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08-27-2010, 12:43 PM   #1
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possible for 2 lenses to be lighter/darker at the same focal length and aperture?

I'm trying to decide whether I should keep my SMC A 28mm 2.8.

Compared to the kit lens at 28mm and f4, the kit is actually more saturated, equally sharp, and more contrasty.

However, the prime gives me a 1 stop advantage at 28mm, and I find that even at f4 (like the kit), more light gets to the lens.

How could this be? In your opinion, is it worth having both?

08-27-2010, 12:48 PM   #2
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If the funds that would be made available by selling the A would be better allocated elsewhere, and you dont feel the need to have/use a prime 28mm, then sell it.

Otherwise I don't see a reason not to just keep it.
08-27-2010, 12:55 PM   #3
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Is it possible that equal focal lengths and and apertures could result in a lighter or darker pictures depending on the lens?
08-27-2010, 01:54 PM   #4
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Sure. Google the term "T-stop". Also realize that in a zoom lens, the stated maximum aperture is just an approximation. It doesn't really suddenly change from exactly f/3.5 to exactly f/4 at one focal length and stay exactly at f/4 until another focal length and then suddenly change to exactly f/4.5 and stay there until the point where it suddenly changes to exactly f/5.6 Instead, it continually changes from f/3.5 to f/5.6. The camera just has no way of reporting the aperture to you in anything but discrete steps like that.

08-27-2010, 02:02 PM   #5
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QuoteOriginally posted by paperbag846 Quote
Is it possible that equal focal lengths and and apertures could result in a lighter or darker pictures depending on the lens?
Yes.

The size of the physical aperture is only part of the equation that determines how much light goes to the sensor/film. The other factors are:
  • Reflection from the front and internal lens surfaces. This is reduced by better lens coatings
  • Light absorption from the glass itself. This is a property of the glass used and the thickness of the elements.
That is why you might sometimes read about exposure in terms of T-stops rather than f-stops. Read the Wikipedia article paragraph for a full description:
F-number - Wikipedia, the free encyclopedia
As you might imagine, accurate exposure should probably take transmittance into account, hence the usefulness of through the lens metering.


Steve
08-27-2010, 02:55 PM   #6
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QuoteOriginally posted by stevebrot Quote
Yes.

The size of the physical aperture is only part of the equation that determines how much light goes to the sensor/film. The other factors are:
  • Reflection from the front and internal lens surfaces. This is reduced by better lens coatings
  • Light absorption from the glass itself. This is a property of the glass used and the thickness of the elements.
That is why you might sometimes read about exposure in terms of T-stops rather than f-stops. Read the Wikipedia article paragraph for a full description:
F-number - Wikipedia, the free encyclopedia
As you might imagine, accurate exposure should probably take transmittance into account, hence the usefulness of through the lens metering.


Steve
Ah ha! And by extension, a prime less would have the advantage of less elements (not all the time of course), leading to primes having in advantage in light transmittance! Thanks so much... I was rather baffled.
08-27-2010, 03:28 PM - 1 Like   #7
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A simple test: Tripod the camera, set in M(anual) mode. Aim at a fixed target. With the kit lens at 28mm and f/8, take an exposure reading (hit the Green button). Shoot. Now replace the kit lens with your A28/2.8 (which is worth what, around US$50?) set to f/8. Shoot. Now you have two exposures of the same target with two lenses with the same shutter and aperture. Are they of equal brightness? Now, without moving the camera/tripod, and with the A28 still at f/8, hit the Green button again. Does it give the same exposure reading? Try the exposure readings comparison again at f/11, f/16, and f/22. Do they differ between lenses?

QuoteOriginally posted by paperbag846 Quote
Ah ha! And by extension, a prime less would have the advantage of less elements (not all the time of course), leading to primes having in advantage in light transmittance! Thanks so much... I was rather baffled.
I'm not sure if this is a factor, but consider that the A28 is an FF lens projecting an image circle that fully covers an FF frame (43mm diagonal), while the kit lens is designed to fully cover an HF/APS-C frame (29mm diagonal) -- and it definitely vignettes on FF below 25mm. IOW, that A28 (or any FF lens) may just pump more light to the viewfinder than an HF/APS-C lens of the same focal length would. Alas, I have no HF primes with which to test this. Any volunteers?
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