Understanding Shutter Speed Settings

I have two images taken consecutively inside a church with daylight streaming in the window and incandescent/tungsten lights on.
One image shows the the shutter speed as 0.4 and the other 1/4 (One reading of this to me is 40% and a quarter of a second respectively)
However I seem to think the fwd slash is irrelevant and the setting is the same in both cases? If so why are the results shown differently. Can anybody explain to a simpleton what the figures are telling me. Many thanks. :shame:

0.4 and 1/4 are not the same. 1/4 is 0.25.

Where are you seeing the different displays? On the camera monitor? Computer software? EXIF reader?

Where are you seeing these different shutter speed numbers?

I am processing the photos in Lightroom 5 and the figures show under the histogram at top right.

1/4 second is 0.25 second.

1/2.5 second (camera marking) is 0.4 second... surely what you saw.

They are not the same, and the / is not irrelevant. The camera does not show the /, but it is implied. If it does not say " (the seconds quote mark is shown on whole seconds), then x is known to be 1/x second. And of course, it has some 0.n equivalent if we divide 1 by x.

You can duplicate and verify this by taking another picture at 1/2.5 second, and then looking at it in Lightroom.

Many thanks Wayne but I know what I saw. Two consecutive images processed in Lightroom 5. One shows shutter speed os 0.4 and the other 1/4. I might take another picture onn 1/2.5 and have a look in Lightroom as you suggest.

JJM said:

So why is the camera showing a decimal reading in one instant and a fraction the next? That's why I am thinking the fwd slash is irrelevant?

Click to expand...

The camera does not show the decimal equivalent. You said you saw this in Lightroom.

1/2.5 and 1/4 are 2/3 stop different, so the scene or the light must have changed. Or maybe Auto ISO changed, affecting shutter speed. Or Auto or P mode may have just shifted a bit, to different numbers. We would have to know apertures and ISO values to compare exposures, but Why is not really this question.

But 1/2.5 and 0.4 are the same shutter time value (both are 0.4 seconds), just shown in two places by two different methods. The camera shows shutter speeds faster than 1 second implied to be 1/ the number shown. It shows 2.5, meaning 1/2.5 second, which is 0.4 second. Lightroom can make other choices.

Just take another picture at 1/2.5 second to duplicate and verify this, to answer your question.

Yes, I can understand why the light conditions/exposure may have changed or Auto or P mode may have shifted. When I said the "camera readiing" I meant the reading as shown in Lightroom 5. I understand fractions and decimals to a certain point but I am curious as to why Lightroom would show a decimal reading and a fraction on two consecutive photos. I might try to look in more detail at the EXIF reading of both images to see what they show?

OK, I took a 1/2.5 second picture, and

ExifTool shows it as 0.4 seconds.

Irfanview Exif shows 1/2.5 seconds.

Interestingly, Adobe Photoshop CS6 Bridge shows it as 0.4 seconds, but opening it in ACR (Adobe Camera Raw) then shows 1/3 second. Sending it on to Photoshop reports 0.4 again.


But camera markings are only nominal numbers anyway, just some marking to keep humans happy. When shutter speeds were invented 100 years ago, it must have seemed too odd to mark it 1/64 second because our binary computers were not yet known. So they marked it 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, and still do. A couple of shifts in the numbering, but today, each one actually works as exactly 1/2 of the previous. And it's good enough for us humans.

But the camera has to use shutter speeds of 1,2,4,8,16,32 seconds, and also 1/(those same numbers), so that each stop is 2x or 1/2x the next one. This 2x difference is the basis of our system.

And for same reason, shutter third stops have to be in numerical multiples of cube root of 2 (1.2599) from the next third stop. (but fstop is different, third stops are cube root of square root of 2, or 1.12246x steps from previous third). Three third stops is exactly 2.0x exposure, or 1.0 stop.

This makes 1/2.5 nominal have to be 0.3968503 seconds (target goal, not implying this much hardware accuracy). ISO and aperture are very similar situations. f/11 is actually f/11.31 (used in calculations). But the camera always knows how to do it right, and it just sort of glosses over the markings it shows us.


Try setting your camera shutter to 30 seconds, and then timing it with the second hand on your watch.

It will be 32 seconds, promise. It has to be, to honor 2x and 1,2,4,8,16,32, etc.

Which is why the interval timers have to set 33 second intervals to be able to handle multiple 30 second shots.


I've recently done some work regarding this, for example a calculator to show difference in two exposures is at
Exposure Calculator, to Compare Any Two Camera Exposure values, aperture f-stop, Shutter Speed, ISO and EV

It inputs entry of the nominal numbers, but it calculates with the exact numbers. Otherwise, things get rounded off, and equivalent exposures just don't come out as 0.000 difference.

Of interest (to me) is that the bottom of the same page has a second similar calculator, but which uses instead the regular nominal numbers, and its default shows a case that reports about 1/6 stop off due to nominal rounding. And worse, some other online calculators round that to third stops and show it as 1/3 stop off. It seems better to just use the actual numbers the camera uses.

WayneF said:

But the camera has to use shutter speeds of 1,2,4,8,16,32 seconds

Click to expand...

....why?

Simply because if we want each stop to be exactly 2x more light than the previous step (the basis of our exposure system), 1,2,4,8,16,32 is the only choice. We call it seconds, so 1.0 second has to be included, and the rest are multiples, each 2x more than the previous.

The camera is marked 1,2,4,8,15,30 seconds, but if that were actually used, the step between 8 and 15 is not exactly 2x more light. And 4 to 15 is not exactly 4x. So the camera uses 1,2,4,8,16,32, regardless of the approximated markings. Same with the actuals of 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, etc. We humans don't really care about the exact value, but we do want 2x steps.

These are just nominal rounded markings for human convenience. We wouldn't like working with actual numbers like 1/813 seconds. So we think of it as 1/800, we can easily double or half that in our head, and the camera deals with the accuracy.

The camera also claims to have third stops and half stops both called 10 and 20 seconds (and 1/10 and 1/20). But of course third and half stops simply cannot be the same value, they are 1/6 stop apart. Third stops are closer, but 1/2 stops actuals are more like 11.3 and 22.6 seconds.

You can time the longer 15 and 30 second shutters to see this is true, they will be 16 and 32 seconds.

Old mechanical shutters (springs and gears) were difficult to calibrate, and they did not exceed 1/1000 second, and few even tried that. High speeds simply were not accurate, and it really didn't matter what we called them.

But modern focal plane shutters use a quartz crystal clock to digitally time the start of the second closing curtain after the start of the first opening curtain, and they are quite accurate now. The same motor drives both curtains at the same speed.

Many thanks again Wayne for your detailed response. You have demonstrated to yourself the vagaries of the system. I guess I shall just have to live with the differences from one image in Lightroom to the next. At least I know what 1/4 and 0.4 represent anyway!

WayneF said:

Simply because if we want each stop to be exactly 2x more light than the previous step (the basis of our exposure system), 1,2,4,8,16,32 is the only choice. We call it seconds, so 1.0 second has to be included, and the rest are multiples, each 2x more than the previous.

Click to expand...

Ah, thanks.