Nikkor 24-70mm issue

[Wayne]

Use a ruler and shoot almost parallel along it, say focus on the 15cm mark of a 30cm ruler at say 2m from the camera at various fstop settings. This makes identifying front or back focus easy.

[kiwi]

Kiwi, are you sure about f/8? I would have used f/5.6 or f/4 to give more definite OOF areas. (Or used a range of apertures)

Yeah, good point, use 2.8 through to 8 on the object. Waynes ruler is a good idea too. Just get something that has good doF markings and get close enough to fill say 1/4 of 1/2 frame with it

[swifty]

Hi DJ,
I don't want to confuse you further but there are a few more points to consider after you've determined whether your 24-70 is focussing properly.
Assuming the focus is OK, there are a few quirks with this lens that might explain some of the lack of sharpness issue.
Firstly there is noticeable field curvature at 24mm. Since you use DX, the effect is less but I definitely could detect it when I use to have this lens and used it on an S5 (also DX camera). It basically means if you're focussing on a flat object, say in the middle, even if the middle is in focus, the corners might not be (not that the corners arn't sharp). Problem goes as you stop down.
24mm is not the lens' strong point. It really shines in the mid to long focal lengths, at least on my copy.
As with all lenses, the sweet spot for sharpness is usually a few stops from maximum but although this is still true for the 24-70mm, I certainly had no problems with sharpness at 2.8 which was where I used it most.
But as others have mentioned, regardless of all these 'issues,' if the lens is not a dud, there should be at least one point in the photo (not necessarily where you AF) that should show good sharpness and the tests the other forum members have suggested should show you whether there are focus problems, that should be eliminated first.

[dbose]

Thanks guys - I will use the ruler and try and shoot at F2.8 through to F8 and see what the results come up. I'll post them in this thread. I do shoot RAW so will use cs5 to sharpen the images.

Kiwi, after all of these I still think it'd be good if I could meet you next week for a quick test - if that's still ok with you.

[kiwi]

Yip. Shoot these tests using jpeg too

[Wayne]

A note.

When shooting these tests, shoot jpeg if you like , but make sure the incamera settings are 0 for sharpness. I prefer to shoot tests raw and simply convert them to full size tiff for examination. That way you can see raw data that has no sharpening applied and it not only determines spot on focus but whether or not your sample of the lens is a sharp or soft copy.

[dieselpower]

Yip, and d90 has fine tuning too I think

Definitely doesn't have it - not in software anyway.

The AF system in the D90 is based on mirror refraction. If the AF sensor is a slightly different distance to the image sensor, then what the AF system reports as in focus and what the sensor grabs are completely different things.

I had issues with my D90 back focusing when I first got it and had to take it to Andersen's at Capalaba to get it re-calibrated (under warranty). It was way, way out was their diagnosis when they gave it back. I was noticing it with ALL lenses, not just one particular one (though my 50mm/f1.8 was the worst).

I used focus charts to test - basically grids on paper, with a target to focus at. I did multiple tests - using AF, manual focus using my eye to tell me when it was right, manual using the indicator in the view finder and live view. Live view got it right 100% of the time (it focuses using the image sensor, not the AF sensor) as did using my eye. The AF system and the indicator in the view finder were both out (the indicator while manually focusing relies on the AF sensor for its information).

Here's what I'd do for a quick, easy test -

1. Set the camera up on a tripod, or make it so it will not move
2. Turn off vibration reduction on your lens
3. Grab a series of batteries and line them up getting progressively further away from the camera, or use a ruler and shoot it at a 45 degree angle.
4. Fill the frame with the batteries/ruler and make note of where you focus using single point AF and your center focus point
5. Take a test photo letting the camera AF
6. Take a test photo manual focussing but using the focus indicator in the view finder
7. Take a test photo manual focusing, ignoring the viewfinder indicator
8. Take a test photo using live view
9. Try with several different apertures to eliminate lens softness
10. Compare results in your computer

It can't hurt to do this with all your lenses to compare the results. If it's front focusing, you'll see the point that is in focus is in front of where you anticipated. If it's back focusing, the point in focus will be behind where you anticipated. Hopefully, it's neither! If it's an AF calibration issue, you should see the auto focused shots are out, the ones with the focus indicator are out, the one where you focused to what you could see (provided your eyesight is good!) should be ok, and the liveview should be spot on.

If you see that it's out with the AF and the focus indicator on all your lenses, then it's an AF calibration issue and will need to go to Nikon for calibration. From what I've read and heard it's a relatively common problem.

What this setup achieves - stops camera movement and eliminates user technique problems. Basically provides a controlled test environment

This may show up the problem, or it may not, but it's something to test and only takes about 10 mins.

[dbose]

Ok here are the retests. I used a flat book. The camera was set up at 45 degrees on a tripod. There are 8 images. The first 4 were using AF (starting from F2.8, F4, F5.6 and F8) with 1/200 th shutter speed and external flash. The next 4 were with manual focus and same settings. All shots were aimed at the middle part of the writing. Other settings were ISO 200 taken in RAW and then sharpened using cs5 and saved in jpg. I can see the sharp bits but still think the images are a bit noisy (?) or could have bee shaper?

[kiwi]

the books dont look perpendicular to the camera ?

[dbose]

The next 4 with manual focus...

[dbose]

the books dont look perpendicular to the camera ?

Yeah you are right - setting it up was difficult and I did it quickly (kids screaming behind my ears)...will try out again...but would these give some of idea regarding the lens? I am back at work from tomorrow and will call you to see if we can meet sometime.

[kiwi]

not much help sorry, tests have to be done properly to form any conclusion. just call at some stage, no worries.

[Wayne]

I think they are too underexposed to really see the point of focus properly.

I suggest the ruler, it really does make this test very easy.

Set the strobe to iTTL so that you gain correct exposure.