How important is an accurate exposure time? Considerations not only for Minox.

Many people who use analog cameras are concerned about whether the shutter speeds are correct and or how they can be adjusted. These are legitimate questions, especially if, like me, you enjoy working with these cameras.

But is this question also relevant in practical terms? How important is the “correct” exposure time? I decided to investigate this issue practically and conducted some experiments with my Minox. The results may be surprising and applicable to other analog cameras:

Two photos, one of which was exposed 50 times longer than the other, produce virtually identical results.

You can find my report on this with all the photos here:

When I sell my cameras I made shutter speed measurements. I measured each speed 10 times and post the mean as well as the standard deviation (so that one can tell how consitent they are). However, when I list them with speed 1 stop or so slow people balk at my cameras and didn't buy. I bet many people selling cameras with worse shutter performance than that and the buyers didn't even notice,

Chan Tran said:

When I sell my cameras I made shutter speed measurements. I measured each speed 10 times and post the mean as well as the standard deviation (so that one can tell how consitent they are). However, when I list them with speed 1 stop or so slow people balk at my cameras and didn't buy. I bet many people selling cameras with worse shutter performance than that and the buyers didn't even notice,

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I believe you every word!

When you say the lowest density shot was 4 stops underexposed, I assume you were metering for the shadows. It has much more shadow detail than my color negative does when it's just 1 or 2 stops underexposed, when I'm metering the whole scene in-camera.

loccdor said:

It has much more shadow detail than my color negative does when it's just 1 or 2 stops underexposed, when I'm metering the whole scene in-camera.

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I agree; maximum underexposure on that strip is around 2 stops at most, probably around 1.5. I've shot a decent amount of Vision3 50D.

@koraks

That's one of the things I find nice about slide, since you meter for the highlights it often results in a few stops of effective film speed when compared to the same speed color negative.

@koraks @loccdor

I took an object measurement across the entire image. In my opinion, there are no extreme lights and shadows in this motif, are there?

What do you think of the overexposed photo taken at 1/20 s? In your experience, is overexposure of 1.6 f-stops plausible in this case?

tjwspm said:

@koraks @loccdor

I took an object measurement across the entire image. In my opinion, there are no extreme lights and shadows in this motif, are there?

What do you think of the overexposed photo taken at 1/20 s? In your experience, is overexposure of 1.6 f-stops plausible in this case?

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No. I would say it's about 3 stops overexposure.

tjwspm said:

What do you think of the overexposed photo taken at 1/20 s? In your experience, is overexposure of 1.6 f-stops plausible in this case?

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I agree with @brbo that the overexposed frames are several stops overexposed. The one on the left, I'd say 3 stops at least, might even be more. At some point you're just pushing density all the way up the HD curve until it starts to slope off.

tjwspm said:

I took an object measurement across the entire image. In my opinion, there are no extreme lights and shadows in this motif, are there?

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I agree it's a low-contrast scene. This makes things easy in one sense, in that metering can be expected to be fairly constant. It also has a very problematic implication for your conclusion that underexposure is less of a problem than people say. If you have a very low SBR (scene brightness range), this also means that shadows are relatively less deep. This can make it seem like underexposure is much less of a problem than it would be in a more brightly lit (and therefore more contrasty) scene. This is also one reason why people find that push processing film works reasonably well on dreary, grey winter days. Since nothing much happens near the toe part of the curve, you don't lose much image data.

So I believe there are some caveats to your otherwise very nicely presented and illustrated (as always!) blog entry.

tjwspm said:

Two photos, one of which was exposed 50 times longer than the other, produce virtually identical results.

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See David Vestal, The Craft of Photography. He made an even more extreme experiment with Tri-X.

50 times means 5.6 stops difference. Under 2 stops and over by 3.6 stops.

The rule of thumb that saves me when doing the mental math is 3 stops =~ 10x. It's really 8x, but a useful approximation for sanity checks.

bernard_L said:

See David Vestal, The Craft of Photography. He made an even more extreme experiment with Tri-X.

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Thanks for pointing that out. I assume you're referring to his comments in Chapter 2, “Light Meters”? I'll read that again carefully. At one point, he points out how strong the influence of the gradation of the paper used is. That reminds me that I'm working in a hybrid workflow, which gives me completely different possibilities for manipulation...

@brbo @koraks

Thank you very much for your comments. I really appreciate this constructive criticism because it helps me improve.

We are talking about this negative strip here:
I took the picture in front of a lit sheet of paper. So it's probably not a perfect reproduction.
Nevertheless, I would like to summarize what I have taken away from the discussion so far:
1. The negative on the far right looks as if it is only 1 to 2 stops underexposed.
2. The negative on the far left looks as if it is about 3 stops overexposed.
Then the 4th image from the left should be the most correctly exposed, right?

I then checked my calculations again and actually found an error in this table, which I corrected:
I previously had based my calculations on the LV values (at ISO 100) rather than the EV values (for ISO 50 film).
Now everything fits together. The frame on the far left is now 2.6 stops overexposed (instead of 1.6), and the frame on the far right is only 3.2 stops underexposed (instead of 4.1). A correctly exposed photo would therefore be between the third and fourth frames. The 3.2 stops are still too much in your opinion, but it's moving in your direction. I have amended the text accordingly with my conclusions regarding tolerance to underexposure.

I am glad that I was able to improve the report with your help. For me, that is a big benefit of this forum.

tjwspm said:

Thanks for pointing that out. I assume you're referring to his comments in Chapter 2, “Light Meters”? I'll read that again carefully. At one point, he points out how strong the influence of the gradation of the paper used is. That reminds me that I'm working in a hybrid workflow, which gives me completely different possibilities for manipulation...

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I am referring to Chapter 9 "Basic Control - Film Speed and Exposure" Harper and Row 1974 updated edition.
There he shows results of exposing the same scene with Tri-X at EI 1600 down to EI 0.37 (zero point thirty seven). Occasionally, on this forum, I've read advice or reporting about speed testing in 1/2 stop increments...

@tjwspm good detective work, well done! Thanks so much for looking into this. And congrats again on your beautifully presented blog series on the minox.