Re: Primer on photographic exposure, etc.
Re: Primer on photographic exposure, etc.
- Subject: Re: Primer on photographic exposure, etc.
- From: Ben Goren <email@hidden>
- Date: Wed, 22 May 2013 15:02:17 -0700
Tom,
I was having trouble visualizing what you described, so I hacked it all together in Illustrator at actual size with precise measurements. What follows is just an export to the Web, so it's still proportional. Assuming it makes it through to the list okay (and I'll post it somewhere if not), on the left side of the diagram is my interpretation of the dimensions you specified:
As you can see, the camera is directly on the reciprocal of the incident angle from about the outer quarter of the subject...and it's exactly that portion of the frame where I was seeing huge specular hotspots when I used the 24mm lens.
If you eyeball it, a 50mm lens would have about a 200mm field of view at that distance -- a bit less than half of the 430mm of the 24mm lens. That would put the specular hotspot right on the edge, probably just outside of the field of view. But, anything that wasn't perfectly flat -- such as the rounded edge of a thick gob of paint -- would have some angles that would throw small specular highlights.
Changing the distance of the light source but keeping it on the 45° axis is just going to change the apparent size of the specular hotspot, but it'll still be there. Changing the distance from the camera to the subject will change the subject magnification and the absolute position and magnification of the specular hotspot on the subject, but it won't change the relative location of the hotspot within the frame.
What *does* (or, at least, *should*) work is to move the light at an angle farther from the camera axis and closer to the plane of the subject. On the right side of the diagram you can see that the specular reflection from the even the outermost portion of the frame misses the camera entirely. You can also see that my earlier gut instinct of 45° + 1/2 FOV was overly aggressive, perhaps...but there's still the question of raised edges that I need to empirically experiment with.
So, that's what I had been struggling with when I tried to pop on a wide-angle lens to get a low-resolution image of a larger work without having to increase the camera-to-subject distance. Now I not only know better...I know why, and perhaps how to deal with it.
I should again hasten to add: I haven't actually experimented with this geometry in the studio, so I don't know what other unexpected problems I might run into. One of my biggest concerns is going to be how much results will differ from what the i1 Pro sees with its 45°/0 geometry...I don't have an intuitive grasp of what's going to happen, so I'll have to try it out.
Cheers,
b&
On May 22, 2013, at 4:09 AM, Thomas Lianza <email@hidden> wrote:
> 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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