Using partial Pol with Oblique

[75RR]

Would like to call on interested members to contribute side by side comparison images of Oblique and Oblique + Pol to test my impression the the use of Pol with Oblique increases resolution.
Since this is the comparison test of two illumination techniques any objective will do as well as any detailed subject.
The amount of cross polarization I leave to each of you - whatever amount seems to improve the image the most, just obviously don't reach Extinction (darkest point).

[75RR]

I will go first by re-posting the scale images as I think they show up the difference well.
Reduced the canvas size and rotated them 90 degrees to allow a side by side presentation.

A single image without moving the focus point might well provide a better comparison,
but as I already have these will start with them. Will put up some others later.

Planapochromat 63x/1.4, 50µm at widest point, Oblique and Oblique + Pol, 4 and 4 image stacks in Photoshop

[75RR]

Plan 40x/0.65, Pleurosigma angulatum, Single image, Oblique and Oblique + Pol, Converted to Black and White

Following a suggestion by gekko, I used a diatom in an attempt to see if there was an actual increase in resolution or just a perceived one due to added contrast.

[zzffnn]

I don't have photos comparisons, but I did experiments with polarizers on and off and in all positions.

There was a perceived resolution increased in mine. I saw that in your photos too, though I am not sure if you moved focus point with your scale. As you said, focus point should not be moved for good comparison.

I called it perceived resolution increased. Because it was caused by depth perception and shadow effects (detail is there without pol or even in plain brightfield field, one just cannot see it clearly due to shallow focus and/or lack of contrast/shadow).

Here is the optical theory as Rik told me over at the other forum: pol creates a shadow in objective back focal plane, thus reducing effective aperture. That reduction increases focus depth slightly. Also, darkening of background by pol creates more shadow, thus increasing contrast as well. End result is that you see more dots and lines more clearly, in a single non-stacked photo.

For stacked photos, focus depth benefit of pol is likely not obvious, but shadow contrast effect is still there and may help.

Another way to do it is cross pol, then slide in 1/4 wave retarder partially. Or simply move around oblique mask slightly. Optically, the effects may be similar, if one gets the light direction close to each other.

[75RR]

Looks like it might be perceived resolution with the Plan 40x/0.65,
will try the diatom test with the Planapochromat 63x/1.4 and see what we get.

[zzffnn]

Please don't move focus point next time. Otherwise, at NA 1.4, the comparison won't be meaningful (a slight focus shift can cause all the difference in images). I understand it is hard to do though. You can switch on/off one polarizer, while avoiding vibration.

Perceived resolution or real resolution, it only matters when photos get better, right?

[75RR]

Please don't move focus point next time.

There was only focus point movement on the stacks (first two images), no other way to make one. :)

Here are the other images:

Planapochromat 63x/1.4, Pleurosigma angulatum, Single image, Oblique and Oblique + Pol, Converted to Black and White

[zzffnn]

Did your scale stack start and stop at same points though? Sorry, I misread your posts previously (I just woke up and confused the diatom singles with scale stacks).

The difference is bigger than I expected though, and it only occurred in that stacked scale image (diatom single frames did not show that much difference). I wonder why? Stacking artifacts that only occurred in oblique, but not in oblique+pol?

Again, pol+oblique did slightly better with diatom single frame, at NA 1.4 immersion. I saw less difference this time, than the dry 40/0.65 comparison.

[75RR]

Did your scale stack start and stop at same points though?

As near as one can be. The fact that they are both 4 image stacks is a good sign.

[zzffnn]

^ Sorry, I misread. I have edited my post above to make it clear.

[KurtM]

Curious: What's the point in attempting oblique when you have DIC?

Motivation for asking is that I have never enjoyed much success with oblique illumination, and certainly never got reliable/repeatable results. Seems to me the problem is inevitably scattered light that "fogs" the image. I have seen many images posted where oblique is used to very nice effect, so perhaps part of my question is HOW you manipulate for oblique? I have tried all sorts of masks, and partially dial-in phase annuli, on a variety of stand makes and models. The results are a more or less pleasing oblique effect, but practically always at the cost of loss in image sharpness which renders it untenable.

[75RR]

Curious: What's the point in attempting oblique when you have DIC?

Only have DIC for 40x and 63x - have a lot more objectives than that!

Motivation for asking is that I have never enjoyed much success with oblique illumination, and certainly never got reliable/repeatable results.

Best and easiest way is to offset the brightfield aperture on your Phase/DIC condenser. I close the iris until I just get a hint of contrast then I slowly offset the brightfield aperture until it looks right.

[KurtM]

Cleared that up nicely, thank you 75! (Wish I had 63x DIC, but mustn't whine...)

[gekko]

Very hard for me to tell. Even if I could see a difference, I could easily think that the resolution is higher when in fact it is contrast that is higher. I am talking about my own shortcoming, with no implication as to other people's ability.

[75RR]

Don't seem to be getting the marked effect I got with the scales. Last one then.

[JimT]

You guys are way above my pay grade but IMHO one does not increase resolution with filters. In my astronomy days (Nights ) filters were great for enhancing the contrast of DSOs (Deep Space Objects) and revealing what otherwise couldn't be seen but resolution was dependent on objective size and quality.

Now-a-days one can do a lot to bring out and emphasize heretofore invisible details but they have to be there in the first place.

Good subject though. Thanks for posing the question.

JimT

[zzffnn]

Don't seem to be getting the marked effect I got with the scales. Last one then.

The one on the right looks slightly better. But it hurted my eyes to look for the difference. Comparing 100% crop may be easier to eyes.

Yes, the scale stack showed a bit more difference.

[KurtM]

Scale vs. diatom = apples vs. oranges? Gekko offers an excellent insight in asking whether he sees increased resolution, or improved contrast? Diatoms have sharply defined features which make them admirable tests for optics, and so in them we look for resolution. But the scale maybe not so much, perhaps more a case of needing contrast to discern structure. In a contest of scale images, for my own self I declare it a tie. For gleaning maximum information they're all necessary since each lighting scheme shows different things to advantage.