Talk:Exposure (photography)

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1 Total amount of light
2 Double exposure
3 Latitude
4 Error in EV interpretation
5 Removed Link

[edit] Total amount of light

What do you mean by "the total amount of light"? --- Miguel April 12th 2006.

The article on light value explains this a bit better. In a nutshell, you got an adjustable amount of light reaching the sensor while the shutter stays open for an adjustable period of time. "Total amount of light" refers to the exposure value, which is the product of the aperture value (which determines how much light is transmitted to the sensor) and exposure duration (except that everything is usually measured on a logarithmic scale, so EV is actually expressed as a sum). Depending on the application, exposure may be specified in foot-candle seconds. --MarkSweep (call me collect) 02:58, 13 April 2006 (UTC)

To get a total amount of light in the focal plane, you need more than EV. You also need to know the luminance of what you're pointing it at. The net result is usually measured as lux seconds (in SI units), the product of focal-plane illuminance times the exposure time. It is straigtforward to calculate a focal-plane illuminance from an f-number:

$E = {\pi L \over 4 N^2}$

where E is focal-plane illuminance in lux, L is scene luminance in candela per square metre (cd/m2), and N is the f-number (assuming unity transmittance, for which f-numbers are usually adjusted anyway).Eq. (6) The result is not really a "total amount", but an amount wavelength-weighted by the luminosity function to model how bright that amount appears to human eyes. Multiply E by exposure time to get total exposure in lux seconds. Dicklyon 22:38, 2 January 2007 (UTC)

[edit] Double exposure

double exposure is when u take one image make a image over it. but u can't over expose it.

[edit] Latitude

Modified a number of areas, in particular info that was not correct on latitude (film/digital/slide etc), correct exposure (there is none! etc). Latitude seems to have been mistaken with "dynamic range".... More work needed here....any thoughts feel free. —The preceding unsigned comment was added by Barryfitzgerald (talk • contribs).

I added a header and reformatted your note a bit; hope you don't mind. Dicklyon 21:37, 2 January 2007 (UTC)

I'm wondering what's behind some of your latitude statements. Do you have sources? In particular, digital at high ISO doesn't have less latitude, it just moves the latitude from the shadow end to the highlight end (at least in cameras that I'm familiar with that capture RAW data independent of the ISO setting). Dicklyon 22:14, 2 January 2007 (UTC)

My experience suggests that at say ISO 100 you can get over 2 stops worth of info back, at ISO 1600 that trashes the image quality...feel free to tart up the text...just put it down quickly. Dpreview supports this in their reviews also.. example:

http://www.dpreview.com/reviews/nikond40/page18.asp

Note reduction in dynamic range etc etc.

Yes, but it's too bad they didn't combine the RAW and high-ISO tests to see what the real highlight range is. These guys are pretty good at camera phenomenology, but don't really know enough about the science of it to design good tests and reports sometimes. Dicklyon 23:06, 2 January 2007 (UTC)

With latitude of film/slide/digital...well it is well known film has good highlight, poor shadow..digital is the reverse...in many ways..and not at all like slide which has little lattitude either side in general.

Yes, but since digital can be underexposed, equivalent to shooting at higher ISO, you can get back a more symmetric range, more like negative film; you need a decent raw file and raw conversion program to take advantage of it, though; as shown by dpreview, Nikon tends to hard-clip their highlights, while Canon does a nicer film-like shoulder in their rendering.

Let's be careful about repeating impressions and folk wisdom, and go for verifiable facts instead. Dicklyon 23:06, 2 January 2007 (UTC)

[edit] Error in EV interpretation

I fixed the lead where it said higher EV meant MORE total exposure. Actually, if all you vary is EV, that's backwards, since higher EV means faster shutter or smaller aperture, which gives you LESS exposure. This is a common confusion, since a higher EV is apppropriate when shooting a brighter (higher luminance) scene. See exposure value.

How about a formula for total exposure from EV and scene luminance? Using the formula above for focal-plane illuminance and multiplying by exposure time t we have total exposure:

$E t = {\pi L t \over 4 N^2}$

But since ${t \over N^2} = 2^{-\mathrm{EV}}$ , we can write the total exposure as:

$E t = {2^{-\mathrm{EV}} \pi L \over 4}$ .

That's the total lux seconds, for L in candela per square meter. Dicklyon 22:58, 2 January 2007 (UTC)

[edit] Removed Link

Why was the link to this page removed: http://wamp.co.uk/index.php?page=grad ? It nicely shows how exposure varies when ND filters are used.--Tiberius47 02:37, 13 January 2007 (UTC)

Better question might be why was this page added. The same guy added this page to several other articles recently. It's a whole page to help you divide by 2 a few times, and it's on a commericial selling site. Shouldn't we just remove all those? Dicklyon 05:54, 13 January 2007 (UTC)

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