Focus Adjustment

[colin C]

I've been good and completed all my chores. I even cleaned two cars inside and out and then I got bored. In the olden days I would have grabbed my camera kit and off to the New Forest for some Wildlife, but I am trying to be good and follow the rules. Walking around in Camo with a big lens on a tripod, it's a bit difficult to justify reasonable exercise. So, what to do?

Last year I bought a secondhand 800mm lens. The condition was a bit ratty and it needed a new lens to body seal and those jobs are now done. I have never had cause to doubt the autofocus of this lens, but as I have an autofocus target and some time on my hands, it wouldn't hurt to check things out.

The first image is the target, placed at the top of the garden. It shows the full size, which is A3, the angle of the graduations and the "square on" target for the autofocus. It also indicates how much of a crop I have used to determine how much (if any) adjustment was required. Having set up the equipment, lens on a sturdy tripod and target on a stool, I zeroed the lens on the target with the aperture wide open (F5.6). I manually adjusted the focus so the target was a blur, focussed and took the shot. I alternately focussed forward and then backward to blur the shot, before letting the autofocus do its thing, just to ensure consistency.

Second image shows a heavily cropped image of the target and it was spot on, needing no adjustment. This was checked in three further images and they were all the same. Good result. I was always happy with this lens, but buying secondhand there was always the possibility of a slight problem.

While everything was set up, I thought I would check my 200-400mm. I had no worries about this lens, I bought it new and had always babied it, so I knew there wasn't a problem. The third image was the staring point and it was near focussing slightly. This was confirmed by further shots and they were all out by the same amount.

The fourth image is the final shot with the 200-400mm after dialling in +10 on the AF Micro adjustment. All now good and I am a happy boy again. I did have some problems making the adjustments, even though I have done this many times in the past, for me and others. I couldn't dial in any adjustment. Whatever I did on the top dial, or the rear dial, it wouldn't adjust. Just before I pulled my hair out I put my finger to the rear screen and I could move the adjustment where I wanted. I forgot the 1Dx MKIII had a touch screen, so I didn't even consider it.

You do have to look very carefully to work out whether the lens is forward, or back focussing and I find it easier to work out where it is going out of focus both sides of the line, rather than where it is focussed. Hopefully, these crops are big enough so that you can get the gist of whaT IS HAPPENING.

IMG_Target a by colin cross, on Flickr


IMG_800mm a by colin cross, on Flickr


IMG_200-400 Start b by colin cross, on Flickr


IMG_200-400 Finish a by colin cross, on Flickr

[Trev B]

Well done you Colin, Interesting. I don't think I would feel qualified enough to start tinkering like that.
If anyone does try it and things go a bit 'Not to Plan' can you cancel it out by Clearing all camera settings in the yellow tab (Spanner)

[colin C]

Yes Trev you can do that, but by going one logical step at a time and checking the results, you can't go far wrong. If it looks like you need a correction of 10 and you go the wrong way by -10 and make things worse, next time apply +20 and that should bring you where you need to be.

Plus corrects near focussing and minus corrects far focussing. I use correction in tens, whether + or - and once I am near enough, probably go for a correction in fives. Anything less than five is just not worth the effort and even out by ten still performs very well. I never had any complaints with the 200-400mm and it was a very sharp lens ........ it will be just a tiny bit sharper now when wide open.

With the 800mm, although it was a bit tatty, it was owned by a pro and regularly serviced by Canon. Which is probably why it was absolutely spot on for focussing as Canon will have a lot finer calibration tools available.

If a good number of your lens are spot on, then the body is OK and you only need to adjust the errant lenses. However, if all lenses are out by a similar amount, then you can calibrate the body.

A useful tool and helps with confidence in your equipment.

[Trev B]

Thanks Colin

[@imagesBV]

..............worthwhile checking, before you just believed it was good, now you know its good!
...........of course I am now denied such fun with the R mirrorless system.

[Alan_H]

Very interesting Colin and brave of you, did you look at your adjustments on the rear screen or were you tethered to a computer

Alan

[Brian Sugden]

Very interesting Colin and brave of you, did you look at your adjustments on the rear screen or were you tethered to a computer

Alan

I too have micro-adjusted my lenses to both my 1DX and 7DIII. It's quite easy to check focusing on the rear screen and there is nothing to be afraid of. You can't do any damage and as Colin says you can re-set, but be careful you don't re-set all your camera settings back to the factory default.

[2Beers]

You can also use reikan focal software which automatically checks the focus

https://www.reikanfocal.com

[Guy24]

Hi Colin,

When undertaking the calibration - at what distance do you have the target. thinking of doing my lenses - but depending what you read the distance varies!!!. Do you also ensure the target centre is at the same height as the camera/lens?

thanks

Guy

[Brian Sugden]

I lay the chart on the floor in my living room, set the camera/lens on a tripod angled at 45 degrees, positioned so that I can focus on the chart and away I go. Not very scientific but it works for me. Occasionally when I'm out I'll check focus by photographing a sign or something similar again at about 45 degrees to check focus.

[@imagesBV]

I think I remember from the original instructions in the magazine that the distance to the subject should be at least 50 times the focal length....worth checking.

[colin C]

the distance to the subject should be at least 50 times the focal length

Interesting.

I try get get a reasonable distance where I would normally use the lens, tempered by the need to have the target a reasonable size in the frame. With the 800mm I was tight to the house with the target at the top of the garden, which is around 40 meters. Doing the math: 0.800 x 50= 40. That was a bit of luck.

On Brians method: I found that a target at 45deg was too steep an angle to easily determine which graduations were sharp and where they were starting to blur. My target is around 22deg, where the shallower angle shows a more gradual sharpness to blur.

[Brian Sugden]

At 45 degrees I find that the level of out of focus is equal both in front and behind the sharp point. I also don't worry about distance from lens to chart. Why did we never have to do this with film?

[@imagesBV]

.......as the python sketch goes.....you were lucky, I didn't have autofocus on my Praktica or Minolta
...though there was a new fangled Dynax model that I couldn't afford....I think Minolta produced the first autofocus camera

[colin C]

Why did we never have to do this with film?

We didn't have auto focus lenses in the days of film. They were only available during the last throws of film cameras and unless we had a lens that was obviously not focussing well, we just accepted it.

Now with digital and the ability to sharpen in camera and post editing, we are more critical with a lens's performance. As we have the ability to test lens focussing and correct it in camera, we tend to use it.

In short; with film we couldn't, with digital we can.