How do you test a zoom for focus accuracy?

I am having some issues with my 120-300 that I need to get resolved. It seems to be back focusing a little, but not all the time. I haven't taken the time to look at the EXIF to see if it is only at a certain focal length or not:o . I am just looking for a way to test it, so that I can verify if it is backfocusing or not. I have only recently started using it, so it may very well be user error also.

Any advice appreciated.

Thanks,
Travis

Print something like this out

http://www.photo.net/l​earn/focustest/scale45​.jpg

Then use the centre focus point on your cam and set it right in the middle of the chart and have the camera at a 45 degree angle.

That should show you if the lens is front or back focussing.

I might be totally wrong so everyone feel free to correct me.

Thanks, that is kind of what I was thinking.

So then I just do the same test at different focal lengths? What aperture is recommended (my guess would be wide open)?

Any other advice?

Travis

Definatley wide open because a smaller aperture will increase the DOF which will help to mask focusing issues. I don't think it's essential to try various focal lenghs but I don't think it would hurt either.

Use a tripod if possible because it will eradicate the chance of human error and when you are shooting wide open small movements back and forth can give the impression of focus problems.

That's about all I can think of!

I'm surprised I came up with that to be honest, learnt a lot from this forum!

So I finally got around to testing my lens. Now my question is; how critical is the 45 degree angle? I had to do quick set-up and didn't measure anything . Also I thought that I was shooting straight on but it appears I wasn't after looking at the shots on the computer .

I won't post any resultant shots until I know for sure that I did this correctly, so any help is appreciated.

Thanks,
Travis

Well even though I am not sure if I did the test correctly or not, here is an example at 300mm f/2.8. I used mirror lock-up and a remote switch.

If anyone can help I would appreciate it.

Thanks,
Travis

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hey Travis where did u get that chart? i see it all over the place and would like to test my lenses just for the hell of it.How would i measure 45degrees anyways?

Read the thread from the top.

To me, tha looks like it's back focussing. But does it really matter that much? Lots of peoples are, i should think and they dont seem to mind or notice.

sando wrote in post #2019690
But does it really matter that much? Lots of peoples are, i should think and they dont seem to mind or notice.

To me, yes it matters. I do alot of my sports shots wide open, so I need the focus to be spot on. If I was always stopping down, then no it probably wouldn't matter much. Also I didn't buy a 2.8 zoom to use it at 5.6 and above. I have seen what this lens can do, so that is what I expect from my copy also .

Travis

bluej511 wrote in post #2019676
hey Travis where did u get that chart? i see it all over the place and would like to test my lenses just for the hell of it.How would i measure 45degrees anyways?

I got the test chart here - http://www.focustestch​art.com/chart.html

To set a 45 degree angle measure an equal distance back from the focus spot and up to the camera. If you remember from geometry, a 45,45,90 degree triangle has two sides of equal length and two equal angles (45°). If I could sketch it here it would help illustrate the point, but I am sure you get it.

Travis

did RikiWriter sell you a dud? Have you consulted him about it?

How good are this test since the focusing distance is very small comparative to sports shots? Obviously, we cannot print a large scale version of this chart to test at a greater distance!!!

JTL

JohnTheLuck wrote in post #2019845
How good are this test since the focusing distance is very small comparative to sports shots? Obviously, we cannot print a large scale version of this chart to test at a greater distance!!!

JTL

The point of the test is to check for front or back focusing. It's assumed that if the focus is spot-on then back focusing or front focusing at any distance should be close to or pretty much non-existent.

BryanP wrote in post #2019854
The point of the test is to check for front or back focusing. It's assumed that if the focus is spot-on then back focusing or front focusing at any distance should be close to or pretty much non-existent.

Is this assumption really true? It seems logical, but is it true?

JTL

BigBlueDodge wrote in post #2019798
did RikiWriter sell you a dud? Have you consulted him about it?

Now, why in the world would you think that?

I have in fact talked to him about it. He has been more than helpful (or vey helpful, if you listen to George Carlin). I don't think that he would knowingly pass on a "dud" lens to someone. Just a feeling I have about him .

The thing is, I can't be sure if the lens is a "dud" or if I am. I have never had any problems shooting the same style shots with my 70-200 2.8 or with my friend's 300L 2.8. So I "think" my long lens technique is OK, but I can't be too sure either .

All I know is that I really want to get this issue resolved, and if anyone here can coach me on my next steps I would REALLY appreciate it. When this lens hits focus it is awesome, but when it misses..... well that's why I am here.

Travis