A random question I belive involves optics

[6x6TLL]

If this is the wrong Forum, I hope a mod can help it find a new home.

I was wondering if anyone could explain why, when I use a say a graduated ND filter on my medium format rig, it's very easy and clear to see the transition between the clear and ND portion of the filter as I move it. Even at wide open aperture (f3.5 or f4). Obvious and clear as day.

However, the same filter in the same holder on the 4x5 or 8x10 setup, and it's usually a struggle to see where the transition falls on the ground glass, even when stopping down to f16 or f22. Even with lenses that are considered bright (f5.6).

I'm assuming that the difference has to do with the optics and format, possibly lens design, but am not sure. I was hoping that some of the knowledgeable people here could help explain it.

Thanks!

[Mark J]

I'm just thinking through this. It's not the optics design in detail.
I think the two main factors are the entrance pupil size ( stop as seen from the front ) and the distance of the filter from the entrance pupil.
The bigger the entrance pupil, the more the grade edge is softened and blurred.
The further the filter is from the entrance pupil, the more the field pupils track across the filter and the easier it is to see the grade.

I think both factors are coming into play here. The entrance pupil is definitely bigger with an 8x10 lens ( 300 ? )
However not so much with a 150 on 4x5.
However the filter is probably pretty close to the entrance pupil with a 150.
On medium format I guess you're using an 80 (?) ... the lens barrel is usually fairly big on these and the filter a bit further away.
On a wide like a 55 to 65, the entrance pupil is smaller and also deeper inside the lens, both of which will help to make the grade 'sharper' .

[6x6TLL]

Thanks for thinking out loud with me on this one, Mark.

So the actual physical aperture diameter is one factor (not the f stop itself, but the actual opening size), and the distance from the aperture to the filter is the second?

I tested specifically on wide lenses, since they are supposed to be easier to see the grad with, and I was testing out a few different matte box solutions to replace my tattered and repaired Lee holder. One thing I wanted to check was to make sure they didn't vignette with the wider lenses.

That would most likely mean that the distance from the aperture itself would be pretty similar between the 40mm Super Angulon (MF) and the 210mm APO-Symmar-S (8x10) as the matte box fits onto the front of the lens, and the Grad ND slots into a tray. I can check again in the morning to confirm.

However, as you point out, the aperture pupil would most likely be larger on the 210mm compared to the 40mm. Something for me to look at and see.

Does anyone else have any thoughts on this?

[Kino]

I wonder if a potential solution would be to construct a black paper envelope that only covers the filter up to the middle of the demarcation point of the graduation.

You could then use this to place the "horizon line" of the ND and remove it prior to shooting, as it should be clearly visible on the ground glass.

A few more fiddly steps for sure, but it might solve the problem.

Don't know, just gathering wool here...

[Mark J]

So the actual physical aperture diameter is one factor (not the f stop itself, but the actual opening size), and the distance from the aperture to the filter is the second?

Yes, correct.

[Jody_S]

I have screw-in grad filters in a couple of sizes, and a set of 4" square grads for bigger lenses. The graduation on the screw-in filters is quite abrupt, as it needs to be over the small, round filter. The graduations on the big 4" filters are much wider. I attributed the difficulty in spotting the demarcation on the ground glass to that, to the very wide graduation. I've never been able to clearly establish where it is when I'm composing, I usually end up looking at the lens from the front and eyeballing it, to cover the top 1/3rd or 1/2 the image.

[sharktooth]

That's an interesting phenomenon.

I wonder if there is some flare effect through the filter if there's no lens shade.