1.6 crop factor and f stop question

gtc · October 6, 2005

i am still waiting on my m42 screwmount to eos adapter so that i can mount my super- multi -coated takumar f4/200 on my canon 20d.i have also ordered a smc 300mm so that i can go birding.

in the meantime a question-with the 1.6 crop factor my sensor will be looking at an image circle within the centre of the lens and crop out the edges,thus removing any problems with edge softness etc.

i am probably off track but have been ruminating on this one for awhile.

as f is calculated on the light passing through the whole lens,including the edges which are a little 'thicker', would i be right in assuming that a cropped image circle would be brighter/faster than if using a full frame sensor or film camera?

if so how would i determine the level of improvement to the glass speed?

if not,why not?

looking forward to having this question settled.

greg

Shay Stephens · October 6, 2005

The answer is a simple no. f/4 is f/4 is f/4.

Why you ask? The answer to that lies in how f/numbers are calculated in the first place.

f/4 is a ratio of the diameter of the aperture in the lens and the focal length of the lens.

It has nothing to do with how the image circle is used to record the final image.

f/4 simply means that the aperture of the lens is 1/4 of the lens focal length (if we were using f/8 that would 1/8). So if you have a 200mm lens, the aperture opening is going to be 50mm which is going to let in a known amount of light and create an image circle with a standard brightness. How the camera cuts into that circle to record the image has no bearing on the resulting brightness of the image circle.

So fire away, content with the knowledge that you don't have to deal with any more math conversions than is necessary :-)

More info on f/stops:
http://en.wikipedia.org/wiki/F-number

gtc · October 6, 2005

so..?
thanks shay,

so the image circle brightness is the same level of brightness across the image circle -eg the edges are transmitting just as much light/brightness as is in the centre of the lens?

if so, this clears things up for me and i can think about something else in idle moments...

thanks

greg

Shay Stephens wrote:

The answer is a simple no. f/4 is f/4 is f/4.

Why you ask? The answer to that lies in how f/numbers are calculated in the first place.

f/4 is a ratio of the diameter of the aperture in the lens and the focal length of the lens.

It has nothing to do with how the image circle is used to record the final image.

f/4 simply means that the aperture of the lens is 1/4 of the lens focal length (if we were using f/8 that would 1/8). So if you have a 200mm lens, the aperture opening is going to be 50mm which is going to let in a known amount of light and create an image circle with a standard brightness. How the camera cuts into that circle to record the image has no bearing on the resulting brightness of the image circle.

So fire away, content with the knowledge that you don't have to deal with any more math conversions than is necessary :-)

More info on f/stops:
http://en.wikipedia.org/wiki/F-number

Shay Stephens · October 6, 2005

gtc wrote:

thanks shay,

so the image circle brightness is the same level of brightness across the image circle -eg the edges are transmitting just as much light/brightness as is in the centre of the lens?

Nope. the light intensity typically drops towards the edges depending on the lens. Wide angle tends to vignette towards the edges more than telephoto.

gtc · October 7, 2005

doesent that mean..

Shay Stephens wrote:

Nope. the light intensity typically drops towards the edges depending on the lens. Wide angle tends to vignette towards the edges more than telephoto.

thanks shay,

sorry to be a pain but you either seem to be agreeing with me or i have missed the point

-so therefore the light measured at full aperture across the whole lens/image circle for a full frame sensor/35mm film would be less bright than the light measured across the reduced image circle of a 1.6 crop sensor?

doesen't this mean that the cropped image circle will be brighter than the larger image circle due to the removal of the duller edges by cropping,and therefore resulting in greater brightness and therefore a faster lens?

greg

DavidTO · October 7, 2005

Dang Shay. You are a veritable fount of information. And most of it's good!

Thanks for explaining that one. I never knew how simple the concept was.

Shay Stephens wrote:

The answer is a simple no. f/4 is f/4 is f/4.

Why you ask? The answer to that lies in how f/numbers are calculated in the first place.

f/4 is a ratio of the diameter of the aperture in the lens and the focal length of the lens.

It has nothing to do with how the image circle is used to record the final image.

f/4 simply means that the aperture of the lens is 1/4 of the lens focal length (if we were using f/8 that would 1/8). So if you have a 200mm lens, the aperture opening is going to be 50mm which is going to let in a known amount of light and create an image circle with a standard brightness. How the camera cuts into that circle to record the image has no bearing on the resulting brightness of the image circle.

So fire away, content with the knowledge that you don't have to deal with any more math conversions than is necessary :-)

More info on f/stops:
http://en.wikipedia.org/wiki/F-number

Shay Stephens · October 7, 2005

Perhaps I don't understand the point you are getting at
If you have a focal length of 17mm, the edges would be brighter with a 1.6x crop, however, if you zoomed the lens on a full frame camera to 26mm you would have the same brightness at the edges since the image circle would now cover the same area as the 1.6x crop camera.

But the overall image would be just as bright on both cameras, and the exposure needed to properly render the scene would be the same with both cameras regardless of the vignetting at the edges.

Vignetting is an optical defect (for good or ill) and if you base your exposure on having properly exposed corners, you will overexpose the overall frame. A cropped frame does not get any brighter, it just hides more of the vignetting at the edges.

Remember, f/4 is f/4 is f/4 ;-)

gtc wrote:

thanks shay,

sorry to be a pain but you either seem to be agreeing with me or i have missed the point

-so therefore the light measured at full aperture across the whole lens/image circle for a full frame sensor/35mm film would be less bright than the light measured across the reduced image circle of a 1.6 crop sensor?

doesen't this mean that the cropped image circle will be brighter than the larger image circle due to the removal of the duller edges by cropping,and therefore resulting in greater brightness and therefore a faster lens?

greg

1.6 crop factor and f stop question

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