metering and reciprocity issue

[kevin_mahoney1]

<p>Using Sekonic L-508, I metered a cd spindle (using an umbrella as my modifier), and read iso 100, f/4, 125th of a sec.<br>

I encountered blinkies when taking the shot. Changing my camera settings to f/5.6, 60th of a sec..NO BLINKIES...why is that?</p>

<p>Thank you in advance for all answers...:-)</p>

[leighb]

<p>I have no clue what "blinkies" are.</p>

<p>What camera and lens are you using?</p>

<p>- Leigh</p>

 

[john_deerfield]

<p>I might suggest the book "Light: the Science & Magic".</p>

<p>Your surface has a direct reflection of light. You need to move the light outside the family of angles to not record the direct reflection.</p>

[curt wiler]

<p>Kevin, we need more information. What is the light source? I think you are asking why the same amount of light would over-expose one image and not the other (reciprocity would NOT be a factor at these speeds). Electronic flash would explain it, however.</p>

[parv]

Leigh, "blinkies" are parts of an image flashing in black or white colors (in my case) to show overexposure or underexposure (when histogram is also shown on the LCD of Sony A700).

[Alan Marcus]

<p>Google<br>

Diffraction Grating</p>

<p> </p>

[tobiasfeltus]

<p>What you are describing is that if you take an incident reading off your lights, then take a picture of a reflective surface, and your image is peaking. <br>

Reciprocity failure has nothing to do with this, and has nothing to do with digital photography, as far as I know. It was a chemical issue with halides during long exposures.<br>

So, your issue is that you are taking a picture of a reflective surface. Your incident reading will produce a middle-grey (18% reflectance, or 50% Black) from a white surface, in theory. An average exposure, thus given, should be the most suited for most situations. If, however, you want blacks that are pure and absorbing, you will probably have to expose at around two stops under. If you want a high-key white background you may have to over expose by two stops.<br>

What you have done is, essentially, used a meter reading that would be good to expose a non-reflective surface, on a mirror.<br>

However, you used the same exposure and one gave you peak warnings. This makes no sense. What kind of lights are you using? if they are fluorescents, then you may simply be encountering some banding caused by the frequency of the bulbs, rather than an exposure issue.</p>

[john_deerfield]

<blockquote>

<p>However, you used the same exposure and one gave you peak warnings. This makes no sense.</p>

 

</blockquote>

<p>Sure it does, as you said, the OP is taking a picture of a mirror (highly reflective surface). If I stand in front of a mirror with a shoe mount flash on my camera, the flash in manual mode and I meter the flash with a flash meter at the mirror, I will get my 18% gray reading, whatever that might be. I set my camera to the flash meter reading. The problem is that my light is within the family of angles. I am going to get a direct reflection of my light from the mirror that is much brighter than the metered reading. I need to move the light outside the family of angles to avoid that direct reflection.</p>

[Alan Marcus]

<p>A CD's or DVD's when viewed or photographed often put on show. We see and can photograph patches of iridescent colors, a reproduction of the rainbow. We also see these patterns in nature. A rainbow of color an oil spill on a wet street, the vivid colors of butterfly wings and the feathers of hummingbirds etc. </p>

<p>Thin films like soup bubbles or oil on water and surfaces with tiny periodic groves like a CD are said to have a surface that is dispersive. We call this a diffraction grating. Such a surface will put on a show of iridescent colors. The patterns seen change and ungulate based on the angle and frequency of incoming light. Now the heart of the digital camera is an chip covered with light sensitive receivers in a grid pattern. When we photograph a CD the grids on the CD and the grids on the chip interact sometimes reinforcing sometimes canceling. Likely, shutter speed play a key role.</p>

<p>Reciprocity is not responsible. Reciprocity failure is a drop in ISO that occurs when the shutter speed utilized is abnormally short or long.</p>

[rodeo_joe1]

<p>I think it's worth repeating that reciprocity (law) failure does NOT affect digital cameras. It applies only to film photography.</p>

<p>Kevin, when using flash, only the aperture affects the exposure, so of course the "blinkies" were reduced when you changed from f/4 to f/5.6. The fact that you changed the shutter speed as well is irrelevent - unless the ambient light was almost as strong as the flash. Changing the shutter speed has no effect on the duration of the flash output, which is usually far shorter than the camera shutter exposure. This is the reason why flash can "freeze" fast motion, even though the camera synch speed is relatively slow.</p>

<p>I suspect why you got overexposed highlights in the first place was because the angle that the meter was pointed at the reflective surface of the CDs wasn't the same as the angle that the camera was aimed at them. As previously mentioned, it's like looking at a mirror, and a slight change in viewing angle can cause a big change in the reflected light. This just shows one limitation of using a handheld meter, and why it's usually a lot more reliable to simply take a test exposure or two and use the camera histogram as a guide.</p>

[pete_s.]

<p>Alan, interesting reading. Thanks for the info.</p>