I was at the meet-up of my local photography club the other night and one of the guys asked me about light meters. The question went something like, “I’m going to be mounting old lenses on my Nikon digital camera, so how do I measure the light?”. As it turns out, this guy was also going to be using 4×5 and 8×10 view cameras in the near future.
There are two different scenarios here – old mechanical lenses on electronic cameras and old style completely mechanical cameras. I’ll tackle each scenario separately.
Metering light when using mechanical lenses on digital cameras and electronic film cameras
Modern lenses are electronic. The aperture is set electronically from the camera body, and the camera know what the maximum aperture (largest aperture hole, smallest f number) of the lens is. If its a zoom lens, this maximum aperture may vary based on the focal length you set the lens to.
The camera needs to know this maximum aperture because all the light metering is done at this maximum aperture, and it needs to make adjustments to the light it reads to cater for the lens aperture. It does not mean this will be the aperture thats used when you make the photo, but thats the aperture that the light is coming through when the cameras light meter is reading the light.
Its a simple concept thats tricky to explain.
Imagine you have a magical gadget that reads the amount of light. You sit this gadget on the palm of your hand, it faces the wide open sky, and it tells you how much light there is right now. Typically this amount of light would be in units called lumens or lux.
Now imagine you stick a cardboard tube over the gadget. The tube has just restricted the amount of light reaching the gadget, but you still would expect it to give you the same answer for the amount of light, because the sky light has not changed. So, you need to tell the gadget how much the cardboard tube has restricted the light. The way you would tell it is to give it the diameter of the tube opening and how long the tube is. Given that aperture is always expressed in terms of focal length (thats what the f means in a aperture setting like f/8), the aperture of the cardboard tube will also be expressed in terms of the length of the tube, so you need to tell the gadget the f number of the tube. The gadget can them factor this information into its calculation of the amount of light.
Well, thats exactly what happens in the cameras light meter. It needs to know the current aperture of the lens expressed in terms of its focal length. The current aperture is always the maximum aperture for an electronic lens.
The light meter needs to know what the aperture of the lens is whilst its reading the light so it can factor this into its calculations. If you put a mechanical lens on your camera, the camera now does not know what aperture the lens is set to, and most cameras will not have any function where you can manually set what the lens maximum aperture is.
Just as a side note, I shoot with a Hasselblad V-system film camera, and one of the Hasselblad view finders I use has a built in light meter. Because this light meter was designed for use with the Hasselblad system, and the Hasselblad V-system lenses are all mechanical, the meter does have a function where I can set the lens maximum aperture. This meter is an exception to the rule though.
So, if your camera (and thus its built in light meter) does not have a facility to set the lens maximum aperture, your only option is to use an external light meter. This being a hand held incident or spot meter, another digital camera, or an app on your phone.
If you are using a pre digital 35mm camera, it will probably have a light meter built in, and often just as sophisticated as a current digital cameras meter. If you are using a medium format film camera, it may have a light meter built in or it may not.
If your 35mm film camera has a light meter built in, and you are using an old manual non electronic lens, you have the same problem as the digital camera scenario above, so the answer is the same.
Metering light when using manual lenses on non electronic film cameras
A large format 4×5 or 8×10 film camera will not have a light meter built in, nor will most medium format film cameras. So, how do you measure the light? Now that you have read the section above, the answer to this problem should be obvious.
You are going to need a light meter. Light meters come in four flavours:
- digital camera
- smart phone with light meter app
- hand held incident meter
- hand held reflective spot meter
By the way, I am assuming you do know how to meter light appropriately. If not, please see other posts on my site for guidance.
Using your digital camera as a light meter
Yes, your digital camera is a great light meter. I am going to assume that you have some sort of digital camera with you, even if you are shooting on large format film. So, if you are just getting started with large format photography, don’t go out and buy a light meter, use your digital camera. Because your digital camera has an electronic lens (and thus it knows the lenses maximum aperture), the meter readings you take with it can be used directly with your film camera. Just set the ISO of the digital camera to the ISO of your chosen film (or the ISO you choose to rate the film as), and you are good to go. If you meter appropriately with your digital camera, then the meter reading you decide on can be used directly on the film camera – set the film camera’s lens to the aperture and shutter speed that the meter in the digital camera is displaying, compose, focus, and take the shot.
The comment I generally get back when I suggest this technique are “but the lenses are different”. The important point that people often forget here is that aperture is always expressed relative to the lens focal length. As I said above, thats what the f means in f/8 for example – focal length. So, f/8 on a 28-70mm zoom on a digital camera means exactly the same thing as f/8 on a 250mm large format 8×10 prime lens. And of course a shutter speed of 1/60 is the same amount of time on any camera or any lens with a built in shutter.
Using a smart phone with a light meter app
There are numerous apps available for both iPhone and Android devices that performs the functions of a light meter. Many are free, some you have to pay for. In most cases they use the phone’s camera/s, with the front facing camera performing an incident meter function, and the rear camera performing a reflective meter function. There is at least one app (for the iPhone only) that also needs a special device plugged into the headphone socket for it to work. Its called Lumo.
The phone may well be a good option for you – its about the same size as an incident light meter, and its something you probably already have with you. Evaluate the apps and compare their performance to your cameras meter.
Using hand held light meters
The other two choices are to use hand held light meters. These come in two flavours, either reflective or incident. As a side note, in case you were not aware, the built in meter on your digital camera is a reflective meter.
In a separate blog post I will talk about the differences between these two meter types.
Which meter should you use ?
Use the one you have already – your digital camera. When shooting film, you probably want an instant digital record of the scene anyway, and you are going to save a lot of film if you do your exposure and compositional learning with the digital camera.
Then, because its cheep, try out a light meter app on your smart phone.
When these first options becomes annoying, go out and buy a hand held meter from eBay and go retro!
