Crop Factor, Low Light and Aperture with m4/3 lenses? Part 2

Started Jun 16, 2014 | Discussions thread
OP Truthiness Regular Member • Posts: 122
Re: Crop Factor, Low Light and Aperture with m4/3 lenses? Part 2

knickerhawk wrote:

Truthiness wrote:

Although we don't have any perfect physical speedboosters, we have some pretty good ones, and I think it's clear that the difference in resolution between two 20cm X 30cm prints generated with the same FF lens on an FF camera and on an m43 camera (with a sensor having the same resolution as the FF camera) using a good speedbooster will be much smaller than the difference suggested by the 69 and 267 times required enlargement factors you mentioned, or do you disagree?

You're right for most cases of course - I never claimed anything else.

However the problem is that this thought experiment has no relevancy for this issue: when you use the booster you downsize the image from 43mm diagonal to 21.6mm, thus you use the original lens as intended, all of it's lens area including all of the original image circle. The situation is quite different when you only use the center part of the image circle and enlarge that.

So, in other words, all we need to do is design all m4/3 lenses with FF image circles and speedboosters built in and your "inherent" advantage for FF magnification disappears. Hmmmm...

I don't think you've thought this through at all.

Let's say L1 = lens we're using, SB = ideal, perfect speedbooster, and L2 = L1 + SB. Now, does L1 have the same optics as L2, ie. is L1=L2 true or false?

What you're doing is improving the optical configuration of L1 to suit better the smaller format by using the SB, using the whole optics of L1 and compress signal it provides with SB to the smaller format, ie. you're comparing lens L1 to lens L2, different lenses with different properties when you should be comparing the one same lens of L1 to itself on different formats.

Now, that is in no way contradicting what I said.

You should be considering the image you get by not using the whole of L1, but only the cropped image.

A more valid though experiment is this:

Take a lens - measure the lp/mm of the image on the sensor plane.

Then use a perfect 2x teleconverter to crop just the center of the image and then measure the lp/mm of the image on the sensor plane. In this experiement you enlarge everything from the center part of the image circle and the resulting image. Essentiall this is the same as if you were comparing different fromats on the same lens.

(This time the TC is not being used to improve the image quality by using all of L1 instead of part of it, but to crop the image, just like a smaller format would do, thus the L1+TC vs L1 is not relevant.)

Since magnification is the root of an inherent and unavoidable evil according to your original posting in the other thread, what do you make of the greater magnification required to project the larger image circle onto a FF sensor compared to the lesser magnification required to project an image circle on a m4/3 sensor?

It is hard to follow what you mean, but if you think that there is "more work" to turn 10 meter tall subject into 24mm height of FF than there is to turn it into 13mm of m43, then I am puzzled on why that would be the case.

If that is not what you meant, could you please rephrase what you mean as I'm struggling to follow your logic.

(Also, it is preferrable to use the work 'enlargement' instead of 'magnification' in this context.)

A related question:

If you take a picture with your camera, lets say of a bird sitting on a tree. If you take picture 1 (P1) with a 300mm lens and picture 2 (P2) with a 50mm lens and both P1 and P2 are perfect ideal lenses, and you want to print the bird, and just the bird, into the size of 1 square meter from each photo. The camera you use is a regular imperfect camera with finite number of pixels.

Which image (or crop of the bird) needs to be enlarged more? Which image has the better image quality, the one which needed to be enlarged more or the one which needed to be enlarged less?

If the 300mm lens where the bird is enlarged less provides better quality output image, why the same principle would not apply in the case we're discussing about?

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