[Leica] RD-1 users: Light loss with fast lenses?

[sethrosner at nycap.rr.com (Seth Rosner)]

Henning, I had completely forgotten this. I have a vague recollection that 
many years ago there was a Taylor-Hobson-Cooke lens calibrated and marked in 
T-stops. Perhaps more than one.

Seth
----- Original Message ----- 
From: "Henning Wulff" <henningw@archiphoto.com>
To: "Leica Users Group" <lug@leica-users.org>
Sent: Friday, March 11, 2005 11:42 AM
Subject: Re: [Leica] RD-1 users: Light loss with fast lenses?


> At 8:50 PM -0800 3/10/05, Peter Klein wrote:
>>Folks: I've noticed an interesting phenomenon with my E-1 and fast OM 
>>Zuiko lenses, and I'm curious if something similar happens with RF lenses 
>>on the RD-1.
>>
>>The widest apertures don't give me as much light on the E-1 as they do on 
>>film. My 50/1.4 is more like a 50/1.8 wide open.  My 50/1.8 is more like a 
>>50/2.
>>
>>Note that I'm not talking about metering error here, nor am I talking 
>>about vignetting at the edges.  I'm talking about using my 50/1.4 to take 
>>a bunch of bracketed shots of a blank wall with manual exposure. If I get 
>>a pixel level of 128 near the image center at 1/30 at f/2.8, then I would 
>>expect to get the same level at 1/125 at f/1.4.  But I don't.  I need to 
>>slow the shutter to 1/80 to get the same shade of grey.  This is 2/3 of a 
>>stop more exposure than expected.  The same lens shows less than 1/3 stop 
>>loss with film.
>>
>>I'm curious if the RD-1 has a similar effect with f/2 and especially f/1.4 
>>lenses.  Could some of you RD-1 owners who have Summiluxes and Noktons 
>>check this out and let us know?
>>
>>I'm sure all this has to do with sensor angle of acceptance vs. lens 
>>characteristics like exit pupil size and angle of the cone of light. There 
>>has been some talk of this on digicam forums, with the usual 
>>indistinguishable combination of heat and light.  I personally believe 
>>that something is indeed going on.  I've seen too many examples of weird 
>>DSLR behavior with film lenses at maximum and minimum apertures.
>>
>>DSLR owners who have used the same fast lens on both a DSLR and film body, 
>>feel free to chime in, too.
>>
>>Thanks!
>>--Peter
>
> Good to hear you're feeling better!
>
> Happens on film, too. If you ever try to be systematic on film, and 
> measure your exposures with a densitometer, you'll note the same thing. 
> The suspicion might come up that the manufacturers have inflated their 
> maximum f-stop numbers for the sake of marketing, but due to the less than 
> optimal diameters of lens elements, especially fast ones (we're neither 
> willing to pay for nor carry lenses with optimal sizes) there is a 
> lessening of effective, or T-stop for very fast lenses. An f/1.4 lens will 
> still be faster than an f/1.8, but neither actually produces twice as much 
> light on the film as the same lens stopped down to f/2.8 resp. 3.5. Note 
> that an f/1.4 lens stopped down to f/2 will produce a denser neg than an 
> f/2 lens wide open; but you are carrying a much larger and more expensive 
> 'f/2' lens in the f/1.4 stopped down.
>
> The aperture marked on the lens is a geometric aperture, not a 
> transmission aperture, from whence comes the 'T-stop'.
> ---------------------------------------------------
>
> I should add that older lenses especially, and then rangefinder lenses on 
> the R-D1 in particular, will have more fall off at the corners as 
> discussed here and at various other places, _and_ will also have somewhat 
> lower exposure levels at the center with fast lenses due to the edge rays 
> necessarily striking the center more obliquely when the lens is used wide 
> open, and thus exhibit some of the same 'vignetting' at the center due to 
> non-perpendicular rays.
>
> It might be that the E-1 is more sensitive to this as it was designed with 
> 'digitally optimized' lenses in mind, and less compromised for the sake of 
> older lenses which did not have their ray bundles as perpendicular to the 
> sensor array as the E-1's own lenses.
>
> This isn't very noticeable on my 20D and might not be that noticeable on 
> the R-D1 as the latter is definitely intended for 'non-optimized' lenses.
>
> -- 
>    *            Henning J. Wulff
>   /|\      Wulff Photography & Design
>  /###\   mailto:henningw@archiphoto.com
>  |[ ]|     http://www.archiphoto.com
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