Archived posting to the Leica Users Group, 2004/02/16

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Subject: digital lenses was: Re: [Leica] Digital Bessa RF
From: Jonathan Borden <jonathan@openhealth.org>
Date: Mon, 16 Feb 2004 16:06:53 -0500
References: <2713435.1076929221244.JavaMail.root@127.0.0.1> <1076958087.7087.9.camel@creo_pc3>

Feli di Giorgio wrote:

>On Mon, 2004-02-16 at 03:00, frank.dernie@btinternet.com wrote:
>  
>
>>The problem is, Jim, that we do not need film dimension to match film quality,
>> digital sensors about 1/4 the size of Minox film match 35mm P&S quality. We
>> only need a full size sensor to utilise the full angle of view of our existing
>> 35mm design lenses
>>    
>>
It is a mistake to assume that an optimal 35 mm lens (e.g. "M" series) 
will be optimal for a smaller imaging area. Consider state of the art LF 
lenses e.g. the Schneider Symmar XL 150mm f5.6 which covers 8x10" -- I 
don't see people clamoring to use this in 35mm -- indeed it would be a 
rather mediocre lens even though its sweet central spot covers > the 
35mm film area.

The issue is that optimizing a lens for 8x10 is much different than 
optimizing a lens for 35mm or a smaller area.

>Frank
>
>That may be true, but I really detest the increase in DOF you get with
>the 1.5 multiplication factor.
>
>  
>
I believe the actual DOF is related to the F stop and focal length -- 
you mean the *apparent* increase in DOF. The way to correct for that 
would be to use a wider F stop. Indeed for a lens optimized to a smaller 
imaging area you should be able to use a wider maximal F stop. All else 
being equal such a lens should also have a *high* resolving power in lpmm.

The question has arisen as to how many pixels are needed to approximate 
35mm film. Assume 75 lpmm that would be approximately 2000 lines x 
2000lines or at least 2 pixels/line = 16 megapixels. Now if your lens is 
not good, or your focus is off or your hands shake etc, you need less 
pixels -- and also remember that there isn't necessarily a 1:2 
correlation between lines and pixels (for example it is widely 
acknowledged that a digital CD recording at 44 khz cannot truly 
reproduce an analog recording even though the human ear cannot hear > 20 
khz) i.e. you probably should have more pixels to capture all the 
information.

In any case this is a back of the napkin approximation that tells us 
that a good 5 megapixel sensor is not enough to equal good 35mm film 
given a high quality lens. The 75 LPmm is for Provia F 100 slide film -- 
for Tech Pan and on a tripod, you are going to need many more megapixels.

Jonathan
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In reply to: Message from frank.dernie@btinternet.com (Re: [Leica] Digital Bessa RF)
Message from Feli di Giorgio <feli@creocollective.com> (Re: [Leica] Digital Bessa RF)