This may lose me the respect of my peers and all right-thinking people, but I have managed to take pictures for over 50 years without any knowledge whatsoever of Fred Parker. Now that I have heard of him, and have skimmed through the so-called exposure guide on his website, he appears to me to be the most spectacular obsessive and obfuscator I have ever encountered, with an argument whose main thrust seems to be that all exposure meter makers are idiots and that the only true way to work out exposure is with his voluminous tables.
People, you do not need any of this. It will not help you take better pictures, it will not give you any genuine insight into sensitometry or image formation.
To repeat very briefly:
An exposure meter measures light intensity, It can be (and many meters historically have been) calibrated in random figures. It is, however, also possible to calibrate in EV figures, which are the shutter speed/stop combinations which a film would need if:
1) It was of ISO 100 speed
2) You had chosen to actually rate the film at this speed
3) The meter was aimed at an 18% gray area.
If 1), 2) and 3) apply, and your camera shutter is calibrated in EV, you could apply the meter reading directly to the camera and get a good exposure. If not, you need to apply a correction (actually an offset) for the actual film speed you are using and the actual zone you are metering and extract a reading in the form of f/stops and shutter speeds. Some meters have dials which do all of this (such as the Lunasix I once owned), all others allow different film speeds to be set but may require you to mentally apply zone compensation. For example, if you meter an area and want it to be zone 3 (instead of the default 18% gray or zone 5), you increase the EV number by 2 (i.e. cut the exposure by 2 stops) and transfer say EV 15 to the meter scale instead of EV 13 (as I do with my Pentax Spotmeter or my Gossen Digiflash, for example).
Let those who have a passion for sensitometry and arcane formulae go into this subject as deeply as they like - from the point of view of practical photography, I guarantee you will be none the wiser afterwards and that what I have written above and in my previous posting is ALL YOU WILL EVER NEED TO KNOW!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
No, the difference in the meaning of EV will not affect the quality of anyone's photographs, nor does it affect the value of the advice Fred gives about scene brightness.
It would be very easy to change his method to be entirely in line with the original APEX method. What he calls EV could be called BV, with the subtraction of five - so what he calls 'EV 0' would become 'Bv -5', for example. The text could also be re-written with the discrepancies removed.
"Can the information be corrected and posted in a format as simple as Fred's?"
Yes, easily. I'd be happy to draft suggested changes if it wouldn't upset anyone.
Best,
Helen
Helen: Thanks for clarifying that. I am glad I don't have to change anything drastically.
I have a concern though: this morning I looked at two old cameras that have EV scales on the shutters. If I set the shutter at f16 and 1/100 sec (the sunny 16 rule), the pointer indicates an EV value of 15, just like the EV chart that I referenced shows.
I am sure your calculations are entirely accurate. But do you think another standard was adopted along the way for consumers?
David: I agree somewhat with your assessment of the Parker site. Although the EV Table B shows in chart form what you stated in your previous post. People learn differently, and presenting the relationship in a graphical format may help some newcomers grasp the relationships easier.
I do see the table as a little crowded too. That is why I copy them and delete a lot of extra stuff (like 800 speed film and EV numbers below say, 12). I use it, along with my light meter, as a starting point for determining the exposure I want.
Helen: Thanks for clarifying that. I am glad I don't have to change anything drastically.
I have a concern though: this morning I looked at two old cameras that have EV scales on the shutters. If I set the shutter at f16 and 1/100 sec (the sunny 16 rule), the pointer indicates an EV value of 15, just like the EV chart that I referenced shows.
I am sure your calculations are entirely accurate. But do you think another standard was adopted along the way for consumers?
The example you give is only half of the equation. You're missing the fact that you must balance the equation...
As Helen has very plainly stated twice now, the equation is:
Ev = Tv + Av = Sv + Bv
look carefully, Your example is: 1/100s, f/16. Shutter speed and f-number. These correspond to the symbols Tv and Av respectively. If you look up (or calculate) the Tv for 1/100sec you find that it is: Tv=7, similarily, for f/16, Av=8 so, Tv + Av = 7 + 8 = 15. Now, to be useful, this must be equal to the other side of the equation...so, we require that
15 = Sv + Bv
Sv is fixed by the film speed. For ASA 100 speed film, Sv = 5. Thus, for this situation to produce something like properly exposed negative, one would need to be in light with brightness value, Bv of 10. In which case you would have:
Ev = Tv + Av = Sv + Bv
15 = 7 + 8 = 5 + 10
If your film speed were say, ASA 400 (which corresponds to Sv = 7), then you could shoot in less bright light at the same shutter speed and aperature.
to wit...
Ev = Tv + Av = Sv + Bv
15 = 7 + 8 = 7 + 8
The Sv for ASA400 film is 7.
I am sure that you will find that Fred Parker has made some simplifying assumptions in producing his "guide". I think thatyou may find that he has adjusted everything for ASA 100 film speed?
__________________ It's just the normal noises in here...
Helen:...
I have a concern though: this morning I looked at two old cameras that have EV scales on the shutters. If I set the shutter at f16 and 1/100 sec (the sunny 16 rule), the pointer indicates an EV value of 15, just like the EV chart that I referenced shows.
I am sure your calculations are entirely accurate. But do you think another standard was adopted along the way for consumers?
...
(Brad has already answered this well, so this is just another way of saying it.)
No, it isn't another standard, it is a misuse of the existing one, perhaps with the intent of simplicity (mistaken in my view, because the use of BV is as simple*). f/16 and 1/125 is indeed EV 15, but it is EV 15 at any film speed, not just ISO 100. On FP's chart f/16 and 1/125 can be between EV 17 and 10, depending on film speed. That is incorrect. f/16 and 1/125 is EV 15. It is not any other exposure value.
What Fred Parker calls EV is actually a scene brightness value. It is numerically equal to EV at ISO 100, but it is not EV plain and simple.
If you scroll down and look at his illuminance table (which is correctly labelled 'EV at ISO 100' rather than just 'EV') you can quickly deduce that he uses an average scene reflectivity of 13.5% to generate the table.
*Kodak avoid this altogether by using letters to designate scene brightness, such as in their exposure calculator dial in the Pro. Photoguide - to which FP's table bears a strong similarity.
Best,
Helen
__________________
Some of my snaps are here and here.
One thing about APUG, it has me crawling about in the dusty recesses of the lumber room of my mind!
Notwithstanding that I am one of the world's worst mathematicians, and that the people who were charged with teaching me logarithms and such like were eventually led away crying, I appear to have discovered the following:
The equations "Ev = Tv + Av = Sv + Bv" etc. which have been much quoted on this thread appear in my Ilford Manual of Photography 6th edition and are stated to have formed part of the APEX (Additive Photographic Exposure) system, which as was mentioned elsewhere was described in the relevant ASA standard of 1960 governing film speed and exposure. Crucially, as I understand it, and "understand" may not be the right word, for the equations to work all the parameters have to be expressed as logarithms to base 2. No doubt there are appropriate tables of logs to base 2 somewhere (as opposed to the usual logs to base 10), and when dentistry without anesthetic carried out by a gorilla loses its appeal, I may well turn to reading them, but I feel the fact remains that APEX was a system which, while sound in principle, virtually no one felt was useful. The good Fred Parker may have thought it a good idea to revive the APEX system, but I can see no earthly reason, as I said before, for any else to join him!
Regards,
David
Sponsored Ad. (Subscribers to APUG
have the option to remove this ad.)
I normally shoot 100 speed film so I guess it has not been an issue.
It seems like when I switch to 400 I could make a new chart. It does not seem like it would be too much trouble to generate it in Excel and print it out.
One thing about APUG, it has me crawling about in the dusty recesses of the lumber room of my mind!
Notwithstanding that I am one of the world's worst mathematicians, and that the people who were charged with teaching me logarithms and such like were eventually led away crying, I appear to have discovered the following:
The equations "Ev = Tv + Av = Sv + Bv" etc. which have been much quoted on this thread appear in my Ilford Manual of Photography 6th edition and are stated to have formed part of the APEX (Additive Photographic Exposure) system, which as was mentioned elsewhere was described in the relevant ASA standard of 1960 governing film speed and exposure. Crucially, as I understand it, and "understand" may not be the right word, for the equations to work all the parameters have to be expressed as logarithms to base 2. No doubt there are appropriate tables of logs to base 2 somewhere (as opposed to the usual logs to base 10), and when dentistry without anesthetic carried out by a gorilla loses its appeal, I may well turn to reading them, but I feel the fact remains that APEX was a system which, while sound in principle, virtually no one felt was useful. The good Fred Parker may have thought it a good idea to revive the APEX system, but I can see no earthly reason, as I said before, for any else to join him!
Regards,
David
Yes David, that is the system I have been describing here, and I hope that I have given all the necessary formulae. I'm sorry to have to tell you this, but your understanding of the maths is deficient: log2 tables are not required, and never were. You can use log10 tables, but you don't have to resort to such advanced mathematics with all the cruel memories of having to sit through maths lessons when you were young. I hated them. The ability to halve and double, add and subtract will see you through, however.
Whether or not the APEX system is of use to any individual is not for me to judge. The simple theory behind it (and it is very simple) comes in handy for me now and then. The rest of the time it does no harm.
Best,
Helen
__________________
Some of my snaps are here and here.
And yes, David B., is right. The creativity is what we all must have as photographers to see our subject and to project our mind's image through our work. The craft, that is the technical side of things, only allow us to better get to the point of taking the picture by allowing us to bypass the rote and rite of the how tos and getting us to the task of pressing the shutter button quicker, or, should I say, more confidently. We practice more and know our equipment better so that it does not get in the way of a great photograph.
First of all, I want to say that this has been one of the most interesting discussions I've seen in a long time. It's really cleared up a lot of confusion, for me at least.
You often see, on the web, EV numbers attached to film speed. From what I'm gathering from this, true EV is exposure only, not related at all to film speed. (Therefore, much info on the web is either patently incorrect or based on misunderstanding, or a poor job of explaining.)
Quote:
Originally Posted by Helen B
2^Av = N^2 (N is f-Number)
2^Tv = 1/T (T in seconds)
2^Sv = S/π (S is ASA film speed, now ISO)
2^Bv = Bl (Bl in foot-lamberts) = B/π (B in candles per square foot)
Ok, not to get too geeky here, but the way I see this, Av, Tv, etc., would be base 2 logarithms. Is this correct? If so, it makes a lot of sense how they are added to get the final EV numbers.
First of all, I want to say that this has been one of the most interesting discussions I've seen in a long time. It's really cleared up a lot of confusion, for me at least.
The possibilities for confusion are endless! When I was writing my previous posts, I double-checked every statement against a Pentax digital spotmeter, a Gossen Digiflash meter and a Rolleiflex T camera with a meter and an EV shutter. Each time, I confirmed that EV 1 = 1 sec. at f1.4 with ISO 100 film. Just now, I picked up an old Weston Master V meter from my desk, set the dials to 100 ASA and I sec. at f1.4 and hey presto! - it's showing EV 1 too BUT the meter has a larger central part to its dial which is labelled LIGHT and has numbers from 1 to 16 which are offset by 1.3 from actual EV values (which appear on this meter in a very small window), so that for example "LIGHT 1" is EV 2.3 (which is what I was talking about earlier when I said meters could be calibrated in arbitrary figures). A brilliant piece of needless confusion on the part of Weston (except that possibly it was intended for the round numbers on the "LIGHT" scale to correspond to round numbers of lux - the "1" position would be 2 lux). I'm going to lie down now.
Linear Mode
