It's Bigma Time !!!

[Adrian.Lines]

I always struggle with stops. Am I right in thinking one stop down is +50% the current F stop, and two stops down halves the light hitting the sensor, so to achieve same exposure you need to double the exposure time.

So when I hear people saying that a lenses performance is at its best two stops down, an F/2.8 would be best at F/5.6....but you say this is not always true. Is that to do with the difference between the sensor size and a true 35mm.

I told you I struggled !!

[paulthomas]

Originally Posted by Adrian.Lines

I always struggle with stops. Am I right in thinking one stop down is +50% the current F stop, and two stops down halves the light hitting the sensor, so to achieve same exposure you need to double the exposure time.

Not quite. A stop is a jump of one Exposure Value which either halving or doubling the amount of light. The f number of a lens is a ratio of the lens's focal length and the diameter of the appature. Larger f numbers mean a smaller appature as you know and thus less light getting in. But the amount light getting in is proportional to the area of the appature which in turn makes it proportional to the square root of the diameter of the appature. So to increase one stop you need to double the area of the appature which means multiplying the diameter by the square root of 2 (approx 1.4) which will in turn cause the f number to be decreased by the same ratio.

Originally Posted by Adrian.Lines

So when I hear people saying that a lenses performance is at its best two stops down, an F/2.8 would be best at F/5.6....but you say this is not always true. Is that to do with the difference between the sensor size and a true 35mm.

An awful lot of it has to do with the quality of the lens itself.

Originally Posted by Adrian.Lines

I told you I struggled !!

I only hope I haven't confused you further

[ron1863]

Originally Posted by glsammy

Everyone to their own, but I can get excellent images at way lower than 1/400sec with the monopod.
Have a look at my Gallery images, not one was taken using a tripod and many hand held here's an example:


This was taken hand held at 1/100sec, through double glazed windows, with the Bigma.

I'm impressed...........what do you use to clean your windows?

[eeyore]

Originally Posted by ron1863

I'm impressed...........what do you use to clean your windows?

photoshop

[glsammy]

Originally Posted by eeyore

photoshop

Yep, that's just about the only cleaning solution used round here!