Ok it's Monday night and I've successfully confused the hell out of myself:bang:
I'm curious as to the benefits of fast lenses.
I understand that they give you a faster shutter speed in low light but surely this results in a really shallow depth of field.
I'm guessing that they aren't faster across the aperture range or are they? My admittedly amateurish test says not.
So do you just accept a really shallow dof on the basis that as it's dark you're not to worried about deep dof or am I missing something
While I'm busy confusing myself is there a way of calculating the dof of a lens at different apertures?
Sorry that this is a bit of a rambling question and I'm sure the answer is either very simple or contains unfathomable maths but this is bugging me.
You can get quite deeply into math, but there is a more commonsense and easy answer.
Consider first that all photography is a compromise. Exposure is never absolute, because not everything is lit the same. Focus is never absolute, because everything is a different distance from the lens, and so on.
Now, with lenses, there is a four-way dance going on.
First is the focal length. In general, all other things being equal, a longer focal length produces shallower depth of field. So a 90mm lens at f/2.8 has a shallower DOF than a 50mm lens at f/2.8 for the exact same situation.
Second is lens aperture. In general, all other things being equal, a wider aperture (lower f-stop number) has less DOF than a smaller aperture (higher f-stop number).
Third is distance from your subject. In general, all other things being equal, if you are closer to your subject, there is less DOF than if you are farther away from your subject.
And fourth is sensor or film image size. In general, all other things being equal, if you have a 35mm film or that size sensor, you have less DOF than if you have a smaller sensor or film size. Larger films and sensors are the opposite, of course.
So...
A 50mm lens set to f/1.4 can see better in the dark, meaning lower light and faster shutter speed, but it will have very low DOF depending on how far you are from your subject and what size sensor you have.
In addition - not to make things overly complicated - lenses are generally at their best optically when they are stopped down a bit. Usually somewhere between f/5.6 and f/8 or so - it varies depending on lens.
Fast & Good lenses are a bit rare, and often quite dear as well. The goal if price were no object would be a lens that is as good at f/1.4 as it is at f/8, but few are. So not only do you get shallow DOF, but often less than perfect image quality.
The only real workaround in such a situation would be faster film (or sensor ISO setting) but of course that brings its own set of compromises to the table, or slower shutter speed, which introduces blur - either from your movement or from your subject's movement. Like I said, compromises - the name of the game.
Calculating DOF can make your head swim, but fortunately there are two methods to help you.
The first is a stop-down preview mechanism. Some SLR cameras have them, some do not. I have found it to be an approximation, as stopping down makes the viewfinder rather dim unless you're shooting wide open, so you're not really seeing what the camera will see in general terms. Of course, there is no stop-down preview for non-SLR type cameras, as you do not view through the taking lens on other types.
The second, and perhaps easier, is to use a calculator like that available on DOFMaster.
http://www.dofmaster.com/dofjs.html
This is really rather simple to use. It asks the four questions I mentioned above - lens focal length, f-stop, distance to subject, and size of sensor or film.
Then it calculates the overall DOF, and also reports the distance in front of where you have focused that will be acceptably sharp and the area behind where you have focused that will also be acceptably sharp.
This is a creative tool for advanced photographers who wish to take more creative control of their work.
It is relatively easy to take a portrait with a 90mm lens at f/2.8 from a short distance and have the background appear all creamy and out-of-focus due to short DOF. But perhaps a hint of detail in the background might be nice - or a bit more. How do you know what would be the best setting to use?
Given constraints over things you cannot or might not be able to control, such as light, film speed (or sensor ISO) and so on, a tool such as a DOF calculator will let you control precisely how you wish to render your out-of-focus objects.
Experience will be your teacher in terms of what looks best - that's subjective anyway, so as you wish it to be is best. But it is good to be able to seize control when you want to.
I hope you find this helpful.
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