Help with exposure compensation

[MrScott]

Hello all.

A while ago I found an old Practika 35mm SLR with some extension tubes, so I'm wanting to have a go at some macro work (My other camera is a Powershot S2 IS so it can't do proper macro). I've got 2 extension tubes and a Galaxy vario converter I found with the camera, but the light meter isn't TTL so I need to work out how much I'm going to need to compensate exposures. This is what I've got:

I'll be using them with a 50mm lens. I've found the maths to work it out for the extension tubes, but I don't know how the converter will affect things. If anyone could help or point me in the right direction it'd be much appreciated, I'm new to this manual 35mm stuff

 

[ChroniC]

Thinking back to what i was taught i believe this is standard exposure compensation.
I dont know how the extension tubes will effect these.

Lets take an example. Say.

Light meter gives a reading of: F11 at 250th of a second.

(F5.6 / 1/60s)-(F8 / 1/125s)-(F11 / 1/250s)-(F16 / 1/500s)-(F22 / 1/1000s)

I believe the sizes of the apeture at each F stop is half or double of the previous, which means you need twice or half the time for the light to allow the film to react.
Im pretty sure thats right, i should know this really, im sure i took an exam on this too. Memory is fading rather quickly

Hope this is helpful.

There is a rule, i will try to remember that applies to old tilt shift cameras with bellows, that used a ruler, and you could calculate the exposure compensation need for the length the bellows where extended, i would ahve though a similar principal applied for the length of the extension tubes and convertor.

For maginification compensation use (which is probably the formula you have)
E = (M+1) squared

so tube 2.5
E= (2.5+1)squared
= 12.25

Since you get twice as much light each time open it up 6 F stops if possible.
Im not sure what happens if you put both rings together is it 5.5 or 2.5 x 3????
Probably not helping you there

The convertor will definetly reduce the level of light reaching the film, at a guess i would have thought a few F stops.
I dont know definitive math for that, so id work out the magnification exposure, and then compensate with a few F stops or double the amount fo light with the flash.

Good luck and id like to know how you get on, give me some feedback because id like to work this out too. Helpful to my course.

 

[MrScott]

Hi, thanks for the help. Those are pertty much the maths I found for it here so it sounds right, sometime today or tomorrow I'll work it all out and take some shots. I'll calculate using the overall extension of the tubes / converter I'm using, then also bracket a couple of stops each way and see what comes out best. I'll make sure I let you know when I (eventually) get the film developed and see how they come out.

 

[morgan]

Lets take an example. Say.

Light meter gives a reading of: F11 at 250th of a second.

(F5.6 / 1/60s)-(F8 / 1/125s)-(F11 / 1/250s)-(F16 / 1/500s)-(F22 / 1/1000s)

I believe the sizes of the apeture at each F stop is half or double of the previous, which means you need twice or half the time for the light to allow the film to react.
Im pretty sure thats right, i should know this really, im sure i took an exam on this too. Memory is fading rather quickly

you've got this the wrong way round...should read

5.6=1/1000
8=1/500
11 =1/250
16=1/125
22=1/60

 

[ChroniC]

you've got this the wrong way round...should read

5.6=1/1000
8=1/500
11 =1/250
16=1/125
22=1/60

Yeah so i have, was about to correct you saying you were wrong, but then i looked an i had ment to put longer times, confusing 1000, as more than 500.
Doh! Im ment to know this