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[OM] Re: Apertures, where is f22 ?

Subject: [OM] Re: Apertures, where is f22 ?
From: swisspace <swisspace@xxxxxxxxxxx>
Date: Wed, 08 Jun 2005 13:18:33 +0200
Hmmm I see, - I thought it would work like this

if the lens is now further away from the object but has a 10percent 
depth of field - say 1m instead of 20cm for the same view. Tthe depth of 
field for the 1m lens is 10cm and at the same apeture then 2cm for the 
20cm lens.

Thats what I like about life you learn something new everyday, 
unfortunately however it is not always advantageous to!

Cheers Ian

Moose wrote:
> swisspace wrote:
> 
> 
>>This is where I hope my investment in the 180f2 comes into it's own, the 
>> 180F2 is also a macro lens well it goes to 1:2 . now on the E-1 it 
>>becomes a 360f2 macro and with the 1.4x teleconvertor a 500f2.8 macro.
>>
>>So if my theory is correct you should get a deeper depth of field for a 
>>given subject, although after just writing this down I am not so 
>>convinced this will be the case, First thing I will be trying though, 
>>when my adapter arrives.
>> 
>>
> 
> Sorry, the same subject area, shot with the same actual focal length and 
> aperture and displayed and viewed at the same size, will have the same DOF.
> 
> We tend to talk about a doubling of effective focal length, because 
> that's an easy way to think about it and deal with it in our heads. 
> However, it is really a halving of the field of view at the same focal 
> length, because of the smaller sensor. So, to cover the same subject 
> area, one must be twice as far away (which does change perspective). 
> However, to get the same display size, the original frame must be 
> magnified twice as much, returning DOF to what it was with 35mm.
> 
> Macro is also different. Our thoughts about what a particular repro 
> ratio means are not really based on the repro ratio, but on the 
> magnified, displayed image. A lens which gives a 1:2 ratio does so 
> regardless of the sensor size, as the ratio is simply between the linear 
> dimension of the subject and that of the image it projects on the focal 
> plane. However, with a smaller sensor, the image fills the sensor at a 
> lower repro ratio.
> 
> If a lens makes a 1:2 image of a 10mm high object on 35mm film, the 
> image is 5mm high on the film. If the image is printed at 4x (roughly a 
> 4x6 print), the final image of the subject will be 20mm high. If the 
> same subject, lens and settings are used on a half size sensor, the 
> subject image is still 5mm high on the sensor.  When it is enlarged to a 
> 4 in. high print, that is 8x and the image of the subject will be wice 
> as high, 40mm high. Thus, a 1:2 repro ratio setting on a macro lens 
> "acts like" a 1:1 setting using 35mm film when photographed on a "2x" 
> sensor. However, since all lens and distance settings are unchanged, but 
> magnification is doubled, DOF decreases.
> 
> A number of shortcuts in thinking that work fine as long as film/sensor 
> size is constant can lead to eroneous conclusionn when format changes. 
> The folks who shoot multiple formats already know this. Like all 
> photographic parameters, there are trade-offs. One thing gets "better" 
> and another gets "worse". No free lunches.
> 
> Moose
> 
> 
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