ISO Question

[dpp]

Hello

This maybe a daft question- actually it probably is.

Anyway here goes.

If I take shots on a gloomy day at ISO 400 and then go inside to an even gloomier room and use flash, can I put the ISO on 800 and use flash. And if I do what will be the difference between using say ISO 800 and ISO 200 with flah indoors.

I am getting confused. By using ISO 200 will the flash be more powerful or less powerful?
What will be the best setting to use regarding ISO indoors

[Jon]

The flash, like any light, will be more effective at higher ISO settings. I won't say "more powerful" because it's only able to produce so much light, but the higher ISO setting will let the camera use it more effectively (faster shutter speed and smaller aperture). The difference between ISO 200 and ISO 800 under the same conditions will be two f/stops.

[scottbergerphoto]

At ISO 800 the minimum distance on your flash may become an issue. The change in ISO to 800, increases your flashes effective distance by a factor of 4. Your flash controls it's output by the length of time it's on. That can only be reduced so much. You might need to increase your minimum distance from the subject to 10-12 feet.

In addition let me clarify Jon's post. With regards to ISO and flash when you go from ISO 100 to 200 the flashes effective distance is increased by 1.4, and when you go from 100 to 400( meant to say 800) it is increased by 4.

Regards,
Scott

[Jon]

and when you go from 100 to 400 it is increased by 4.

Regards,
Scott
. . . er, by 2. Typo, I'm sure.

[robertwgross]

. . . er, by 2. Typo, I'm sure.

We knew what Scott was thinking, even though his fingers were not cooperating that instant.

Each one stop increase in ISO should yield about 1.4 times the effective distance.

---Bob Gross---

[scottbergerphoto]

Thanks guys. I meant to say that 100 to 200 = 1.4 and 100 to 400=2, then 100 to 800=4x(Should read 2.8X). The original poster indicated ISO 800.

eg. At ISO 100 at 550EX at f/1.0 will go 55 M
At ISO 400 it will go 110M and
At ISO 800 it will go 220M (should read 154M)
Regards,
Scott

[Scottes]

ISO 100 to 800 is 3 stops, so 2.8.

Scott, time for a new keyboard. :-)

[PacAce]

ISO 100 to 800 is 3 stops, so 2.8.

Scott, time for a new keyboard. :-)

We knew what he meant. His fingers just aren't cooperating today, is all! ;)

[Scottes]

We knew what he meant. His fingers just aren't cooperating today, is all!
Let's hope that he doesn't have to use the word "supercalifragilisticexpialidocious" in his next post... The forum could crash again.


Hey, wait a second... How can we pick on him when CDS's typing is such easy game? Sorry, Scott. We'll be good.
:-)

[robertwgross]

eg. At ISO 100 at 550EX at f/1.0 will go 55 M


So now we all go rush around and look for our f/1.0 lens.

I wish.

---Bob Gross---

[dpp]

Guys

Thanks very much indeed, I have leaernt so much from this forum its incredible, I am a newbie and like anything new that you study it baffles you, I apologise for the stupid questions and thank you all for your patience and understanding

[scottbergerphoto]

So now we all go rush around and look for our f/1.0 lens.

I wish.

---Bob Gross---Robert,
As I recalll it was you, who not so long ago corrected me, when I posted a Guide Number without referencing f/1.0 as the f stop used when referring to a Guide Number. If you want to have it both ways, you'll have to go to a different kind of forum:lol: .
Scott

[scottbergerphoto]

My apologies to those of you with cameras that lack that invisible f stop between 400 and 800. On my very special camera, there is one . That's why I came up with a factor of 4 and not 2.8. For those of you with normal cameras and not under the influence of mind altering drugs:p , going from 100 to 800 is in fact 3 stops and the factor is 2.8.

I hope this resolves any confusion my posts created. I am sufficiently humbled that I won't answer any flash questions or make fun of Robert Gross for being slightly more of a curmudgeon then me for the next five minutes;) .

Often wrong, but never in doubt,
Scott

[Scottes]

...I won't answer any flash questions or make fun of Robert Gross for being slightly more of a curmudgeon then me...

Well that just takes the fun out of life.

I mean, flash is everything. :wink:

[robertwgross]

Robert,
As I recalll it was you, who not so long ago corrected me, when I posted a Guide Number without referencing f/1.0 as the f stop used when referring to a Guide Number. If you want to have it both ways, you'll have to go to a different kind of forum:lol: .
Scott

As I'm sure we all know, the Guide Number gets divided by the f/stop of the lens. However, that f/stop is very rarely 1.0.

A newbie might assume that the Guide Number gets divided by f/1.0 no matter what. For a typical f/3.5 lens, that might lead to flash underexposure.

I was sitting out one night, ready to do some night wildlife photos, so I had all of the gear rigged to fire. However, I had to go through the math to make sure that I had enough light. Unfortunately, I had nothing to write with, so I was doing all of the meters to feet calculations, and divide by the f/stop, and all that... in my head. An f/1.0 lens would have made life easier.

---Bob Gross---

[PhotosGuy]

However, I had to go through the math to make sure that I had enough light. Unfortunately, I had nothing to write with... Why do all that math? Just figure out the right exposure for 11 feet one time. For 16', open up one stop. For 8', close down one stop. 2X for 22' & 5.6', etc.
Or didn't I understand the question?

[robertwgross]

Most 550EX users know the guide number in meters (55). However, mark me as an American, but I don't always think in meters of distance. I can estimate distance to a subject spot by stepping it off. Then I can convert to meters and do the math from there.

In fact, since it was going to be night, I put up aiming sticks for 20, 40, and 60 feet. The 550EX and camera will work out the exposure as long as it is within the maximum range. I had to know that it was within that range.

---Bob Gross---

[charlesu]

I don't think we have made this confusing enough. Shall we add a discussion on the law of inverse squares? I can just see the new guys tossing out their flashes now, or swearing to only use them in Program mode. Heck, I don't blame them.

LOL

Here's a very simple explanation of what this is and why it's important.

http://www.jimloy.com/physics/inverse.htm


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