Polarisation Filter

[Doremus Scudder]

I use my cellphone to check shutter speeds. I just record the sound and measure the signal using Audacity, a free app. https://www.audacityteam.org/

Here's what the signals look like on a 90mm lens at 1/4, 1/8, 1/15 and 1/2, top to bottom. I've selected 1/8 shutter test the second from the top and highlighted the signal begin and end. The time appears in milliseconds at the bottom 0.124ms which when converted to a fraction is almost exactly 1/8 of a second. (1/8=0.125) The time indicated in the circle is changed to a fraction to check the actual speed. It's good up to about 1/60th. It gets a little flaky above to measure easily. But most of your shots will be at low speeds which are fairly easy and accurate to check.
Attachment 223205

Yep, I use a similar method, the Shutter Speed app. I have Audacity, so maybe I'll use it as you describe next time and see how the two methods stack up.

Plus, when I have my lenses CLAd, I make sure the tech tests the shutter speeds optically at f/22, a few times at each speed setting, and then sends me the average for each speed in ms. I'll then make a sticker for the lensboard if there are significant discrepancies. I like to round off to the nearest 1/3 stop; that's way close enough for B&W.

I like to have my shutters tested at my average taking aperture because the effective speed changes with aperture, being faster for wide apertures and slower for smaller ones.

Best,

Doremus

[Alan Klein]

I'm sorry you told me that. Now I have to retest all my lenses. I have no idea what aperture I used during testing.

[Rod Klukas]

And, though someone may have mentioned it, a warning that you should always focus through your filter. If you do not when the filter is introduced, the focus at the film plane will move 1/3 the thickness of the introduced filter. This is a problem which increases as the focal length shortens.

Unfortunately Polarizers are thicker than a UV or even a light colored, same brand filter. When using a Red 25 or 29, for instance, I often focus through a UV of the same brand and model line, filter. Then switch to the darker filter to make the shot. The filters need to match though, for best sharpest results.

Rod

[Doremus Scudder]

And, though someone may have mentioned it, a warning that you should always focus through your filter. If you do not when the filter is introduced, the focus at the film plane will move 1/3 the thickness of the introduced filter. This is a problem which increases as the focal length shortens.

Unfortunately Polarizers are thicker than a UV or even a light colored, same brand filter. When using a Red 25 or 29, for instance, I often focus through a UV of the same brand and model line, filter. Then switch to the darker filter to make the shot. The filters need to match though, for best sharpest results.

Rod

This has been discussed in another thread recently. I learned quite a bit from the discussion.

The upshot seems to be that focusing with the filter in place is not usually necessary when the filter is in front of the lens (i.e., between lens and subject) at "normal" working distances since the focus shift is usually tiny compared to the lens-to-subject distance. Depth of field and depth of focus are usually more than enough to handle the slight shift (which is usually less than normal focusing tolerance when focusing by eye).

However, when working close up, the focus shift introduced by a filter might be significant (depending on magnification), so focusing with the filter in place is a good idea in that situation.

Of course, focusing through the filter if possible, would be the wisest option and reduce the possibility of compounded errors. Using a weaker filter as ersatz for a denser one, even if not quite the same thickness/refractive index, would certainly reduce the error some (if not mostly).

It's all a question of tolerances anyway, especially when we're maximizing the depth of field.

Best,

Doremus

[Drew Wiley]

Amen. Habits which many find perfectly adequate for their own purposes, I happen to find abominable with respect to the significantly greater degrees of magnification in larger prints which often potentially accompany my own needs.

[r.e.]

And, though someone may have mentioned it, a warning that you should always focus through your filter. If you do not when the filter is introduced, the focus at the film plane will move 1/3 the thickness of the introduced filter. This is a problem which increases as the focal length shortens.

Unfortunately Polarizers are thicker than a UV or even a light colored, same brand filter. When using a Red 25 or 29, for instance, I often focus through a UV of the same brand and model line, filter. Then switch to the darker filter to make the shot. The filters need to match though, for best sharpest results.

Rod

As Doremus says two posts up, focus shift is discussed in this recent thread, especially in relation to filters mounted on a rear element: https://www.largeformatphotography.i...r-Lens-Element

A lot of filmmaking, including most professional filmmaking, is done without autofocus. I use my Blackmagic camera only with manual lenses. As you suggest in your post, focusing manually through a filter is frequently impractical. Neutral density filters, used constantly with video, are an obvious example. I've never heard anyone, in a video context, suggest that one should focus through a UV or clear filter before adding an ND or any other filter. I also don't recall that advice in the context of large format photography. It's hard to imagine users of Lee Filters and similar systems adopting that procedure.

That said, I don't question that what you are saying has an historical foundation. I'm just curious what it is.

[LabRat]

Add focusing after stopping down with filter on, and things get dimmer...

And for those who like shooting very old lenses, often colors focus differently in those under corrected optics...

Steve K

[Alan Klein]

This has been discussed in another thread recently. I learned quite a bit from the discussion.

The upshot seems to be that focusing with the filter in place is not usually necessary when the filter is in front of the lens (i.e., between lens and subject) at "normal" working distances since the focus shift is usually tiny compared to the lens-to-subject distance. Depth of field and depth of focus are usually more than enough to handle the slight shift (which is usually less than normal focusing tolerance when focusing by eye).

However, when working close up, the focus shift introduced by a filter might be significant (depending on magnification), so focusing with the filter in place is a good idea in that situation.

Of course, focusing through the filter if possible, would be the wisest option and reduce the possibility of compounded errors. Using a weaker filter as ersatz for a denser one, even if not quite the same thickness/refractive index, would certainly reduce the error some (if not mostly).

It's all a question of tolerances anyway, especially when we're maximizing the depth of field.

Best,

Doremus

Do you have to re-tilt as well as refocus?