What does EV mean in filter compensation?

[LisaImmarco]

<p>Hello,<br>

I just purchased a Contax T3 with a 30.5mm lens (Zeiss). I also purchased a Zeiss Jena Green 1 filter. It's a yellow green filter, and although I can't find the filter factor for it, it seems to be about 1.5. I am new to colored filters for black and white photography, so I originally thought that meant I compensate 1 and a half stops. That's how 'green' I am (pun intended). Now I know it means half of that, correct? I.e. a filter factor of plus 2, means I open up 1 stop?<br>

The T3 compensates in decimals, either plus or minus. So, would a plus .70 EV, be equivalent to about 2/3 of a stop, hence, the correct compensation for a 1.5 filter factor?<br>

I am not a technogeek, but if someone could help me out here, also know where I can find info on filter factors for the Zeiss Jena lenses I would be grateful. Does the compensation always equal to half of the factor? Any simple (not space rocket) tutorials out there?<br>

Also, if anyone knows a good source for a 30.5 high quality Green 2 (dark green) filter, screw on, that would be appreciated.<br>

Thanks in advance.</p>

[LisaImmarco]

<p>I forgot to mention that the manual calls the decimal compensation 'ev,' but doesn't explain further.</p>

[steven_clark]

<p>EV is measured in stops (Exposure Value) so an EV1.5 filter factor should be 1 and a half stops. If the words "stops" or EV appear with a number it is this.</p>

<p>Sometimes filter factors are measured as a multiplier. In that case 2x is one stop 4x is 2 stops 8x is three stops. This might be the one most often called "filter factor"</p>

<p>Neutral density filters also get measured in a logarithmic scale (like decibels). 1.0 is 10x and every 0.3 is about a stop. So a 3.0 ND filter is about ten stops, or exactly 1000x. This one would most often be called "density".</p>

<p>Given that there's at least 3 ways these numbers could be represented it might be easier if you saw two numbers so you could figure out which ones are which.</p>

[Bill C]

Hi, just like Steven says.

 

Regarding your yellow-green filter - the most "standard" one I know of is a Wratten 11. Per Kodak Tmax film data sheets, the filter factor is

3X, or alternately, open the lens aperture by 1 2/3 stops. Ideally, you should get your exposure corrections per film data sheets since they

sometimes vary by film. But since film makers typically only list factors for their own filters, you can't always do this.

[LisaImmarco]

<p>Steven,<br>

That's what I thought. But I compensated at 1.5 EV and the film was somewhat overexposed. It could be because it was already 3200 ISO.<br>

I'll just have to experiment. Thanks for explaining.</p>

[LisaImmarco]

<p>Steven,<br>

I compensated at 1.5 EV and the film was somewhat overexposed. Since I don't have the filter compensation guides for older Zeiss Jena filters I don't know whether which system they're using.<br>

I'll just have to experiment. Thanks for explaining.</p>

[glen_h]

<p>In the case of many filters, there is no exact correction.</p>

<p>For neutral density filters, the correction should be right, but for strong colored filters it will depend on the color of the scene under consideration.</p>

<p>Traditionally 'optical density' was measured logarithmic base 10, and that convention carried over to much of photography. Fortunately, log10(2) is close to 0.3, so it is easy to convert from optical density (log base 10) to stops, dividing by 0.3. </p>

<p>It isn't so unusual to give factors of 2, 4, or 8, and expect photographers to know that those mean 1, 2, and 3 stops. Other factors are less obvious. In any case, the conversion is log base 2, which on many calculators you do as ln(x)/ln(2) or log(x)/log(2).</p>

<p> </p>

[Alan Marcus]

<p>Filters absorb some of the exposing light so when a filter is installed we must somehow increase the exposure over what is needed when no filter is mounted. Modern cameras measure the exposing light after it has transversed lens and filter thus the camera’s logic automatically makes the necessary adjustment. However there are times when it is necessary to make these adjustments manually.<br>

How to adjust exposure to compensate for “filter factor”:</p>

<ol>

<li>

<p>Multiply exposure <strong>time</strong> without filter by the “filter factor”. Say the filter factor is 4 and the exposure without filter is 1/100 of a second @ f/8. Now multiply 1/100 x 4 = 1/100 x 4/1 = 4/100 = 1/25. Thus the revised exposure is 1/25 of a second @ f/8.</p>

</li>

<li>

<p>For hand-held exposure meter, divide ISO by filter factor. Say the ISO setting is 200 and the “filter factor” is 4. 200 ÷ 4 = 50. Reset ISO to 50 and the meter logic will automatically compensate (this method for hand-held meters, actual ISO does not change).</p>

</li>

<li>

<p>Count of your fingers by 2’s. Since the f/number system is based on a 2x change, we can calculate the number of f/stops change this way. Say the “filter factor” is 8, we count on our fingers 2 – 4 – 8, that’s thee fingers. If the filter factor is 16 we count 2 – 4 - 8 – 16 that’s 4 fingers. Now we open up the aperture bases on the number of fingers counted. </p>

</li>

</ol>

<p>The EV system is also based on a 2x change. Thus 2 EV = 1 f/stop whereas 4 EV = 2 f/stop etc. </p>

[glen_h]

<p>I believe that silicon sensors will be reasonably close, especially if there is an IR filter. Silicon goes down to the near IR.</p>

<p>CdS, a popular light meter sensor some years ago, is mostly blue sensitive, </p>

<p>CdSe is sensitive to most of the visible spectrum, but seems to be less commonly used.</p>

[allen_gara]

<p>Alan, that's the best explanation I've seen on using filter factors. Lisa- in case you haven't figured it out, filter factors are multipliers, not adders. 1.5 mean +2/3 stops roughly, not +1.5 stops. EV adjustments are the same as stop adjustments. 1 EV is the same as one whole shutter or aperture stop.<br>

<br />All that being said, I usually don't use the filter factor. I take a reading of a grey card or blank light wall, then put the filter on (or over the sensor if it's not TTL or within the lens front) and adjust compensation until I get the same reading. I get a lot of filters used, and am too lazy to google search for the original manufacturers data. </p>

[LisaImmarco]

<p>Thank you all so much for your responses! A very belated thank you and Merry Xmas to all of you!</p>

<p>Lisa</p>