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

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

First: I hate this WYSIWYG editor - ruined one attempt totally. The 149 post limit (or 150?) is silly too. Come on DPR...

If your logic doesn't survive a platonic thought experiment, what sort of logic is it?

Your thought experiment was not only faulty, but also not relevant for the topic. The logic used was not really much more complex one 1+1=2.

What this does is that it perfectly downsizes the image drawn by the lens from 43mm diagonal to 21.6mm, thus you can duplicate the 43mm siized image diagonal perfectly as well.

You said earlier:

the more you need to enlarge the image, the better quality the image lens draws needs to be for one to get the result one wishes

What we have seen here is that it's not really the enlargement that is the issue.

It is precicely that, the enlargement factor.

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.

Now, let's talk about two different lenses - one designed to project an FF image circle, the other a smaller m43 image circle. Earlier, you said:

Regardless, there is a limit how good a lens can be, thus a larger format in principle has a higher ceiling for image quality in this regard as well.

You have not explained here what physical principle connects the first half of your sentence with the second half, especially taking into account two lenses designed to project different sized image circles?

All physical lenses have aberrations, do they not? The more these aberrations are enlarged, the lower the resulting image quality will be, thus there is need for better quality lenses when the enlargement is large. Also this is usually the case. I am quite certain the average m43 lens draws more linepairs per millimater on image plane than the average full frame lens because it needs to do that to be competetive.

How good a lens for different formats can be? There are many variables involved, some beneficial to small, some to the big - smaller elemets are easier to make close to perfect, on the other hand tolerances for larger formats are much larger, not just for the optical elements, but also for their mechanical positioning. Can I give you precise and simple answer how much smaller image circle influences which points in lens design and manufacturing, no I can not. But it is not just a reasonable statement due to the significant differences in enlargements, manufacturing tolerances and so on. I think there was a topic on this issue in the Techology/Science forum recently with much more qualified people than me involved.

Additionally there is the issue of diffraction. There is an absolute limit for image quality when it comes to resolution and the larger the format, the higher the ceiling is.

Without the perfect idealized speedbooster you can not enlarge from 21.6mm diagonal to 43mmm diagonal without the imperfections of the lens becoming more visible.

While this is true, the thought experiment is a simple illustration of the fact that we must not forget to take into account the differences in image circle size each lens is designed to project.

You're expanding the issue. All I claimed is that you need to have a better lens in a smaller format to get the same image quality. I see no real point in going to the direction you're driving as it is beside the point. I think that would be overly complex and fruitless.

A question: What is the mathematical relationship between sensor size and how much more visible the imperfections of a lens become?

If we use the very same lens, then exactly by the the same factor the crop factor is. Seriously, what else could it be? a(crop)=a(native)*cropfactor.

There must be a mathematical relationship, after all, if enlargement is the actual cause of the differences in lens performance w/r/t different formats that we're discussing.

Indeed, and it was presented above. All the enlargement does is that it enlarges everything, both details and aberrations. There really is nothing more to it.

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