The Foundation of Exposure (Page 2): Thanks for that!

jerryc41 Loc: Catskill Mts of NY

selmslie wrote:

Here are the two files you can upload:

Thanks for that!

joel.photography

I was a physics major,so I can handle this math. But how does it help you take better pictures in the real world?

Heather Iles Loc: UK, Somerset

joel.photography wrote:

I was a physics major,so I can handle this math. But how does it help you take better pictures in the real world?

Yes, I am puzzled as I don't understand physics.

BboH Loc: s of 2/21, Ellicott City, MD

Down loaded your Excel spreadsheet - it consisted of 5 row and 5 columns - is that the nature of it?

aellman Loc: Boston MA

selmslie wrote:

This subject has been covered from other angles but the following description might help make the subject clearer. I have summarized this from a PDF that I posted on my web site. The PDF and associated Excel spreadsheet in my next post.

To record a gray card as gray you need to balance ISO with exposure value (EV). EV is a combination of aperture and shutter speed so together ISO, aperture and shutter speed make up the exposure triangle.

The fundamental assumption underlying EV is that an exposure of 1 second at f/1 equates to an EV of 0.

An exposure of one second is given a value of 0. Each doubling of the shutter speed increases the value by one:

1 sec	1/2	1/4	1/8	1/15	1/30	1/60	1/125	1/250	1/500
0	1	2	3	4	5	6	7	8	9

An aperture of f/1 is given a value of 0. Each full stop of reduced aperture increases the value by one:

f/1	f/1.4	f/2	f/2.8	f/4	f/5.6	f/8	f/11	f/16	f/22
0	1	2	3	4	5	6	7	8	9

The shutter speed and aperture values are added together to get the exposure value (EV). You can see how aperture and shutter speed are combined as EV in this table from the back of a Rolleiflex TLR:

F:	2.8	3.5	4	5.6	8	11	16	22
1 sec	3	3.5	4	5	6	7	8	9
/2	4	4.5	5	6	7	8	9	10
/4	5	5.5	6	7	8	9	10	11
/8	6	6.5	7	8	9	10	11	12
/15	7	7.5	8	9	10	11	12	13
/30	8	8.5	9	10	11	12	13	14
/60	9	9.5	10	11	12	13	14	15
125	10	10.5	11	12	13	14	15	16
250	11	11,5	12	13	14	15	16	17
500	12	12.5	13	14	15	16	17	18

You can expose with an EV of 10 at several offsetting shutter speed and aperture combinations. In some film camera lenses (like Hasselblad) EV is mechanically linked so that you can change both shutter speed and aperture while keeping the EV constant.

ISO can also be expressed it as a logarithmic series:

25	50	100	200	400	800	1600	3200	6400	12800
2	1	0	-1	-2	-3	-4	-5	-6	-7

When everything is combined:

1/500 sec      9
f/16           8
EV (subtotal) 17
ISO 400       -2
Total         15

Recommended Exposure Values

You will often see published EV tables based on ISO 100. So long as you adjust for ISO, these can be used as a reality check or for actually getting a starting exposure in unfamiliar situations. For example:

EV	Lighting Conditions*
Light sand or snow in full or slightly hazy sunlight (distinct shadows)
Typical scene in full or slightly hazy sunlight (distinct shadows)
        Rainbows: Clear sky background
Typical scene in hazy sunlight (soft shadows)
        Side lighting in full sunlight
        Rainbows: Cloudy sky background
Typical scene, cloudy bright (no shadows)
        Back lighting in full sunlight
Typical scene, heavy overcast, Areas in open shade, clear sunlight
        Sunset
Subjects in deep shade
Neon lights at night
Night sports, ice shows
Night street scenes, window displays, circuses
Fairs and amusement parks at night
Home interiors
Christmas tree lights outdoors, night vehicle traffic, subjects lit by bonfires
Floodlit buildings, monuments and fountains
Fireworks
Distant view of lighted buildings

* Taken from the Wikipedia article on Exposure value where you can find additional information.

This subject has been covered from other angles bu... (show quote)

I feel seasick.

Jerry G Loc: Waterford, Michigan and Florida

Very interesting article. I have been taking pictures for over 50 years and finally understand EV. In the past I have never bothered to find out what EV was, it was just a useless number on my light meter. Now that I know that it is a logrithmic value for light intensity it will remain a useless number on my light meter. I am not saying it isn't important, but I have never seen a light meter that only gives the ev. It's like using a computer, we don't need to understand code to know how to use photoshop. Again I not saying it's not important, it's just with modern cameras or fifty year old light meters it's more than we need to produce quality photos.

BebuLamar

Stan W. wrote:

Huh? What?

In the chart it listed 0.125 Cd/m^2 as EV 0 @ ISO 100. That is correct for meter calibrated to a K factor of 12.5. This K factor is used by Sekonic (it's listed in the specification) and I heard also used by Canon and Nikon for their built in meter but I saw no official listing as such but I think my Nikon's are calibrated to that by testing. With Pentax spotmeter and Minolta they used a K factor of 14 for their meters (also listed in their specification) so when the meter indicates 0 EV @ ISO 100 it means 0.14 Cd/m^2. I use this value as I have Minolta meter.

The value listed for Illuminance related to an incident meter using a flat receptor (or with the dome retract in the case of Sekonic) and in this case Sekonic and Minolta both use a C factor of 250 which gives 2.5 Lux for EV 0 @ ISO 100.

selmslie Loc: Fernandina Beach, FL, USA

joel.photography wrote:

I was a physics major,so I can handle this math. But how does it help you take better pictures in the real world?

If you already understand what is going on it probably won't help you. The thread might help others who are just beginning to understand exposure.

Heather Iles wrote:

Yes, I am puzzled as I don't understand physics.

The point is that we think in logarithmic terms whether we realize it or not. Adding and subtracting logarithms is the same as multiplying and dividing raw numbers.

Shutter speeds of 1/60, 1/125, 1/250, 1/500 are each separated by one whole stop - a factor of two. In logarithmic terms (base two log), they are each separated by one unit.

The same is true for the sequence of ISO values 100, 200, 400, 800 ...

It is also true for aperture steps of f/2, f/2.8, f/4, f/5.6, f/8 ... - one stop imcrements. The complication is that aperture numbers are a linear measurement so numeric the values are separated by the square root of two or about 1.4.

But if you think of these separations as logarithms - as your eyes, sensor or film respond to them - the steps are easily combined. You no longer need to think in terms of fractions and square roots.

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

It's not really all that complicated. If you shoot in manual, you will learn in a month or 2 how 1 setting affects the others. Those of us who started with fully manual film cameras, learned quickly, as it got very expensive, to ruin pictures with wrong exposure settings. I learned the basics of exposure without ever hearing "the exposure triangle". beginners, today. don't have to worry bout the expense of a ruined roll of 36 exposure Kodochromes. I for one don't worry about making mistakes with my pictures, The vast over whelming number of my photos are mistakes but nobody but me sees them.

selmslie Loc: Fernandina Beach, FL, USA

BboH wrote:

Down loaded your Excel spreadsheet - it consisted of 5 row and 5 columns - is that the nature of it?

That's all there is to it.

Feel free to unlock the sheet and examine the Excel formulas. The PDF explains what's behind them.

yssirk123 Loc: New Jersey

Thanks!

BebuLamar

selmslie wrote:

The point is that we think in logarithmic terms wh... (show quote)

Also if you memorize the Aperture, Shutter speed and ISO values in the chart for example f/5.6=5, f/16=8, 1/500=9, 1/125=7, ISO 200=-2, ISO 1600=-4 then you can figure out the equivalent exposure in your head. It may seem difficult to memorize them at first but after a while it's quite easy. And then you can go on to the 1/3 stop values.

Heather Iles Loc: UK, Somerset

selmslie wrote:

The point is that we think in logarithmic terms wh... (show quote)

Thanks selmslie for your response. You have the patience of a Saint and one day I may get my head around these figures. Did anyone say that photography was easy?

Heather Iles Loc: UK, Somerset

BebuLamar wrote:

Also if you memorize the Aperture, Shutter speed and ISO values in the chart for example f/5.6=5, f/16=8, 1/500=9, 1/125=7, ISO 200=-2, ISO 1600=-4 then you can figure out the equivalent exposure in your head. It may seem difficult to memorize them at first but after a while it's quite easy. And then you can go on to the 1/3 stop values.

Thank you. Now this is easier and I will try to memorize it and have made a note of it.

As they say, there is more than one way of "skinning a cat".

selmslie Loc: Fernandina Beach, FL, USA

Jerry G wrote:

... I am not saying it isn't important, but I have never seen a light meter that only gives the ev. ...

I have several that give me EV directly:
- the Pentax digital spotmeter displays EV to the nearest 1/3 stop and uses a couple of rotating rings to adjust for ISO, shutter speed and aperture
- the Minolta digital spotmeter provides either EV to the nearest 1/10 stop or shutter speed/aperture readouts after I select the ISO
- the Gossen Luna Pro digital incident/reflected meter provides either EV to the nearest 1/10 stop or shutter speed/aperture readouts after I select the ISO
- the Gossen Digisix 2 incident/reflected meter displays EV to the nearest 1/3 stop after setting the ISO and uses rings to select the shutter speed/aperture

Other meters offer an EV readout or just provide a way to balance the ISO, shutter speed and aperture.

If the meter uses a rotating dial, that dial is a slide rule calculator. Slide rules are based on logarithms.