RX vs RXII

[photostranger]

It will not let me edit all the sudden, but "half as much light" on the table above should be "twice as much light". I changed it around from stopping down to stopping up to fit the 20% increse scenario.
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On another topic I do not understand how stopping down would work out. If a +100% differntial is twice the original value, wouldn't -100% differential be half?

Instead, it is zero, which is obviously not true! The ratios still stay clear, as one stop down would equal 0.5:1 or 50%.

This is not as easy as i thought it was.

 

[John_bird]

Hi Shawn afraid your maths is a bit shakey 100% increase equates to doubleing to reduce by 100% is to reduce to zero, to halve is to to reduce by 50% and as we are talking about a loss not a gain I think you will find I am correct.
Hope this clarifies(my maths is better than my spelling) John.

 

[photostranger]

john-
though i am pretty sure that 1/5 stop is equal to 20% increase. You are right that my math is shakey on the differential portion, but none the less the ratios are correct.

-sk-

 

[photostranger]

Also, I just realized something. We *are* talking about I both increase *and* decrease because neither the RX or RXII take *precidence.*

Unless you are saying that if you look into the RXII first you get a 1/2 stop reduction (because we'd be reducing) and if you look into the RX first you get a 1/5 stop reduction (because we'd be increasing). Ofcourse this is silly! It does not matter which takes precidence because both reduce or increase at the same rate, 20%.

 

[ou1954]

Posted by Shawn Kearney on Friday, July 18, 2003 - 8:47 am:

john- though i am pretty sure that 1/5 stop is equal to 20% increase. You are right that my math is shaky on the differential portion, but none the less the ratios are correct.

-sk-

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I think that any discussion about variations of 1/5 stop or any fractions of stops is imprecise, but probably reasonable for the discussion in this thread.

The fact is, as we all know, that the light collected is inversely proportional to the square of the F-Number.

What we don't know is just how far one would move the F-Stop indicator to cause a 1/5 stop change. There is no reason to expect that the change is F-stop is linear with position. The only thing we know for sure is that, with reasonable manufacturing care, the F-stop settings marked on the lens are probably close, but in-between settings are just guesses.

On the other hand, since this thread is dealing with pretty small numbers, I wouldn't ague very strongly that moving an F-Stop indicator 1/5 of the way between two indicated F-numbers is a reasonable approximation of an actual effective change of 1/5 stop.

DAW

 

[John_bird]

Hi again Shawn what I am saying is that the mirror in the RX only reflects slightly less than 80% of the incidental light falling on it which equates to stoping down the lens by slightly less than 1/2 stop. this effect is visable. if it equated to 1/5 stop it would be almost unnoticipal. This does not bother me for the reasons in my other post to this thread If you order from B&H now you will find a tax bill for approx £72 will land on your door step in about 3 weeks, still a great deal(thats what I did) and if the view finder bothered you I am sure that a compitant repair man could change the mirror for one for an RX2.
Hope this reply dose not sound to pedantic by for now John
P.S if you want to talk about my likes and dislikes of the RX against my old RTS2 my email address is john.bird467@ntlworld.com

 

[photostranger]

To settle this once and for all, I wrote kyocera and got the following response. It turns out I am right, but not at all for the reasons which i thought I was. The real reason why 20%=a little MORE than 1/4 stop is WAY above my head:

Shawn,

Thank you for your e-mail.  You brought me back to photo school.  I knew when I saw the press release on the RX II that this was going to be a bit of a problem.

The answer lies in logs.

Numerically, if the writer means that the RX transmitted 100 units of light, now the RX II transmits 120 units, we have a 20% increase in the number of units. 

We begin with a log value of 2.0 representing 100 units of light and add 20 units for a total of 120 units of light.  The log value of 120 is 2.079.  The difference of one full stop in log luminance is .3.  We have an increase in log value of .079 divided by .3 = 26%.  I have to make a leap here and say that 26% of .3 = a tiny bit more than 1/4 f/stop.

It is never a good idea to mix logs with percentages. 

Blake Ziegler
Contax Technical Director

 

[photostranger]

Don-

You can visualize 1/5 stop by stopping down to 1/2 stop, and compairing the previews. Then you can guestimate 1/2 of 1/2 stop, 1/4 stop, and figure it is indistiguishably close to 1/5. It is not perfect, but for all intensive purposes, it works. Especially since 1/5 stop is likely not even visible.

I think after my reply from kyocera, there is little doubt that the difference betwene the RX and the RXII's viewfinders is muniscle(sp).

 

[andrew_le]

>I don't discount the discussion on this for a moment, but may I risk being >subjective here amidst the mathematics? The RX11 now has, for me, that >focusing 'snap' (see previous threads) that is so noticeable in the Aria >(and indeed, from memory, the RTS 11). This may not amount to much when we >crunch the numbers, but it has that extra edge that makes all the >difference in practice. If the RX already satisfies users on this count >(and from the correspondence it clearly does), then there is little to be >gained in changing.

 

[photostranger]

andrew- obviously your post is more useful than mine, this is why I disclaimed I had never used the RXII.

I think at this point the discussion is about "what is an f-stop" than "RX v. RXII"