Testing Lenses: Stopped-Down MTF Curves

[gr82bart]

https://www.lensrentals.com/blog/2017/11/testing-lenses-stop-downed-mtf-curves/

An interesting technical read.
Regards, Art

 

[AgX]

Any decent lens MTF test would show results with open aperturte as well stopped down at various apertures.
Stopped down tests are nothing to put in a headline.
In this case the lab for certain reasons only did open aperture tests in the past and now added stopped down tests.

 

[ic-racer]

Stop down a little, OK. Stop down too much and all lenses will test the same.

 

[AgX]

What I learn from that testing is that Sigma no longer is the 2nd class lens supplier as it was looked upon in the heydays of film.

 

[ic-racer]

What I learn from that testing is that Sigma no longer is the 2nd class lens supplier as it was looked upon in the heydays of film.

I think many lenses from the "heyday" have been surpassed. I recall one of the first lenses to be designed with MTF in mind was the Zeiss 28/2.0. Though, that was designed back in the 1980s. I cherished that lens (even bought a second) but my modern Nikkor AF 28/1.8 might be better (if you can stand the plastic).
(Nikkor top, Zeiss bottom; only look at upper most curve, 10 cycles. Other lines don't match up, 20,30, 40, etc).

 

[Sirius Glass]

Any decent lens MTF test would show results with open aperturte as well stopped down at various apertures.
Stopped down tests are nothing to put in a headline.
In this case the lab for certain reasons only did open aperture tests in the past and now added stopped down tests.

Yep. No news here. Zeiss and many others have done their MTF curves at all apertures.

 

[alentine]

https://www.lensrentals.com/blog/2017/11/testing-lenses-stop-downed-mtf-curves/

An interesting technical read.
Regards, Art

Great link.
Thanks.

 

[Theo Sulphate]

MTF is worth only so much.

Once, when I needed a Canon EF lens and had none at all, I bought the only one available at the shop: a 75-300mm f/4-5.6 EF III. Many consider this the worst lens Canon ever made. It seems to be all plastic and the zoom is rough. Holding it doesn't give the impression of any quality at all.

Yet, I've made many nice photos with it and I wouldn't sell it.

Here are its MTF curves:

 

[Nodda Duma]

MTF data combined with information on stray light control / veiling glare and distortion will tell you a lot about the optical performance for in-focus objects, but as Theo indicates won't tell you the whole story.

Now throw in ray fan / OPD plots as well and that will paint me a pretty complete picture. Unfortunately, ray fans are difficult to interpret without a grounding in optical aberration theory. Which is too bad because they provide such good info.

 

[NJH]

His articles are very good, he also showed in one of his blogs the equivalent effect of missing exact focus by a tiny amount, the MTF results looked awful!

 

[AgX]

That autor hinted at field curvature and the resulting need to focus not only at the subject to be in focus but also at the part of the field where that subject will be placed.

Here at Apug we already once discussed that focusing in the center and then swinging the camera already implies focus shift due to pure geometry, field curvature not even considered.
Precise focusing out of center though will be difficult with an average SLR.

 

[NJH]

Ah yes that article was an interesting read. This one?
https://www.lensrentals.com/blog/2016/09/fun-with-field-of-focus-part-1/

I remember reading it when I was trying to understand Makro Planar 120mm performance at or near infinity (Hasselblad / Rollei). I found a test where the tester says its infinity performance is all about strong field curvature such that one needs to stop down to at least F11 but he didn't clearly say which way the field curves, he produced a pretty convincing set of test images though against the Sonnar 150. If the curvature is as one may assume for such a lens, towards rather than away, then naturally one has no choice but to stop down a lot until there is enough depth of field to counter act the field curvature.