Light falloff with Cold Light head on 4x5?

[PKM-25]

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.

I have a spare cold light head and was thinking of taking the diffuser out of it and doubling up my existing one to spread the light out more...

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

 

[ic-racer]

Yes, higher magnification stresses the system and brings out flaws. Seems like you are checking all the right things. If it is a gradual falloff, you might just have to burn the corners when doing large prints.

 

[PKM-25]

Yes, higher magnification stresses the system and brings out flaws. Seems like you are checking all the right things. If it is a gradual falloff, you might just have to burn the corners when doing large prints.

Yeah, 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 the extra diffuser though unless you think 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...

On the other hand, 4x5 is simply amazing in it's ability to enlarge, it looks like it practically *needs* to go to 20x24 just to open up the minute details I see on the the neg, pretty cool thus far...

 

[DREW WILEY]

Cold lights always need to be oversize relative to film format. For instance, I use a 14X14 V54 head
for 8x10 film. A good diffuser will help, but is not a substitute for the ideal grid size. I've noticed that many cold lights marketed for a particular model of enlarger per mechanical fit are not in fact ideal with respect to light distribution or eveness. The second problem is whether your enlarger lens itself is giving even distribution (doubtful). You should be able to compensate for both problems simply with edge and corner burn-in; but if you can't, you might need to look at the diffusion system itself and how to improve that.

 

[PKM-25]

Cold lights always need to be oversize relative to film format. For instance, I use a 14X14 V54 head
for 8x10 film. A good diffuser will help, but is not a substitute for the ideal grid size. I've noticed that many cold lights marketed for a particular model of enlarger per mechanical fit are not in fact ideal with respect to light distribution or eveness. The second problem is whether your enlarger lens itself is giving even distribution (doubtful). You should be able to compensate for both problems simply with edge and corner burn-in; but if you can't, you might need to look at the diffusion system itself and how to improve that.

Thanks for your reply, my spare diffuser is in storage about 23 miles away. I have pre-Winter stuff to do in it anyway so I will go get it. As far as the lens goes, it is a new condition Rodagon 150mm 5.6, I have not really heard of any falloff problems with it and it seems to be well regarded. Since I am going to be shooting and printing a lot of 4x5, I have no issues with doing whatever it takes to get the illumination as even as possible before I resort to corrective burning and dodging.

The light is often too strong at smaller prints too, so maybe the double diffuser is the ticket....

 

[MattKing]

If you have an additional diffuser, you might be able to combine it with a mask to even out the illumination, assuming there is a place to put the mask.

 

[PKM-25]

If you have an additional diffuser, you might be able to combine it with a mask to even out the illumination, assuming there is a place to put the mask.

I thought about that too. Theoretically, the mask could fit in between the two discs if it is of the heat resistant variety like a Rosco product. I don't think it will take that though, I bet I get super close if not right on with just doubling the disks.

And it is not like having a mask in there will create issues for 6x6 or 35mm.