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Exposure problems

Stefan Steib

Active member
Paul
as a User of Lightmeters for years also I think you simply run into the fact that about all MF Backs are a bit "cheating" on sensitivity. This was confirmed by DXO see here - depending on your back and compared to which camera this may be as much as 1,5 Stops.
See the graph I downloaded from DXO.
This is not really a problem most of the time when you just use Histograms -
see Siebel (and as we all do this now!) and other comments :

" I sold all my meters and have never looked back. Shoot a test frame, check the histogram and adjust from there. This is WAY faster and more accurate than any meter."

It is accurate to achieve best results, but it is NOT accurate in terms of absolute Values.......

Greetings from Munich
Stefan
 

MGrayson

Subscriber and Workshop Member
I've often wondered what DxO were going on about, so I checked their website. They define ISO not relative to some fixed optical density, as there is no such thing on a sensor, but relative to total saturation. This has the odd effect of lowering the "measured ISO" when a sensor increases its headroom. Or equivalently, I could double the measured ISO of my sensor by clipping the signal when the sensor wells are half full.

In that light, the often reproduced chart of manufacturers "cheating" shows, instead, manufacturers offering extended highlight retention.

Matt
 

Stefan Steib

Active member
Hi Matt

I think I know what you mean, if you take a look at tonal range and on the Signal to Noise level all 3 cameras are on par again.
But the dynamic range shows the same discrepancy, which I cannot explain with your Theory !

Regards
Stefan
 

MGrayson

Subscriber and Workshop Member
I have no horse in this race, I'm just trying to understand what DxO's numbers mean. I suppose I should now look at how they define dynamic range, which depends on shadow noise, which can be measured per-pixel, per square mm, or per-image percentage. Each choice will favor a different sensor. You really can make these charts say (almost) whatever you want by a judicious choice of coordinates and definitions.

On the other hand, I abhor the "I don't care what the numbers say, the Artist must rise above mere technology!" argument used whenever the measurements disagree with a cherished position. (See the laughable "6 stops better DR" thread from Lu-La). But I digress.

Has the OP's issue been resolved? It certainly seemed like a flash sync problem and not an issue of manufacturers mislabeling ISOs or faulty light meters.
 
J

jeffacme

Guest
"Absolute Values" interesting but irrelevant to the process I am advocating.

Having a thorough understanding of your system and how it behaves creates a device independent atmosphere that accounts for the variation described and puts the user in total control and aware of changes that affect the final result.
 

Stefan Steib

Active member
Matt @ all

quite easy to find out. I´m sure Paul has "ANY" 35mm digital body that he can compare with what he gets from the back. If this shows normal exposure at the same settings and the same equipment...........

Regards
Stefan
 

Shashin

Well-known member
The digital world has not changed the fact that a handheld meter is best when working systemically with your system. Shutter inefficiency, shutter error, lens transmission, actual ISO of your back, etc. will impact what your meter says and what the correct exposure is, although the shutter will have little affect in Paul's case.
 

johnnygoesdigital

New member
All these charts are impressive, but I agree with Jeffacme, about having a through understanding of one's camera system. The fact that Paul states it's only with strobes, seems to indicate, that use of the inverse square law might be appropriate in this scenario. Or in other words...move the lights. Keep us posted, and perhaps take more photographs of the model to be sure:)
 

jlm

Workshop Member
re the histogram. this is really sort of an incident meter that happens to show where the highs and lows fall. Unlike a reflected meter reading, the histo does not relate the readings literally to specific parts of the scene. You can guess, but that is not the same thing. I find it useful in the same way my incident meter is useful
 

SergeiR

New member
It was never meant as a criticism of the method used, nor is it my intention to force into anything, but it is rather an open discussion between photographers in which one tries to give and exchange information, the way this forum works and should be for.

My apology if you felt so, Sergei.

Best regards
Thierry
(odd, my reply didn't post earlier today.. go figure).

Not at all, Thierry - i am not taking it as any sort of attack or uber-criticism. I have strong belief that only way to learn is to ask questions (albeit - right ones.. as my school math teacher used to say "right question - half of the answer") and to discuss things. And i like learning , and there is a lot to learn and experiment with in photography, specially now, when digital gives us ability to see results faster (he said, after spending few hours today, twiddling with 4x5 negatives :)).

Despite doing workshops meself, I am constantly attending seminars on photography (ok, not constantly, but whenever i got cash to burn on chance of learning new tidbits), and i wouldn't mind picking lock or two on histograms :)
 

SergeiR

New member
re the histogram. this is really sort of an incident meter that happens to show where the highs and lows fall. Unlike a reflected meter reading, the histo does not relate the readings literally to specific parts of the scene. You can guess, but that is not the same thing. I find it useful in the same way my incident meter is useful
Actually on modern digital backs allow you to get histograms of particular scene bits. Nearly down to pixel, which is what we were talking about above. But it get some time.. And not available to everyone.

Oh and as for DxO - i wouldn't trust them too much. They have their own way of doing stuff, and they do readings on jpegs.. and we all know how it all works with jpeg "SOOC"..

But i can tell you from experience with calibrating ZD back and Leaf back and D700, and Olympus E-3, and E-P2, that they do have different graphs of response for "standard" ISO steps (25-50-100-200-400). I.e they are indeed a bit off , but then so was film in old days. Cheer up, meter/memorize compensations and shoot :) Technical stuff is too boring anyway :)
 
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