A confusing problem with lens f stops and timer f stops

Hi all,

So I was recently doing some contact sheets. My blacks appear at f/4 at 4s. So when I tested the same negatives at f/8 at 16s, the contact sheet becomes much much darker. I know I’ve closed the lens 2 stops and given 2 stops more exposure but for some reason it gets much darker.

Any reason for that?

Many thanks,
Fatih

So I have done further tests, I get the similar densities at f/11-8s and f/4-4s

So closing the lens for 3 stops but giving only 1 stop more exposure?

I don't know what it is, but something's going wrong here.

You don't say anything about the development. You need to be more specific. Nothing "appears" until it is developed.

The aperture may not be working correctly.
Which lens, and does it have any features that allow you to limit the range of apertures?
Try the test at f/5.6 and 8 seconds as well.
By the way, technically speaking "exposure" is the product of time and intensity, so pedantically speaking, you are talking about longer exposure times, not "more" exposure, when you refer to time differences.
That probably won't matter in this context, but using the better terms may help avoid confusion.
One further possibility: the timer may be malfunctioning.
I was recently printing in our Darkroom Group's darkroom and used an older, electromechanical timer. Things seemed "off", so I checked the timer and it was actually giving me ~40 seconds when set to 60 seconds and ~14 seconds when set to 24.

xkaes said:

I don't know what it is, but something's going wrong here.

You don't say anything about the development. You need to be more specific. Nothing "appears" until it is developed.

Click to expand...

So I develop in normal Ilford MG 1+14, for 1:30. The paper which is exposed at f/4 and 4s gives correct blacks, but f/8 at 16s gives a much darker print.

I mean by correct black as the film leader is as black as the area of the paper which does not have the film on it, described in how to create contact sheets.

MattKing said:

The aperture may not be working correctly.
Which lens, and does it have any features that allow you to limit the range of apertures?
Try the test at f/5.6 and 8 seconds as well.
By the way, technically speaking "exposure" is the product of time and intensity, so pedantically speaking, you are talking about longer exposure times, not "more" exposure, when you refer to time differences.
That probably won't matter in this context, but using the better terms may help avoid confusion.
One further possibility: the timer may be malfunctioning.
I was recently printing in our Darkroom Group's darkroom and used an older, electromechanical timer. Things seemed "off", so I checked the timer and it was actually giving me ~40 seconds when set to 60 seconds and ~14 seconds when set to 24.

Click to expand...

It is Schneider APO 60/4. I can see the aperture is closing correctly but then I am not quite sure if it is really halving the light.

I am certain about the timer because I have tested it against my iPhone countdown timer.

I'll test at f/5.6 and 8 seconds but I think that will be quite dark as well.

Apologies for exposure confusion, yes it is longer exposure time (or more exposure to the light?)

The only explanation I have at the moment (if everything is working correctly) when the exposure times are short, the lamps do not reach full output hence a short exposure to the paper. Maybe I should never go below 8 seconds to make sure lamps reach to maximum output.

Fatih Ayoglu said:

It is Schneider APO 60/4. I can see the aperture is closing correctly but then I am not quite sure if it is really halving the light.

I am certain about the timer because I have tested it against my iPhone countdown timer.

I'll test at f/5.6 and 8 seconds but I think that will be quite dark as well.

Apologies for exposure confusion, yes it is longer exposure time (or more exposure to the light?)

The only explanation I have at the moment (if everything is working correctly) when the exposure times are short, the lamps do not reach full output hence a short exposure to the paper. Maybe I should never go below 8 seconds to make sure lamps reach to maximum output.

Click to expand...

You should also check the reciprocity farther from maximum aperture - something like the correct times for f/8 and f/16.

The wide open aperture marking on a lens is often optimistic. For an f4.0 lens the wide open aperture may range from 4.5 to 5.0. It's a marketing thing. After wide open the lens' aperture markings will usually be reasonably accurate.

A good enlarging meter will quickly show what is going wrong. To belabor a point: "When you can measure what you are speaking about, and express it in numbers, you know something about it; When you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind." - Lord Kelvin/William Thompson.

If the timer is mechanical or electro-mechanical it can still cause irregularities if the if mechanism slows down or speeds up in it's travel. The same problems can occur with electronic timers that use a pair of knobs to set the time. Digital timers are usually accurate. Counting "1-elephant";"2-elephant";... should be accurate enough to show if the problem is in the timer or the lens' aperture mechanism.

The amount of light for the longer time / small stop must be changing or be non-linear. It could be the timer, but that is a major drift. Simple incandescent bulb, colour head, or something else? It could even be unsafe light in the darkroom bring the paper threshold up in the longer exposure.

Change the timer and test, then do it in full dark. You only have to do the long exposure - if it isnt the same as before you have the answer. If it still happens then it is the light source.

Fatih Ayoglu said:

So I develop in normal Ilford MG 1+14, for 1:30. The paper which is exposed at f/4 and 4s gives correct blacks, but f/8 at 16s gives a much darker print.

I mean by correct black as the film leader is as black as the area of the paper which does not have the film on it, described in how to create contact sheets.

Click to expand...

It's not clear from your statement what you're actually comparing.

You describe f4 at 4 seconds gives you "correct blacks", but you say f8 at 16 seconds gives you a "much darker print". Those aren't really the same thing.

For example, it's normal to do a test to determine the minimum exposure to achieve maximum black in the print (at base+fog in the negative).. That exposure level, however, may result in a very dark print if the negative was underexposed and./or underdeveloped.

Nicholas Lindan said:

The wide open aperture marking on a lens is often optimistic. For an f4.0 lens the wide open aperture may range from 4.5 to 5.0. It's a marketing thing. After wide open the lens' aperture markings will usually be reasonably accurate.

A good enlarging meter will quickly show what is going wrong. If the timer is mechanical or electro-mechanical it can still cause irregularities if the if mechanism slows down or speeds up in it's travel. The same problems can occur with electronic timers that use a pair of knobs to set the time. Digital timers are usually accurate. Counting "1-elephant";"2-elephant";... should be accurate enough to show if the problem is in the timer or the lens' aperture mechanism.

Click to expand...

My experience and experiments agree with @Nicholas Lindan: The wide-open f-stop of a typical lens is not accurate. BTW, @Nicholas Lindan runs the Darkroom Automation company which sells enlarger meters (among other things). I have one and recommend it. In fact, I was using it 30 minutes ago.

Mark

The timer might be accurate in terms of seconds (compared to your phone timer) however what I would do to double the time is that instead of lighting the paper for a continious 8 seconds I do 2 rounds of 4 seconds to match my tests, because in reality your enlarger's lighting bulb fades in and out ( might not be noticeable to the naked eye tho) e.i. once you turn on the switch, it takes a bit of time to get to maximum temp/brightness and a bit of time to go out once you switch off .... so do a test to see if this fixes the problem, otherwise the issue is more likely the marketing thing that Nicholas pointed out ... update us with the results.
Cheers

It could be the bulb in your enlarger is getting brighter after some seconds as it heats up. Maybe time to replace it.

If the OP is using a cold light head then all bets are off on 2x the time == 2x the exposure; these heads have very long warm up times. They have heaters that are supposed to mitigate warm up but this rarely works well.

Regular incandescent lamps, like the venerable PH21x (US) and P3/4 (UK et al.), have a 50 millisecond (1/20 of a second) warm up time that can be dismissed in the OP's circumstances.

Mondo1 said:

The timer might be accurate in terms of seconds (compared to your phone timer) however what I would do to double the time is that instead of lighting the paper for a continious 8 seconds I do 2 rounds of 4 seconds to match my tests, because in reality your enlarger's lighting bulb fades in and out ( might not be noticeable to the naked eye tho) e.i. once you turn on the switch, it takes a bit of time to get to maximum temp/brightness and a bit of time to go out once you switch off .... so do a test to see if this fixes the problem, otherwise the issue is more likely the marketing thing that Nicholas pointed out ... update us with the results.
Cheers

Click to expand...

That depends on the enlarger light source. A cold light might show those affects, but I use an incandescent bulb and I do not experience a warm up lag in brightness.

Sharktooth said:

It's not clear from your statement what you're actually comparing.

You describe f4 at 4 seconds gives you "correct blacks", but you say f8 at 16 seconds gives you a "much darker print". Those aren't really the same thing.

For example, it's normal to do a test to determine the minimum exposure to achieve maximum black in the print (at base+fog in the negative).. That exposure level, however, may result in a very dark print if the negative was underexposed and./or underdeveloped.

Click to expand...

I agree with this. I’m inclined to suspect that you have misunderstood the advised procedure. Could you explain in full detail what the advice was, and what you actually did?

BobD said:

It could be the bulb in your enlarger is getting brighter after some seconds as it heats up. Maybe time to replace it.

Click to expand...

I've yet to come across an incandescent bulb that does this. I frankly find it a far-fetched idea and I would instead start with @Nicholas Lindan's suggestion to measure the brightness of the baseboard projection. Without a negative in the carrier, even a digital camera with a spot meter function can be used, or an old-fashioned 'analog' spot meter. Not as nice and accurate as a dedicated enlarging light meter, but it'll be good enough to spot the kind of gross problems that seem to play a role here.

Good morning all,

Thank you very much for your replies.

I use De Vere 504 MKV head, twin lamp 2x250W, dichrom head. So if we are talking about the real temperature of the lamps, they get very hot and yes the beginning and the end of exposure have some sort of fading, which I believe is normal with these kind of lamps.

I will measure EV values on the base board with a Sekonic light meter (in reflection mode) to see if each half stop closing the aperture gives 0.5 reduction on the EV values.

Sharktooth said:

It's not clear from your statement what you're actually comparing.

You describe f4 at 4 seconds gives you "correct blacks", but you say f8 at 16 seconds gives you a "much darker print". Those aren't really the same thing.

For example, it's normal to do a test to determine the minimum exposure to achieve maximum black in the print (at base+fog in the negative).. That exposure level, however, may result in a very dark print if the negative was underexposed and./or underdeveloped.

Click to expand...

snusmumriken said:

I agree with this. I’m inclined to suspect that you have misunderstood the advised procedure. Could you explain in full detail what the advice was, and what you actually did?

Click to expand...

Ok so what I have done is this;

First I set the the lens at f/4, timer at 16s, focused the lens, gave the exposure on a scrap paper with Stouffer 21 step Wedge tablet, then I have lowered it to f/4 at 8s and finally to f/4 at 4s, because each time, I thought I could give shorted exposure. Results are below. Arrows are the steps I cannot see the difference with the following black part.



Then I thought, ok because f/4 at 4 seconds should be equal amount of light exposure to the papers with f/8 at 16 seconds (2 stop closing aperture, 2 stops giving more light), I should print my contact sheet at f/8 at 16 seconds. Result below. As you can see it is very dark, not something I have expected based on the above test. Because even at f/4 @16 seconds the image on the top right have some whites.



At this point I thought something is wrong so I have printed at f/4 and 4 seconds. Result below. Now that seems more correct. The images on the top are overexposed but the images on the below should be correct prints at Grade 2 (paper is Fomavariant 312, filter setting is 5M)



Now I have confused and thought, ok f/8 @16s gives a very dark image, how about f/8 @8 seconds. Then I thought still it is too dark, so I have closed the aperture 1 more stop to f/11 and printed with 8 seconds. Now the result is more or less the same to f/4 at 4 seconds.





Please correct me at any stage.

PS we are not discussing my photography skills, and because I have forgotten to put the glass on the negatives, they are blurred, I have corrected that.

Many thanks,
Fatih

And here is the EV measurements;

f/4	EV8.1
EV7.7
f/5.6	EV7.2
EV6.8
f/8	EV6.2
EV5.7
f/11	EV5.2
EV4.7
f/16	EV4.1

So the drop on the EV is consistent with each 1/2 stop closing the aperture and as expected.

What I have realised is, if I take the measurement as soon as when I switch on the lights, it reads as EV6.5 vs EV8.1 and then after 1 second or so, the reading stabilises. So I think the issue is, the bulbs require some time to reach the correct output.

Well, in that case I stand corrected on my earlier post and marvel in the horrible behavior of these particular bulbs....I truly did not dare to suspect anything as abysmal as this warmup behavior was even physically possible with relatively low-power incandescent bulbs.

And if this really is the issue, then obviously if the exposure time is too short, then the exposure stops before the lights get to full power or the paper does not get a real set time exposure, in my case 4 seconds, like for 25% of that time, the light is not in full power.

However if the exposure duration is 8 seconds, then it is 12.5% of the time and if the set exposure is 16 seconds, only 6%.

Looks like it is time for me to invest in good ND filters to extend the exposure duration to stay in optimum aperture settings of my lenses.

Or look into what kind of weirdness is going on with this particular light source. As mentioned earlier, a normal warmup time for an incandescent bulb is a few dozen milliseconds. What you're witnessing isn't normal.

I think with a typical set-up with no digital timer or LED head or lens with perfectly calibrated aperture (no idea whether that's available), we just have to accept that reciprocity in enlarging isn't perfect and make final adjustments at working aperture. I sometimes do my first test strips wider open to save time if the exposures get long, with larger prints or everexposed negatives, but final test strips always at working aperture.

koraks said:

Or look into what kind of weirdness is going on with this particular light source. As mentioned earlier, a normal warmup time for an incandescent bulb is a few dozen milliseconds. What you're witnessing isn't normal.

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I will change the lights to a new ones and retest to see if the bulbs are the issue