To infinity and beyond??? Focusing question. 18-200 vr

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Okay to take a picture of stars or star trails or mountains, houses with stars, etc.(aka stellar body photography), you put your camera on a tripod, open your aperture all the way...or not depending on the final shot, adjust your ISO for effect desired, then set your focus on infinity...to get the stars in focus...that is the source of info that i have been gleaning from the internet...If you want the stars in focus, the lens has to be on infinity...which makes sense.

Okay...I did that, and the intent of the shots were to get stars in focus.:

Focus at infinity:


looks good...until you zoom in to 100%:


Now here is the same lens at around 4 feet according to the focus ring.:



And then here is the 100% closeup:


(there is some star tracking on this one, but that was expected)

The shots are the same; VR is off, 18mm, amount of seconds...about 27 for first, and 21 for second (which would account for some tracking, but not out of focus)...and the same ISO...everything else is the same, the only difference is the focus.



Now I KNOW the stars are not 4 feet in front of me.:biggrin: So what is up. Did/does the 18-200 VR have this issue? If it is time for a visit to the NOT LOCAL camera shop...what can I expect repair wi$e?
 
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Possibly a couple of things here. First, the first time did you set the lens at infinity or on the stops at max distance? There is a difference. Many lenses will "over focus" on the stops. In other words won't be in focus.

Second, are you sure you set the ring on 4 feet and not 4 meters on the second shot? The reason I ask is that the hyperfocal distance at 18mm and f3.5 is about 15 ft. IOW if you focus at 15ft, then anything beyond will be in focus. 15ft and 4m are just a hair difference on the focus adjustment so if you tried setting at 4m you may have actually been focused at or beyond hyperfocal range so infinity was in focus.

Check out the DOF calculator here
 
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Possibly a couple of things here. First, the first time did you set the lens at infinity or on the stops at max distance? There is a difference. Many lenses will "over focus" on the stops. In other words won't be in focus.

I put it right in the middle of the infinity symbol. There is no line marking actual infinity like on Canon lenses, So I am/was assuming that the middle of the ellipse is infinity.
Second, are you sure you set the ring on 4 feet and not 4 meters on the second shot? The reason I ask is that the hyperfocal distance at 18mm and f3.5 is about 15 ft. IOW if you focus at 15ft, then anything beyond will be in focus. 15ft and 4m are just a hair difference on the focus adjustment so if you tried setting at 4m you may have actually been focused at or beyond hyperfocal range so infinity was in focus.

Check out the DOF calculator here

:biggrin: It's been too long to remember. I do remember that I changed the focus from the infinity symbol down, and it was blurry until I reached numbers at least...IOW from 15 ft-ish to the ellipse the stars were blurry.

Thanks for the DOF calculator, I think it is already on the help me link page, but if it isn't I'll add it...I have never used one before, but now I might need to. :biggrin: I did use it on the lens and the numbers on the lens do seem to focus as the chart predicts they should, it's just I think that the infinity marking...or lack there of is wrong.

I have had a lot of issues with that infinity mark...is there a way to find actual infinity on it...focus on the moon I guess.

So what is the best way to find actual infinity of a lens that is marked only with an elipse...focus on a high cloud, or the moon?
 
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Weeeeellllll now this is interesting.....after doing a little hands on research, the Infinity mark for this particular zoom lens 18-200 vr looks like it is for the 200 end of the lens.

An object at 75-100 ft away was in focus near to the edge of the infinity mark. when it is at 70mm, it is about 1 mm away from infinity mark, and so on down the lens so that at 18mm it is closer to the 5-6 ft mark...oh great here comes more math to try to understand the focus ring on the camera again...:biggrin:

So let me see if my making sense of this lens makes sense to you...the Hyperfocal length of this lens at 200mm and f5.6 is 1160.6 ft. I think that if I focused on something 1160.6 feet away, at 200 mm, the lens' focus ring should be right at the center of the infinity symbol...but if I focused on the same object at 18mm f5.6, the lens' focus ring should read between 15 and 5 ft (according to the DOF calculator it will be 9.45 feet, but there isn't a mark on the ring for that...)

So If I wanted to use the lens to do stars at night, I would first set the Aperture and focal distance...then focus to infinity for that aperture and focal distance...then take the picture...
 
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It's not that simple. You can't just set a focus ring to infinity and expect everything beyond xx amount of feet to be in focus. It is my understanding that there is an intentional design characteristic to compensate for minimal expansion/contraction within the lens itself. Also to allow overshoot beyond infinity to assist in proper AF.

You may want to find infinity yourself and make a physical or mental not on your distance scale.
 

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