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09-09-2009, 07:54 AM   #16
DAZ
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QuoteOriginally posted by Firebird Quote
I did some research after reading DAZ's post.

If my understanding is correct, a specific f-number allows the same amount of light regardless of lense with the subject located at infinity.

However if the subject is not located at infinity, there is a working f-number. Which may explain my variations in low light tests as the subject was less than 5 feet away.
The thing to remember is the f/stop number is a ratio and with an optically/mechanically perfect lens it would be the same as a T/stop. The f/stop is usually the more important number as it will determine things like DOF and on most modern lenses is close to the T/stop.

Many things can cause the f/stop to differ (or appear to differ) from the T/stop. Prime lenses will usually be closer then zooms as they are usually simpler both optically and mechanically. Zooms are also more likely to have a fudge factor of just getting close because of they are more complicated mechanically. Because the lenses are moving the blades mechanically to an f/stop mechanical slop makes a difference. That means an f/8 on one lens may not be exactly an f/8 on some other lens. In fact an f/8 on a given lens may not be the same T/stop every time. These kind of variations can be relevant to people doing things like video that are changing lenses and need a seen to stay the same level but for most using TLL metering small changes are not all that important as these small changes are either not noticeable or can be corrected PP.

The biggest different between f/stop and T/stop I see is not a real difference but an apparent one do to metering. I am not saying that the meter is not accurate (it is probably more accurate then most people need) but how and what is metered. Unless you are metering a plain white wall using a constant light and center weight averaging it is almost impossible meter the same every time. These small errors can swamp the difference that you are looking for and there are other errors in the camera system as big or bigger.

For those with a special need for this kind of information this may be critical but for most it is just an interesting academic discussion. This is why you donít see this information in reviews any more.

DAZ

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