Is this true? (Technical question)

[crunch]

Like I said, the built in exposure meter said I was underexposing by one third stop, but this could be user error. I'll try again later and see.

[sfisher]

Originally Posted by crunch

Just to re-iterate if it got lost in the long posts above: the better lens seems to let LESS light through!

This might seem less surprising when you consider that the 28-90 has 10 elements, while the 24-105 has 18. If you assume 99% transmittance for each element, that's 90.4% transmittance for the 28-90 and 83.5% for the 24-105. If my maths are correct, this translates to approximately an 1/8th of a stop difference between the two lenses.

[sk66]

Personally, I think there may be something to the original claim, for numerous reasons.

First, just general lens design/efficiency...My D300 works best with an ev of -.3 for most of my lenses, but I have one which works better with an ev of 0. This could be due to coatings, number of elements, efficiency of light transmittal etc. (That's not even considering filters.)

Now add that most "better" lenses tend to have larger diameter objective lenses. A larger diameter objective lens will allow more light into the lens to start with. Yes, it will be reduced by a given f-stop diameter, but if the limiting factor is what's getting into the lens to start with and not the f-stop restriction, the larger diameter lens should perform better.

Now consider camera ISO sensitivity steps. If a camera can only do full steps (many will do 1/2 - 1/3 steps) or is set to do only full steps, any of these variables could cause the camera to "step" up or down in ISO if the "nominal" is near the limit of the original ISO.

So, will a "better lens" allow lower ISO perfomance for a given situation...I'd say *it could* in a particular situation/ combination....

[vandergus]

If you assume 99% transmittance...

Where exactly does this assumption come from.

[vandergus]

Not quite. Your L glass has better CONTRAST. Its not that the image taken with it is darker, but it has a better contrast range. It just looks darker.

Therefore, no difference.

When people say a lens has better contrast, they are talking about local contrast, the type of contrast that contributes to the apparent sharpness of an image, not global contrast. To my eye, the shot on the left looks lighter than the shot on the right, not less contrasty. But I suppose the images are similar enough to allow for some personal interpretation.

[sfisher]

Originally Posted by vandergus

Where exactly does this assumption come from.

From this site:

http://www.hoyafilter.com/products/hoya/hoya-02.html

I realise that most of the figures quoted here are for light reflectance rather than absorbance, but I thought 99% transmittance would be a reasonably estimate for a good muticoated piece of lens glass.

I was simply trying to estimate how much difference the extra elements would make to the light transmittance. I wasn't trying to make any precise statements.

[sk66]

Originally Posted by crunch

Like I said, the built in exposure meter said I was underexposing by one third stop, but this could be user error. I'll try again later and see.

I could see where this might actually be the case.

Consider two lenses, same focal length and aperture. One lens is "tack sharp" at that aperture, the other lens not quite so sharp. The lens having difficulty with sharpness is due to diffraction errors. The light IS getting thru, but it isn't falling where it should in the scene. (To block the stray light and fix the problem we "stop down"). The result here might be that the "overall scene" appears lighter because the "stray light" is falling across the entire scene. It would also have less clarity/contrast for the same reason.

Kind of like taking a picture with excess moisture in the air...things appear more washed out and "brighter" overall....

[crunch]

Originally Posted by sk66

Consider two lenses, same focal length and aperture. One lens is "tack sharp" at that aperture, the other lens not quite so sharp. The lens having difficulty with sharpness is due to diffraction errors. The light IS getting thru, but it isn't falling where it should in the scene. (To block the stray light and fix the problem we "stop down"). The result here might be that the "overall scene" appears lighter because the "stray light" is falling across the entire scene. It would also have less clarity/contrast for the same reason.

This is probably what's happening. However, I think we've entirely discounted the fact that a quality lens is "better" than a cheaper lens as it lets in significantly more light at the same aperture, shutter and ISO settings. This is simply not true and should answer the OP beyond any doubt.

[sk66]

Originally Posted by crunch

This is probably what's happening. However, I think we've entirely discounted the fact that a quality lens is "better" than a cheaper lens as it lets in significantly more light at the same aperture, shutter and ISO settings. This is simply not true and should answer the OP beyond any doubt.

Not necessarily...see my post above.
*If* the limiting factor is the amount of light gathered by the objective lens, and not the aperture setting, a "better" lens with a larger objective lens *should* perform better..

[vandergus]

*If* the limiting factor is the amount of light gathered by the objective lens, and not the aperture setting, a "better" lens with a larger objective lens *should* perform better..

Yes "if", but it's not. The aperture determines how much light gets through the lens.

This isn't that complicated. Everyone go back and read fletch's post for a summary of how exposure works. Crunch's test was a good example of how exposure can vary between lenses. It's not very much and it doesn't even correlate to price. That pretty much sums it up.

I'm not trying to squash the discussion, just trying to keep it moving in a forward direction.