crop factor with SLr lenses?

Brian Standing · January 19th, 2008, 10:03 AM

Thanks, Mel for that very detailed and helpful explanation.

Here's what's still not clear to me. Sony announces that if you use a lens designed for a DSLR-Alpha on the Z7, you get a "crop-factor" (which I believe is a more accurate term than "multiplier") of 7x. We know that the DSLR-Alpha series uses a sensor that is smaller than 35mm. So, it seems that if (assuming an appropriate adapter exists) you attach an SLR lens designed for 35mm still film, you will get a "crop-factor" greater than 7x.

Or is there something I'm still missing?

Jack Markham · January 19th, 2008, 10:55 AM

Well, the size of the film or digital sensor does not matter. A fifty millimeter lens is a fifty millimeter lens. The lens is, in fact, is designed with an image circle to cover a specific size sensor. But all this means is that only the center, the best part, of the image circle of a 35mm lens is used with a 1/3 inch sensor.

It is the ratio of the angle that the diagonal measurement of the sensor subtends compared to the larger angle subtended by the larger sensor that is referred to as "crop factor" when a lens that is designed for a larger sensor is used on a smaller sensor.

To say this in another way, if you were to scribe box the size of a 1/3 inch sensor in the center of a 35mm slide, that would be the part of the image the 1/3 inch sensor would see with the 35mm lens.

Brian Standing · January 19th, 2008, 03:00 PM

Yes. I understand that you are simply cropping out the center of the image, not changing the magnification of the lens.

All of which comes back to my original point. Setting aside the mechanics of how it works, the question is this:

According to the Sony literature, if you put a 50mm DSLR-Alpha lens on a Z7, you get the equivalent field of view that you would get if you put a 350mm DSLR-alpha lens on a DSLR-alpha camera.

So, what field of view do you get if you put a 50mm 35mm SLR lens on a Z7?

1. DSLR-a lens designed for APS-C sensor on a 1/3" CMOS sensor = 7x crop factor.

2. Film SLR lens designed for a 35mm film frame on a 1/3" CMOS sensor = ? (I'm thinking it should be around 10x. Am I right?)

January 19th, 2008, 10:03 AM	#16
Brian Standing Major Player Join Date: Dec 2003 Location: Madison, Wisconsin Posts: 621	Thanks, Mel for that very detailed and helpful explanation. Here's what's still not clear to me. Sony announces that if you use a lens designed for a DSLR-Alpha on the Z7, you get a "crop-factor" (which I believe is a more accurate term than "multiplier") of 7x. We know that the DSLR-Alpha series uses a sensor that is smaller than 35mm. So, it seems that if (assuming an appropriate adapter exists) you attach an SLR lens designed for 35mm still film, you will get a "crop-factor" greater than 7x. Or is there something I'm still missing? __________________ http://www.prolefeedstudios.com/blog/ Documentary for the masses!

January 19th, 2008, 10:55 AM	#17
Jack Markham New Boot Join Date: Oct 2004 Posts: 19	a focal length is a focal length Well, the size of the film or digital sensor does not matter. A fifty millimeter lens is a fifty millimeter lens. The lens is, in fact, is designed with an image circle to cover a specific size sensor. But all this means is that only the center, the best part, of the image circle of a 35mm lens is used with a 1/3 inch sensor. It is the ratio of the angle that the diagonal measurement of the sensor subtends compared to the larger angle subtended by the larger sensor that is referred to as "crop factor" when a lens that is designed for a larger sensor is used on a smaller sensor. To say this in another way, if you were to scribe box the size of a 1/3 inch sensor in the center of a 35mm slide, that would be the part of the image the 1/3 inch sensor would see with the 35mm lens.

January 19th, 2008, 03:00 PM	#18
Brian Standing Major Player Join Date: Dec 2003 Location: Madison, Wisconsin Posts: 621	Yes. I understand that you are simply cropping out the center of the image, not changing the magnification of the lens. All of which comes back to my original point. Setting aside the mechanics of how it works, the question is this: According to the Sony literature, if you put a 50mm DSLR-Alpha lens on a Z7, you get the equivalent field of view that you would get if you put a 350mm DSLR-alpha lens on a DSLR-alpha camera. So, what field of view do you get if you put a 50mm 35mm SLR lens on a Z7? 1. DSLR-a lens designed for APS-C sensor on a 1/3" CMOS sensor = 7x crop factor. 2. Film SLR lens designed for a 35mm film frame on a 1/3" CMOS sensor = ? (I'm thinking it should be around 10x. Am I right?) __________________ http://www.prolefeedstudios.com/blog/ Documentary for the masses!