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05-27-2010, 11:12 AM   #1
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Shutter Speed and Aperture Comparison

Hello all. I am new to this forum and new to dslrís . I would like to thank all of you for the information I have gleaned to date. It has played a big part in my decision to by Pentax and my enjoyment in learning to use the k7. My first lens purchase (other than the kit lenses that came with the camera) was the Asahi 55/1.8. I absolutely love using this lens! I am now interested in a 55 or 50 f1.2 but I am unsure as to the gain in shutter speed /low light capability between these and my current 55/1.8. Assuming ISO 400 and all other variables the same except that both lenses would be wide open at 1.8 and 1.2 respectively, what would be the difference in shutter speed? Also, what would be the comparative shutter speed if one was a 55/1.8 and the second was a 50/1.2? Is there a formula or chart that would show this info.?
In viewing back ground blur for 1.2ís through 1.8ís, I find them all appealing. My only real reason for wanting a 1.2 would be a noticeable jump in low light capability and faster shutter speeds at a given ISO to avoid blur and noise.
I have done some searching on this forum and the web, but canít seem to find a clear answer. I suspect that I am so new to this that I do not know how to phrase my search. Any help would be greatly appreciated.

05-27-2010, 11:25 AM   #2
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The 1.2 is a full stop faster than the 1.8 lens, so you will gain a stop of shutter speed or a stop of ISO, or a combination of the two.
For example, (and presuming both lenses are shot at maximum aperture), if the f/1.8 lens is giving you 1/60th, the f/1.2 will give you 1/125th, or the f/1.8 might require ISO 800, while the f/1.2 will allow ISO 400.
05-27-2010, 11:26 AM   #3
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That's more or less one stop, so a 2X difference. Focal length doens't matter - it's about aperture. But DOF gets correspondingly shallower too. Really, I doubt most people who have f/1.2 lenses shoot at f/1.2 mainly to get faster shutter speeds very often - it's more abou the artistic effect of the shallow DOF.

I'd suggest a basic book on photography that covers the fundamentals of exposure, like Bryan Peterson's Understanding Exposure, or whatever your local library or bookstore might have handy, to get a grasp of these issues.

Last edited by Marc Sabatella; 05-27-2010 at 05:27 PM.
05-27-2010, 11:29 AM   #4
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The difference between 1.2 and 1.8 is a little more than a full f-stop, so your shutter speed would be twice as fast.

Full f-stops are 1.0 1.4 2 2.8 4 5.6 8 11 16 22
So it's a logarithmic scale. The size of the opening doubles every time you go to a lower number, therefore the shutter speed gets cut in half.

Make sense? Sorta?

05-27-2010, 11:51 AM   #5

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Hi indy,

I'll agree with the points that both WF and Mark have already made. As a beginner, you should also be made aware of DOF concerns from a practical standpoint also. The DOF with an f 1.2 normal (short tele with the crop factor) lens is razor thin, and focusing one effectively is very difficult. When fractions of an inch in focus accuracy become a factor, then it must become a concern in weighing against the benefits of added light gathering capacity. Focusing in low light can be very difficult, and the added DOF of a slower aperture is a useful hedge against minor focusing mistakes that a very fast aperture lens will magnify. Add in the extra prices demanded with ultra fast lenses, and the slower alternatives don't look nearly as bad in comparison.

Very fussy shooters who want the DOF control that an f1.2 lens allows are willing to put up with the difficulty of shooting a lens like this, but the great majority of photographers, although they'd probably like the bragging rights of owning one, would probably find a slower alternative much more practical.

05-27-2010, 12:06 PM   #6
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Thank you all! I appreciate the quick responses in and insight.
05-28-2010, 06:25 AM   #7
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Bryan Peterson's "Understanding Exposure" is a good book that covers some of the basic concepts of photography, including how aperture and shutter speed relate to each other.

And it's much cheaper than a 50mm f/1.2

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