Fast lenses question

[jawarden]

Hi all,

This is my first post here. I've been spending some time recently getting to know my lenses. I have been struggling a bit with my 90MM lens for my Contax G2 (especially in lower light situations, where maximum aperture is needed and I'm stuck with no depth of field at close distances), and that has me thinking about the differences between the not-so-fast Contax lenses and other lenses on the market.

I understand that fast lenses have a larger aperture opening when set at their max, i.e. an f2 lens is faster than a f2.8 when they are set at those apertures. I also get that the f2 setting will produce a narrow depth of field. My question relates to the other settings of these two lenses.

Let's assume two 90mm lenses that are identical except for the speed; one is a faster f2 and the other is an f2.8. Let's set them both to f8, and use the same shutter speed for each to capture an image. Will they produce the same exposure or will the faster lens overexpose the image?

Do you see what I'm getting at? Are fast lenses faster at all apertures, or do they just offer the additional apertures?

Thanks,

Jeff

 

[antiquark]

Let's assume two 90mm lenses that are identical except for the speed; one is a faster f2 and the other is an f2.8. Let's set them both to f8, and use the same shutter speed for each to capture an image. Will they produce the same exposure or will the faster lens overexpose the image?

They will produce (theoretically) identical exposures at f/8.

Do you see what I'm getting at? Are fast lenses faster at all apertures, or do they just offer the additional apertures?

No, they're only faster at the largest aperture.

 

[mike.TTL]

Jeff, the answer is you only get the additional apertures.

Same number = same aperture, regardless of the lens' maximum aperture. So if you had a 90mm f2.0 and a 90mm f2.8 and set both at f8, then took a picture with each with all other parameters the same (same shutter speed, same film), you'd get exactly the same exposure.

Edit: Beaten by antiquark!

Mike

 

[marcr1230]

the aperture ratings are standard - f2 is f2 wherever you find it,
f8 is f8
the lenses could be different optical designs or with different flare reduction coatings, so you may see contrast or resolution differences, or different egde/corner sharpness. All in all though the aperture is a standard measure

 

[Luddite Frank]

Simple answer - " f = f = f "... f /8 = f /8.

A "faster lens just "opens wider" / has a larger maximum aperture relative to its focal legnth...



Luddite Frank

( Footnote - few (if any ) lenses achieve their greatest sharpness / definition at full aperture ("wide-open")... usually a lens reaches peak definition stopped-down 1/3 to 1/2 way...)

 

[jawarden]

Thanks very much. In my case then a faster lens might not make an improvement. Already my depth of field is too small (at about a meter from the subject and f4 the dof is only 3/4"), so if an eye is in focus, the ear is not. Stepping back another meter helps the dof issue so I guess more experimenting is in order. Thanks for the help.

 

[f16sunshine]

The aperture numbers on a lens refer to a ratio of the Focal lensgth v. the diameter. For example a 50mm with a diameter of 25mm would be 50/25
or f2. Regardless of the fax aperture this ratio does not change. So if a 1.2/50mm and a 2.8/50mm are both shot at f8. That ratio of FL to opening diameter is 50/6.25mm =8 for f8 on both lenses. This is not to say both will transmit the same amount of light as there are other factors. For practical purposes with modern lenses , no need to consider anything else in light transmition.

 

[Luddite Frank]

That's a paradox of focal legnth - the longer the focal-legnth, the shallower the D-O-F.

If we're talking 35mm format, for many years the 85mm to 105mm lens was considered an "ideal" focal legnth for portraiture, as it got the camera far enough away from the subject for flattering perspective, and yet the (relatively) shallow D-O-F helped "soften" peripheral features (such as ears)...

A few weeks ago, I was shooting flowers in my wife's garden, and trying to get some really close detail of azalea and phlox blossoms... I was using a screw-mount Pentax SLR and a 35-105 /Macro-Zoom lens, and also my 1934 Leica III rangefinder, with both the 90mm f/4 Elmar, and the 135mm f/4.5 Hektor lenses. When I got the prints back, the stuff I'd shot with the SLR was pretty-much what I was going- for...

The pics I shot with the rangefinder illustrated to me just how shallow D-O-F really IS up- close, lens wide-open... if my focus target had been the pistil / stamen (center parts) of a blossom that measured maybe 1/2 inch across, I had many shots where the perimeter of the petals was in-focus, but center bits were not. I was shooting wide-open, and getting as close as the lenses would focus - about 3.5 to 4.5 feet. The D-O-F gremlin got me big-time.

I did another series of flowers a week later, with the old Leica and the long lenses, this time stopping the lens down to at least f/ 8 to f/12 (1/3 to 1/2 way)... it will be interetsing to see if the results are any better.

(I used a tripod for all the flower shots...)

If you're shooting hand-held in low-light, a fast lens will help, but only to the limit of your film-speed.

LF

 

[35mmdelux]

If youre wrestling w/ DOF at f/2.8, try the Noctilux at f/1.0.