Autufocus errors

[nikonian]

Originally Posted by SieSmith

Seems to be more like 6 or 7mm if focussed at 2m (using DoF calculator in following link).

http://www.cambridgeincolour.com/tut...h-of-field.htm

This will show up very well if focus position is even slightly off.

Afraid your calcs are incorrect.
300mm @ f4.5 with a C of C of .015 at a distance of 2 meters gives a Hyp of 1.33 that calculates a DOF of .01 meters which equals 30mm

[SieSmith]

Originally Posted by nikonian

Afraid your calcs are incorrect.
300mm @ f4.5 with a C of C of .015 at a distance of 2 meters gives a Hyp of 1.33 that calculates a DOF of .01 meters which equals 30mm

Not my calcs. Like I said I used the online calculators in the links provided. Could you let me know where I'm going wrong typing the values into the boxes?

Better let these boys know they've got it wrong as well then.
http://www.dofmaster.com/dofjs.html

[nikonian]

Originally Posted by SieSmith

Not my calcs. Like I said I used the online calculators in the links provided. Could you let me know where I'm going wrong typing the values into the boxes?

Better let these boys know they've got it wrong as well then.
http://www.dofmaster.com/dofjs.html

I was just testing to see who would be the first to spot the deliberate mistake. Actually the dofmaster does show the DOF is .01 meters. I ran through two progs that I wrote plus the dofmaster and they all agreed to .01 meters Then for some reason I brought a 3 into the calculation. .

[nikonian]

Now if you do not agree, i will meet you at 6am in MESNES St pistols cocked.

[Ashleyj]

Originally Posted by nikonian

... .01 meters which equals 30mm

Err, no! 0.01 meters is actually 10mm. 30mm is 0.03 meters.

[nikonian]

Originally Posted by Ashleyj

Err, no! 0.01 meters is actually 10mm. 30mm is 0.03 meters.

That is the whole point, as I stated . 3 crept in when I converted .01 meters to millimeters.

[Ashleyj]

Originally Posted by nikonian

That is the whole point, as I stated . 3 crept in when I converted .01 meters to millimeters.

Sorry, missed that.

[KeithT]

I have been taking pictures for well over thirty years now. In that time lens manufacture has obviously improved for the better. Most of the images I have taken over the years have all been pin sharp where I have needed it to be and I put this down to always shooting with centre focus point. Now I know cameras have multi focus points these days, but even with my old 5d classic as with my MkII upgrade, I never stray from that way of working and have no desire to do so. As long as my subject matter is spot on the rest usually takes care of itself. If I want off centre focus I usually focus and recompose the shot.

The only time I have had a problem with shallow dof was with my Canon 50mm 1.4 (now sold on). I had a lot of trouble taking reasonably sharp portraits of both my granddaughters at f1.4. Either they would move slightly and blur the image, or I would do the same as I fired the shutter. So difficult did I find it that I had to stopdown to f2.8 before I conquered the problem. There was nothing wrong with the lens, it was all down to technique and my own lack of controling it at that aperture.

[SieSmith]

Originally Posted by nikonian

I was just testing to see who would be the first to spot the deliberate mistake. Actually the dofmaster does show the DOF is .01 meters. I ran through two progs that I wrote plus the dofmaster and they all agreed to .01 meters Then for some reason I brought a 3 into the calculation. .

No problem nikonian, easy done. You only had me up half the night trying to figure out where I was going wrong!

Originally Posted by nikonian

Now if you do not agree, i will meet you at 6am in MESNES St pistols cocked.

I think it will have to be a 6am pistol shoot out , although I think I know why. I am entering the subject distance in cm not m, so get a slightly more precise and less rounded result.


Now the original point was about front and back focus and tweaking the focus point to "calibrate" the lens. What I would like to ask is "What settings do you use for this calibration?".
Do you use a close, medium or long subject distance? Do you use a certain aperture? Is it something you only do once, or is it a procedure you should do before a particular shoot, given that you know the setup you're going to be using.
Or do you just test your lens at all settings and take an "average" to which you microadjust it?

Sorry, that ended up being more than 1 question, but anyone got any advice?


Just to pick up on a point that Keith raised:-

Originally Posted by KeithT

...If I want off centre focus I usually focus and recompose the shot.

I tend to do this also, Keith, but can't this lead to problems as well, if not done correctly and with care? If someone focuses then recomposes with the narrow DoFs we've been discussing, it could appear to them that the focus on the lens is out, even though it is really their technique that is at fault?

Cheers, Sie

[nikonian]

I made up this gizmo to check for front and rear focus errors. Used my d90 to check all my lens`s NIKON and TAMRON. I was ready to scrap the lot then I thought I will try the Nikon 3100. What a difference,all focussed spot on the middle with the focus running off either end at open aperture. When stopped down to F5.6, a full clean showing. Any body want to buy a D90?