MTF charts

[Gary Kinard]

I am totally puzzled here. I understand the MTF charts and know how to read them.
My question is how do we know if the lens is performing at that level?

I have heard Rodger comment on this before and it made me curious.
I have a 300mm f2.8 and a Sigma 500mm prime.

When I test them side by side the 300 ll with a 1.4 T.C. and the bare Sigma lens.
I find the Sigma sharper and the 300 just lacks the detail compared to the Sigma.
Even though the 300 + 1.4 is suppose to be better compared to the MTF charts Canon publishes.

What am I doing wrong? Is there a way to tell if they are in the right ball park?
I am just completely confused now.

Also I read that the 7D with the pixel density will cause the MTF reading to go down? By quite a bit. DXo mark has this listed.

Is the 7D so telling on a lens. That I can get soft images with a T.C. compared to the MTF charts?

Forgot, the Sigma 500 is on a Pentax K-5, the 300 is on a 7D. Is that a reason for the difference?

Gary

[Gary Kinard]

No advice here. Interesting
I Googled it and came up with a program called Quick MTF http://www.quickmtf.com/
I had some quality charts printed, and in the process of getting lighting set up to shoot the charts.
Although still over my head it is slowly starting to make some sense.
Anyway i will shoot four lenses and get with the engineer that wrote the program to help me interpret the results.
If he can put up with me long enough.. I think he is getting frustrated at my lack of knowledge on the subject.
But a fun little project. I am hoping to come away with enough info to tell when I buy a new lens if it is going to perform to my standards.
Who knows?

[Roger Clark]

Gary,
Sorry for not responding. Somehow I missed your post during a busy work week.

Before doing MTF calculations, have you done a microadjustment test? For example, make an image normally (using the AF points), and then use live view and focus with live view and make an image. This is best done with a tripod and a static subject. Is the live view focused image sharper? If so, you need microadjustment.

Roger

[Gary Kinard]

No problem Rodger, I had the lens and camera calibrated at Canon here. In Bangkok it is very cheap. 15 US dollars. I had the camera checked and they said they checked everything and it was ok. The Canon service center is impressive to say the least. When I got back I did a quick micro adj check, but did not use live view. I will try it later.
I am really getting to the point where I just do not think the 300mm with T.C. is an option? For birding, for me anyway. Really i find the same thing with the 500. It looses so much with a 1.4? Bare both are excellent. At least both of mine are.

I'll keep this thread updated with my findings. The lenses that I will check are the Sigma 500mm f4.5, Sigma 300mm f2.8 prime, Canon 500mm f4L IS, Canon 300mm f2.8 ll.

Gary

[Roger Clark]

Gary,
I can't say anything about the sigma 300 f/2.8 or sigma 500 f/4.5, but the canon 300 f/2.8 and 500 f/4 are quite impressive bare and with 1.4x and 2x TCs in my experience. But if you do microadjustment inside at one temperature then photograph outside where the temperature is quite different, to microadjustment might also be quite different. Microadjustment varies with temperature. See: http://www.clarkvision.com/articles/microadjustment/

So that is why I suggested testing focus using live view. Live view focusing is slow but accurate and needs no calibration. If you take an image with phase detect AF and it is soft, and then take one with live view and it is sharp, then microadjustment is needed. If both are soft, then it is the lens or lens+tc.

Roger

[Gary Kinard]

Ok, thanks for the tip. I had planned to take the equipment into a room. leave it there for a few hours, Record the dry bulb and web bulb temps at testing time. And test all on an equal footing. I really do not know if that is necessarily important? At least all equipment will be equalized and the same temp. Don't know how important it is but also thought I would set up the camera, lens and wire it to a computer for the tests so I would not touch the camera or lens. Just for expansion and contraction. I will set it up and run thru a few practice runs to make sure I don't run into anything wierd or unexpected. Again I think that is way overboard but since I am retired. Time is virtually a non issue. Hummm
Really just killing time and doing it for my own use mainly. Since I plan on buying another god awful expensive lens soon. To fuel this insane hobby... LOL I am out of here.

Heading to BKK for some R&R

Thanks again for the tip. I will make sure to use live view.

Gary

[Gary Kinard]

Shot a few test shots to make sure my lens was fixed. What a pain in the rear. I only have two Canon lenses and I did a quick test against the Sigma 500mm prime to make sure it was sharper with a T.C.
Having some more charts printed for the MTF 50 shots. Interesting to play with. Just for my own benefit though.

If interested you can look here for the very informal test shots I made today.
http://birdsthatfart.com/1/post/2013/01/sigma-300mm-f28-canon-300mm-f28-sigma-500mm-f45.html

Now that that is ove I am going to take some pics and have fun. Later