Re: Primer on photographic exposure, etc.
Re: Primer on photographic exposure, etc.
- Subject: Re: Primer on photographic exposure, etc.
- From: Thomas Lianza <email@hidden>
- Date: Wed, 22 May 2013 11:09:44 +0000
- Thread-topic: Primer on photographic exposure, etc.
Hello Ben,
The field of view, strictly speaking, has nothing to do with the
specification of angle of illumination and collection. In general terms,
the specifications of the illumination geometry for an instrument have
absolutely no relationship with an image capture system. Also, the
illumination angle is specified from the normal (90 degrees) of the
surface, so when you say "68°/0, much steeper than I've been using" you
are actually illuminating at a much lower physical angle relative to the
surface itself.
If you are using 45/0 illumination with sources such as the Einstein, the
most important factor is the distance of the luminaire from the optical
axis. This distance should be at least 1.5X greater than the maximum
field captured by the imaging system. An example using a 24mm lens, 43mm
image circle, 10x reduction. The recorded field dimension would be 430mm.
The distance from the optical axis to the edge of the field will be 215mm.
The light source should be no closer than 322.5mm and set at an equal
height to maintain 45 degrees. This assumes that you are using a source
of small physical extent. The general rule of thumb is that the source
size should as maximum represent no more than 8 degrees total extent as
viewed from the object plane. In the case of the Einstein sources you
mention, the basic diffuser is hemispherical with an extent of
approximately 50mm. To maintain the 8 degree maximum subtense
requirement, the source would need to be more than 126mm from the optical
axis, which is smaller than the 322mm mentioned earlier, so you would have
no issues pulling the source back to that distance.
In short, the illumination constraints are based upon the maximum field to
be illuminated and the maximum subtense of the source to object. If you
put a large diffuser on your Einstein, you would have to back off
proportionately to maintain the 8 degree subtense.
Regards,
Tom Lianza.
On 5/21/13 2:04 PM, "Ben Goren" <email@hidden> wrote:
>On May 21, 2013, at 10:22 AM, Ben Goren <email@hidden> wrote:
>
>> On May 21, 2013, at 9:27 AM, José Ángel Bueno García
>><email@hidden> wrote:
>>
>>> As you might know, the angle of view is not a problem but the
>>>direction of light yes.
>>
>> What it took me a while to figure out is that the two are very closely
>>related.
>
>Thinking about this further, optimum geometry for copy work is probably
>45° + (1/2 angle of view)/0. For a 50mm lens, that would be 68°/0, much
>steeper than I've been using. For 24mm, that's 87°/0 -- almost perfect
>side lighting and easily explaining my problems. For 180mm, it's 52°/0,
>close enough to 45°/0 that nobody is going to notice.
>
>Cheers,
>
>b&
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