Tamron 17-50mm f/2.8 front/back focusing

chickenandavocado · 02-16-2012, 02:20 AM

interesting, mine with +0.3EV seems to get it right outdoors.

DaveHolmes · 02-16-2012, 02:31 AM

Mine works...

Marc Sabatella · 02-16-2012, 07:10 PM

Also, not sure how you're testing, but if you're expecting a white sheet of paper to look white, that's not how it works.

vanyagor · 02-21-2012, 06:20 AM

Originally posted by Marc Sabatella

Also, not sure how you're testing, but if you're expecting a white sheet of paper to look white, that's not how it works.

I adjust exposure based on the histogram mostly.

Disclaimer: Despite all the above mentioned problems, I still love the lens and I can work around these problems just fine. However it does look like these problems are not just user error ... I hope to be wrong here.

veato · 02-21-2012, 06:40 AM

What's the simplest method I can use to test my 17-50 for front/back focussing? It's something I've never done before.

vanyagor · 02-21-2012, 07:37 AM

Originally posted by veato

What's the simplest method I can use to test my 17-50 for front/back focussing? It's something I've never done before.

You can print out tests designed for that purpose from internet.

However the simplest way is just to direct your camera at an angle (~45 deg) to the subject (a page of text, laptop keyboard or a ruler for instance). Take the example of a laptop keyboard: Auto Focus on a certain letter. Take the picture. Check if that letter is indeed in focus. Double check with focusing manually (manual focusing should give correct results all the time if your focusing screen has been correctly adjusted, which you can assume is the case). For a "clean" test use a tripod or rest the camera on the table. Though in my experience hand held test gives consistent results too.

For a page of text direct the camera at an angle to the surface, focus on a certain line of text, then check if that line is in focus in the resulting image...

Check this for different focal lengths.

Jonathan Mac · 02-21-2012, 08:50 AM

I'd like to add that focus testing, while very useful, should always be combined with some real-world shots to see how the lens performs outside of a test scenario.

The Tammy 17-50 is a crazy lens, capable of great results but has insane, unpredictable (and sometime uncorrectable) AF problems, combined with underexposure that varies, for no discernible reason, from +0.3 to +1.7.

Marc Sabatella · 02-21-2012, 09:10 AM

Originally posted by vanyagor

I adjust exposure based on the histogram mostly.

Yes, but that doesn't address the perception of underexposure. A histogram for a correctly exposed image in all cases will have the average/peak/whatever about half a stop left of center - that's what ISO standards for exposure call for. If you happen to have been shooting a 13% grey card, this will look exactly correct. if you happen to have been shooting an 18% gray card, then a correctly exposed image will look half a stop dark. If you happen to have been shooting a white piece of paper, then it will look several stops too dark. If you happen to have been shooting a real world scene, then the extent to which it looks good will depend on the extent to which the portion of the image you are most concerned about resembles a 13% grey card.

So yes, when adjusting exposure, by all means, look at the histogram. But if you're expecting anything but about half a stop left of center on average to be the starting point, that is simply not correct. Shooting a controlled subject (like a grey card) should result in a histogram about half a stop left of center. Only if you are seeing something other than that can you say there is an exposure issue. If you see the issue shooting wide open, then the issue is with the camera and the amera alone - the lens can play no role in that. If exposure wide open is good but it looks less god stopped down, then that siggests the aperture stops are not adjusted perfectly.

FruitLooPs · 02-21-2012, 01:24 PM

Also if you are going to attempt a DIY focus test without a chart, make 100% sure that it is focusing on the area you're expecting (as the focus points can actually be slightly different to what you see in the viewfinder). I would go with a chart, if you're focusing on a page of text on a 45degree angle just a minor inconsistency in the center point focus will have you thinking you have front or back focus - hence the test images having a lone central point for focus.

I'm still pondering what to do with my lens, it disappoints me often but when it hits the mark it really hits it.

RickyFromVegas · 02-21-2012, 04:49 PM

My copy does the exact thing.
@50mm, -10, @17mm, +3.

I keep it at -4 as a compromise between the two. HAHAHa.

I should get the lens checked out, though...right?

veato · 02-21-2012, 05:25 PM

Can anyone link me to a decent chart for the job. I know I could search myself but I want one someone has experience of using to point me in the right direction.

vanyagor · 02-21-2012, 06:09 PM

Marc, thanks for your in depth comment. I have a few questions.

Originally posted by Marc Sabatella

Yes, but that doesn't address the perception of underexposure. A histogram for a correctly exposed image in all cases will have the average/peak/whatever about half a stop left of center - that's what ISO standards for exposure call for.

Can you explain what a correctly exposed image means in this case. I thought that a correctly exposed image is when the whole histogram is within the range (while the location of the maximum/or average does indeed depend on the scene). In fact one has to look at color histogram too, not just the luminance histogram. Is that correct?

Originally posted by Marc Sabatella

If you see the issue shooting wide open, then the issue is with the camera and the amera alone - the lens can play no role in that. If exposure wide open is good but it looks less god stopped down, then that siggests the aperture stops are not adjusted perfectly.

Can you explain why wrong exposure wide open implies that it is camera's fault? Could it be that the aperture just doesn't open fully for whatever reason (i.e. being lenses fault, not cameras)?

Going back to specifically my issues, I can not claim yet it has problems with exposure. I need to test it further. Maybe those were caused by unusual scene circumstances.

vanyagor · 02-21-2012, 06:18 PM

Originally posted by FruitLooPs

Also if you are going to attempt a DIY focus test without a chart, make 100% sure that it is focusing on the area you're expecting (as the focus points can actually be slightly different to what you see in the viewfinder).

Right! I forgot about this because I always shoot with central point only AF.

Originally posted by FruitLooPs

I would go with a chart, if you're focusing on a page of text on a 45degree angle just a minor inconsistency in the center point focus will have you thinking you have front or back focus - hence the test images having a lone central point for focus.

If I understand correctly a chart is the same as a page of text, except instead of random text it has markings which give you a measure of front/back focus.

For the purpose of just finding out if your lens is back focusing or not you don't really need a chart. You just need to have you camera (and the test subject) fixed (tripod and table top serve perfectly well). A good measure of front/back focusing is how much AF correction you need to apply to fix it, so you don't really need a test chart imho. Please correct me if I'm wrong.

vanyagor · 02-21-2012, 06:24 PM

Originally posted by RickyFromVegas

My copy does the exact thing.
@50mm, -10, @17mm, +3.

I keep it at -4 as a compromise between the two. HAHAHa.

I should get the lens checked out, though...right?

What a coincidence! What is your lens serial number, maybe yours and mine were from the same batch!!! Mine is 019744 My copy was Made in Japan.

I used to set it to -10 since I was mostly shooting portraits at f=50mm and when I had to go wide I set it manually to +3. Now after repair it's consistent for all focal length so I have it set to -10 and the camera remembers this setting for the lens. So I'd say sending it in for repair was not useless. And the turnaround time was short enough. If your lens is on warranty I would send it in.

vanyagor · 02-21-2012, 06:25 PM

Originally posted by veato

Can anyone link me to a decent chart for the job. I know I could search myself but I want one someone has experience of using to point me in the right direction.

Sorry no link from me. But if your search this forum for "back/front focusing" you will find those links. It has been discussed a lot.

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02-16-2012, 02:20 AM	#16
chickenandavocado Veteran Member Join Date: Oct 2010 Location: Hong Kong Posts: 327	interesting, mine with +0.3EV seems to get it right outdoors.

02-16-2012, 02:31 AM	#17
DaveHolmes Veteran Member Join Date: Jul 2010 Location: Yorkshire Posts: 1,501	Mine works...

02-16-2012, 07:10 PM	#18
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	Also, not sure how you're testing, but if you're expecting a white sheet of paper to look white, that's not how it works.

02-21-2012, 06:20 AM	#19
vanyagor Senior Member Join Date: Nov 2011 Location: Boston, MA Photos: Gallery Posts: 177 Original Poster	Originally posted by Marc Sabatella Also, not sure how you're testing, but if you're expecting a white sheet of paper to look white, that's not how it works. I adjust exposure based on the histogram mostly. Disclaimer: Despite all the above mentioned problems, I still love the lens and I can work around these problems just fine. However it does look like these problems are not just user error ... I hope to be wrong here.

02-21-2012, 06:40 AM	#20
veato Veteran Member Join Date: Oct 2011 Location: Nottingham Posts: 543	What's the simplest method I can use to test my 17-50 for front/back focussing? It's something I've never done before.

02-21-2012, 07:37 AM	#21
vanyagor Senior Member Join Date: Nov 2011 Location: Boston, MA Photos: Gallery Posts: 177 Original Poster	Originally posted by veato What's the simplest method I can use to test my 17-50 for front/back focussing? It's something I've never done before. You can print out tests designed for that purpose from internet. However the simplest way is just to direct your camera at an angle (~45 deg) to the subject (a page of text, laptop keyboard or a ruler for instance). Take the example of a laptop keyboard: Auto Focus on a certain letter. Take the picture. Check if that letter is indeed in focus. Double check with focusing manually (manual focusing should give correct results all the time if your focusing screen has been correctly adjusted, which you can assume is the case). For a "clean" test use a tripod or rest the camera on the table. Though in my experience hand held test gives consistent results too. For a page of text direct the camera at an angle to the surface, focus on a certain line of text, then check if that line is in focus in the resulting image... Check this for different focal lengths.

02-21-2012, 08:50 AM	#22
Jonathan Mac Pentaxian Join Date: Apr 2009 Location: Madrid, Spain Posts: 10,894	I'd like to add that focus testing, while very useful, should always be combined with some real-world shots to see how the lens performs outside of a test scenario. The Tammy 17-50 is a crazy lens, capable of great results but has insane, unpredictable (and sometime uncorrectable) AF problems, combined with underexposure that varies, for no discernible reason, from +0.3 to +1.7.

02-21-2012, 09:10 AM	#23
Marc Sabatella Veteran Member Join Date: Sep 2006 Location: Denver, CO Photos: Gallery Posts: 10,685	Originally posted by vanyagor I adjust exposure based on the histogram mostly. Yes, but that doesn't address the perception of underexposure. A histogram for a correctly exposed image in all cases will have the average/peak/whatever about half a stop left of center - that's what ISO standards for exposure call for. If you happen to have been shooting a 13% grey card, this will look exactly correct. if you happen to have been shooting an 18% gray card, then a correctly exposed image will look half a stop dark. If you happen to have been shooting a white piece of paper, then it will look several stops too dark. If you happen to have been shooting a real world scene, then the extent to which it looks good will depend on the extent to which the portion of the image you are most concerned about resembles a 13% grey card. So yes, when adjusting exposure, by all means, look at the histogram. But if you're expecting anything but about half a stop left of center on average to be the starting point, that is simply not correct. Shooting a controlled subject (like a grey card) should result in a histogram about half a stop left of center. Only if you are seeing something other than that can you say there is an exposure issue. If you see the issue shooting wide open, then the issue is with the camera and the amera alone - the lens can play no role in that. If exposure wide open is good but it looks less god stopped down, then that siggests the aperture stops are not adjusted perfectly.

02-21-2012, 01:24 PM	#24
FruitLooPs Veteran Member Join Date: Dec 2007 Location: New Zealand Photos: Gallery \| Albums Posts: 440	Also if you are going to attempt a DIY focus test without a chart, make 100% sure that it is focusing on the area you're expecting (as the focus points can actually be slightly different to what you see in the viewfinder). I would go with a chart, if you're focusing on a page of text on a 45degree angle just a minor inconsistency in the center point focus will have you thinking you have front or back focus - hence the test images having a lone central point for focus. I'm still pondering what to do with my lens, it disappoints me often but when it hits the mark it really hits it.

02-21-2012, 04:49 PM	#25
RickyFromVegas Veteran Member Join Date: May 2011 Location: Las Vegas Photos: Gallery Posts: 366	My copy does the exact thing. @50mm, -10, @17mm, +3. I keep it at -4 as a compromise between the two. HAHAHa. I should get the lens checked out, though...right?

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02-21-2012, 05:25 PM	#26
veato Veteran Member Join Date: Oct 2011 Location: Nottingham Posts: 543	Can anyone link me to a decent chart for the job. I know I could search myself but I want one someone has experience of using to point me in the right direction.

02-21-2012, 06:09 PM	#27
vanyagor Senior Member Join Date: Nov 2011 Location: Boston, MA Photos: Gallery Posts: 177 Original Poster	Marc, thanks for your in depth comment. I have a few questions. Originally posted by Marc Sabatella Yes, but that doesn't address the perception of underexposure. A histogram for a correctly exposed image in all cases will have the average/peak/whatever about half a stop left of center - that's what ISO standards for exposure call for. Can you explain what a correctly exposed image means in this case. I thought that a correctly exposed image is when the whole histogram is within the range (while the location of the maximum/or average does indeed depend on the scene). In fact one has to look at color histogram too, not just the luminance histogram. Is that correct? Originally posted by Marc Sabatella If you see the issue shooting wide open, then the issue is with the camera and the amera alone - the lens can play no role in that. If exposure wide open is good but it looks less god stopped down, then that siggests the aperture stops are not adjusted perfectly. Can you explain why wrong exposure wide open implies that it is camera's fault? Could it be that the aperture just doesn't open fully for whatever reason (i.e. being lenses fault, not cameras)? Going back to specifically my issues, I can not claim yet it has problems with exposure. I need to test it further. Maybe those were caused by unusual scene circumstances.

02-21-2012, 06:18 PM	#28
vanyagor Senior Member Join Date: Nov 2011 Location: Boston, MA Photos: Gallery Posts: 177 Original Poster	Originally posted by FruitLooPs Also if you are going to attempt a DIY focus test without a chart, make 100% sure that it is focusing on the area you're expecting (as the focus points can actually be slightly different to what you see in the viewfinder). Right! I forgot about this because I always shoot with central point only AF. Originally posted by FruitLooPs I would go with a chart, if you're focusing on a page of text on a 45degree angle just a minor inconsistency in the center point focus will have you thinking you have front or back focus - hence the test images having a lone central point for focus. If I understand correctly a chart is the same as a page of text, except instead of random text it has markings which give you a measure of front/back focus. For the purpose of just finding out if your lens is back focusing or not you don't really need a chart. You just need to have you camera (and the test subject) fixed (tripod and table top serve perfectly well). A good measure of front/back focusing is how much AF correction you need to apply to fix it, so you don't really need a test chart imho. Please correct me if I'm wrong.

02-21-2012, 06:24 PM	#29
vanyagor Senior Member Join Date: Nov 2011 Location: Boston, MA Photos: Gallery Posts: 177 Original Poster	Originally posted by RickyFromVegas My copy does the exact thing. @50mm, -10, @17mm, +3. I keep it at -4 as a compromise between the two. HAHAHa. I should get the lens checked out, though...right? What a coincidence! What is your lens serial number, maybe yours and mine were from the same batch!!! Mine is 019744 My copy was Made in Japan. I used to set it to -10 since I was mostly shooting portraits at f=50mm and when I had to go wide I set it manually to +3. Now after repair it's consistent for all focal length so I have it set to -10 and the camera remembers this setting for the lens. So I'd say sending it in for repair was not useless. And the turnaround time was short enough. If your lens is on warranty I would send it in.

02-21-2012, 06:25 PM	#30
vanyagor Senior Member Join Date: Nov 2011 Location: Boston, MA Photos: Gallery Posts: 177 Original Poster	Originally posted by veato Can anyone link me to a decent chart for the job. I know I could search myself but I want one someone has experience of using to point me in the right direction. Sorry no link from me. But if your search this forum for "back/front focusing" you will find those links. It has been discussed a lot.