exposure issues using a DSLR as a lightmeter

[Andrea Gazzoni]

I found myself using a Canon 5D MarkII as a lightmeter during a recent trip.
When the film came back from the lab, I discovered that almost every exposure (expired Velvia 100F) was off by 1 to 2 stops.
First guess was bad film.
Second guess bad chemistry at the lab.
So I've done some other tests a few days ago, by loading two film holders with both fresh and expired film.
Shot some 120 rolls too.
Still used my DSLR readings (settings "neutralized"), transferred them to the view camera, and...
both new and old film are still badly underexposed.
The digital shots are ok.

I've already done this before with a $150 compact digital camera, all went fine.

Any suggestion? Except using a dedicated spotmeter...mine needs to be calibrated

thanks
Andrea

[Light Guru]

Well a couple of things come to mind.

First of you are shooting expired film. This alone could cause various issues in exposure.

Second many film photographers don't actually shoot shoot film at the speed listed on the box. Form example I like to shoot most ISO 100 films at ISO 50. But because every film type is different testing is required to find out what ISO a certain film pro forms best at for you.

[Andrea Gazzoni]

really could a 2 years expired film, always kept in the freezer, loose as much as one stop of sensibility?
this has never happened before to me

any difference between say F22 1 sec on a DSLR and the same reading applied on the view camera, right?

[vinny]

use the $150 digicam and throw the dslr away.
sell the dslr and buy a new meter.
two year exp fuji won't lose speed that way. I routinely shoot stuff that's 10+ yrs expired with zero issues.

[Light Guru]

really could a 2 years expired film, always kept in the freezer, loose as much as one stop of sensibility?
this has never happened before to me

any difference between say F22 1 sec on a DSLR and the same reading applied on the view camera, right?

2 years expired probably not depending on how it's been stored but you never said how expired in your original post so ass far as anybody knew it was 5 or 10 years expired.

A 1 second exposure on digital could definitely be different on film. Film has reciprocity failure with times of a second or longer digital does not have reciprocity failure.

But also like I said in my last post many don't actually shoot ISO 100 film at ISO 100 they rate it lower by doing testing.

Also you should know that slide film has an extremely low tolerance for not being exposed right. Expecting your first couple sheets or rolls to be perfect is probably not going to happen you need to learn how that particular film type behaves and shoot accordingly.

[paulr]

DSLRs' actual ISO rating usually differs from the nominal one. Here's a chart for your camera. It's not off by as much as you're reporting, but this could be part of the issue. So could the difference between the t-stops of the dslr lens and the LF lens.

[mdarnton]

I've noticed that I'm much more satisfied with the way underexposed shots look on the back of my DSLR than they do on film. I get something I like on the camera, and invariably the histogram is way to the left. I guess, then, if I were going to use the camera as a meter, I'd work for a centered histogram, rather than a nice look on the preview. How do the histograms for the "metering" look, if you still have those?

[Leigh]

A DSLR is NOT an exposure meter.

It's a system, of which the exposure analysis is one component.

- Leigh

[David_Senesac]

I would try calibrating the DSLR meter against the compact digital meter by framing the same subjects from same location, using the same exposure frame modes. Make some digital captures and review captures with full screen information. If that shows something off then get more basic and frame something nearby all the same color like a wall or even a grey scale card. If that is still off then one of the camera meters, probably the DSLR is doing something wrong or is in a mode you are not aware of. Of course be sure the DSLR exposure meter is not in spot metering mode. Generally evaluating readings from any meter whether in a dedicated lightmeter or within a camera needs to be done suspicously with an understanding they will vary quite a bit depending on mode and where they are pointing.

I've been using my Canon G10 more for evaluating exposure EV values besides my lightmeter before exposing Provia in dim conditions. So far it seems to be in the ball park versus readings with my lightmeter but I really don't trust it enough yet across all light levels. In other words just because it reads what I'd expect in late morning sunny front lit conditions doesn't convince me that it will also do so at lower levels of the scale in dim cloudy conditions.

I just went outside at 3pm PST on this clear sunny November SF Bay Area day with the sun altitude about 30 degrees. I took an image from a few feet away of a bright light tan wood wall, aperture at f8.0 ISO 100 exposure compensation 0. with the resulting Av mode shutter speed 1/320 second. That is about EV14.5. Image came out ok as expected.

I then pointed my Polaris lightmeter in Incident mode directly at the sun that read EV14.6. This is also my preferred method of evaluating landscape lightning and tend to then expose film about a half stop below. Then changed to 5 degree Reflective mode and read EV15.6 pointing at the subject wood wall which is about what to expect on a bright light subject versus Incident mode. Though not as high an EV as pointing at a white wall. Image came out ok as expected.

Then framed a more neutral subject and had to change the exposure compensation to -1/3 in order to get a reasonable result. Shutter speed showed 1/240 second or EV13.9. All these values were about what I would expect. If I was exposing a sheet of Provia 100F, just from the Incident reading of the sun would probably set my exposure to EV14.1 that is pretty close to what the G10 showed.

[Nathan Potter]

Using the DSLR as an exposure meter can work fine. You just need to determine your own ISO value for the film you wish to use. This is done by simply following a standard zone system procedure for the film calibration. See any number of books on the subject.

Nate Potter, Austin TX. Holliston MA.