Is a Prime Lens Brighter than a Zoom?

[bgelfand]

A friend asked me a question that I cannot answer - is a prime lens brighter

than a zoom lens set at the same focal length?

 

He reasons that the prime lens, having fewer elements, and therefore fewer

air/glass interfaces, would pass more light than a zoom lens which has more

elements.

 

His argument makes sense from a theoretical point of view. However, when TTL

metering with my 50 mm prime, I obtain the same exposure value that I do when

TTL metering through my 35-70mm zoom set to 50mm. My hand held meter has no

correction for the number of elements in the lens and gives the same exposure

value as the camera TTL meter.

[tonybeach]

The T-stop difference wouldn't be enough for your meter to register (less than 1/6 of a stop).

[don_cooper]

If you're referring 'brightness' as how the image looks through the viewfinder, then the brightness depends solely on the maximum aperture of the lens at the focal length being used. A modern lens on a modern camera always has the aperture wide open when viewing/focusing and only stops down to a smaller aperture at the time of taking the photo. So a prime lens with an aperture of F2.8 will have the same level of viewing brightness as a zoom lens that has a constant F2.8 maximum aperture. However if the zoom lens has a variable aperture (F2.8-F5.6)then the zoom will be of equal brightness at F2.8 but as you zoom out the maximum aperture is less at longer focal lengths (F5.6) and consequently will be darker in the viewfinder. That's why fast zoom lenses are so prized and expensive. A brighter image in the viewfinder makes manual focusing easier and autofocus faster.

[Kent Shafer]

This question and Don's answer make we wonder why all zoom lenses aren't variable aperture.<p>

 

Take the 70-200mm f/2.8 zoom, for example. Since an f/stop is the ratio of the focal length to the diameter of the aperture, the maximum diameter of this lens's aperture at 200mm must be 200mm/2.8 or about 71mm. A 71mm aperture diameter at the 70mm zoom setting would amount to f/1.0, which of course this lens does not offer.<p>

 

Could the answer be that while the physical characteristics of the 70-200 zoom could theoretically provide f/1.0 at the 70mm setting, the designers chose not permit it because the image quality wouldn't be acceptable?<p>

 

Thanks,<p>

Kent

[rainer_t]

From ...

 

http://en.wikipedia.org/wiki/Aperture

 

-- "The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter."

 

Kent -- "Could the answer be that while the physical characteristics of the 70-200 zoom could theoretically provide f/1.0 at the 70mm setting"

 

No. You're correct, that a 200mm f/2.8 lens needs ab effective aperture of 71mm .... but this aperture is the optically visible aperture (if you look into the lens through the front element). The physical diameter of the diaphragm is smaller.

 

If you now zoom from 200mm to 70mm you'll see the effective aperture shrinking (while the physical diameter of the diaphragm could even remain the same) ... so that at 70mm the effectife aperture is only 25mm.

[vivek iyer]

"A friend asked me a question that I cannot answer - is a prime lens brighter than a zoom lens set at the same focal length?

 

He reasons that the prime lens, having fewer elements, and therefore fewer air/glass interfaces, would pass more light than a zoom lens which has more elements."

 

The earliest zoom (variotar) is a simple triplet!

 

Check out the optical construction of a 200/2 VR Nikkor!

 

The assumptions were wrong.

[Dieter Schaefer]

Anthony already answered the question correctly. From <a href="http://en.wikipedia.org/wiki/F-number">wikipedia f-stop</a>:

 

<i>T-stops

 

Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness. Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly (The T in T-stop stands for transmission),[2] but modern cinematographic lenses now usually tend to be factory-calibrated in T-stops. T-stops measure the amount of light transmitted through the lens in practice, and are equivalent in light transmission to the f-stop of an ideal lens with 100% transmission. Since all lenses absorb some quantity of light, the T-number of any given aperture on a lens will always be greater than the f-number. In recent years, advances in lens technology and film exposure latitude have reduced the criticality of t-stop values. Remember: F-stops are for geometry, T-stops are for transmission.</i>

[radfordneal]

Zooms do tend to have more elements than single-focual-length lenses, but of course there's no guarantee of that. More elements would tend to reduce light transmission, but again, that's not guaranteed, since among othe things, the coatings used will affect transmission. The effect of all this with modern lenses tend to be small, but I've noticed it at times, I think. It's hard to be sure, since there's also no guarantee that the f-stops you set are properly calibrated, the same for both lenses, so any differences seen might just be due to inaccurate f-stop settings.

[craig_gillette]

At a given focal length, "brightness" should be consistent if the "aperture" is the same. If you have a 70-200/2.8 lens at 135mm and a 135mm f2.8 prime, the metering should be the same or at least very close. May not happen as the lenses may transmit light differently, etc. With a ttl metering system, the "correct" exposure should accomodate the differences. It's also taken wide open, so the stop down mechnisms need to be set correctly to actually change light by the appropriate ratios. If they are too far apart, then the differences could easily impact exposures based on external metering, handhelds, auto-flashes, etc.

[bgelfand]

Thank you all for you answers. I appreciate the insight.