nd and pinhole help

[tezmed]

After going out last night for a photgraphic evening and practising manual settings and using bulb mode etc my tokina 11-16mm on my 650d. With a smallest aperture of f22, Iso on 100 (min poss) i was able to stretch the 30 sec body capabilities into bulb mode and get a decent exposure with a time of 2 minutes (120 secs) as below...

Im trying to get my head around the light stop loss of various nd filters, (of which i possess a fair few.... I've got nd2/4/8's and coloured nds (not nec for this) and grads etc......

how do i calculate the loss caused by putting 1 or 2 etc infront of the lens??? As i understand it if i can get 1 lightstop i can double my timings, but i need to find out what constitutes 1 stop exactly or il fail miserably...

also does anyone know the correct aperture size of a pinhole lens?? I was given one for xmas and am keen to experiment with it to see what i can achieve with it etc, but i dont know how many stops below f22 the pinhole lens is precisely... has anyone any ideas of what aperture they are or any way of working out what it is etc??

cheers

 

[Dale_tem]

Nice shot!

I use an exposure calculator on my phone, put on your settings, tell the app the ND filter you are putting on and it will tell you the exposure tisme

 

[tezmed]

I use an exposure calculator on my phone, put on your settings, tell the app the ND filter you are putting on and it will tell you the exposure tisme

okay il have to have a look out for this on my phone /ipad

cheers

 

[HoppyUK]

Yes, one stop is any halving or doubling of exposure, eg 1/125sec to 1/250sec; ISO400 to 800; f/4 to f/5.6 (note f/numbers go up/down by the square root of two, ie 1.4x).

Filter density is expressed in different ways, like exposure factors of 2x, 4x, 8x and 16x are one, two, three and four stops. They may also be marked 0.3, 0.6, 0.9, 1.2 in optical density. Stacking more than one filter, you can add stops together, eg three stops plus two stops is five stops. Or 0.9 plus 0.6 is 1.5, but you can't just add together filter factors - have to convert them first.

 

[Simon photo]

When you say pinhole lens im guessing your saying pinhole body cap?
The quickest way would be to find out who made the hole, is it drilled? Laser?
One way around this would be to scan the hole at a very high resolution and measure the hole in photoshop. I did this for a camera i made and calculated an aperture of around f165
You can also find out the focal length by measuring the distance from your film plane (sensor) to hole.
Here is a link to all the info you need to calculate things.
An exposure chart can be made when you have the right info.