# Are you really getting all the benefits of wide apertures.....?



## GuyF (Jun 5, 2012)

There's an interesting article in the May issue of the British Journal of Photography that compares the low-light capabilities of the 5D mk3 and Nikon D4. Both cameras use their own 24mm f1.4 lenses in the article. As I've just bought a mk3 and Sigma 85mm f1.4 I thought the article would be pretty informative but was quite surprised to read:-

"Fast lenses are only part of the solution, as the microlens structure of sensors has the effect of limiting the maximum effective f stop - you get the reduced depth of field, but not the true lens speed. To compensate for this, camera firmware detects apertures faster than f2.8 and applies additional gain. You must add the gain mapping applied at all apertures to compensate for vignetting."

It goes on to say that shooting at f1.4 and ISO 6400 will probably give you ISO 25600 in the corners. Yikes!

Well, you learn something new every day.

I guess those shooting Leica with a Noctilux f0.95 must feel robbed!


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## Mt Spokane Photography (Jun 5, 2012)

GuyF said:


> There's an interesting article in the May issue of the British Journal of Photography that compares the low-light capabilities of the 5D mk3 and Nikon D4. Both cameras use their own 24mm f1.4 lenses in the article. As I've just bought a mk3 and Sigma 85mm f1.4 I thought the article would be pretty informative but was quite surprised to read:-
> 
> "Fast lenses are only part of the solution, as the microlens structure of sensors has the effect of limiting the maximum effective f stop - you get the reduced depth of field, but not the true lens speed. To compensate for this, camera firmware detects apertures faster than f2.8 and applies additional gain. You must add the gain mapping applied at all apertures to compensate for vignetting."
> 
> ...


 

Here is another article on the effect.

http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml


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## Forceflow (Jun 5, 2012)

That really sucks... that certainly is something I did not want to learn


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## neuroanatomist (Jun 6, 2012)

There are two issues at play here.



GuyF said:


> You must add the gain mapping applied at all apertures to compensate for vignetting. ... It goes on to say that shooting at f1.4 and ISO 6400 will probably give you ISO 25600 in the corners. Yikes!



This sounds like it's referring to peripheral illumination correction, which is applied to jpgs if the setting is enabled, and by RAW converters if desired. Or, you can live with the vignetting (which with the 24/1.4L II is actually closer to four stops at the edges, i.e. ISO 6400 gets pushed to the equivalent of ISO 102400 in the corners!!). Vignetting affects most lenses to some degree, is apparent on both digital and film, fast lenses are generally worse (although some f/5.6 zooms are quite bad, too).



Mt Spokane Photography said:


> Here is another article on the effect.
> 
> http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml



This one, referencing the data from DxOMark, is a different issue. This is specific to fast lenses, generally faster than f/2. The clandestine exposure boost applies to the RAW image data, and is usually not more than 1/2 stop. The effect is worse with smaller pixel sizes.


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