Blue Filter for Focuser?

[tgtaylor]

I have the Peak Model II focuser, Will this filter, which fits both the Model I and II, improve focus significantly? In other words, is it worth the $30? https://www.bhphotovideo.com/c/produ..._Critical.html

Thomas

[Oren Grad]

With aerial-image focusers like the Peak Model II, your accuracy may get worse with a blue filter, not better. See pp 76-77 in Ctein's Post Exposure, currently available free from Ctein's website:

http://ctein.com/booksmpl.htm

[Larry Gebhardt]

Also from Post Exposure see pages 145 to 154. The focus shift from blue light will depend on your lens, light source, and paper. If you find a serious focus shift issue I think you would want a UV cut filter for the lens and not a grain focuser filter. That way the green and blue light would be focused at the same plane for VC paper.

What paper and light source do you use?

Edit: I was asking about light and paper because I'm assuming you noticed a focus problem. If that's not the case I wouldn't worry much about it. I focus with white or green light and have yet to see any issues with either focus or focus shift related to blue light.

[Drew Wiley]

These were designed for the era of blue-sensitive graded papers and enlarging lenses which were not necessarily well Apo corrected. I find no need for the blue attachment with more modern enlarging lenses. But otherwise, yes, as you go upwards in price scale with Peak you get better focus, with the expensive Critical Focus Magnifier being conspicuously the best. It too came with an accessory blue filter.

[Pere Casals]

Also from Post Exposure see pages 145 to 154. The focus shift from blue light will depend on your lens, light source, and paper. If you find a serious focus shift issue I think you would want a UV cut filter for the lens and not a grain focuser filter. That way the green and blue light would be focused at the same plane for VC paper.

+1

This happens if the illumination throws UV and lens has transmission there. EL Nikkor enlarger lenses are transmissive in the UV, for this reason they are used for UV photography, but EL Nikkors are corrected 380 to 700nm...

We also can block UV in the light source with a bare clear sheet of common Polycarbonate:



Plexiglass (PMMA "methacrylate") instead has to be of a special type to block UV: Plexiglass UV 100

It would be interesting to measure lp/mm with and without UV, to check what Ctein said, I've doubts comming from my own tests. This can be done by projecting an usaf 1951 slide with the enlarger on a DSLR facing up without the lens, with and without UV.

[Drew Wiley]

Most of the UV capable of disturbing focus is removed in modern colorheads or anything which can be classified as a diffusion device. It's a non-issue with respect to focus, even with EL Nikkors unless you're trying to engineer a head for deliberate UV enlargement, in which case you have far bigger problems to contend with, like not even looking into a magnifier mirror scorching your eye with high UV!

[LabRat]

No, as most lenses are corrected ok for tricolors...

May make focusing harder due to hard to see blue light and dimmer...

Not an issue...

Steve K

[Drew Wiley]

I have had in my hands for free some funky old chrome Componons, perhaps 50's vintage, that exhibited focus shift per blue versus red or green, at least at wider stops. I got rid of them and never actually printed anything with them, having far better lenses on hand. But if your lenses are pre-70's or 3-element "student grade", its worth checking this potential problem out. But the eye itself is much less sensitive to blue than to green, so focusing through a deep blue filter is difficult unless your light source is strong. Ordinary enlarger lamps are yellowish tungsten, so don't transmit blue very efficiently.

[Leigh]

Nobody has mentioned the fact that enlarger lamps produce NO UV in the first place.
So why would you need to attenuate it.

The blue filter is meant to attenuate RED light, which is a significant portion of the lamp output.

- Leigh

[ic-racer]

It is to improve precision in which you eye can focus on the reticle due to the pronounced chromatic aberration of the human eye. Has nothing to do with the enlarger's projected image. Works even when the device is not under the enlarger.