help needed with ISO values in fstop terms

Hope someone can shed some light (pun intended).. I am deciding on an f4 lens or a 2.8 lens for a 7d which has very good higher iso performance and got too wondering how many fstops iso increases are worth. Did a load of reading and searching for charts and didnt find anything conclusive as film speed doesnt equate to digital camera iso, or at least thats what I've read.

So can anyone tell me please what ISO on an F4 lens wide open would be needed to give an equal exposure as an F2.8 lens wide open on ISO 100 with the same shutter speed

ISO 200. The difference between f 2.8 and f 4 is one stop so you would need to increase your ISO by one stop to compensate for the smaller aperture. Obviously, you can only increase your ISO to a certain point before that option runs out. An f2.8 lens gives you one extra stop before that happens.

thanks for the quick reply.. no offence meant, I did also think that iso 100 - 200 - 400- 800 etc gave you an fstop at each jump, but I was reading a while back, so not sure where now, that digital camera iso values werent so simple, instead that some other value like 800 or 1600 was 1 stop faster than 100 I hope your right, because apart from dof niceness i can use shutter speeds of a 2.8 lens wide open just by bumping the iso to 200, that will be a tiny increase in noise

He's right. the ISO standard specifies an increase of 1 EV for each doubling of the ISO number. This aspect of the original film standard applies to digital also. 1 EV is also the difference between f-numbers, so going from f/4 to f/2.8 is an increase of exposure by 1 EV.

Aside from the good accurate info you've been given, I'd suggest you read this "Newbie Guide" thread:

https://photography-on-the.net/forum/showthre​ad.php?t=414088

It's true that there are significant changes in how we approach ISO in the digital realm, but the fundamentals of exposure haven't changed. We just have more flexibility in how we accomplish exposure.

The book "Understanding Exposure" by Bryan Peterson is an interesting example of how views of this have evolved from the film days. His first edition was about shooting with film. His second edition, the one I have from '04, did include digital, but didn't incorporate some of the understanding that we have in using digital ISO, so I found it intersting to read it and then to, in my head, "re-interpret" things.

Now there is a Third Edition out that, from what I understand, does a better job with the transition to digital. I haven't read it for myself though.

If you haven't read the book, order it right away! Or, I've heard that their is an online version available as a pdf, so if you Google "understanding exposure pdf", you should find it assuming what I've heard is correct.

Enjoy your photography!

Thanks guys and really no offence meant, I had known from film, but perhaps I read a wrong piece of info on digital iso or read it wrong or forgot, old age etc

oRGie wrote in post #13277959
Thanks guys and really no offence meant, I had known from film, but perhaps I read a wrong piece of info on digital iso or read it wrong or forgot, old age etc

Heh! No offence at all, this is a journey we are all on!