Darkroom Automations Enlarging Meter question
I just got one of these meters. Very interesting tool so far. A question. I was measuring the light drop off on the baseboard with the meter, when I start playing with the aperture of the lens. Should each stop of the lens match up with a single stop on the meter? ie: if the meter reads 7.5 at F11, then shouldn't it read 8.5 at F8? I'm find that this is the not the case with my EL-Nikkor 50/2.8. I haven't tried my other enlarger lenses yet. If it should be the case, and it is not then what could be problem? The click stops on the lens are not correct?
Since the hot spot in the middle goes away as you stop down, the corollary is that not all points on the baseboard will respond equally to a change in the lenses aperture. Bottom line is the meter will be correct. I'll let Nicholas elaborate further.
I was keeping the meter in exactly the same place and stopping the lens down, not moving the meter around when I was testing that. I therefore expected the meter to read a single stop difference, but it did not. As for the evenness of the light I found that overall the light on the baseboard is within 1/2 a stop (according to the meter) over the entire area of illumination. I'm not sure if that is good or bad.
You don't mention what sort of discrepancies you are seeing, so it is hard to comment except in general terms.
The aperture in an enlarging lens isn't a precision mechanism. Of my lenses the El-Nikkor's are accurate to a bit better than a 1/10th of a stop between click stops, the Rodagons and Apo-Rodagons not so good, and my (albeit pre-WWII) Focotar is pretty awful.
The fall off will change as the lens is stopped down, but by f5.6 on a f2.8 lens this effect has ceased for all practical purposes. Still, you should have the meter directly under the lens when making these measurements.
Light evenness of 1/2 a stop is average for a condenser head. Good diffusion heads are about 1/4 stop.
Nicholas O. Lindan
I guess what I want to know is whether my understanding is correct. That is stopping down the lens one stop should in theory change the meter by one stop. I tested two of the lenses I have. An EL-Nikkor 75mm. That lens after F4 was completely correct if I kept the meter directly under the lens. That is each click resulted in a change of 1 on the meter. As I moved out from the center it tracked less precisely. With the EL-Nikkor 50/2.8 I got the following with the meter under the center of the lens (using delta mode). 0, 1.84, 1.77, 2.63, 3.63, 4.58. The 75mm lens I typically use at F11. The 50mm lens I typically use at F8.
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Some of the questions in this thread made me wonder how well the aperture markings on a typical enlarging lens translate into different light intensity at the print.
I did the following experiment to determine the accuracy of the aperture calibrations on my 50mm f/2.8N EL Nikkor.
I installed the lens on my Beseler 23CII condenser enlarger and set my Minolta Flashmeter IV onto the baseboard at the center of the projected light—no negative.
The meter was set to 2 seconds and ASA 1000. I adjusted the elevation until the meter read exactly f/45 with the lens set to f/2.8.
By taking readings at each stop on the lens I got the following as: lens aperture, meter reading
f/2.8, f/45 + 0.0
f/4, f/32 + 0.1
f/5.6, f/22 + 0.1
f/8, f/16 + 0.1
f/11, f/11 + 0.0
f/16, f/8 + 0.0
The aperture calibrations of this lens appear to produce reasonably accurate 1-stop differences in light intensity at the baseboard.
I forgot about using a light meter! Obviously its not precise enough for measuring print tones (the area of coverage is too large), but for a general measure of light it should be fine. I'll check with my light meter this evening and see what readings I get.
That looks about right. The maximum aperture on a lens is usually a bit optimistic, so the 0.84 stop difference between f2.8 and f4.0 is to be expected.
Originally Posted by dfoo
After this the stop-to-stop differences are: 0.93, 0.86, 1.00, 0.95
The mean difference is 0.935 stops, the SD is 0.058 stops
I'm sure this is within the nominal manufacturing tolerances for the lens and I wouldn't worry about it. Some lenses, like your 75mm, are better, some are worse. As one normally meters at the printing aperture any errors in the lens' aperture click-stops aren't of much importance.