[lavascript]

I'm kinda struggling with how to word my question/assumption... (As well as what forum to post it in...)

The number of pixels in an image is inversely proportional to the focal length, right?

In other words, I take two shots. One is shot at 2MP at 100mm. The other is shot at 8MP at 50mm, but a 2MP image is cropped out of the center. I would end up with basically the same image as the first, correct?

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I'm trying to do some math to help me with a lens upgrade decision. I'm considering swapping my Sigma 17-70 for a Tamron 17-50. On my 10MP K10D, I'm thinking that if I really needed that 70mm, I could still get a 7.2MP image at "70mm." Is that sound logic? (10.2*(50/70))

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Last edited by lavascript; 01-04-2011 at 01:53 PM.

[eddie1960]

I'm trying to do some math to help me with a lens upgrade decision. I'm considering swapping my Sigma 17-70 for a Tamron 17-50. On my 10MP K10D, I'm thinking that if I really needed that 70mm, I could still get a 7.2MP image at "70mm." Is that sound logic? (10.2*(50/70))

there is some logic there aside from DOF differences that will be present. Essentially though you can't obtain all the qualities of the 70mm focal length through cropping though (shallowness of the DOF i.e.). You could however always add a DA 70 down the road

[lavascript]

there is some logic there aside from DOF differences that will be present. Essentially though you can't obtain all the qualities of the 70mm focal length through cropping though (shallowness of the DOF i.e.). You could however always add a DA 70 down the road

Yes, yes, I know, "FOV equivalency" and whatnot. That's why I put "70mm" in quotes, meaning that it wasn't actually a true 70mm. But focusing strictly on my FOV/resolution logic, is it sensible?

[eddie1960]

I haven't sat down and done the math, but yes you should be able to crop down to a 70 mm equivalency and maintain your resolution in the 7+MP range
as long as you don't mind losing the ability to print at larger sizes. I crop occasionally and still have printed up to 13x19 though with little effect if I'm viewing the print from a normal distance for a larger print

[Lowell Goudge]

While I understand the logic, you are looking at smaller lenses and shorter lenses to get the same image resolution by cropping in, but. there is something you need to consider.

in your example you are discussing cropping out such that a 100mm shot is only a 2MP print , cropped out of an 8MP print from a 50mm lens,, then the pixel count is inversly proportional to the square of the focal lenght since pixel count is relitive to image area.

having said that. there is a limit with what you can achieve by cropping tighter.

The biggest issue is that cropping tighter in a smaller (dimensionally) image on the sensor, means that you are enlarging the final print more. this means that object blurr from camera shake (and even with shake reduction there are limits) will be amplified by the enlargement, As a result the images simply won't be as sharp.

I think it wouold be preferrable to just keep the same lenses and accept higher slightly pixel count and slightly more detail

[lavascript]

While I understand the logic, you are looking at smaller lenses and shorter lenses to get the same image resolution by cropping in, but. there is something you need to consider.

in your example you are discussing cropping out such that a 100mm shot is only a 2MP print , cropped out of an 8MP print from a 50mm lens,, then the pixel count is inversly proportional to the square of the focal lenght since pixel count is relitive to image area.

having said that. there is a limit with what you can achieve by cropping tighter.

The biggest issue is that cropping tighter in a smaller (dimensionally) image on the sensor, means that you are enlarging the final print more. this means that object blurr from camera shake (and even with shake reduction there are limits) will be amplified by the enlargement, As a result the images simply won't be as sharp.

I think it wouold be preferrable to just keep the same lenses and accept higher slightly pixel count and slightly more detail

Well, obviously keeping both lenses would be preferable. Wouldn't that apply in pretty much every situation? I'll start a new thread in the lens forum about my potential lens swap.

My question here was about my math and logic. And from what I can gather from the replies, it seems to be correct.

Thanks.

[Lowell Goudge]

Well, obviously keeping both lenses would be preferable. Wouldn't that apply in pretty much every situation? I'll start a new thread in the lens forum about my potential lens swap.

My question here was about my math and logic. And from what I can gather from the replies, it seems to be correct.

Thanks.

While the math is correct, the picture quality will suffer by cropping down to the same pixel count, for the reasons I have stated, you will be enlarging more to get back to the same finished image size, and the enlargement will increase issues of camera shake, depth of field and focus error. All these will need to be considered as well before you can successfully produce the same quality of image

[Class A]

Your first example is correct but your second isn't. Only ~5.2MP remain when you crop a 50mm shot to a 70mm shot with a 10.2MP sensor (10.2*(50/70)^2).

The increase in blur is the same as when shooting at 70mm instead of 50mm. The DOF of the cropped 50mm shot will be the same as the DOF of the 70mm shot. Cropping and zooming are equivalent (modulo resolution).