Go Back   Steve's Digicams Forums > Digicam Help > General Discussion

Reply
 
Thread Tools Search this Thread
Old Feb 19, 2010, 7:46 PM   #1
Senior Member
 
Join Date: Feb 2006
Location: California
Posts: 2,295
Default 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
lomitamike is offline   Reply With Quote
Sponsored Links
Old Feb 19, 2010, 8:07 PM   #2
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,572
Default

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.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Feb 19, 2010, 9:23 PM   #3
Senior Member
 
VTphotog's Avatar
 
Join Date: Mar 2005
Location: Extreme Northeastern Vermont, USA
Posts: 4,234
Default

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
VTphotog is offline   Reply With Quote
Old Feb 19, 2010, 9:34 PM   #4
Senior Member
 
Join Date: Feb 2006
Location: California
Posts: 2,295
Default

Quote:
Originally Posted by TCav View Post
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 is offline   Reply With Quote
Old Feb 19, 2010, 9:37 PM   #5
Senior Member
 
Join Date: Feb 2006
Location: California
Posts: 2,295
Default

Quote:
Originally Posted by VTphotog View Post
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 is offline   Reply With Quote
Old Feb 19, 2010, 9:41 PM   #6
Senior Member
 
Join Date: Feb 2006
Location: California
Posts: 2,295
Default

100 macro
Attached Images
  
lomitamike is offline   Reply With Quote
Old Feb 19, 2010, 10:07 PM   #7
Senior Member
 
Join Date: Feb 2006
Location: California
Posts: 2,295
Default

Top photo is the 100 f/2, then leaving everything on the camera as is, I changed lenses to the 100 macro (bottom photo).
Attached Images
  
lomitamike is offline   Reply With Quote
Old Feb 19, 2010, 10:19 PM   #8
Senior Member
 
mtngal's Avatar
 
Join Date: Nov 2005
Location: Frazier Park, CA
Posts: 16,056
Default

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.
mtngal is offline   Reply With Quote
Old Feb 19, 2010, 10:53 PM   #9
Senior Member
 
Join Date: Feb 2006
Location: California
Posts: 2,295
Default

Quote:
Originally Posted by mtngal View Post
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.
lomitamike is offline   Reply With Quote
Old Feb 20, 2010, 1:08 AM   #10
Senior Member
 
mtngal's Avatar
 
Join Date: Nov 2005
Location: Frazier Park, CA
Posts: 16,056
Default

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.
mtngal is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 1:04 PM.