50mm 1/1.7 lens vs a 55mm 1/3.5 lens
 

pardon my ignorance but for minimizing depth of field, which would be the better lens? and whats the difference in the two lens anyways, what does the 1.7 or 3.5 refer to exactly?
I shot an email to Declan (knowledgable guy that he is) but thought i would open it up to the crowd as well in case anyone had any thoughts...
cheers
Q

The values you are refering to is the f-number.

Check this out
http://en.wikipedia.org/wiki/Aperture

A summary:
Low f-number=fast lens=big aperture=brighter=less vignetting=shorter DOF=expensive

For a 35mm adapter you would probably want a fast lens. 1.7 is "semi"-fast, 1.4 is really fast, and 1.2 is extremely fast. Around 2.0 is mostly standard on SLRs.

Dof
 

so, according to your post, and to the article, i should use that 1.7 with a high shutter speed to achieve minimum DoF (the least amount in focus)?
Q

Yes, the faster lens (the one with the wider aperture) will give shots with more differential focus if used at those wider apertures. Both lenses will give the same dof if used at the same aperture of course.

Most good 35 mm SLR lenses have dof scales that relate to the aperture ring, and you can see there how limited the dof field is at wide apertures.


You're correct that high shutter spoeeds will force you to use wide apertures, but upping the shutter speed isn't really the best way to go with video and CCD chips. better to use neutral density filters to soak the light.

The f/1.7 lens is approximately 2 stops faster than the f/3.5 lens.

tom.

Yes, the faster lens (the one with the wider aperture) will give shots with more differential focus if used at those wider apertures. Both lenses will give the same dof if used at the same aperture of course.

Most good 35 mm SLR lenses have dof scales that relate to the aperture ring, and you can see there how limited the dof field is at wide apertures.


You're correct that high shutter spoeeds will force you to use wide apertures, but upping the shutter speed isn't really the best way to go with video and CCD chips. better to use neutral density filters to soak the light.

The f/1.7 lens is approximately 2 stops faster than the f/3.5 lens.

tom.

Keep in mind also that you should practice using the lenses stopped down 1-2 f-stops. Most/all photographics lenses do not produce their best picture quality at maximum aperture. Telephoto/zoom lenses usually need to be stopped down more to gain maximum image quality.

The wider aperture available, the better but if that f3.5 55mm is a Micro-Nikkor, don't throw it away.

You can do some amazing close work with that provided it is not vignetting into the corners of your image. There are some Micro-Nikkor images at www.dvinfo.net/media/hart. Any of the .jpgs with ohara008.JPG or similar filenames were done with this lens.

Quito

IMHO after doing this for a while, getting faster lenses is important for two reasons, but should be tempered.

First, because the whole apparatus combined does lose a fair amount of light. add up the 35mm lens, adapter, and the camera, and you could be a fair amount. The faster the lens, the less issue you have here.

Second, it's true the faster the lens the shallower the depth of field. But check out some of the DOF calculators to see the resulting DOF - you may find yourself shooting with 1" of DOF on a 50mm f1.2 lens. It's great for some applications, but I wouldn't say 1" of DOF is that useful.

A lot of times you'll be wanting to stop down on both the lens (to hit the sweet spot as well as get the DOF relevant for your scene) and also stopping down in the video cam to hit it's sweetspot as well.