"True" infinity distance for focus

[kidblue]

I'm trying to determine super-sharp infinity focus on my M6. After searching, I'm finding conflicting reports on just "how far" infinity can be judged. The safest bet is undoubtedly The Moon or something multiple miles away, but I guess my real question is whether there is any discrepancy between the "last" meter mark on the lens and the infinity mark.

In other words, if my last meter markings on the lens are "20m" or something similar before the "infinity" mark, is everything past 20 meters infinity or are we talking about minute adjustments between 20 meters and "true infinity", like a mile or more?

Is infinity 50+ meters or much farther?

Thanks,
Noah

 

[Roger Hicks]

Depends on the focal length, the format size and the aperture.

With a 20/4 on 35mm, 20 metres is 'infinity'. With a 300/2, it isn't. The markings... well, they're just markings, and the manufacturer can choose how to space them.

Cheers,

R.

 

[ferider]

Adding to what Roger wrote: as a rule of thumb, multiply the focal length by 1000. After that you are "good enough". So for example, with a 50mm, everything 50m and beyond is "infinity". Of course this also depends on lens speed, enlargement, etc., it's just that, a rule of thumb.

dofmaster.com can calculate hyperfocal distances for you. For example, with a 50/1.4, on 35mm film, the hyperfocal distance is 59m. Everything beyond is in focus (for the goven CoC).

I point at a star to adjust infinity alignment.

Roland.

 

[kidblue]

Thanks for the tips. The way some of the anecdotes made it seem was that infinity had to be a zillion miles away because of some minutia between the final focal marks and the infinity mark.

My M6 finds infinity a little short on multiple lenses. Just a hair short of the mechanical stop while focusing is "infinity". When I turn the lens all the way to the mechanical infinity it's just barely off. Is this what I should be very carefully adjusting via the big daddy screw near the top of the lens mount?

 

[Steve M.]

In practical terms, most lenses will be in focus at infinity as long as it's achieved roughly around 150 feet. That's about the distance of a light pole I use by our home for just such purposes, and I've never had any issues w/ the literally hundred or so lenses and cameras I've used to set it there. This may not jibe w/ what's written, but again, in actual use it works perfectly for me. Probably because at those distances the lens is stopped down a bit from wide open.

 

[kidblue]

Anecdotally, I'd like to point out that 150+ feet is a lot farther than it looks. I've been using the steeple of a house across the street and it's only 60 or 70 feet away, even with the altitude.

Is that tiny adjustment done on the "big screw" truly as easy as it looks for the sake of finding infinity?

 

[Roger Hicks]

Anecdotally, I'd like to point out that 150+ feet is a lot farther than it looks. I've been using the steeple of a house across the street and it's only 60 or 70 feet away, even with the altitude.

Is that tiny adjustment done on the "big screw" truly as easy as it looks for the sake of finding infinity?

Yes, but a right-angled screwdriver makes it a lot easier. And it is quite a tiny adjustment.

At 60-70 feet I'd expect the focus on the RF to be slightly closer than 'infinity' with many coupled lenses.

Cheers,

R.

 

[tlitody]

Infinity in practical terms is determined by the ability of the lens to resolve detail.As you open the aperture the theoretical possible circle of confusion becomes smaller and therefore infinity moves further away. With a perfect 35mm lens at F1.4, the hyperfocal distance would be 460 meters. So practically speaking anything 500meters plus away is infinity.
Thing is that lenses aren't perfect so in reality it won't be that far. At F4.0 with a perfect lens(no aberations) hyperfocal distance would be 57 meters but in reality it will be less because the lens isn't perfect.

But for your testing of your 35mm lens assume 500 meters and you'll have it covered at all apertures. But it varies with focal length. A 50mm lens and you are looking at 1000 meters at F1.4 and with an 85mm lens 3000 meters. But again these are theoretical limits. Reality won't live upto the theory.

 

[kidblue]

Thanks for the tip regarding the adjustment - I'm anxious about it, but it really does look simple.

I had been using a really soft Russian 28mm lens (LTM on a M adapter) and found it to be really soft, even at f5.6+. I thought it was the lens being old and crappy, to be blunt. I found checking both another 35mm LTM lens and a proper M 50mm that infinity was too "early" on all of them. It's as if I hit infinity and then it goes a hair past it. Is this common and really easy to fix?

 

[Roger Hicks]

Is this common and really easy to fix?

Yes and yes. I've even adjusted it in the field. Well, actually, on top of a castle, in Spain as far as I recall.

Cheers,

R.

 

[kidblue]

I'm just a dude in Los Angeles. Not nearly as sexy. Did you just stick a well-fitting screwdriver in there and gingerly turn?

 

[Roger Hicks]

Did you just stick a well-fitting screwdriver in there and gingerly turn?

Yup. And bear in mind that if you overdo it, you can always turn it back the opposite way. It really is quite easy, but you don't need to do it often: once it's done, mostly, it stays done. I've had to do it maybe half a dozen times in 35 years of using Ms.

Cheers,

R.

 

[kidblue]

Not to beat a dead-horse, but this is really to ensure no back-focus and closer focus, correct? Because even if infinity "overshoots" a bit, it's still "infinity" as "seen" by the lens, right?

 

[Roger Hicks]

Not to beat a dead-horse, but this is really to ensure no back-focus and closer focus, correct? Because even if infinity "overshoots" a bit, it's still "infinity" as "seen" by the lens, right?

No. It's to ensure that the RF and the lens agree. Scale focusing will still work but that's not why you bought an RF camera. Nothing AT ALL to do with either back focus or close focus.

Cheers,

R.

 

[ampguy]

first

first

I would try a thinner adapter. You can get a pair of inexpensive calipers that measure thickness to 3 digits at harbor freight.

The only way to know if you are exactly at infinity, is to have a system that goes beyond infinity. This is built into more advanced focusing systems like AF DSLRs, and even the Hexar AF.

Thanks for the tip regarding the adjustment - I'm anxious about it, but it really does look simple.

I had been using a really soft Russian 28mm lens (LTM on a M adapter) and found it to be really soft, even at f5.6+. I thought it was the lens being old and crappy, to be blunt. I found checking both another 35mm LTM lens and a proper M 50mm that infinity was too "early" on all of them. It's as if I hit infinity and then it goes a hair past it. Is this common and really easy to fix?