Do I need to adjust the Exposure for Dawn/Dusk photots taking a couple of minutes?

[wilko61]

Just wondering if I need to adjust an exposure pre-dawn or post-dusk.

I'll give a bit of a work flow example to help explain my question.

I use Aperture prior with a high ISO and maybe large aperture to get an initial reading, then using a smart photo app, adjust the calculation for ISO 100 and change the f-stop
Change to Bulb mode, set up my intervalometer and key in the shutter exposure time and set off the shutter, adjust f-stop down.
However, if for example it is dawn, and the exposure is going to be for say 2-3min duration, during this time, the ambient light shall be increasing as the sun gets closer to the appearing over the horizon and hence the image shall be overexposed. The initial exposure reading is really a point in-time, but the actual exposure is over-time, possible 2-3min even longer.
Is there a way to calculate what time to decrease to exposure by, or is it a guess, best hunch etc. (This exposure time would have to increased during sunset as the light is decreasing)


Thanks

Wilko

[ameerat42]

If I understand it, you are taking a series of exposures, either for a sunrise, or a sunset.

You said you use Aperture priority And an intervalometer, AND that you set the initial exposure.

Well, to me, Av is a semi-auto mode, and that will vary the exposure time (shutter speed) with changing light, would it not?
Therefore I do not see a problem.
Am. (Or Am not?)

[arthurking83]

....

You said you use Aperture priority And an intervalometer, AND that you set the initial exposure.

......

The way I'm understanding the OP, is that they're getting an initial reading in Av mode from which to base the longer exposure Ev value for the actual long exposure.


If the exposure needs to be set with a pre determined value, then your guess is going to be as good as anyone or any calculation process. There isn't an accurate way to determine the required exposure for a given period, if the required exposure value changes within the exposure time. Too many variables to consider .. so guessitimations are as good as any other source.

Are you using any filters to help with exposure balance and time?(eg ND/GND filters).

I have used Aperture priority for some long exposures, up to about 2mins, and they all seem to expose quite accurately all by themselves.

Exposure times have varied from 1.5mins to over 2mins, and the camera simply does it's own thing .... even tho the meter is incapable of calculating an actual exposure time.


If you are going to maintain the workflow you're currently using, it may be a good idea to become familiar with using a black card to minimise the tendency for the sky to blow out if the exposure requirement changes.

That is, you hold a black(or any other colour) card in front of the lens and over the area of the sky during a part of the exposure period. Once again, the exposure requirement is going to be based on guesswork.

2-3mins at the time of the sun's cresting over the horizon will make a significant difference to exposure requirements along the horizon. The upper sky shouldn't be as sensitive to exposure differences in that time frame tho.

I suppose the real question is why you would want to set the exposure time at 2-3mins, and not simply allow the camera to expose correctly, or follow the guideline of the camera's metering system?

[wilko61]

If I understand it, you are taking a series of exposures, either for a sunrise, or a sunset.
You said you use Aperture priority And an intervalometer, AND that you set the initial exposure.
Well, to me, Av is a semi-auto mode, and that will vary the exposure time (shutter speed) with changing light, would it not?
Therefore I do not see a problem.
Am. (Or Am not?)

Hi AM,

You are right. I use AP because then I can 'quickly' get a shutter reading for a high ISO, say up around 800-1600 and the shutter speed might be say 25sec. The reason for the high ISO is so I can get an initial shutter-speed above 30sec.

I use the smart phone app to recalculate for a change in ISO and f-stop back to say ISO100 and f/11, then app shall give me the new shutter-speed, normally this shall be in the minutes. This is fine if the light is going to be 'constant', but because we are in 'pre-dawn', the light is progressively getting brighter, hence after 2-3 minutes I guessing we would be a couple of f-stops out. This is my question is there a way to allow/calculate for the change in light when making the exposure? EG if the calculation was ISO 100, f11 and 2:30min exposure, when I take the actual photo should I actually use ISO 100, f11 2:00min, therefore subtract 30sec off the exposure to allow for the brightening of the sky.
Obviously this would be reversed for dusk, we would have to extend the exposure for the reduction in light.

The shutter-speed in AV (<=30sec) is 'set' at the time the shutter is depressed, but what happens during this exposure-time, as the light (sky) brightens the image would over-expose?, I think I'd need to actually reduce the exposure by having a shorter shutter speed. (This is my dilemma I think!)

I'm sure if seascapers where shooting pre-dawn and using an ND filter to extend their exposure, they would have to take this into consideration. They'd look up their chart for say 'x'min exposure, but know that it's getting brighter so they only use a 'y'min exposure which is a shorter exposure time.

Sorry for my raving here

Cheers

Wilko

- - - Updated - - -

Thanks Arthur for your informative response. I was posting the same as yourself and you seem to have cleared up my question.

I suppose I 'm making an assumption, that because when using AV mode the shutter speed stops at 30 secs then the shutter-speed isn't calculated beyond that (aren't I an ass :o ). From you reply, "... I'll just let the camera do it's thing", next time I'm out I'll do some trials with some bracketing too just in-case. I appreciate the extra workflow addition, using the black card, I've heard of it's use (by Dylan), now I'll have to investigate the application.

As yet I'm actually to shoot any images of this nature, I'm really just trying to get my thoughts in order before I go out.

Thanks too AM for taking the trouble to reply. I appreciate both responses

Cheers

Wilko

- - - Updated - - -

So another related question the, if I'm in Aperture Priority mode, is the camera continuing to make an adjustment to the shutter duration for the light entering the lens, once the shutter is open or is the only calculated at the time the shutter is open? (ie is it possible for shutter duration to change once the shutter is opened to account for an increase/decrease in light?)

Cheers

Wilko

[ameerat42]

I see. I rather thought the exposure times would be less than the several mins you indicate - like, max 30 sec.
I think that you will ultimately become the local authority on this very thing, based on the experience you gain.

Now I am interested to see what you come up with.
Buona fortuna!
Am.

[wilko61]

Wow AM the pressure is on then! I think my guesstomometer shall be working over-time, stay tuned.

Cheers

Wilko

[Steve Axford]

Er - you do what any of us do when we are trying to predict what will happen in the future. You make a best guess and call it science. I do it all the time with time lapse photography. I don't know what is going to be in the frame in an hour's time, so I guess. The more you do it the better you get, but it is you that has to do it. Nobody has bothered to write a book on it and if they had, they are probably wrong for your situation. Just think about it - it will depend on the terrain (mountains etc), weather (clouds etc), season, etc etc
But, it's not going to change dramatically in a minute or two. Underexpose by a stop or two should be heaps.

[Mark L]

I suppose I 'm making an assumption, that because when using AV mode the shutter speed stops at 30 secs ....

With the 60D, 30secs is the max time for any mode other than Bulb. Your 5D Mk II possibly offers more.

[arthurking83]

Apologies, went back(2009!) and had a peek at the images and they were in Manual mode, not Aperture priority as I assumed. Aperture priority is the mode I 'always' use so I assumed that these images were captured that way too.

Doofus me, had forgotten about the 30s limit in anything other than Manual mode! :o

[wilko61]

Thanks Steve, Mark and Arthur for the updates. 30sec seems to be the max for the 5DMkII also. But shooting in manual/bulb mode shouldn't be an issue for me. My 'lack' of experience for the guesstimation was my concern :o for an adjustment to reduce/increase the shutter open time.

Cheers

Wilko