Checking new lenses for autofocus tuning

I just picked up a very nice used AF-S 24mm f/1.8G. It looks almost unused. My testing showed it was softer, even at the center, than expected wide open but it did well after stopping down to 2.8 or 4. Lab tests in the reviews showed that it should have still had excellent center sharpness wide open so I did the autofocus fine tuning thing. Sure enough, it needed a +14. It's a sweetheart now... plenty sharp even at f/1.8.

I wonder if that's why someone sold this lens? It really did look barely used; hardly a mark on the mount. On the other hand, unless you did a careful test, you might never notice any autofocusing error in a wide angle like this. Anyway, I'm glad to know it's performing at its best now.

The testing is fun the first time but gets tedious on additional lenses, but I make myself do it each time I get a new one. I don't use a testing tool; I set up a still life or shoot a cluttered desktop with lots of small detail and with a good target to focus on in the center. I use a tripod and/or flash to eliminate camera movement. I view the focus area at three or four hundred percent and compare all the shots in pairs split-screen until I find the one that looks sharpest. Surprisingly, it's usually fairly easy to pick the winner.

One thing I haven't done is compare the tuning at different distances. Do you think a lens with the autofocus fine-tuned across the room (or closer) will still be at its best near infinity? Like I mentioned, I find the testing tedious, but maybe I'll be inspired to do a tuning at different distances test... or not...

With a prime, keep in mind the depth of field is shallower when your subject is closer than it is when your subject is further away. When it comes to zoom lenses, a longer focal length tends to have a shallower depth of field wide open than a wider focal length.

hark said:

With a prime, keep in mind the depth of field is shallower when your subject is closer than it is when your subject is further away. When it comes to zoom lenses, a longer focal length tends to have a shallower depth of field wide open than a wider focal length.

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I'm such a knucklehead sometimes! I hadn't thought about that... autofocus tuning a wide prime at close range would not only be optimum, but very likely the only way to do it... it would likely be impossible to notice small AF errors at longer subject distances.

I did a little reading on the subject and the well-known FOCAL software instructions has a tool for suggested distances for testing specific cameras with various focal length lenses. The distances are quite close. For example, 3' 11" for testing my 24mm lens on a D750...

desmobob said:

I did a little reading on the subject and the well-known FOCAL software instructions has a tool for suggested distances for testing specific cameras with various focal length lenses. The distances are quite close. For example, 3' 11" for testing my 24mm lens on a D750...

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Interesting find with the FOCAL software info.

With a telephoto zoom, you can also take readings at a couple of focal lengths that are most commonly used and average out the value given when performing the tuning.

I spoke a little too soon in regards to autofocus tuning my new-to-me 24mm f/1.8G. I took it outside for some testing today and shot it and my AF-S 18-35mm f/3.5-4.5G head-to-head. It turns out the 24mm lens is still very soft, even in the center, at f/1.8, 2, and 2.5. The 18-35, wide open, was putting it to shame.

Rather than return it and pay postage, a re-stocking, fee, and then have to find (and take another chance on) another used lens, I decided to ship it off to Nikon for service. Considering all the great values in used lenses I've enjoyed over the years, I guess one bad deal won't hurt me too much. Even with Nikon's repair fee, I'll still be a fair bit below the price of having purchased a new lens by having this one fixed by Nikon.

I've only dealt with Nikon's service department one other time, and my experience was wonderful. When I sent in an F4s film body for a minor repair it was right at the end of Nikon's 20 year service/parts commitment for that model and they still had a lot of parts left. They offered to do a complete overhaul for a small additional charge and I basically ended up with my old chassis and all new internals and electronics. Too bad I finally gave in to digital shortly after that... I still have that camera, though.

An update:

Nikon got the lens back to me very quickly (it helps that UPS Ground service from Melville NY gets to me next-day in upstate NY). They received it on the 14th, entered it into the system on the 15th and sent it back to me on the 17th. They adjusted, aligned, calibrated, checked, etc. and the charge was $209 plus tax and shipping. Another head-to-head test vs. the AF-S 18-35mm f/3.5-4.5G shows marked improvement.

At this point, I'm wondering if this lens was worth purchasing. And I only question that because my AF-S 18-35mm f/3.5-4.5G is just an incredible performer. And that's not even considering the fact that it's a zoom. But the AF-S 24mm f/1.8G offers more speed and a very, very smooth and nice bokeh wide open. I'm eager to get out and use it "in the real world" and enjoy it.

I guess, being older, I have some kind of ineradicable bias against zoom lenses. I do love that 18-35 but assumed true salvation would be found in the well-regarded Nikkor f/1.8G primes. I purchased the AF-S 35mm f/1.8 and love it. I'm sure I'll love the 24mm f/1.8G just as much. And knowing how good and very handy the 18-35mm zoom is, will I use the 35 and 24 often enough to justify the expenditure? Very likely not. But it's a hobby and if buying a couple of lenses adds to my fun, I guess I can eat mac and cheese and ramen noodles for another month or two (or six) to recover from the expense...

I have the 18-35mm f/3.5-4.5G, and yes it is an incredibly sharp lens. Plus it's lightweight so it's a great lens to use with ease and yields impressive results. I also have the 16-35mm f/4 which is longer in length and has VR. Both are stellar. I only had the 24mm f/2.8 from my film days. It was okay but not one I'd rave about unless it was stopped down.

I think that AF-S 18-35mm f/3.5-4.5G might be one of my all-time favorites and a "best-value" Nikkor... lots of bang for the buck!

desmobob said:

I think that AF-S 18-35mm f/3.5-4.5G might be one of my all-time favorites and a "best-value" Nikkor... lots of bang for the buck!

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I opted to add the 16-35mm because I really wanted/needed the slightly wider perspective that 16mm offers. The 18-35mm isn't weather sealed, but now I have the 16-35mm that I can use especially for those inclement days. Only briefly did I consider selling the 18-35mm, but it's too good of a lens, and if I decide to do real estate photography, it will be a valuable back-up lens.

I got to snap a few photos with the Nikon-serviced AF-S 24mm f/1.8G and it seems to be working fine now. I'm relieved, pleased and eager to make use of it.

Some friends visiting the lake for the holiday weekend:

My 24/1.8 is sharp as a tack, while my 35/1.8 is less so, mostly noticeable at closest focusing distance. But I don't really concern myself because these focal lengths meet my needs and are better than my film lenses of old.