Know your Gear

To get the absolute most from your gear and to "properly apply" the exposure triangle you need to know two basic things...

Your camersa's base ISO (easy) and your lens's sharpest aperture (not as easy, but not hard).

Base ISO...Easy, it's the lowest ISO setting that's "native"(i.e. a number, not "Low-1" or some such)
Why is this important? Because that is the ISO the camera is designed around and optimized for. At every other ISO setting you are giving up some performance capability.
I'll use my D7000 as an example: (higher readings/numbers are better)

Dynamic range(DR): at ISO100 =14 (f)stops, at ISO1600 = 9.5 stops. That's almost five stops of loss, ~ 30%. With less DR capability highlights clip earlier and shadow detail is lost earlier.

Signal Strength over Noise (SNR): at ISO100 = 41db, at ISO1600 = 28db. That's more than 25% loss in signal strength. With lower SNR noise becomes more visible in the image.

Tonal Range: at ISO100 = 9 bits, at ISO1600 = 7 bits. 

Color Sensitivity: at ISO100 = 24 bits, at ISO 1600 = 17 bits. Almost 25% loss.

You don't have to know all of the specifics, I don't. I used DXO lab test results to get the information. The only thing you have to know is that if you are not using the camera's base ISO you are giving up performance...quite a lot in some instances. The further away from base ISO, the more you are giving up.

Now onto the lens's sharpest aperture. As a general rule f/8 will work for most lenses, but it is not necessarily the sharpest for your particular lens. And by "sharpest" I mean the lowest aperture where maximum image clarity/detail/contrast/color is achieved.

I'll use some examples (Nikon Lenses)

My 24-70 and 70-200 f/2.8 lenses: best aperture ~f/4. These are very expensive "pro" lenses and they still are not at their "optimum" until stopped down a bit.

My 28-300 f/3.5-5.6: ~ f/8. This is also an "expensive" lens but for a different reason. Still, if used at f/8 I can get images comparable to the 70-200 f/2.8 which cost more than twice as much money.

The 18-200 f/3.5-5.6 I used to own: this one was a bit trickier as it's best aperture varied with the zoom setting, it was f/8 at min/max zoom and f/11 in the middle.

And now the "fabulous Nifty Fifty", 50mm f/1.4: it's best aperture is approximately f/5.6!

So why is it important to know the "best aperture" for your lens? Because at any other setting you are giving up something, either image quality or light. As you set the lens to a higher aperture number you are reducing the amount of light the lens can gather, and if you go far enough (generally above f/11) you will start degrading image quality due to diffraction. As you set the aperture lower you are giving up image sharpness/contrast/clarity/color. The losses generally begin in the corners/edges of the image and move inwards.

So, if you set your camera to it's base ISO, set the lens to it's best aperture, and then use whatever shutter speed is required for the exposure, you will then get the absolute highest quality image possible from your gear.... It's where the "f/8 and be there" rule of thumb came from, because f/8 "works" as one of the "best apertures" for almost every lens.

Now, you may not always be able to use the absolute best settings....maybe you need more SS. Well now you can decide "what" to trade first and why...Maybe there isn't a large DR in the image...easy, give up a couple stops of ISO. Maybe that's not enough....OK, maybe now I need to "compromise" because if I go any higher on the ISO the image will be "unusable" noisy. Easy enough, open up the aperture. Maybe the compromises are too much and this is the type of situation I'm in all the time for the type of photography I want to do....NOW it's time to buy new gear.

Sometimes you might not WANT to use the absolute best settings... Maybe I don't want to be able to see every pore in the models skin...easy enough, open up the aperture. And maybe for artistic reasons I care more about the "bokeh" in the background than I do the sharpness of my subject....well, I don't really understand this one but ok, open up the aperture more. Realistically, it becomes a balancing act. How much bokeh can I gain for the sharpness lost.....should you go to f/1.4 with that 50mm, probably not.

There's a lot more that can come into making the choices and as you learn more you will be able to make the choices more easily/intuitively. But it really is as easy as base ISO/best aperture/ matching SS. If that won't work or isn't what you want, what do you want to trade off first...Image quality in noise/color/DR then trade ISO. If it's image sharpness (again usually starting in the corners of an image) then trade off aperture.

Sometimes you can regain much of what is lost in post, but it will probably never be as good as if it were captured "correctly" to start with.

What is the base ISO for your camera? 
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