Do I Have a Defective Lens, Or, How Far Away Is Infinity?

Hello,

I bought my first DSLR recently. It's a Nikon D70. It came with two lenses. The camera and one of the lenses were refurbished by Nikon, according to the store I bought the kit from. The other lens is brand new.

I need some info/guidance regarding the AF Zoom-Nikkor 70-300mm f/4-5.6 G lens. I have been having trouble getting sharp focus in manual focus mode. The problem is probably me, but perhaps there is a malfunction of some kind with the lens or the camera. (Incidentally, the kit I bought also included a AF Zoom-Nikkor 28-80mm f/3.3-5.6 G lens)

I noticed the problem on Saturday when I took the camera out for the first time ever. With my Canon G2, I could take crisp landscape photos in MF mode just by setting focus to infinity and aiming the camera at far away subjects.

When I tried this with the D70, I was dismayed when I downloaded the first photos that the ones I took with AF were tack sharp every time, and the MF shots were blurrry. I was trying lots of different focal lengths and aperures and both without and without a tripod. Whenever I switched to MF, extended the zoom to 300mm, and set focus to infinity, I got blurred pictures. I also noticed when I backed the focus ring off the infinity mark just a tad I did get crisp photos. It's like the actual infinity "mark" is too far to the left.

But I dunno. I'm a newbie.

Monday night I tried taking photos of fireworks from the roof of my apartment building. The bursts were anywhere from 1/4 mile to 3 miles away. I set the D70 to MF mode. I set the shutter to 1" and I tried different apertures from f6.3 to about f/16 IIRC. My ISO varied from 200 to 800.

I played around with the camera for the better part of an hour. I used the self timer for most shots, and the D70 was mounted on a tripod. I was amazed when I saw later on that not one of my fireworks photo came out clear.

What might I be doing wrong, or is there possibly a MF defect with this factory refurbished D70 and/or lens?

Thanks In Advance

I haven't actually tried that particular lens, but I will mention a few things regarding infinity focus.

First, only some "zoom" lenses are actually parfocal, that is, they truly maintain the same focus as you change focal lengths. On the ones that are not, the infinity position on the focusing scale may not be as precise because the design must allow the lens to achieve infinity focus (which will not be at the same position) for all focal lengths.

Second, some lenses are also designed with a "safety margin" that goes slightly beyond infinity focus. This is designed to compensate for the greater thermal sensitivity of certain low dispersion glasses, which can effectively change the focal length (and thus the focus characteristics) due to temperature variations. Lenses with ED, LD, APO or UD designations, which include elements with some amount of fluorite (a mineral with low dispersion), or even artifical fluorite, tend to exhibit this characteristic most strongly, and longer focal lengths often further increase this phenomena.

Finally with the advent of plastic construction and molded, rather than machined, lens barrels, many lenses are no longer adjusted at the factory for infinity focus (which used to be a manual operation involving a collimator and a technician setting the infinity stop). Instead, in era of autofocus and "new" (cost-saving) production technologies, manufacturers are increasingly relying upon production tolerances without final finishing or adjustment.

I would first check to see if the lens focuses accurately, and see if it achieves infinity focus correctly, for the range of focal lengths, even if this is not exactly at the infinity symbol on the focussing scale. Shoot with your lens at the widest possible aperture, to minimize depth of field and thus most clearly reveal any focusing errors.

You might want to compare your lens to one at a store to see if they focus on far away(over a couple of miles) subjects at close the same point of the focusing scale. If you do this, or if you compare your experience with someone else who owns the lens, on this forum, just make sure both lenses are adjusted to the same focal lengths.

Hope this helps.

Tony

That lens (and a lot of others) focuses beyond infinity. I have one, and I also fell into the same trap - turned the focus all the way to the stop, and got blurry results. I've heard that they do this to allow tolerance for expansion/contraction with temperature. Tony's suggestion that it's part of the cheap way of making stuff may very well be the right answer.

Manual focus with an AF camera is tough. The lenses don't come with focus scales, the focus ring action tends to be too fast, and the viewing screens on cameras like the D70 are not designed to help.

For what you were trying to do, I think you'd need to let it AF to some distant object, then switch to MF and try to not bump the focus ring.

Ken Plotkin

Thanks a lot for the detailed answer! Wow! I will use your advice and I wanted you to know it means a lot to me. I did not know it's okay to switch focusing modes before taking the shot.

I have one other question if you would like to fill me in. I was also using a Panasonic FZ15 that provided the equivalent of a 35mm-432mm range, but after using the Nikkor 300mm, I would bet the "shorter" Nikon lens is zooming in on far away subjects more closely than the FZ ever did, and I don't understand how that can be so.

Thanks to all in advance!

The Nikkor 70-300 has a 35 mm equivalent field of view of 105-450, which makes it a little longer than the 432 equivalent of your Panasonic.

But focal length specs are kind of like like horsepower ratings on cars - what you get isn't necessarily what they claim. I have never checked this myself, but I've seen some very credible posts here from people who have made careful measurements.

Ken Plotkin

Why manual focus when the autofocus works? Use AFL (auto focus lock) if you have to.

Your eyesight and the diopeter correction may be out of adjustment too.
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Pete Biro

I thought using MF set to infinity would help with shooting fireworks and other high speed subjects. Doing so certainly helped my compact cameras catch action!

Dave

your fireworks issue.

You can't really use autofocus on fireworks, but is should be easy to manually focus by eye, because the distance doesn't change significantly from shot to shot.

Shutter speed and exposure, on the other hand, is quite another issue. For that, there are a number of books that cover the subject. You may be under or overexposing your shots, which will make them look out of focus, or may be using too slow a shutter speed. It may not be the lens at all.

Sometimes point & shoot cameras are much easier to use - that's why they are called p&s's. They also lack the flexibility to take better photographs.

It does make sense, but as you discovered there is no calibrated infinity setting on your 70-300. You need to focus on a distant object - either manually or via the camera's AF - then lock that.

Pete's suggestion about AF lock is very good. Look up the settings for the AF-L button on your camera. Set the custom setting for that button so it serves as a focus button. Then you can point the camera at some suitably distant (and hopefully illuminated) object, push the button, release it and you're set. Just don't bump the focus ring.

Ken Plotkin

Sounds like a plan! But can you tell me when I am going to see fireworks again too?

Dave

Most firework displays fire the pyrotechnics straight up. so if you use the AF to focus on the launch site, then switch to manual you should be okay?



This shot is pre-Nikon, didn't that Dimage 7 barrel at wide angle!!!

Hopefully I can get back to the next big River Thames fireworks with the D70 and a Nikkor or two!!

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Kevin England
http://www.pbase.com/england
http://www.photoboxgallery.com/england
Dee Seventy User

That's a spectacular shot! I hope I can can do as well the next time!

I guess owning a D70 and a couple of lenses does not an experienced pro make!

Dave