Tools of the trade

[ChromaticAberration]

This is a fairly straightforward question and I apologize immediately in the eventuality of this having already been discussed around here.

"How do you pick, in a cropped sensor, the best focal length for different subjects?"

Call me a noob but all this babbling about 20, 28, 50, 55, 85, 105, 135, 200mm focal lengths in vintage lenses got me thinking what should be the apropriate decision when we are adapting those focal lengths to a cropped sensor where everything is multiplied (in my case) by 1.6 times. I mean, what are the arguments behind the decision of taking a 50 instead of a 55 or a 105 instead of a 135?

Thanks in advance

[visualopsins]

If you've used film enough to know what focal length lens you would select for a given subject, divide that focal length by crop factor.

focal lengths and 'traditional' uses are:

20-35 wide
50 normal
85 portrait
135 telephoto

transposing by 1.6x crop factor:

12.5-20 wide
28-35 normal
50-55 portrait
85 telephoto

[RioRico]

And if you're not experienced in FF (full-frame) work, forget you ever heard of crap.factor and just learn which focal lengths do what. No, putting a full-frame lens onto a HF (half-frame) camera does NOT multiply the lens' focal length, it just chops off the edges of the image circle.

So, a 50mm lens on a FF camera at a specific aperture setting and distance gives a certain FOV (field of view) and DOF (depth of field). That same lens on a HF sensor has its FOV reduced by one factor, its DOF increased by another factor. And if you change shooting distance to compensate for the different FOV, then perspective changes too. So any 'equivalences' are superficial -- the optical effects go deeper.

Forget crap.factor. Think of variations from 'normal'. A 135/FF frame is 24x36mm with a diagonal of 43.3mm -- that's 'normal'. My Pentax K20D's sensor has a diagonal of 28.1mm, also normal. Normal on a 6x6 MF cam is 79.1mm. In each format, the 'normal' focal length give a FOV similar to human vision -- with one eye shut! That's our baseline for evaluation.

We may arbitrarily chose factors to classify lenses. 1/3 less than normal is wide; 2/3 less than normal is very wide; 1.5x of normal is portrait-size; 2x of normal is long; 4x of normal is quite long. Simple, eh?

But this ignores a reality of optics -- at the same aperture setting and shooting distance, longer lenses have thinner DOF and shorter lenses have thicker DOF. At f/4, a P&S's normal lens has very thick DOF and a 4x5's is very thin. Folks coming from P&S's expect their lenses at 'normal' focal lengths to act similarly, and may be bitterly disappointed that everything shot with their new dSLR isn't sharp. Bother.

That's why I said to learn what a lens does in the format you're using. Wide, normal, long, these have different effects in different formats.