Post
by jdemott » August 31st, 2013, 11:33 am
Yes, with qualifications. Early on in the development of digital photography, photographers began using digital cameras as convenient light meters that have the added advantage of letting you take a test shot as well.
First, the biggest qualification is that the ISO settings on both cameras have to be the same. If you have ISO 100 film in the SLR, then you would need to compare a reading of ISO on the digital camera (I'm not familiar with the SX150IS, but I assume you can also tell what ISO is being used). If the ISOs are different, you can convert the differences by multiplying or dividing. An ISO of 200 is twice as fast as ISO 100, so a shutter speed of 1/500 at ISO 200 will require twice as long, or 1/250, at ISO 100. Apertures (f-stops) are a little harder. The sequence goes 2.0, 2.8, 4.0, 5.6, 8.0, 11.0, 16.0, 22.0. Note that the numbers double every other step, but that actually represents a factor of four--f/4 has one-fourth the light of f/2; f/2.8 is one half the light of f/2.
Second, you need to have the same field of view. Obviously, you need to have both cameras pointed at the same place. But also, the cameras need to be zoomed to the same view. If you have a wide angle lens on the SLR whiel the digital camera is looking at a smaller part of the scene, then you will only get the reading for the smaller area (sometimes this is what you want to do; it is called spot metering when you measure the exposure at an important area of the photo). Cameras can use different metering systems, generally referred to as matrix, center-weighted, and spot. Whatever metering method your Canon uses is the metering result you will get.
Third, you would have to adjust for any filters. If you have a polarizing filter on the SLR for example, that would reduce the light coming to the film, but a reading from the Canon without a filter would indicate more light (i.e., would suggest a faster shutter speed).
Fourth, another big qualification, is that you have to hope both cameras are reasonably accurate. Even the best cameras are never exact in their shutter and aperture settings (plus or minus one-sixth of a stop wouldn't be surprising). ISO settings are supposed to be an international standard but the testing labs that have measured various digital standards have found that the manufacturers exercise some subjectivity in their ISO claims.
You don't say why you don't trust the meter in your SLR, but if you're planning to use the Canon as a light meter, the best thing would be do do some test shots, carefully recording the settings on both cameras and then examining the results. One of the most common uses of pocket cameras as light meters is to zoom in on different places in the scene to see what the lightest and darkest areas in the photo will be and then make a judgment about what you want to do with the film camera settings. I.e., the digital camera acts as a spot meter for an SLR that may not have spot metering capability.
Hey everyone. Just curious. As I typically shoot with 35mm SLRs, I also bring my little Canon SX150IS digital point and shoot with me too. Can I use the light meter from the Canon SX150 to get the proper exposures for use in the SLRs? Like, say if I wanted to capture a squirrel sitting on the Multnomah Falls platform with my Minolta. Can I just point the Canon SX150 (while on AF) towards the squirrel, then half press the shutter for the proper aperture and shutter speed? Will this be the same aperture and shutter speed needed for a manual SLR camera?
It's a good thing too, now that I'm old and lazy. 
