Does anyone know how to estimate light levels

You could use your digital camera, with all of the settings done manually. Take a photo of the plants in each, with the same settings, and compare the brightness of the photos. At best you will see if one tank has considerably less light than the other.

Two identical tanks, except one is twice as deep as the other, will have different light intensities at the substrate by a factor of 2 to 4, depending on the type of bulb being used. But, your eyes won't be able to tell the difference unless the tanks are side by side, with both light fixtures having the same bulbs in them. Your eyes are a digital camera set on "auto".

That 2 to 4 difference will only be critical if the light intensity in the shallower tank is still just adequate for the plants you have. If it is ten times what is needed, both tanks will do well.

Hoppy, I knew as soon as I saw a thread about light intensity you'd be all over it

I think your advice still applies, but the question wasn't about two different tanks, it was about varying intensities within the same tank.

ray-the-pilot said:

I've noticed that in my 52 gal aquarium, plants in one part of the aquarium are doing better than the same species in another.

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The digital camera thing is a good idea, I've never thought of that.

-Ray-the-pilot, would it be possible for you to post a full tank shot to show the areas in question? also, what sort of lighting do you have set up?

Without the proper measuring equipment, which is not cheap, I can't see how you would accurately do this. Your plants are probably the best indicator of this. I have an Oceanic 50 which is lit via 2 x 55W ahs lighting. The lights are not quite as wide as the tank is, and you can see a drop off at the edges. My L. aromatica doesn't like it on the ends, so I put it in the middle. The ends are occupied by Crypt. wendtiis, and Crypt spiralis, which are not as picky about lighting.

Sorry, I read the question as referring to two tanks, not one. Within the same tank, the light intensity will vary by a huge amount, because the intensity falls off as either the square of the distance from the bulb or directly proportional to the distance from the bulb, or somewhere inbetween. Then, the plants shade the areas below them, and no light fixture can give uniform light over the whole tank surface. We don't see the differences in intensity because, as I said, our eyes are running on the "auto" setting, adjusting for intensity variations so they all look about the same.

Taking a non-flash photo of the tank with a digital camera, then adjusting the image to where only the most intensely lighted parts of the tank are bright, will show up areas that are not nearly as bright. But, this is a crude method, and interpreting the results is guesswork.