Wet plate focus confusion.

[Paul G Hammond]

Hi LFPF,

This is my first post so I hope everyone is well in this strange period of time.

I own a tailboard camera and have been hitting the wet plate process hard since we have been in lockdown in the UK. My camera can shoot 14" x 14" but I mainly shoot whole plate for ease and cost. I recently purchased a FRANKE & HEIDECKE Heidosmat 1:3/ 250 as I wanted to experiment with a faster lens for portraits as my flash wasn't powerful enough with my other lens.

So my question is why would there be a discrepancy between the focus on the ground glass and the image plate?

My work flow has been:

Focus the ground glass so the eyes are tack sharp (I use a mannequin head for practising as it doesn't move). I remove the ground glass and replace with the darkslide and shoot the plate. After development and fixing the plate the image is out of focus on the eyes.
I have worked out that the focus at this point is 5mm forward (part of the neck is in focus), so it seems that I need to pull the focus back by 5mm after I have focused on the eyes to get a sharp plate.

I have tested that distance between the front of the camera to the ground glass and them the distance to the plate and they are the same.

I know that Wet Plate sees light differently so could it be this? Or is it the lens? I can't work out why the image is sharp when focusing and out of focus when shooting.

If anyone can help me understand what's going that would be great..

Thanks in advance

Paul

[Two23]

Try this: Focus on a sharp point, remove lens and then stick a rod straight back through the opening all the way to the ground glass. Mark the distance from the GG to the front edge of the lens board. Replace GG with an opened holder that has a tin (or glass plate) in it. Do the same thing--stick a rod all the way through and mark/measure the difference. It should be the same. This is called "checking the register."


Kent in SD

[Bill Rolph]

Are you stopping down after focusing? Closing the aperture can induce focus shifts, particularly with certain lens designs.

[goamules]

This is a projector lens. Have you shot any film with this lens? There is no telling how corrected this projector lens is for photography at all. I'd say that is your main problem, not wetplate, and not the registry of the film plane to the ground glass. All those things are made to be perfect for photography. Your projector lens is not.

[Paul G Hammond]

Hi thanks for the quick replies.
Yes, I knew it was a projection lens when purchased, but it was fast and cheap and I hoped would do the job for messing around with trying to reduce the amount of power needed. I've already checked the register and this is pretty much perfect.

Can you advise why the image is sharp on the ground glass, but is out by 5mm when developed? That's the confusing part for me.

Thanks for any information.

[Ron_in_Alaska]

It's all about the type of light you need to make wet plate work. UV focuses at a different point than the light used in a projector. I had to reposition my lens after it was focused to fix the problem. By measuring the distance difference you could make a special plate holder just for that lens. I never went that far.

[Bill Rolph]

If the focus shift is due to chromatic abberation, you could try focusing with a blue filter over the lens. Collodion processes, depending on the combination of halides used, are sensitive from 380 to roughly 520nm, with the maximum sensitivity at about 430nm. A #47 filter over the lens will isolate this band for focusing.

[Two23]

If the focus shift is due to chromatic abberation, you could try focusing with a blue filter over the lens. Collodion processes, depending on the combination of halides used, are sensitive from 380 to roughly 520nm, with the maximum sensitivity at about 430nm. A #47 filter over the lens will isolate this band for focusing.

Neat idea. I had also thought of figuring out how far the focus is off and making a permanent shim.


Kent in SD

[Mark Sawyer]

Neat idea. I had also thought of figuring out how far the focus is off and making a permanent shim.

The amount it is off by will vary with different focusing distances.

[Paul G Hammond]

Wow, thanks Ron, Bill, Michael, Goamules & Two23.

This amount of information is amazing, I tip my hat to you all. It's great that it all layers upon itself to describe what I've been suffering and confused with.

The amount it is off by will vary with different focusing distances.

This was going to be my next question as a quick fix, but it make senses that this would change depending on focusing distances. I will look to invest in a #47 filter to focus through and see if this helps.

Wet Plate photography isn't as popular in the UK as it is in the USA so it is wonderful to reach out and receive solutions from you all.

Thanks again.