View Full Version : rules of thumb

10-25-2006, 04:03 AM

I'm new to the whole dSLR photography thing and was wondering if there are any general rules of thumb that apply to photograhy so i have somewhere to start and get an idea as to what settings to use (eg shutter speeds, f-stops ect.) The only one that I could find was to use the focal length of the lens to roughly determine the required shutterspeed, eg: a focal lenght of 300mm requires a shutter speed of 1/300. I dont know if this is correct or not but if you caould share you wisdom with me about similiar rules of thumb it would be very much appreciated!

Thanks in advance!!

D Thompson
10-25-2006, 05:40 AM
On a good sunny day you can shoot @ f16 and your shutter speed will be a reciprocal of your ISO. Let's say ISO=100 then shoot at 1/100 @ f16. You can adjust from that if you need to shoot at a specific aperture or shutter speed.

10-26-2006, 12:25 PM
On a good sunny day you can shoot @ f16 and your shutter speed will be a reciprocal of your ISO. Let's say ISO=100 then shoot at 1/100 @ f16. You can adjust from that if you need to shoot at a specific aperture or shutter speed.
:cool: Sunny 16 Rule

The previous lens thread talking about lens speed and exposure and all that stuff made me think about posting a little something - the Sunny 16 rule.

If you want to understand exposure, you have to be aware of light levels. All those F stop and shutter speed numbers may not mean a damn thing to someone who doesn't realize how much light the scene they are shooting is offering. If you don't have a meter (like a lot of my cameras), its actually pretty simple to estimate exposure fairly accurately. It all starts with Sunny 16.

This rule says that in full sunlight, if you set your aperture value to f/16, your shutter speed will be the approximate reciprocal of your film speed (ISO). What??

If my camera is set to ISO 200, and its sunny out, I'll shoot at f/16, 1/250th. That will give me proper exposure. Then I can go from there. Under the same lighting conditions, if I move my aperture value open one full stop, to f/11, then I am doubling the light entering the camera, so I have to halve my shutter speed, to 1/500th. Simple, right? If I move it to f/8, my shutter goes to 1/1000th. Same thing if I change my ISO value. Remember, sensitivity doubles or halves as I move up and down the scale, too.

Now, what if it isn't sunny? Well, then I can alter things based on the situation. I know that if I'm on a beach or snowy mountain, the scene will actually be brighter than normal sun, so I'll alter my starting point to f/22. If its cloudy, I'll drop it to f/8, or gloomy overcast, to f5/6. But that sunny 16 the easy thing to remember and work from there.

10-26-2006, 12:27 PM
:cool: Another

Shutter speeds make sense. We can wrap our heads around them. F stops are just weird. It's a form of math we aren't used to. Rick explained a little about the focal length ratio in another thread, but I'll expand on it and perhaps it will make sense why 'fast' lenses cost so much more.

Every F stop is an abbreviated number. F4 really means 'focal length divided by 4'. If I am using a 100mm lens, or a zoom lens at 100mm, the opening in my aperture at f4 is 25mm across. That's a circle about an inch in diameter. The area of that circle is pi*r squared. Therefore, the open area of your aperture is .79 square inches.

Ok, so if I want to let in half the light, I just need to close the opening of my aperture by half, right? So, f/8? That would make my aperture diameter half of 25mm- 12.5mm or half an inch. It's also 1/8th my 100mm focal length. Simple. Now, the area of that circle of light at f/8 is 3.14*(.25*.25). So that comes out to... .20 square inches? What? That doesn't make sense. That's a quarter, not a half?? It should be .40 ish, right? NO! This is where those weird aperture numbers come into play. Circles don't play by halves. To halve the area of a circle, you need to reduce it in DIAMETER by .7 times. To double the area, you need to increase the diameter by 1.4 times. Therefore, starting with F/1, we get this order - F1, F1.4, F2.0, F2.8, F4.0, F5.6, F8, F11, F16, F22 and so on. Your camera probably has half stops in between as well.

The cool thing about all these numbers is that F4 on a 300mm lens lets in exactly the same amount of light as F4 on a 16mm lens with the same shutter speed. The actual aperture opening on the 16mm lens is MUCH smaller, but its looking at a much bigger cone of light. The 300mm lens aperture is much bigger, but the cone is a tiny slice of the size of the 16mm's.

Now, if you realize this is a ratio to focal length, you can think about how big a fast telephoto lens has to be. For a 400mm lens to have a 2.8 aperture, the aperture has to be 143mm across! That's 5.6 INCHES. Lens engineers can do a lot with geometry and optical design, but they can't overcome physics. This is why long, fast lenses are HUGE, and very expensive. Lots of glass, lots of engineering..