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-   -   Testing a lens for FF/BF (https://forums.steves-digicams.com/general-discussion-11/testing-lens-ff-bf-166416/)

lomitamike Feb 19, 2010 7:46 PM

Testing a lens for FF/BF
 
I down loaded a couple of charts to test a macro lens I'm interested in and I'm seeing how imperfect things are. Not necessarily with the lens but maybe the AF marks in the viewfinder or even the sensors alignment. I am having a hard time getting a symmetrical photo of the chart. It appears to be off to one side or crooked no matter how hard I try to get it straight using the AF marks in the viewfinder for centering.

I tried another body with the same lens then both bodies with a different lens (same focal length but not macro). I don't want to be too critical but want to be sure I'm getting a good copy. Lenses are not cheap.

Another thing I'm seeing is that close focusing (1 to 1 or close to that) with a macro lens I seem to be getting a FF on one side and BF on the other. Again I'm going to great efforts to align the AF marks to have the photo be perfectly symmetrical. I can't get it straight!

It could be lens distortion or the viewfinder, maybe the sensors alignment is not straight. Who knows?

Has anyone else experienced this? It's driving me crazy.

Mike

TCav Feb 19, 2010 8:07 PM

If you used a focus test chart and got FF on one side and BF on the other, it's possible that the image sensor is out of alignment, but I would not zoom in so close as to get a 1:1 image. I think I'd keep it such that I still got the entire width of the chart in the image.

And just for the heck of it, you could try the charts with the camera vertically as well. Just curious.

VTphotog Feb 19, 2010 9:23 PM

First question- How are you aligning your camera to the chart? Eyeballing it doesn't always work in these situations. Measure.
Second question - Have you tried taking a picture of your camera in a mirror? If your sensor has angular misalignment, it should show up in this situation, regardless of any VF problems. If it is straight, suspect the VF screen.

brian

lomitamike Feb 19, 2010 9:34 PM

Quote:

Originally Posted by TCav (Post 1054346)
If you used a focus test chart and got FF on one side and BF on the other, it's possible that the image sensor is out of alignment, but I would not zoom in so close as to get a 1:1 image. I think I'd keep it such that I still got the entire width of the chart in the image.

And just for the heck of it, you could try the charts with the camera vertically as well. Just curious.

I haven't been able to find information on the proper distance from the lens to the chart to test a 100mm lens. I use an angle finder to get 45* angle and the 2 second timer. I usually try to get close to the minimum focus distance for the lens being tested, between 1:2 to 1:1 for the macro.

Just got done testing vertically and it seems to FF and BF on opposite sides as shooting horizontally. Weird.

I'll post some examples.

lomitamike Feb 19, 2010 9:37 PM

Quote:

Originally Posted by VTphotog (Post 1054377)
First question- How are you aligning your camera to the chart? Eyeballing it doesn't always work in these situations. Measure.
Second question - Have you tried taking a picture of your camera in a mirror? If your sensor has angular misalignment, it should show up in this situation, regardless of any VF problems. If it is straight, suspect the VF screen.

brian

Brian,

Do you set the camera on a tripod in front of the mirror and aim focus on the center of the lens?

lomitamike Feb 19, 2010 9:41 PM

2 Attachment(s)
100 macro

lomitamike Feb 19, 2010 10:07 PM

2 Attachment(s)
Top photo is the 100 f/2, then leaving everything on the camera as is, I changed lenses to the 100 macro (bottom photo).

mtngal Feb 19, 2010 10:19 PM

Make sure that your camera is square to the chart - that's harder to do than you would think. If you aren't square, the sides of the chart aren't the same distance from the lens. If the last two are full frame, I suspect you are very slightly off. Shooting focusing charts is very difficult to get right, I usually get frustrated and find them often inconclusive. Much better to go and shoot some real pictures and see if the focus does what you expect.

lomitamike Feb 19, 2010 10:53 PM

Quote:

Originally Posted by mtngal (Post 1054401)
Make sure that your camera is square to the chart - that's harder to do than you would think. If you aren't square, the sides of the chart aren't the same distance from the lens. If the last two are full frame, I suspect you are very slightly off. Shooting focusing charts is very difficult to get right, I usually get frustrated and find them often inconclusive. Much better to go and shoot some real pictures and see if the focus does what you expect.

Thanks for your reply mtngal.

I think that the AF marks in the viewfinder are not perfectly centered. I have them perfectly straight in the viewfinder and they never come out straight in the end.

I don't believe that I will ever be using a 2.8 aperture at 100mm, 1:1. I'm just trying to find out if this amount of BF at these extremes is not that unordinary. At 1:1 the DOF for 100mm is about 3mm. Thats thin.

mtngal Feb 20, 2010 1:08 AM

One thing about AF sensors - they are normally quite a bit bigger than the indicator in the viewfinder. At least that's what I found on my camera when I was shooting focus charts because I was sure I had a lens that was front focusing and was trying to adjust it. I liked the chart you used in the second picture because of that - I printed it light enough that the camera wouldn't focus on the grey (tested it to make sure that it wouldn't focus on it), then shot the pictures.

I don't think I've ever used my macro lens at anything larger than f8 - if I'm doing macro I'm usually shooting at f16 or 18. I also don't use AF when shooting macro (in fact, when I upgraded my macro lens, I chose one without AF because I never used it on my first one) so I've never thought about it's focus capabilities. Kind-of interesting to think about it and see what you came up with as far as charts.


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