Strange infrared behavior

[jk4u59]

I'm not convinced you can bring your 7.5mm back into focus with your IR conversion with modifications.

If it were just a simple matter of IR phase shift, I shouldn't be able to focus my 7-14 Panny at the 7mm end. Although my 470nm EM-1.1 refuses to autofocus at 7mm (8-14 is fine), it'll still manually focus and capture wonderful images there. The 7-14 works fine on all my other bodies at all focal lengths, both IR and normal spectrum.

So I'm thinking your problem is not just phase shift, it's flange distance +/- filter thickness +/- phase shift.

You can mess with the flange distance via spacers, but I wouldn't do this unless you never plan on using this lens on any other camera.

It's a lot of trouble to bring one lens into compliance. Not sure it's worth the effort.

You're right. But if you check my latest post, i found a site that reports lots of focusing problems for the fisheye lenses of my same model (i.e. 7.5 mm MFT), due to a small miscalibration of its rear element (small in dimensions but important in spoiling the results!).
Anyway, before unscrewing anything I'll carefully check the behavior of my lens at infinity in visible light, using my OM-D E-M5II body.

 

[bargainguy]

Right, I didn't know that your lens has a known issue which can be corrected. The fact that it was in happening in infrared just threw everything off farther.

If you can bring your lens back into alignment and keep it there with nail polish or Loctite, all the better!

 

[jk4u59]

Hi bargainguy,

you said:

It's a lot of trouble to bring one lens into compliance. Not sure it's worth the effort.

I agree, but the procedure I found seems not that difficult or non reversible (some screws and several empirical tests moving the rear element, then some sealing in the end), might be worth to try...

 

[RAH]

Perhaps better: following your advice, I searched the net for something like "Samyang MFT fixing" and landed to this site:

Yeah, I figured the much smaller m43 version of the lens might be different in build than the APS-C version. Too bad. Maybe you could do a web search on resetting the m43 scale? ...

Now why didn't I think of that?!?

 

[jk4u59]

Wait! Before screwing down a more or less working lens I want to be sure it's worth the risk. Today I performed some tests in visible light, with the fisheye mounted on my (standard) E-M5II, focusing at infinity and at the maximum aperture: well, looking at the result (a crop of centre image, seen at 1:1 factor) I wasn't very satisfied. I cannot say it was out of focus, but neither that the image was sharp... anyway, I did it too in a hurry, on late afternoon and with scarce light conditions. So, I plant to repeat the same tests with better care, taking note of the various parameters (being a full manual lens, metadata don't report the aperture)

 

[PakkyT]

looking at the result (a crop of centre image, seen at 1:1 factor) I wasn't very satisfied.

Makes you wonder if the focus has always been very slightly off near infinity but because it mostly worked fine you never really noticed it. Then when put on the IR camera which pushed it to not work at all at infinity, now you are giving the lens a more critical look, where you are now noticing the not so perfect performance even on a normal camera?

Maybe the adjustment outlined in that article you posted could improve both uses.

 

[jk4u59]

Yes, please check my last post: I want to repeat tests in visible light but in better light conditions and carefully controlling the setup and taking note of the parameters. I forgot to disable the stabilizer, for example, and used a cheap tripod instead of the other I have, cumbersome but more stable. Then I'll compare the centre image, at 1:1 factor, with a reference shot taken with the Zuiko 9-19 at its minimum focal lenght, and the same other parameters.
Depending on what I see I could also decide to try opening the Samyang's back... let's see.

 

[RAH]

Yes, this is exactly what happened to me with the m43 Samyang FE lens - I have never been sure whether I am getting proper infinity focus because it achieves infinity focus (it seems) at the stopping point, so how do you know if turning it further would further improve it? This is one reason I bought the Olympus 8mm FE. I have an 8mm version of the Samyang for Canon mount - much larger - and it was fine this way, achieving infinity somewhat before the stopping point.

 

[Petrochemist]

Generally a minor difference in the right direction has no adverse effect. Any change in glass thickness will change the optical distance from the rear of the lens to the sensor which changes how the lens focuses. This is similar to adapting a lens from a legacy mount where the adapter is slightly the wrong length.
If it's slightly short the lens has to move further away to reach infinity focus, using up some of the normal focusing movement but normally of very little concern. A fraction too long means the lens often can't focus to infinity a very noticeable issue for most photographers - most adapters are deliberately made slightly short to ensure manufacturing tolerances don't cause failures. Short focal length lenses are more prone to issues with sensor distance.
In addition to this factor there is the added complication of IR focusing differently than visual light.
Most lens fortunately focus IR with slightly more extension than for visual light but if the lens design is such that IR requires less extension than visual light a lens might be unable to focus to infinity when shooting IR. I have one lens on my Sigma system that does this, needing smaller apertures to make distant subjects look reasonably sharp. This is despite having the internal IR blocking filter removed & only replaced with air (which effectively shortens the optical distance from lens to sensor)

 

[jk4u59]

Hi,

this afternoon I managed to repeat the focusing tests, this time with better conditions: I took three shots with the Samyang 7.5mm MFT focused at the end of its helicoid (hopefully, infinity...), at f/4.5 (full opening), f/4 and f/5.6.
Then I took a similar shot for comparison with the M.Zuiko 9-18mm at its minimal focal lenght and full aperture.
I collected the 100% crops of centre image in a single picture, here attached with the relevant shot data. I also added the shameful IR image...

Only, I'm not sure at all about the results: the focus might not be considered perfect, but it's hard to complain very much (that's for the visibile light, of course: in IR it's simply awfully wrong). The reference picture taken with the Zuiko 9-18 is not that better...

What do you think?