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Help with some basics please . . .
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PostPosted: Mon Jul 07, 2008 3:41 am    Post subject: Help with some basics please . . . Reply with quote
OK.

I find myself confused in calculating exposures . . .
Let me explain in better detail . . .
I have on the way a set of Neutral density filters, so that time lapse pictures should be easier, but then I need to figure out how slow of a shutter spped I can go on a sunny day if I'm trying the waterfall shot again.
So I need to be able to say for example - that if the smallest appeture on a lens I have in my bag allows me 1/15th of a second and I have the following filters ND2, ND4, and ND8 how slow can I go?
And what about the effect of film speed ?

I don't need one number, I need to know how to calculate the numbers if I know my film speed, fstop, shutter speed and the effect of filters in use +/- on stops . . . Confused

Thankjs in advance,
Jim


PostPosted: Mon Jul 07, 2008 8:30 am    Post subject: Reply with quote
ND2 is one stop
ND4 is the double (two stops)
now I can't remember if ND8 is the double of ND4 that is, four stops, or if it is just one stop slower, that is, three stops.
I seem to remember ND8 is three stops but I am not 100% sure.

In any case you can quickly find out by setting your camera to AE mode, check shutter time, place filter in front of lens and recheck shutter time.


PostPosted: Mon Jul 07, 2008 8:47 am    Post subject: Reply with quote
It's 3 stops. ND2 half the light, ND4 one fourth, ND8 one eighth (one half of one half of one half -- 1+1+1).


PostPosted: Mon Jul 07, 2008 8:51 am    Post subject: Reply with quote
A stop doubles light or reduces it by half.
X2 = 1 stop or half the shutter speed eg from 1/120th to 1/60
X4 = 2 stops or one quarter etc. 1/30
X8 = 3 stops or one eighth etc 1/15th
Smile


PostPosted: Mon Jul 07, 2008 8:52 am    Post subject: Reply with quote
I love logical units. 2=1, 4=2, 8=3.
Love it.


PostPosted: Mon Jul 07, 2008 12:34 pm    Post subject: Reply with quote
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


But they are perfectly logical.
Half, quarter, and eighth.
Or start with a a shutter speed of 1/120 put on a 8x ND and 120 divided by 8 = 15
What could be more logical?


PostPosted: Mon Jul 07, 2008 12:38 pm    Post subject: Reply with quote
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


Laughing Looks like it, doesn't it?

Actually, it is quite logical, it's the reciprocal value. Wink


PostPosted: Mon Jul 07, 2008 12:49 pm    Post subject: Reply with quote
LucisPictor wrote:
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


Laughing Looks like it, doesn't it?

Actually, it is quite logical, it's the reciprocal value. Wink


I always thought it was the power:

2^1 = 2
2^2 = 4
2^3 = 8


PostPosted: Mon Jul 07, 2008 12:58 pm    Post subject: Reply with quote
peterqd wrote:
LucisPictor wrote:
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


Laughing Looks like it, doesn't it?

Actually, it is quite logical, it's the reciprocal value. Wink


I always thought it was the power:

2^1 = 2
2^2 = 4
2^3 = 8


I thought they just forgot the "1/". 1/2, 1/4,1/8 Wink


PostPosted: Mon Jul 07, 2008 1:05 pm    Post subject: Reply with quote
zewrak wrote:
peterqd wrote:
LucisPictor wrote:
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


Laughing Looks like it, doesn't it?

Actually, it is quite logical, it's the reciprocal value. Wink


I always thought it was the power:

2^1 = 2
2^2 = 4
2^3 = 8


I thought they just forgot the "1/". 1/2, 1/4,1/8 Wink


Well... yes.


PostPosted: Mon Jul 07, 2008 2:17 pm    Post subject: Reply with quote
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


2^1=2
2^2=4
2^3=8

Anyone born in the last 40 years and growing up exposed to computers should instantly recognise the powers-of-two sequence 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 etc, no?

Actually shutter speeds have the same sequence, but rounded off for a pre-computer generation

2, 4, 8, 15, 30, 60, 125, 250, 500 ...


PostPosted: Mon Jul 07, 2008 2:19 pm    Post subject: Reply with quote
Excellent recognizing ! Thanks!


PostPosted: Mon Jul 07, 2008 2:26 pm    Post subject: Re: Help with some basics please . . . Reply with quote
j.lukow wrote:

So I need to be able to say for example - that if the smallest appeture on a lens I have in my bag allows me 1/15th of a second and I have the following filters ND2, ND4, and ND8 how slow can I go?


ND8 takes you three stops slower so 1/2 sec.

ND8+ND4+ND2 takes you 3+2+1=6 stops slower so 4 sec (with free artistic vignette effect thrown in).


PostPosted: Mon Jul 07, 2008 2:38 pm    Post subject: Reply with quote
ChrisLilley wrote:
zewrak wrote:
I love logical units. 2=1, 4=2, 8=3.
Love it.


2^1=2
2^2=4
2^3=8


Is there an echo in here? Wink


PostPosted: Mon Jul 07, 2008 3:10 pm    Post subject: Reply with quote
Laughing Laughing


PostPosted: Mon Jul 07, 2008 5:47 pm    Post subject: Re: Help with some basics please . . . Reply with quote
ChrisLilley wrote:

ND8+ND4+ND2 takes you 3+2+1=6 stops slower so 4 sec (with free artistic vignette effect thrown in).


Laughing Laughing Laughing

And saving the expense for a Holga.


PostPosted: Mon Jul 07, 2008 7:01 pm    Post subject: Reply with quote
Jim
If you are using a camera with TTL metering you don't need to calculate anything. Just thread the filter on and trust the cameras meter. Think of a ND filter not as a filter but as a lower lighting situation. Less light will fall on the meter but, the meter will still give a proper reading (manual setting or AE/Tv). If of course you are using a camera without metering. Or a camera that has a limited EV range ( your minoltas will be fine). The methods previously described to calculate will work perfectly. I remember your post from the river. It's great you are following up on that theme. Cool Looking forward to your results.


PostPosted: Mon Jul 07, 2008 11:41 pm    Post subject: Reply with quote
Thanks Andy,
I thought this whole thing was related to the F16 rule etc.
I need the math so that I can apply it in other areas . . .
The old deal of (correct me if I'm wrong) F16 = 1/100th w/ ASA 100, if so film speed changes it how, as does adjusting the Fstop . . .

If an explanation would be long, then a couple good links would work Smile

thanks,
Jim


PostPosted: Mon Jul 07, 2008 11:57 pm    Post subject: Reply with quote
Hi Jim
Happy to get into an expanded explanation. But I doubt I could do it better than this link.

http://www.fredparker.com/ultexp1.htm#Introduction


BTW you do have it correct.

In a nutshell it goes like this: your back is to the sun and your subject facing it
with the aperture set to F16 the shutter speed will correspond to the film speed

Check the link it is really well worded and comprehensive.
Cool


PostPosted: Tue Jul 08, 2008 12:19 am    Post subject: Reply with quote
j.lukow wrote:

I need the math so that I can apply it in other areas . . .
The old deal of (correct me if I'm wrong) F16 = 1/100th w/ ASA 100, if so film speed changes it how, as does adjusting the Fstop . . .


Its all powers of two, halving and doubling.

One stop smaller has half the light (f/2.8 to f/4)
One shutter speed faster has half the light (1/250s to 1/500s)
One ISO speed slower has half the light (200 to 100)

The shutter speeds are rounded of powers of two, as explained above

ISO is also (100 200, 400, 800, 1600 ...) and for film, 25 and 50 also exist

The aperture is slightly more tricky as its expressed as a ratio of the focal length to the diameter of the aperture. But the amount of light let in by a circular hole is proportional to the area, not the diameter. Area of a circle is pi r squared. So aperture numbers go up in powers of root two (roughly 1.4): 1, 1.4, 2, 2.8, 4 and so on (again with some rounding off for convenience).


PostPosted: Tue Jul 08, 2008 2:42 am    Post subject: Reply with quote
Thanks!

That clarifies much of it.

I feel Embarassed cause I've heard of Fred Parker's site - I just haven't gotten to it yet. Guess it slipped my mind Rolling Eyes


Jim
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