lenses have to move something to change focus. Usually the barrel moves closer or further from the sensor. This mean there are physical limits to the focus range. The camera brand decides what these limits are, and how long the focus throw will be, when designing the lens. Then there are optical limits - a lens design can only focus so close.

If you want to use infinity, it is best that you learn where infinity is on your specific lens and camera, and then dial that in, manually, when you want infinity. When you are out in a dark forest, at -20C, fumbling with tripod, headlamp, and remote, it is too late. You should know where to focus ahead of time. I can't tell you where to focus your 20mm lens on your camera, as it depends on the specific lens (how it was calibrated, if it was repaired, CLA-ed, what damage it has, etc.) and camera (how its sensor, AF module, focus screen are aligned). The only thing we can tell you is trends, as some lenses are known for being more likely to have certain problems than others. I don't think any of those lenses is notorious for bad focusing. Good luck

[Dave's clichés]

I have set infinity focus on live view and then with a wooden cocktail stick and a bit of white paint marked the barrel with the paint, it leaves just a small dot and when out star shooting there's no doubt that the focus is infinity correct, unfortunately this can't be done with a lens where the focus scale is behind a window.

[08amczb]

When I would like to focus on the stars I use live view magnification and manually adjust the focus. Use the maximal magnification.

[Kendigitize]

What the others said about infinity is pretty much it. It's tough to focus on stars in the dark. Live view helps. I've also set my focus earlier in the evening and put a small piece a tape across the focus ring to keep it set on infinity, make sure the camera focus is set to MF mode! Out of the lenses you list, I would use the 20mm 2.8 for the Milky Way. I use the 28mm 2.8 with great results, just needs a little more field of view for my tastes. I always go up 1 stop to help eliminate coma. But all your lenses will be great for astrophotography.

[promacjoe]

Note: true infinity is a distance that cannot be reached, Nor can it be seen through a telescope.

As others have said, When it comes to infinity focus, no two lenses are the same. When doing astrophotography, you normally focus at the moon, If available, and everything else is said to be in good focus.

[Wingincamera]

When I got my Bower 14mm f/2.8 (also sold as Rokinon & Samyang) I found the distance marks, including infinity, were not close to the actual distances. What I did was place a piece of masking tape to partial cover the lens's marks. Then using a tripod & good light and target dialed in my own distance marks from about 18", 6' and infinity. Now I can quickly dial in the distance I want. You should be able to do this with other lenses as needed.

[northcoastgreg]

All my older, manual focus lens are either at infinity or very very close at the hard stop. That includes the M 20/4, the K 28/3.5, the K 35/3.5, the M 50/1.7 and K 50/1.2. I've had one DA zoom that seemed to reach infinity at the hard stop as well: the DA 12-24. All my other AF lenses focus past infinity, with my DA 15 probably being the worst offender in this regard.

[pixie]

Thank you everybody for your input. Very valuable information from all of you. Everybody here knows their lenses very well, it is impressive. As for the O-GPS1, I see them online for $199 at Adorama, or used ones on eBay for not much less. I guess it isn't worth saving some $40 to buy used and not having warranty....

[whk1992]

Most of my lenses - even manual focusing ones - are reaching the true infinity slightly earlier than the hard stop. I think only my Super Takumar 50mm f1.4 8-elements and SMC 24mm are having perfect infinity at hard stop - all other lenses from my tokina 17mm to SMC 300 are reaching infinity more or less earlier than infinity. The worse example is my Pentacon 30mm f3.5 which I like a lot - it has the true infinity something like at 3m mark - something about 1 cm earlier than the infinity mark.

Same here. My 20-40 Ltd. goes beyond infinity (?!) before reaching its hard stop.