Histogram and metering (30D)

[keithallenlaw]

In reviewing my histogram on some studio shots i took yesterday I came across an interesting
question. I was spot metering in a natural light setting and reviewing the histogram when I
realized this. Does the histogram reading change to match the metering mode? Put another
way, was I reviewing the "the spot" or does it still read the whole scene no matter whats
being metered. I thought this was interesting. Anyone?

-keith

[Rumpole]

The histogram reflects / analyses the photo you have already taken so your metering mode will not be relevant except that it may affect how your photo is exposed.

Hope it helps.

Cheers
KH

[jfrancho]

The histogram is based on the entire image. In the case of raw format, the histogram is based on the embedded .jpg thumbnail. This thumbnail is created using in camera parameters, so it makes some sense to think about how these are set. For a long time I had the in camera parameters set very flat, but now I have been setting them closer to what I want my final product to be. White balance can also have a dramatic impact on the histogram, particularly to clipping in the highlights (think of the impact of using ETTR with bad histo data!), so it make some sense to have that set to shooting conditions, use auto, or CWB, if shooting in raw format. Both parameters and white balance are obviously of paramount importance if shooting in .jpg or raw+.jpg mode.

[keithallenlaw]

Originally Posted by Rumpole

The histogram reflects / analyses the photo you have already taken - in what meter
mode? Any? Or just evalutive?

Hope it helps.

Cheers
KH

[jfrancho]

The histogram is based on the resultant image, irrespective of metering mode. Metering is just a way to measure the scene for determining the exposure. The actual finished exposure will determine how the histogram looks. Metering is a tool to determine what settings to use for a desired exposure. The histogram is a tool for analyzing a finished image to determine whether the metering resulted in that desired exposure.

[keithallenlaw]

Originally Posted by jfrancho

The histogram is based on the resultant image, irrespective of metering mode. Metering is just a way to measure the scene for determining the exposure. The actual finished exposure will determine how the histogram looks. Metering is a tool to determine what settings to use for a desired exposure. The histogram is a tool for analyzing a finished image to determine whether the metering resulted in that desired exposure.

That seems to be a very reasonable answer to my question. Just what I
suspected. It would be more helpful though if it did read the area being metered
IMO. Thanks for you time, cheers!

[jfrancho]

Originally Posted by keithallenlaw

t would be more helpful though if it did read the area being metered IMO.

There is a way to do this if you use raw format and Adobe Camera Raw in Photoshop, kind of. You can use the srop tool in ACR to isolate the area metered. The histo will reflect only the data in the crop area. Since it's a raw file, you aren't actually maing any permenant changes. I've used this to verify that the metering worked as expected, such as situations where exposure compensation was used to overcome white or black backgrounds. It also helps with a scene that has more dynamic range than the camera can capture. You can use the crop tool to isolate the subject from the over-/under- exposed areas, and adjust raw settings for a "nice" looking histo (that can mean different things for different pictures!). It is similar to using Ctrl-click and drag on the Exposure and Shadows sliders in ACR, just a different way of looking at the histo. Again, this stuff mostly applies to using raw format, where some of the tools for analyzing image data are more robust. Remember, the histogram is not going to be represented in true "units of light" or lumens or whatever the camera metering system or a seperate incident meter uses. The histo just represents luminosity in relative terms - clipped and not clipped, from shadows to highlights, left to right, from the darkest values the camera can capture to the brightest values the camera can capture, and everything in between. I hope this makes sense, and perhaps someone more aquainted with image analysis can offer some other simple tips and techniques. I'm not always good at articulating my written comments .

[keithallenlaw]

Originally Posted by jfrancho

There is a way to do this if you use raw format and Adobe Camera Raw in Photoshop, kind of. You can use the srop tool in ACR to isolate the area metered. The histo will reflect only the data in the crop area. Since it's a raw file, you aren't actually maing any permenant changes. I've used this to verify that the metering worked as expected, such as situations where exposure compensation was used to overcome white or black backgrounds. It also helps with a scene that has more dynamic range than the camera can capture. You can use the crop tool to isolate the subject from the over-/under- exposed areas, and adjust raw settings for a "nice" looking histo (that can mean different things for different pictures!). It is similar to using Ctrl-click and drag on the Exposure and Shadows sliders in ACR, just a different way of looking at the histo. Again, this stuff mostly applies to using raw format, where some of the tools for analyzing image data are more robust. Remember, the histogram is not going to be represented in true "units of light" or lumens or whatever the camera metering system or a seperate incident meter uses. The histo just represents luminosity in relative terms - clipped and not clipped, from shadows to highlights, left to right, from the darkest values the camera can capture to the brightest values the camera can capture, and everything in between. I hope this makes sense, and perhaps someone more aquainted with image analysis can offer some other simple tips and techniques. I'm not always good at articulating my written comments .

Hey that was real insightful, thanks. Works for me as i'm a total raw shooter anyway...cheers!