How to read a handheld light meter?

[Cujo]

I get what it does.
My question is my photography teacher says when the reading comes out to 2.808, you would round up to the nearest F-stop whole number, which in F-stops would be 4.0 (1.4, 2, 2.8, 4, 5.6, 8, 11).
I stopped him and said 2.808 rounded up mathematically is 2.81. Wouldn't it be better to use 2.8 as your aperture since it is closer to 2.81 than 4.0 is?
So he says #1 this isn't math this is photography (Really? This whole time I thought I signed up for an algebra class!), and #2, there is no 2.81 aperture! Then he goes out of the classroom and brings back the giant beginners aperture poster to show me in front of the whole class that the next aperture after 2.8 is 4.0 not 2.81 and there is no 2.81 aperture. (I think I know this by now, I wasn't saying 2.81 was an aperture, I was saying the next "closest" aperture to 2.81 is 2.8, not 4.0)
He's the teacher. But teacher's aren't always right. I think that if it came out to 2.808, you should round to the closest F-stop, not necessarily round up to the “next” F-stop, so in this case, stay at 2.8. If he was right, wouldn’t a meter that has enough technology to come out with a number as pin point as 2.808 also have enough technology to just round up for you, directly saying 4.0 or another typical F-stop number? If the meter can say 2.808 and there is no aperture of 2.808, then why can’t the meter also say 3.403 or 3.9? Why even give a 2.808 reading, what’s the point? He says if it says anywhere from 2.80 to 2.805, you stay at 2.8. But once it hits 2.806, you go to 4.0.
I KNOW the next typical F-stop number after 2.8 is 4.0, and if we were just rounding up, then 4.0 would be accurate because there are no other numbers in between! But are we really always rounding up when 4.0 is so far away and 2.81 is so much closer to 2.8 than 4.0 is? In the least, I would think that a reading of 2.808 should have the picture taken at 2.8 because that is the "closest" aperture.
I am glad my gut was right from reading all these answers. After he was done explaining, he looked at me and said You see now? I sheepishly told him "I think you're wrong, I'm going to do my own research".
The problem is most of the class just goes with what he says, so a few people did speak up after I explained what I thought because they understood what I was saying. They kinda backed me up that mathematically it was closer to 2.8, but after his "explanation" that (THERE IS NO 2.81!), they all ok'd with his answer which left me the only one still not satisfied.
Funny side note, he took the picture at F4 after getting the 2.808 reading, showed everyone that it was accurately exposed... I told the girl sitting next to me that it looked a little dark to me and could use a 2.8 and she agreed.
My gut also tells me that he is one of those people that is "always right" and me going back to him with my research is going to be pointless.

 

[JohnH]

Your teacher the NOT brightest strobe in the studio. That's for sure. You can tell him I said so. He needs a refresher course in basic teaching skills. Photography IS all about MATH. Even the aperture F number is, in reality, expressed as a fraction, F stop over focal length. It could be a simple exercise in math to prove you are right in your argument.

However, to be practical, I would first expose as my gut tells me to and then shoot a bracket of over and under if the scene is static enough to allow it.

 

[B0B]

apertures are in steps of the square root of two (as double the size of hole in the aperture admits 1,4 times the light)
so, teach has confused themself by being partly mindful of that.

there are often third stop intervals (Zeiss do this) but most lenses go in half stops, but again obeying the root two principle;
so f2,8 is one stop (double the light transmission) from f4,0 (though many makers engrave f4,5 instead)and the half stop between is not (2,8+4)/2 = 3.4, but is normally given as f3,5 (more or less by tradition).

This you seem to have grasped. The point is, the lens ring may not be that accurate, the iris hole isn't a true circle (except some old Zeiss ones) and if you move the meter on tenth degree the reading may change half a stop anyway.

So in this the Teach is right, forget the math, third stop accuracy is irrelevant normally.

Grab the closest to the reading to one significant decimal - so 2,5 and 2,7 and 2,9 are all f2,8

2,99 is getting my lens fingers twitching to set the half stop (f3,5) but that's cos I use slide film and underexposing slightly is good, as indeed it is for digi pictures taken in RAW.

The third decimal is beyond the difference between batches of film or variations in sensors, probably beyond the effect of a finger mark on the lens!

My Gossen actually shews proper stop and third stop numbers but a second display gives the 'nudge' value small figures as well, which some scientist might one day need (not me though)

 

[deepblue2]

I can tell you that if I had a reading of 2.808 on my light meter (and yes, I use one) I would set my aperture to f2.8.

 

[stevep]

Of course you would use 2.8. I use a hand meter all the time, and it often gives readings that I cannot exactly match with the camera. I don't give a flying rat's butt what that teacher says, 4.0 is simply way too far away from 2.808. You will be technically underexposing your photo by one stop. While with digital it is always better to underexpose instead of overexpose, but the difference between 2.8 and 2.808 is so miniscule to be meaningless. If the meter read 3.9 or 4.3 or something like that, I would use 4.0.

I have great respect for most teachers, but I think this guy fits the old joke, "those who can do... DO, those who can't do... teach".

steve

 

[Jorge]

I'm just now turning my aperture ring and it has two secondary stops between 2.8 and 4, so tell your teacher to learn some more maths and select the aperture you consider better