SLR F-ratio's...

[Vega]

I'm trying to get my head around SLR lenses and f-ratios...

Im trying to work out what f-ratios are best to use for say widefield work. (I'm guessing lower f numbers are better). I heard somehwere that stepping down f-ratios a peg or two can illiminate Coma. When they say stepping down, does this mean up in f numbers? eg from 2.8 up to 5.6 :scratch:

Another thing, my Canon 18-55mm lens says its an f3.5-5.6 yet the actual 400D camera gives me options up to f22 with this lens attached :scratch: .

Cheers

Matt

[jgs001]

I can't answer all of that Matt, however, on the lens itself, that's the smallest F number (widest aperture) across the range of focal lengths. At 18mm, you can use an aperture of f3.5, but at 55mm the biggest aperture is reduced to f5.6. You can set the aperture much smaller than this higher f number and the lens will cope with it up to f22 I think my 18-55 will do.

[barkis]

The f number is governed by the aperture of the lens. The focal length is fixed, but by closing down the lens aperture with an iris, the f ratio goes higher incrementally, depending on the relationship of lens diameter, and the focal length. The smaller the aperture, the higher the f number. Also it becomes slower photographically, ie, a longer exposure is required for a given subject. Stopping down a lens aperture, will increase the depth of field too, and it is true that a lens given to coma can be rectified by stopping it down.

Most of what I have said pertains to a film slr, I don't know how it stacks up against a dslr.

Ron.

[Naz]

The f number is governed by the aperture of the lens. The focal length is fixed, but by closing down the lens aperture with an iris, the f ratio goes higher incrementally, depending on the relationship of lens diameter, and the focal length. The smaller the aperture, the higher the f number. Also it becomes slower photographically, ie, a longer exposure is required for a given subject. Stopping down a lens aperture, will increase the depth of field too, and it is true that a lens given to coma can be rectified by stopping it down.

Most of what I have said pertains to a film slr, I don't know how it stacks up against a dslr.

Ron.

It applies just the same to a DSLR Ron, the laws of optics still apply. usually you get an improvement in lens performance when stopped down a couple of stops from wide open. Sharpness and contrast improve, best to take a series of shots, starting at the lowest and closing the lens down a stop or two for each exposure. I wouldn't try any below f6 to f8. The exposures become to long and you won't get any improvement.

Archie

[Vega]

Cheers all, its starting to make a little sense. Anyone recommend the sweet spot f-number for a milky way shot at 18mm ?

Matt

[Robd]

Matt,

The sweet spot will depend on the lens. Generally it will be close to wide open, just close down a stop or 2 - f4 to f5.6. the best bet is to do as Naz suggests and take a series of shots at different apertures since two lenses of same make and size etc. can differ slightly in their sweet spot.

Rob