Light falloff with Cold Light head on 4x5?

[Kodachrome25]

I am using an Aristo V54 D2-Hi Cold Light head on my 45MX with a RH Designs Stop Clock Vario. I have been doing 35mm and 6x6 without issue for some time using under lens Ilford Multigrade filters.

But last night, I did my first 16x20 from a 4x5 neg in a glass holder and got noticeable falloff on the sides and corners, as much as half a stop. I had the lamp lowered all the way down to where it was touching the top of the glass and used my 150mm 5.6 Rodagon versus my 135, so I thought I was golden in terms of adequate coverage and even illumination, lens was at F/16. It is gradual from about the outer 20% of the image, could use about 1-2 more seconds on a 14 second exposure than the rest of the shot. I might try out adding a spare diffuser I have unless it is thought that it is the lens versus the light source. I am going to need to use my 135 in order to get to 20x24 on the baseboard, so I feel like nipping this on the light source end if I can swing it...


Is this the solution or am I facing another issue I am not identifying yet?

[Bob Farr]

Hi,

Is the light source large enough to cover 4x5?? From your description this is m first question.

Bob

[Drew Wiley]

I already replied on APUG, but here again are your probable issues: 1) a 135 lens will have
some falloff issues, 2) sheet film itself generally has a little more fbf density around the
edges due to development issues (unless you are especially good at developing sheet film),
and 3) unless cold lights are oversized relative to format, they too will have some reduction
in output toward the perimeter. You can grind a custom diffuser to accommodate for all
this if it is in fact predictable. Otherwise, expect to routinely corner and edge burn your
prints.

[Kirk Gittings]

Hmm.......burning the corners a bit is standard procedure. I used to use the same setup exactly for a couple of decades though only with a 150 Schneider lens and never saw any real problem-I use a color head now-same slight issue solved by corner burning.

[Drew Wiley]

Corner and edge burning is so easy that I doubt most b&w printers even think about it.
With color work an uneven field can sometimes lead to recip issues, just like long exposures
using film, where the filtration needed changes with time. But with cold lights the bends in
the tubes also seem to get a little erratic with age or simply if the temperature is too low,
just like fluorescent ceiling fixtures. I have a warmer above my big cold light which I turn
on in advance certain times of the year.

[Kodachrome25]

Hi folks,

I went and got my extra diffuser, I will see how it works when I put it in. I was only using the 150, not the 135. As far as neg. density, it looks good overall, I develop it in a 3010 Expert Drum with a CPP2. My cold light head is usually pre-warmed about 15-25 minutes prior to a session. I guess corner burning is new to me because large format is, so I will work accordingly.

Thanks guys!