[starjedi]

I got FA31mm recently. I had some simple sharpness tests. While the center sharpness definitely is very good comparing to my other prime lens, I found the left bottom corner is kind of soft. The lens itself does not suffer from wobble issue.

Here is the shot F1.8, 1/1000 with tripod, wireless mirror lock up, Live view focused at center. I used the electronic level in camera body together with the bulleye on my ballhead to help locate the equal distance. The scaling factor is 1.27 to get lp/mm value here.



Crop: LeftBottomCorner F1.8


Crop: RightUpperCorner F1.8


The left bottom corner is soft as I noticed comparing to other corners, especially comparing to the right upper corner at wide open: And with the aperture decreasing, it improves and begin to match up a little bit.

And this is F8 1/60 F1.8


Crop: LeftBottomCorner F8


Crop: RightUpperCorner F8



I did the same tests on my DA 50 1.8 & Tokina 90 2.5 & DA 18-135mm . At wide open, FA 31mm sounds to be much soft comparing to Tokina 90 2.5 and even a leg behind DA 50 1.8 at corner. Surprisingly it ties to DA18-135mm at wide openat this focal length. I indeed tried to focus in different place and focus only on corner under live view 10x but every time I focus, the left bottom corner is still notable soft than other places despite that I focused on the left bottom corner. The left bottom corner cannot achieve the same resolution or apparently far below the readings from other corners.

If quantifying those numbers, at wide open,
Worst corner Averagecorner center
Fa31f1.8 40 59 78
Da50f1.8 60 65 75
Da18-135f4@30mm 50 55 80

Should I return this lens? Still several weeks to go. Did someone experience the similar issue when buying new fa31?

---

Last edited by starjedi; 12-10-2014 at 08:49 AM.

[nicoprod]

Are you sure you are equidistant from all 4 corners? Because f/1.8 does not forgive.

[Digitalis]

Are you sure you are equidistant from all 4 corners? Because f/1.8 does not forgive.

no it doesn't, also the uneven lighting isn't helping much either. When testing lenses with a two dimensional target, having the target illuminated with even lighting is critical for proper evaluation.

[Dave's clichés]

There are many types of decentering tests some which are more technical than others, however the real world test (the one that you use in real life) is "infinity focus", take images of distant views at max. aperture and without changing the focus distance make sure that all four corners are covered, foliage and distant trees etc. are very demanding in terms of resolution and the focal plane of infinity is always the same and certain!

[starjedi]

no it doesn't, also the uneven lighting isn't helping much either. When testing lenses with a two dimensional target, having the target illuminated with even lighting is critical for proper evaluation.

Thanks for your guys' responses. I indeed used the electronic level in camera body together with the bulleye on my ballhead to help locate the equal distance. At 2m distance, the DOF is really gorgeous as 29cm. I am used to focus objects with dof <1cm.

My test also includes that if I separately manually focus on the corner. The left bottom corner cannot achieve the same resolution or apparently far below other corners.

[starjedi]

I got FA31mm recently. I had some simple sharpness tests. While the center sharpness definitely is very good comparing to my other prime lens, I found the left bottom corner is kind of soft. The lens itself does not suffer from wobble issue.

Here is the shot F1.8, 1/1000 with tripod, wireless mirror lock up, Live view focused at center. I used the electronic level in camera body together with the bulleye on my ballhead to help locate the equal distance. The scaling factor is 1.27 to get lp/mm value here.



Crop: LeftBottomCorner F1.8


Crop: RightUpperCorner F1.8


The left bottom corner is soft as I noticed comparing to other corners, especially comparing to the right upper corner at wide open: And with the aperture decreasing, it improves and begin to match up a little bit.

And this is F8 1/60 F1.8


Crop: LeftBottomCorner F8


Crop: RightUpperCorner F8



I did the same tests on my DA 50 1.8 & Tokina 90 2.5 & DA 18-135mm . At wide open, FA 31mm sounds to be much soft comparing to Tokina 90 2.5 and even a leg behind DA 50 1.8 at corner. Surprisingly it ties to DA18-135mm at wide openat this focal length. I indeed tried to focus in different place and focus only on corner under live view 10x but every time I focus, the left bottom corner is still notable soft than other places despite that I focused on the left bottom corner. The left bottom corner cannot achieve the same resolution or apparently far below the readings from other corners.

If quantifying those numbers, at wide open,
Worst corner Averagecorner center
Fa31f1.8 40 59 78
Da50f1.8 60 65 75
Da18-135f4@30mm 50 55 80

Should I return this lens? Still several weeks to go. Did someone experience the similar issue when buying new fa31?

Another interesting thing I want to point is that the manufacture in he box is still Hoya... Sounds an old stock..

[ppppsssstttt]

i liked this test: white binder reinforcers on a black background and slightly off focus

LensRentals.com - Testing for a Decentered Lens: an Old Technique Gets a Makeover

[starjedi]

Another interesting thing I want to point is that the manufacture in he box is still Hoya... Sounds an old stock..

Thanks very much for your guys kind help on my case. After numerous tests, I moved forward for asking replacement and finally got a new copy today. Several observations:

1) The new copy is impressive sharp from center to edge at wide open based on the same method for testing my previous copy.

2) the hood shows some slight wobble for new copy unlike firm hood of old copy. But it sounds to be fine as far as I am awared.

3) The new copy is stated as made by Ricoh instead Hoya in the box.

So I am happy to keep it! Thanks again! The only thing is whether I should worry about the hood wobble?

[Digitalis]

I should worry about the hood wobble?

I would keep an eye on it and see if it gets any worse over time - if it does get worse: send the lens in for service before warranty ends just to be on the safe side.