Backfocus, frontfocus, lensfocus!

[LowKey]

Here's a weird one for you guys and gals. My D70 had severe backfocus problems, so I did some adjusting (Instructions on the www) and things were better. I then managed to scrape up the cash to buy a used 80-200 f2.8 lens. Great stuff! - however at close range the lens appeared to be soft when I shoot wide open. At further distances it is nice and sharp.

So - with the devil in me I decided to shoot some focus test shots to see where the problem was. The results were interesting to say the least. The question now is quite simply: where is the problem?? Camera or lenses?

And before you ask - I shot the tests twice, and both times the results were the same

Sigma 90mm macro - spot on centre
18-70mm Kit lens - 10 - 15mm backfocus
Nikon 70-210 f4-5.6 - Front focus
Nikon 80-200 f2.8 - back focus
Nikon 70-300 f4-5.6 - spot on centre (no picture)

[gstark]

Just a couple of questions about the technique that you used to shoot these targets ....

How far away from the camera was each of the targets as you shot them? You weren't, for instance, closer than the minimum focus distance for each lens? Remember too that the each lens will have a different minimum focus distance.

What focus indicator and method was in use in the camera as you shot each of these targets? It's not evident from the photos, but a couple of them look as if you're using the centre focus point, but that's not where your central marker is positioned. That will certainly cause the exact effect that we're seeing here, but we simply can't tell because we don't have all of the data.

[atencati]

Also, did you shoot handheld or on legs. While these tests are helpful they are also very precise. I would guess from the shots that it is not the camera but maybe differences in DoF and focus distance as opposed to focus issues.

Good luck

Andy

[robboh]

Here's a weird one for you guys and gals. My D70 had severe backfocus problems, so I did some adjusting (Instructions on the www) and things were better. I then managed to scrape up the cash to buy a used 80-200 f2.8 lens. Great stuff! - however at close range the lens appeared to be soft when I shoot wide open. At further distances it is nice and sharp.

I cant see pixspot pics here at work

However, I have the 80-200 2-touch f2.8 and was also rather disappointed with it at min focussing distances at wider apertures. To give you an idea, my consumer-grade 70-210 f4-5.6 seems to be sharper at min focus distance/wide open. I also confirmed that this WASNT a backfocus issue or DOF issue.

Since then, Ive also found a comment on Bjorn Rassletts site mentioning the same thing; the 80-200 f2.8 2-touch isnt particularly sharp at combinations of min focal distance and wide apertures, especially at the long end of the zoom.

My kit lens also appears to suffer from a touch of backfocus, as does my 50mm f1.8, however my new 70-200VR appears BANG on and is knife sharp at f2.8/wideopen/min focus distance.

Finally, there is some debate over the validity of the 45deg backfocus tests due to the angle causing the AF sensor to get confused and also that you dont know which bit of the sensor has actually picked up the focus.

Those that think that way seem to think that the optimum way to test for backfocus is a tripod with a big H (eg the backfocus chart) which is perfectly parallel with the sensor plane. IE tape the target to the wall and get the camera lined up horizontally, vertically on the targe and make sure that the camera is parallel to the target. Use AF to get inital focus, focussing on the crossbar of the H. Then set the lens to manual focus and move the tripod back and forwards slightly, taking pics.

I also think a set of dominos (say 5) set up left to right and staggered front to back would be a good test. Focus on the middle of the 5 and this will give the AF sensor a flat plane to focus on and variable distance to check for back/front focus.

HTH
Rob.

[Onyx]

Lowkey - I was in similar position and did as you did, self-adjusted with Tim Jackson's focus chart.

1) I wouldn't trust the kit lens' results.
2) as mentioned - be mindful of min focus distances for each lens.
3) AF-S motor in cheaper lenses (such as kit lens) aren't exactly precise. Double tap-half depress the shutter release (in AF-Single mode) with the camera and subject perfectly still and you'll note the focus ring tends to move ever so slightly.
4) Live with it - as long as you've tested and identified the focus characteristics of each lens, keep in mind which does what and compensate accordingly when shooting.

[LowKey]

Just to clear a few things up:
The tests were shot at minimum focus distance plus six to eight inches to make sure that the chart was not too close to focus.

Camera was mounted on a tripod, test chart was propped up on a table and held in place with cellotape and Prestik. Lighting was 150 watt halogen light, two feet from the chart. AF - S, centre sensor in all cases.

The pics that you are seeing is a 100% crop of the left side of the chart (to save bandwidth).

However, I have the 80-200 2-touch f2.8 and was also rather disappointed with it at min focussing distances at wider apertures. To give you an idea, my consumer-grade 70-210 f4-5.6 seems to be sharper at min focus distance/wide open.

Robboh, funny you should say that - I found exactly the same! Here in South Africa, I paid the equivalent of $1000 for a used 80-200 f2.8 (New is $2000), yet my 70-210 is sharper!! BTW, I like the idea of the dominoes, I may end up doing that.

1) I wouldn't trust the kit lens' results.

Thanks Onyx, I don't really trust it - bit of a hit and miss

Louw