Setting the camera to Manual exposure is not the same as setting the built in flash to operate at a manual output.

Setting the camera to Manual and not changing any flash settings will still give you P-TTL flash, including pre-flash.

If you want the built in flash to operate at a manual output you need to select that in the flash menu.

Put the camera flash mode to M (manual), and dial in the output you require. Range is Full power down to 1/128 power.

What camera are you using ?

What ND filters/holder are you using ?

If you are using something like the Lee system, rather than a screw-in filter you need to be covering the area where the holder meets the front of the lens during the exposure. A baseball cap or piece of cloth or even your hand will do.

Using too small a square/oblong filter can cause problems as well as light is being reflected from the filter edges.

Can you post an image of your set-up

If you are using Adobe CC, you will have had, or shortly will have, an update to Photoshop to version 24.0.0 and ACR to v 15.0

While most of your settings will be maintained, there is a little "gotcha" which always seems to happen when there is a major update like this one is.

Open a raw file in Photoshop and it opens in Camera Raw. At the bottom of the frame there is a link which tells you what your output settings are. if you were previously using 16 bit output, you may find it has changed back to the default 8 bit. Click the link and change your setting to 16 bit, then open the image. You should now be back to 16 bit for all subsequent uses in ACR.

Yes they have gone up considerably in price since I bought mine (hasn't everything).

Have a look at these resources. I really recommend the two videos on shooting seamless white background by Zach Arias. I learned a lot about general flash photography, even if seamless white background is not your goal.........

https://nofilmschool.com/2015/03/light-shoot-seamless-white-background-start-fin...

https://sekonic.com/classroom/

https://www.picturecorrect.com/portrait-photography-lighting-the-5-classical-typ...

Yes there is. Pentax Rumours have been all over it for some days now. It will be AW rather than WR.

I picked up a 100mm 2.8 macro as a freebie with my recent K-1 II purchase. I will be happy to continue using that

You don't "need" to splash out on a flash meter immediately. You can always start off by using the histogram on your camera to assess the exposure after you have taken a shot. Start off with full power on the key light, -2 stops for the fill, ISO 100 and f8/f11 depending how far the subject is from the lights and you can adjust from there.

Lets say you use two lights for the subject, key and fill. You will set the fill light to say 1 or 2 stops less than the key light. Then holding the flash meter in position where the subject will be standing/sitting trigger your flashes by remote. Based on what ISO you have told the meter you will be using it will tell you what aperture to use. If you want to use a wider aperture than it says, then wind down the power of the flash units and try again until you get the desired aperture. Use Manual exposure on the camera with the shutter speed set to sync speed and away you go. Perfect exposure each shot.

I use the Sekonic L-308. It also works as an ambient incident meter for daylight shooting. Handy if you are shooting a black cat against a black wall !

Hi Morten

First, I dont know anything specific about the Yongnuo or Godox flash units, but can give some general advice. You are correct that the pre-flash in Pentax P-TTL will cause issues when using optical triggering. Using manual flash on all three units will get round that problem. In fact you are far better off using manual flash as you will consistently get the desired result from your key and fill lights without having to worry about flash ratios and compensation. I would advise you invest in a hand held flash meter which will save a lot of trial and error. Sekonic do a relatively inexpensive one.

As an alternative to optical triggering, you could look at radio triggers. I use Cactus triggers, but sadly they are no longer in business. Radio triggers avoid any issues of line-of-sight that you can encounter with optical triggering.

Good luck and let us know how you get on.

ps I have seen some flash units advertised as having "anti pre-flash" protection, but have never used them myself.