Is lens-specific meter compensation necessary?

[yanglee]

I apologize ahead of time for my ignorance and for repeating a

question if it has already been asked. If I set a 6-element 50mm lens

to f16 1/100 and another 16-element zoom to 50mm f16 1/100, would both

give me the same exposure? Would one expect the 16-element to block

out more light than the 6-element lens and so should be compensated

accordingly? If so, how can one use a handheld spot meter for

different lenses?

 

Thanks for your answers or a pointer to another existing discussion.

[kang_kim]

Hi,

Even though I don't have the exact answer for your question, I feel that your question is a

reasonable curiosity(sorry for my bad english). My short answer is; There should be some

differences between lenses, but not serious. And in photography, nothing is 100 % perfect

or predictable, which might be a very good thing for some reason. Sorry for my ambiguous

answers. Here are some facts from my experiences. Between the color-labs, there is about

a third stop difference in exposure and there is about a quater stop difference between

same iso rated films(different brands). I think you should use your light meter as a quide

for about 90% and you need to back it up with your experiences; such as your lenses,

films, color-labs, weathers, your moods, and so on. There is a lot of ambiguousness in

photography. And I think that is not a bad thing.

[Ed_Ingold]

The loss at each glass-air surface is only about 0.2% in modern, coated lenses. You would need 50 elements to affect the exposure by 1/4 stop. You have other things to worry about.

 

The amount of extension in many lenses can affect the exposure by 1/2 stop (2 stops for a 1:1 macro lens). In some zoom lenses, the relative aperture changes with the focal length. Filter factors must also be considered, and the factor for a polarizing filter varies with the subject and orientation of the filter.

[JosvanEekelen]

Good question. In SLR cameras the light metering is done through the lens (TTL), this corrects for differences in transmission of light between lenses. In practice you don't have to bother about the extra light absorbtion. What you are referring to is the effective aperture (not sure whether this is the correct english term, in Dutch it is called "effectieve diafragma").

[dan_fromm2]

It is an empirical question. Mount the lenses on a camera with a TTL meter, pick, say, a shutter speed, and see what aperture the camera's meter recommends. You may well see a difference between the lenses.

 

Have you ever heard of "T-stops"? These reflect transmission, not geometric aperture, are used mainly on lenses for motion picture and TV cameras. Bum exposure can cost a film producer a lot of money.

 

I once had a Beaulieu 4008ZM with an 8-64/1.9 Angenieux zoom. Wide open, the lens t-stopped around t/3.3. To get good exposure when metering with an external meter, I had to set the aperture, calibrated in f/stops, about 1.5 stops wider than the meter suggested.

[clauder]

to try this out, you will have to pay attention on what the cam is metering though... you will want the field of view (or metering area) filled with exactly the same color/reflectance material (a grey wall i.e.), and makes sure that it is extremely evenly lit... (as the camera takes a reflected light reading, and the diffrent lenses give diffrent field of views)

[clauder]

ok, forget my previous post, I saw that you thought of setting it to the same focal length... sorry! need some coffee I think!

[yanglee]

Thanks everyone for your answers. I guess the conclusion is that most likely the contribution of each additional element is not significant. But I am going to try this empirically with slide film, I will post the result once I accumulate enough photos to make a conclusion.

 

Thanks.

 

David Lee

[david_henderson]

Edward Ingold's answer is correct though if you test two lenses at the same fstop you might well see a small difference in apparent exposure with slide film due to slight variations in actual aperture between lenses. So if you do get a slight difference it won't mean that f16 isn't f16 and so on.

 

Would Jos van Eekelen please note that the same is actually true if you meter externally or non TTL. I don't need to correct for exposure variations between my non TTL medium format lenses, because the lenses are corrected for most differences in transmission.

 

Edward Ingold. If you're still watching this thread could you expand on your comment on polarisers please- I'm interested.

[Ed_Ingold]

Blue sky is polarized in a band centered at 90 degrees to the sun, and diminishes closer or further from the sun. The amount of polarization also increases with altitude and the absence of water vapor and particulate. For example, the sky in the Colorado Rockies (away from Denver) can appear nearly black through a polarizer. In Charleston SC, in the summer, you hardly notice a change.

 

There is a considerable amount of skylight reflected from foliage, buildings and rocks - perhaps 30 percent. This is almost completely attenuated by a polarizer. This reflection imparts a bluish cast, and is the prime reason I use a polarizer (the sky thing is easily overdone).

 

The net variable effect is probably not more than 1/2 stop in total exposure except in extreme (e.g., high country, Colorado) conditions. If you fail to compensate, you merely underexpose a little, which makes the colors appear more saturated anyway.

[david_henderson]

Thanks. I use slide film for my colour work and a half stop less exposure is a lot to me. There have been a number of occasions where I've experienced frames being darker than I anticipated, and it's been crossing my mind that the use of a blanket 2 stop adjustment factor for my polarisers isn't appropriate and as I don't use ttl metering much I wouldn't pick up the issue except in the results. I am aware of the effect of different angles relative to the sun on the extent of polarisation, and I've photographed at altitude sufficiently to have worked out the need to be careful. What I haven't been factoring in is the effect of particularly clean or dry air on the darkening affect. In retrospect most of the difficulty I've had has been in intense light with strongly saturated blue and cloudless skies, and I guess that ties in with what you're saying.

[kelly_flanigan1]

Older non mulitcoated zooms can have a horrid light loss; this is why a pro cameraman has his lenses for movies "T-stopped".This has been done even with still lenses; going back 1/2 century. Also multicoated zooms can have radically more light loss too; when they age and get dust sucked in ; since they are not sealed as well; and pump/suck dirt in when zoomed. The reflection loss is dependent on the number of air to glass surfaces; NOT the number of elments. A Tessar has 4 elements; but only 6 air to glass surfaces. The old Auto Nikkor Telephoto-Zoom F1:4 `4.5 FL=85mm to 250mm lens for the old Nikon F has 8 groups iof 15 elements The old 43 to 86mm F3.5 has 7 groups of 9 elements. Both cam out in the single coating days; both "T-stopped" less than the max f stop numbers. <BR><BR>In a dusty area; the T stop numbers rise with time. A single coated zoom of F4.5 might T stop at F6.3; then be F8 when dirty.<BR><BR>The uncoated; single and mulitcoated refection numbers quoted are often best case lab numbers; with perfectly clean lenses. With time a lens can get fine dust inside; the zooms are a sorid lot in this matter. Here the real world engineering T stop measurements are often WAY different than a sterile' "best case" fantasy numbers that Physics chaps use; and many optical texts.<BR><BR>In a radioactive lens; one has the further light loss due to the radioactive elments aging. This can be 1/2 stop ; 2/3'rds stop loss or more. With a Accoustic Optic Modules; a too high of beam density in watts/mm2 can break down the crystal structure; and blacken the transparent cyrstal. Here one may have to scoot the crystal over a grunt; and use a nonblackened portion. <BR><BR>

[helenbach]

If you compare the f-stop of a lens (relevant for depth of field) with the T-stop (relevant for light transmission) you generally find about a third of a stop difference for primes and between half and two-thirds of a stop difference for zooms. However, in practical use the amount of overall flare tends to even out the midtone and shadow exposures (comparing prime to zoom). That brings surface defects, such as tiny scratches, into consideration.

 

The decision on whether or not to compensate for differences in transmission is probably best made on individual experience, requirements and practice - if it doesn't cause you a problem, don't bother about it.

 

Best, Helen