exposure issues using a DSLR as a lightmeter

[paulr]

I love my dslr, but really wish the metering system were designed for people who knew what they were doing. as it is, the histograms only reflect the currently selected jpeg processing. There is no way to know the only thing that really matters, which is if any of the color channels have clipped in the raw file.

Some clever people have figured out hacks (like UniWB) that trick the camera into doing this, but then you completely sacrifice the usefulness of image reviews on the back screen. In order to get useful histograms you can't even see the pictures you just took.

How hard would it be to offer a raw histogram feature? I realize there are some complications, since nothing actually clips on the shadow end ... but you could pick a standard based on some level of average s/n ratio, and it would be so useful.

A "light meter mode" would also be useful. Not to lots of people, but this would just be a software item too. No actual cost to implement.

[paulr]

I'm a little surprised that no one has mentioned (I didn't check all the links in posts) that DSLRs are calibrated to give a 12% grey whilst a typical meter is calibrated to give 18% grey.

Is this true? New to me. Where can I read about the thinking behind this?

[Ed Bray]

Is this true? New to me. Where can I read about the thinking behind this?

Without being flippant, google it. There are loads of references to the fact that both Canon and Nikon calibrate their DSLR meters to 12% grey.

Here's one

There are a lot more.

[paulr]

That's interesting. According to the Thom Hogan post, it sounds like camera and meter manufacturers have been using the ANSI standard nearly forever, and so the 18% gray card has been off since before digital cameras showed up.

[thrice]

Also, if you're using longer focal lengths make sure you calculate for bellows extension.
http://www.largeformatphotography.in...ws-factor.html

[Nathan Potter]

It doesn't matter what the DSLR is calibrated to, or if it is calibrated at all. If you want perfect film exposures you need to use the film to calibrate the DSLR meter. The simplest way I use is to attach a logD 3.0 step tablet to the LF film then use the DSLR to meter an 18% grey card. Then over expose the LF film by about 5 stops so the grey card exposure falls at about log D 1.5 on the log D 3.0 step tablet. Use your normal development for the film/DSLR you have chosen to calibrate. The image of the step tablet on the film will show exactly where zone II or zone VIII falls if that is your choice for shadow and highlight texture values. There are other procedures that amount to the same thing and as I said previously check a number of publications describing such techniques.

Nate Potter, Austin TX., Steuben ME.

[Simon Liddiard]

It doesn't matter what the DSLR is calibrated to, or if it is calibrated at all. If you want perfect film exposures you need to use the film to calibrate the DSLR meter. The simplest way I use is to attach a logD 3.0 step tablet to the LF film then use the DSLR to meter an 18% grey card. Then over expose the LF film by about 5 stops so the grey card exposure falls at about log D 1.5 on the log D 3.0 step tablet. Use your normal development for the film/DSLR you have chosen to calibrate. The image of the step tablet on the film will show exactly where zone II or zone VIII falls if that is your choice for shadow and highlight texture values. There are other procedures that amount to the same thing and as I said previously check a number of publications describing such techniques.

Nate Potter, Austin TX., Steuben ME.

I had to google this... now I'm facing a challenging weekend

[Ed Bray]

It doesn't matter what the DSLR is calibrated to, or if it is calibrated at all. If you want perfect film exposures you need to use the film to calibrate the DSLR meter. The simplest way I use is to attach a logD 3.0 step tablet to the LF film then use the DSLR to meter an 18% grey card. Then over expose the LF film by about 5 stops so the grey card exposure falls at about log D 1.5 on the log D 3.0 step tablet. Use your normal development for the film/DSLR you have chosen to calibrate. The image of the step tablet on the film will show exactly where zone II or zone VIII falls if that is your choice for shadow and highlight texture values. There are other procedures that amount to the same thing and as I said previously check a number of publications describing such techniques.

Nate Potter, Austin TX., Steuben ME.

Hi Nate,

Ideally we would all calibrate our meters/lenses/development regime/film stock to ensure we can get repeatable accurate results that work for us.

Unfortunately when people buy a lightmeter they often 'ass u me' that the lightmeter is calibrated at the factory to give an 18% grey (Zone 5) when they are taking a reflected reading of any particular plain tone and base their exposure on that without any further testing. If a DSLR meter is used to take the light level reading (when calibrated to approx. 12% grey) then it is typical that the reading expected to be Zone 5 will a Zone 6 and therefore lead to approx -1 stop less of exposure given. If the film/development/lens and DSLR had been previously used to obtain a working EI for that user then the issue is moot as the difference would have been factored out during the testing regime.

If any photographer changes their meter or method of metering then any previous testing carried out will not be accurate although with B&W or Colour Print film there may be sufficient latitude for an acceptable result, whilst we are all aware of how little latitude Transparency film has and the difference could be sufficient to give unacceptable results.

[Nathan Potter]

Ed, yes I agree with what you have said. I did not know that about the 12% vs 18% grey calibration. I never used a DSLR until about a year ago and quickly discovered that it over exposed my images by about 2/3 of a stop.

If the OP has calibrated his film previously using a different meter he still may be in trouble after compensating for the 12% differential if the two meters have not been calibrated with each other. Of course it depends upon how fussy one is about accuracy of exposure.

Nate Potter, Austin TX., Steuben ME.

[Greg Miller]

Without being flippant, google it. There are loads of references to the fact that both Canon and Nikon calibrate their DSLR meters to 12% grey.

Here's one

There are a lot more.

This isn't as big a deal as may seem. The % is the reflectance of the card, not the luminosity of the gray. Somebody decided the world averages to 18% reflectance. But so many specific scense are drastically different. We've been living with this long before digital.