Use "mark" function and gray card to set exposure?

[VideoMaker]

Up to now I have relied on 70% zebra and WFM to manually set exposure. Generally this works well but it is (for me) still a bit subjective because I am having to estimate what the scene content is, and/or which part of the scene should be exposed at 70% luminance. Today I decided to play around with the mark function and my 18% gray card as an alternative for setting exposure. I'm looking for feedback from others as to whether this seems to be a vaild approach for metering interior architectural shots. Here is what I did:

In good outside lighting I zoomed in until the gray card filled the screen. I then did an automatic exposure which yieled 45% luminance in the mark rectangle. So now my assumption is that a perfectly exposed gray card is 45% luminance. Based on that, I believe I should be able to place my gray card in a critical place in the scene and perpendicular to the camera, and adjust the exposure to yield 45% luminance when zoomed in on the gray card. Am I correct in assuming this should give me optimum exposure at the place where I placed the gray card?

 

[VideoMaker]

I did some experimenting today and it looks like my theory is flawed but I'm not sure why. If I zoom in to fill the frame with my gray card I might get, say 45% luminance. If I keep the same aperture and zoom out to where the card is just beyond the edges of the marker rectangle, the luminance reading might drop to 25%. I was expecting a small change in the range of 1-3%. Give this, I'm not sure what value the "marker" function provides. Anyone using the marker? If so, how?

 

[ggrantly]

All other things being equal (that is a big if), you should have increased exposure as you zoom out due to the lens. The iris lets more light in at wide angle.

Grant

 

[VideoMaker]

Grant, are you saying that if I have the lens zoomed and aperture at say, f2.4, and zoom out still at f2.4, it is expected and normal that the exposure will change? I assumed that constant aperture means constant exposure (all other things being equal). More specifically, assume I have a perfectly lit green screen of infinite dimensions. If I am at f2.4 it is normal to see a significantly different luminance value when zoomed in vs zoomed out?

All other things being equal (that is a big if), you should have increased exposure as you zoom out due to the lens. The iris lets more light in at wide angle.

Grant

 

[Barry_Green]

No, that shouldn't change anything, but... what is your marker pointed at? If you're zoomed in so that the entire marker box is pointed at the gray card, and then you zoom out and the marker box is seeing more of the scene than just the gray card, then yeah, it will likely be averaging its exposure report across that entire scene, instead of just the gray, and that could change its reported brightness level.

 

[VideoMaker]

Barry, I just retested and got the expected results. I must have been doing something wrong yesterday. In today's test I zoomed all the way in on the gray card until it completely filled the frame. I did an auto exposure which set the aperture to f5.6 and 45% luminance in the Marker readout. I then switched to manual exposure f5.6 and zoomed out until the gray card extended just beyond the edges of the marker. The luminance still read 45% as I had hoped it would.

I am hoping this will be useful for interior shots where I may not have a good reference object for the zebras, or a good feel for what the WFM should look like. My thinking is to place the gray card in an area of the room where I want optimum exposure and set the aperture for 45% luminance using Marker on the gray card. I've kind of ignored the Marker feature until now but this seems like it should work. Does anyone see anything wrong with this approach?

No, that shouldn't change anything, but... what is your marker pointed at? If you're zoomed in so that the entire marker box is pointed at the gray card, and then you zoom out and the marker box is seeing more of the scene than just the gray card, then yeah, it will likely be averaging its exposure report across that entire scene, instead of just the gray, and that could change its reported brightness level.

 

[matt s.]

What if your gray card is not in the highlight area of the scene? This could cause blow outs over exposure in areas where you did not place the gray card correct? your idea sounds good but I would have to double check with the 105 zebras to be sure there is nothing overexposing in the shot.

 

[VideoMaker]

Matt, your point is a good one. Generally I DO have some blown highlights around windows, which is ok. But it would be good to do a sweep of the room with the 105 zebras. For this type of work I normally have time to go through a setup like this.

What if your gray card is not in the highlight area of the scene? This could cause blow outs over exposure in areas where you did not place the gray card correct? your idea sounds good but I would have to double check with the 105 zebras to be sure there is nothing overexposing in the shot.