M7 ISO values between 800 and 1600?

[sarah_michelle_larsen]

<p>Hi !<br>

Just want to be sure. There is only "whole" values of ISO on the back of my M7. There is a jump from ISO 800 to ISO 1600. What are the 2 missing values? Is it 1000 and 1250?<br>

If that is correct, then there really is a big jump between 1250 and 1600 when I compare from 800 to 1000?<br>

Thanks.</p>

 

[ray .]

<p>Each mark is 1/3 of a stop. So from 800-1600 the first mark up would be 800 + 1/3 X 800, or 1067, the second 1334, then 1600. But just think of the marks as 1/3 of a stop increments. </p>

[sarah_michelle_larsen]

<p>I see..<br>

Thank you very much Ray.<br>

Sarah.</p>

[Rob F.]

<p>1000 and 1250 were right on the nose, Sarah!</p>

[ray .]

<p>Rob, aren't they just 1/3 stop increments in any case?</p>

[phc1]

<p>Forgive me if I'm teaching you to suck eggs, but exposure is measured in stops, in this case with film speed. Each stop equals twice as much light as the last stop.</p>

<p>So the usual incrementation is 25 - 50 - 100 - 200 - 400 - 800 - 1600 - 3200 - 6400, there being one stop between each step.</p>

<p>Aperture and shutter speed work the same way, so that 125 at f8 on ISO 400 film will give you exactly the same exposure as 500 at f5.6 on ISO 200 film.</p>

<p>Cheers, Paul.<br>

<a href="http://www.paulhardycarter.com">paulhardycarter.com</a></p>

[t_nu_tamm]

<p>If you divide this one stop into three equal parts, then those in-between values should be 1008 and 1270. The numbers will form a geometrical scale with a coefficient circa 1.26. But, yes, it's more practical to think just in fractions of stops...</p>

[t_nu_tamm]

<p>Oops, seems that "Geometric progression" is the word.</p>

[pc_b]

<p>Hard to top Tõmu's "self adjusting" precision...<br>

The real factor for 1/3-increments (i.e. of ASA numbers in DIN-width steps....) is the cubic root of 2: about 1.26, as already mentionned above.</p>

<p>For half stop increments it is the square root of 2: about 1.414.</p>

<p>The official ASA numbers were heavy-handedly rounded to provide numbers that are easy to memorize.</p>

<p>1/3-stop increments are chemically, physically, exposure-wise etc. of equal width even though the numbe seem to indicate otherwise.<br>

In short, nothing to worry about: the machines are doing it for you.</p>

<p>Enjoy your picture-taking Rolls Royce and 'drive' it everywhere!</p>

[george_hickey1]

<p>This seems like a good place to post this.</p>

<p>I just bought an M7 The ISO dial will only turn the two stops each way for exposure compensation and not all the way around to manually set an ISO. I am holding down the release button. Am I doing something wrong? Is the camera defective?</p>

<p>Real world this is not a problem as I no longer roll my own film so I'm using DX coded cartridges and never push my film more than the two stops I can do with the exposure compensation.</p>

[sarah_michelle_larsen]

<p>Thank you all. So if I want to shoot at ISO 1250 I set the dial 1/3 of a stop from the ISO 1600 mark.</p>

[ray .]

<p>Yes, and if you have the manual the info is in there too.</p>

[Rob F.]

<p>Ray: yes, they are just 1/3 stop increments. And while the figures of 1008 and 1270 are mathematically precise, it's conventional to express them as 1000 and 1250, so I wanted to let Sarah know that she was correct to assume that the incremental settings do refer to those conventional numbers. The latter are approximations, just as the use of 1/8 second as the next stop longer than 1/15 is an approximation. (Who would want to worry about 1/7.5?) The responders who referred to the cube root of two, (about 1.26) as the exact ratio are just as correct. If you multiply 1.2599 x 1.2599 x 1.2599 you get an answer (1.9999) very close to two. So that number represents evenly spaced one-third stops.</p>

<p>Best, Rob</p>

[pc_b]

<p>George, your M7 is not broken, don't worry.</p>

<p>Up to this point, you have only actuated the exposure compensation turning the double dial(!) with two fingers on the structured rim. Now, to dial in a 'personal ISO', you need to place your thumb flat on the (metal) center of the dial and turn it: only the inner part of the dial will turn as the outer dial ('rim') is locked by the button mechanism. Et voilà!<br>

(I doubtfully hope my written explanation helps.)</p>

<p>Cautionary notes: these two dials interact. For example, rating your 400 ISO film as 200 ISO and compensating +1 gets you to an actual E.I. of 100 ISO (_if_ you make use of the internal exposure meter or take pictures 'on Auto'). <br>

Be careful about setting it to the DX-position: it need to be right on. Any click benath will cause general under or overexposure liek the outer dial. You will be warned though by a nervously blinking dot in the viewfinder display.</p>

<p>Positive side-effect of the interaction effect: you can expose at 25000 ISO by going 6400 _and_ -2. Or to ISO 2 (you'll figure out how).</p>

<p>Enjoy your picture machine!<br>

Cheers, Pete</p>

[pengkui_luo]

<p>FYI: Full ISO/ASA scale in 1/3-stop increment:<br>

<strong>25</strong>, 32, 40, <strong>50</strong>, 64, 80, <strong>100</strong>, 125, 160, <strong>200</strong>, 230, 320, <strong>400</strong>, 500, 640, <strong>800</strong>, 1000, 1250, <strong>1600</strong>, 2000, 2500, <strong>3200</strong>, 4000, ...</p>